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Greenhouse Effect

Greenhouse effect

The greenhouse effect is a natural phenomenon that is responsible for the relatively high temperature maintained on Earth's surface and in its atmosphere. The name comes from the process by which greenhouses are thought to collect and hold heat.

The greenhouse mechanism

A greenhouse is a building in which plants are grown and kept. It usually consists of a large expanse of window glass facing in a generally southerly direction. Sunlight that strikes the windows of the greenhouse passes through those windows and strikes the ground inside the greenhouse. This process is possible because glass is transparent to sunlight, that is, it allows sunlight to pass through.

Sunlight that strikes the ground inside a greenhouse either may be reflected or absorbed by the ground. Sunlight that is absorbed by the ground may later be re-emitted in the form of heat waves. When it bounces back towards the windows of the greenhouse, it is not able to pass back through the windows. In either instance, the sunlight undergoes a change in form once it enters the through the windows. The windows are not transparent, but are opaque, to the reflected and reradiated energy. The energy trapped inside the greenhouse is then used to raise the temperature inside the greenhouse. It is this effect that makes it possible for a greenhouse to stay warm even though the outside temperature is quite cold.

The greenhouse effect in Earth's atmosphere

An effect similar to the one just described also occurs in Earth's atmosphere. The atmosphere does not have a glass window, of course, although the gases that make up the atmosphere act something like a window.

Words to Know

Atmospheric window: A range in the wavelength of radiations, from about 350 to 750 nanometers, that can pass through Earth's atmosphere without being absorbed.

Equilibrium: A process in which the rates at which various changes take place balance each other, resulting in no overall change.

Frequency: The number of waves that pass a given point every second.

Infrared radiation: Another name for heat; a form of radiation with wavelengths in the range from about 700 nanometers to 1 millimeter.

Nanometer: One-billionth of a meter.

Radiation: Energy emitted in the form of waves or particles.

Visible light: A form of radiation with wavelengths in the range from about 400 to about 750 nanometers.

Wavelength: The distance between any two successive crests or troughs in a wave.

Imagine a burst of solar energy reaching the outer edges of Earth's atmosphere. That solar energy consists of many different kinds of radiation, such as visible light, ultraviolet radiation, infrared radiation, X rays, gamma rays, radio waves, and microwaves. These forms of radiation are different from each other in that they all travel with different wavelengths and different frequencies. (The wavelength of a wave is the distance between any two successive crests or troughs [pronounced trawfs] in the wave. The frequency of the radiation is the number of waves that pass a given point every second.) For example, X rays have very short wavelengths and very large frequencies. In contrast, radio waves have very long wavelengths and very small frequencies. The important point, however, is that all forms of radiation, whatever their wavelength and frequency, travel together in a burst of solar energy.

The energy that reaches Earth's atmosphere can experience one of three fates, depending on the kind of radiation and the kind of gases present in the atmosphere. First, about one-third of all the solar energy that reaches Earth's atmosphere is reflected back into space. As far as Earth is concerned, that energy is simply lost to space.

Another one-third of the solar energy is absorbed by gases in Earth's atmosphere. Various gases absorb various types of radiation. Oxygen and ozone, for example, tend to absorb radiation with short wavelengths, such as ultraviolet light. In contrast, carbon dioxide and water absorb radiation with longer wavelengths, such as infrared radiation. When these gases absorb various types of radiation, they convert the energy of sunlight into heat. This phenomenon accounts for some of the heat stored in Earth's atmosphere.

Yet another one-third of the solar energy reaching our atmosphere is able to pass through the atmosphere and strike Earth's surface. This process is similar to the way sunlight is transmitted through windows in a greenhouse. The solar energy that passes through Earth's atmosphere does so because there are very few gases that absorb visible radiation.

Scientists sometimes refer to a "window" in Earth's atmosphere (somewhat similar to a greenhouse window) through which radiation can pass. That atmospheric window is not an object, like a piece of glass, but a range in radiation across which atmospheric gases are transparent. That range is from about 350 to 750 nanometers (a nanometer is one-billionth of a meter). For comparison, the wavelengths of visible light range from about 400 nanometers (for blue light) to 750 nanometers (for red light).

Reflection and absorption. The solar energy that reaches Earth's surface also can experience a number of fates. It can cause ice to melt and water to evaporate; it can be used to convert carbon dioxide and water to carbohydrates in plants (photosynthesis); it can heat parcels of air and water, causing winds, waves, and currents; and it can heat Earth's surface. The last of these fates is the most important. About one-half of all the solar energy that passes through the atmosphere is absorbed by soil, rocks, sand, dirt, and other natural and human-made objects.

All of which is to tell you something you already know. If you put your hand on a patch of dark soil at the end of a sunny day, the soil feels warm. In fact, if you place your hand just above the soil, you can feel heat being given off by the soil. The reason is that objects that are heated by sunlight behave in the same way as the ground in a greenhouse. Those objects give back energy picked up from sunlight, but in a different form. Instead of reradiating the energy in the form of visible light, the objects give the energy back off in the form of heat.

But what happens when heat reradiated from Earth's surface travels back upwards into the atmosphere. Heat is a form of infrared radiation. As pointed out previously, carbon dioxide and water are both good absorbers of infrared radiation. So the very gases that allowed visible light to pass through the atmosphere are now able to absorb (trap) the infrared radiation (heat) reradiated from Earth's surface.

The sum total of all these reactions is that energy from the Sun is captured by both Earth's surface and the gases in the atmosphere. As a result, the planet's annual average temperature is about 55°F (30°C) higher than it would be without an atmosphere.

Human activities and the greenhouse effect

Natural phenomena, such as the greenhouse effect, reach a natural state of equilibrium over many hundreds or thousands of years. (A state of equilibrium is a process in which the rates at which various changes take place balance each other.) Two factors that affect the greenhouse effect are the shape of Earth's orbit and its tilt with regard to the Sun. Both of these factors change slowly over hundreds of thousands of years. When they do, they alter the effects produced by the greenhouse effect. For example, suppose that Earth's orbit changes so that our planet begins to move closer to the Sun. In that case, more solar energy will reach the outer atmosphere and, eventually, the planet's annual average temperature will probably increase.

In recent years, scientists have been exploring the possibility that various human activities also may influence the greenhouse effect. The most important of these activities is thought to be the combustion (burning) of fossil fuels, such as coal, oil, and natural gas.

No one needs to be reminded today of the important role of fossil fuels in our society. We use them for heating homes and offices; for powering our cars, trucks, railroads, airplanes, and other forms of transportation; and for operating industrial processes. But the combustion of any fossil fuel always results in the release of carbon dioxide and water into the atmosphere. By some estimates, the release of carbon dioxide into the atmosphere from fossil fuel combustion reached almost 2.5 billion tons (2.3 billion metric tons) in 1999.

The result of this human activity is that Earth's atmosphere contains a higher concentration of carbon dioxide today than it did a century ago. The most reliable scientific data show that the concentration of carbon dioxide in the atmosphere has increased from about 320 parts per million in 1960 to nearly 360 parts per million today.

Effects of increasing concentrations of carbon dioxide

Many scientists are concerned about the increasing levels of carbon dioxide in Earth's atmosphere. With more carbon dioxide in the atmosphere, they say, more heat will be trapped. Earth's annual average temperature will begin to rise. In early 2001, in a striking report released by the Intergovernmental Panel on Climate Change (a United Nationssponsored panel of hundreds of scientists), scientists concluded that if greenhouse emissions are not curtailed, the average global surface temperature could rise by nearly 11°F (6°C) over the next 100 years. The scientists also stated that man-made pollution has "contributed substantially" to global warming and that Earth is likely to get a lot hotter than previously predicted.

Such a rise in temperature could have disastrous effects on the world. One result might be the melting of Earth's ice caps at the North and South Poles, with a resulting increase in the volume of the ocean's water. Were that to happen, many of the world's largest citiesthose located along the edge of the oceansmight be flooded. Some experts predict dramatic changes in climate that could turn currently productive croplands into deserts, and deserts into productive agricultural regions. Half of all Alpine glaciers would disappear. Coral reefs would be destroyed, and vulnerable plant and animal species would be pushed to extinction.

In 1997, in Kyoto, Japan, representatives from more than 170 nations met to try to agree to decisive actions to reduce their emissions of

greenhouse gases. A treaty, called the Kyoto Protocol, was drafted at the meeting that proposed that 38 industrial nations cut their greenhouse-gas emissions by 2012 to 5.2 percent below levels in 1990 (the United States is the biggest producer of greenhouse gases, producing about 25 percent of the gases associated with global warming; Japan and Russia are the next biggest producers). More than 150 nations signed the treaty, but no industrialized country ratified it, and the treaty cannot take effect until a substantial number of industrial nations ratify it.

In November 2000, in The Hague, Netherlands, officials from around the world met to write the detailed rules for carrying out the Kyoto Protocol. Unfortunately, after less than two weeks, the talks collapsed in disarray with no deal reached to stop global warming. The main reason for the collapse was the argument between the United States and European countries over ways to clean up Earth's atmosphere. Officials attending the meeting did agree to meet once more to tackle the issue of global warming.

Differences of opinion. As with many environmental issues, experts tend to disagree about one or more aspects of anticipated climate change. Some authorities are not convinced that the addition of carbon dioxide to the atmosphere will have any significant long-term effects on Earth's average annual temperature. Others concede that Earth's temperature may increase, but that the changes predicted are unlikely to occur. They point out that other factorssuch as the formation of cloudsmight counteract the presence of additional carbon dioxide in the atmosphere. They warn that nations should not act too quickly to reduce the combustion of fossil fuels since that will cause serious economic problems in many parts of the world. They suggest, instead, that we wait for a while to see if greenhouse factors really are beginning to change.

The problem with that suggestion, of course, is that it is possible to wait too long. Suppose that fossil fuel combustion is causing significant changes in the climate. It might take a half century or more to be certain of the relationship between fossil fuel combustion and warmer planetary temperatures. But at that point, it also might be too late to resolve the problem very easily. The carbon dioxide would have been already released to the atmosphere, and climate changes would have already begun to occur. As evidenced by the collapse of climate talks and the failure to ratify the Kyoto Protocol, there is no general consensus among scientists, politicians, businesspeople, and the general public as to what, if anything should be done about the potential for climate change on our planet.

The battle between industry and environmentalists over the issue of global warming continues with no clear vision for the future. In March 2001, U.S. President George W. Bush all but put an end to the possibility that the United States would follow through with the Kyoto Protocol when he said his administration would not seek to curb the emissions of carbon dioxide from power plants. This was a sharp reversal from his position during the presidential campaign in 2000. Ignoring recently published scientific reports, Bush stated that he made his decision "given the incomplete state of scientific knowledge of the causes of, and solutions to, global climate change."

[See also Carbon cycle; Carbon dioxide; Forests; Ozone; Pollution ]

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greenhouse effect

greenhouse effect An effect occurring in the atmosphere because of the presence of certain gases (greenhouse gases) that absorb infrared radiation. Light and ultraviolet radiation from the sun are able to penetrate the atmosphere and warm the earth's surface. This energy is re-radiated as infrared radiation, which, because of its longer wavelength, is absorbed by such substances as carbon dioxide. The greenhouse effect is a natural phenomenon, without which the earth's climate would be much more hostile to life. However, emissions of carbon dioxide from human activities (e.g. farming, industry, and transport) have increased markedly in the last 150 years or so. The overall effect is that the average temperature of the earth and its atmosphere is increasing (so-called global warming). The effect is similar to that occurring in a greenhouse, where light and long-wavelength ultraviolet radiation can pass through the glass into the greenhouse but the infrared radiation is absorbed by the glass and part of it is re-radiated into the greenhouse.

The greenhouse effect is seen as a major environmental hazard. Average increases in temperature are likely to change weather patterns and agricultural output. It is already causing the polar ice caps to melt, with a corresponding rise in sea level. Carbon dioxide, from coal-fired power stations and car exhausts, is the main greenhouse gas. Other contributory pollutants are nitrogen oxides, ozone, methane, and chlorofluorocarbons. Many countries have now agreed targets to limit emissions of greenhouse gases, e.g. by switching to renewable energy sources. See also pollution.

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Greenhouse Effect

Greenhouse Effect


In the Earth's atmosphere, there are five important greenhouse gases that occur naturally: carbon dioxide, methane, ozone, halocarbons, and nitrous oxide. In correct proportion, these greenhouse gases provide important protection for the Earth's surface. However, if the greenhouse gases become too concentrated in the Earth's atmosphere, then they create a greenhouse effect that overheats the Earth. Although a few scientists continue to dissent, there is near unanimity among climatologists that current global warming is caused by the dramatic increase in atmospheric carbon dioxide since the advent of the Industrial Revolution and the extraordinary increase in the combustion of fossil fuels. In her essay, "The Greening of Science, Theology, and Ethics," Audrey Chapman has argued that ecological ethicists must understand the science behind concepts such as the greenhouse effect in order to contribute meaningful ethical analysis.


See also Ecology; Ecology, Ethics of; Ecology, Religious and Philosophical Aspects; Ecology, Science of


Bibliography

chapman, audrey r. "the greening of science, theology, and ethics." in science and theology: the new consonance, ed. ted peters. boulder, colo.: westview press, 1998.

richard o. randolph

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greenhouse effect

greenhouse effect Raised temperature at a planet's surface as a result of heat energy being trapped by gases in the atmosphere. As the Sun's rays pass through Earth's atmosphere, some heat is absorbed but most of the short-wave solar energy passes through. This energy is re-emitted by the Earth as long-wave radiation, which cannot pass easily through the atmosphere. More heat is retained if there is a cloud layer. In the last 100 years, more heat has been retained due to increased levels of carbon dioxide (CO2) through the burning of fossil fuels. Tiny particles of CO2 form an extra layer, which acts like the glass in a greenhouse. The effect is compounded by damage to the ozone layer caused by chlorofluorocarbons. Scientists of the Intergovernmental Panel on Climate Change (IPCC) estimate that a doubling of the present carbon dioxide emissions could lead to global warming on the scale of an average surface temperature rise of between 1 to 3.5°C. See also Earth Summit

http://www.epa.gov/globalwarming/kids/greenhouse.html

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‘greenhouse effect’

greenhouse effect The effect of heat retention in the lower atmosphere as a result of absorption and reradiation of long-wave (more than 4 μm) terrestrial radiation by clouds and gases (e.g. water vapour, carbon dioxide, methane, and chlorofluorocarbons), which make the atmosphere transparent to incoming short-wave radiation but partly opaque to reradiated long-wave radiation. The insulating effect is not strictly analogous to that of greenhouse glass, since the higher temperature in a greenhouse is owing partly to the reduction in air movement. The atmospheric effect is to alter the balance of incoming and outgoing radiation in the Earth's energy budget. Marked increases in atmospheric carbon dioxide, generated, for example, by the combustion of fossil fuels, could result in a global increase of atmospheric temperatures if not offset by other (perhaps natural) changes.

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greenhouse effect

greenhouse effect The effect of heat retention in the lower atmosphere as a result of absorption and reradiation by clouds and gases (e.g. water vapour, carbon dioxide, methane, and chlorofluorocarbons) of longwave (more than 4 μm) terrestrial radiation. The insulating effect is analogous to that of greenhouse glass (i.e. it is transparent to incoming short-wave radiation but partly opaque to reradiated long-wave radiation) and alters the balance of incoming and outgoing radiation in the Earth's energy budget. Marked increases in atmospheric carbon dioxide, generated for example by the combustion of fossil fuels, could result in a global increase of atmospheric temperatures if not offset by other (perhaps natural) changes. See also ATMOSPHERIC WINDOW.

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‘greenhouse effect’

greenhouse effect The effect of heat retention in the lower atmosphere as a result of absorption and reradiation of long-wave (more than 4 μm) terrestrial radiation by clouds and gases (e.g. water vapour and carbon dioxide). The insulating effect is not strictly analogous to that of greenhouse glass, since the higher temperature in a greenhouse is due partly to the reduction in air movement. The atmospheric effect is to alter the balance of incoming and outgoing radiation in the Earth's energy budget. Marked increases in atmospheric carbon dioxide, generated for example by the combustion of fossil fuels, could result in a global increase of atmospheric temperatures if not offset by other (perhaps natural) changes.

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greenhouse effect

green·house ef·fect • n. the trapping of the sun's warmth in a planet's lower atmosphere due to the greater transparency of the atmosphere to visible radiation from the sun than to infrared radiation emitted from the planet's surface.

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greenhouse effect

greenhouse effect the trapping of the sun's warmth in a planet's lower atmosphere due to the greater transparency of the atmosphere to visible radiation from the sun than to infrared radiation emitted from the planet's surface.

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greenhouse effect

greenhouse effect: see global warming.

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Greenhouse Effect

Greenhouse Effect

The greenhouse effect

The greenhouse effect and climate change

Effects of climatic change

Resources

The greenhouse effect is the retention by Earths atmosphere in the form of heat some of the energy that arrives from the sun as light. Certain gases, including carbon dioxide (CO2) and methane (CH4), are transparent to most of the wavelengths of light arriving from the sun but are relatively opaque to infrared or heat radiation; thus, energy passes through the Earths atmosphere on arrival, is converted to heat by absorption at the surface and in the atmosphere, and is not easily re-radiated into space. The same process is used to heat a solar greenhouse, only with glass, rather than

gas, as the heat-trapping material. The greenhouse effects happens to maintain Earths surface temperature within a range comfortable for living things; without it, the Earths surface would be much colder.

The greenhouse effect is mostly a natural phenomenon, but its intensity, according to a majority of climatologists, may be increasing because of increasing atmospheric concentrations of CO2 and other greenhouse gases. These increased concentrations are occurring because of human activities, especially the burning of fossil fuels and the clearing of forests (which removes CO2 from the atmosphere and store its carbon in cellulose. A probable consequence of an intensification of Earths greenhouse effect will be a significant warming of the atmosphere. Some scientists contend that this human-related atmospheric warming is already occurring and is accelerating. This in turn would result in important secondary changes, such as a rise in sea level and variations in the patterns of precipitation.

The greenhouse effect

The Earths greenhouse effect is a reasonably well-understood physical phenomenon. Scientists believe that in the absence of the greenhouse effect, Earths surface temperature would average about -0.4°F (-18°C), which is below the freezing point of water and more frigid than life on the surface of the Earth could tolerate over the longer termexcept, perhaps, organisms deriving their energy from hot deep-sea vents. The greenhouse effect maintains Earths surface at an average temperature of about 59°F (15°C). This is about 59.5°F (33°C) warmer than it would otherwise be.

On average, one-third of incident solar radiation is reflected back to space by the Earths atmosphere or its surface. Earths local reflectivity (albedo) is strongly dependent on cloud cover, the density of tiny particulates in the atmosphere, and the nature of the surface, especially vegetation and ice and snow.

Another one-third of incoming solar radiation is absorbed by certain gases and vapors in Earths atmosphere, especially water vapor and carbon dioxide. Upon absorption, the solar electromagnetic energy is transformed into thermal kinetic energy (i.e., heat or energy of molecular vibration). The warmed atmosphere then reradiates energy in all directions as longer-wavelength (7-14 æm) infrared radiation. Much of this reradiated energy escapes to outer space.

Much of the solar radiation that penetrates to Earths surface is absorbed by living and nonliving materials. This results in a transformation to thermal energy, which increases the temperature of the absorbing surfaces and of air in contact with those surfaces. Over days and years there is little net storage of energy as heat; almost all of the thermal energy is re-radiated by the surface as electromagnetic radiation of a longer wavelength than that of the original, incident radiation. The wavelength spectrum of typical, reradiated electromagnetic energy from Earths surface peaks is within the long-wave infrared range.

Some of the electromagnetic energy that penetrates to Earths surface is absorbed and transformed to heat. Much of this thermal energy subsequently causes water to evaporate from plants and open-water surfaces, or melts ice and snow. A small amount (less than 1%) of the absorbed solar radiation causes mass-transport processes to occur in the oceans and lower atmosphere, which disperses of some of Earths unevenly distributed thermal energy. The most important of these physical processes are winds and storms, water currents, and waves on the surface of the oceans and lakes.

If the atmosphere was transparent to the longwave infrared energy that is reradiated by Earths atmosphere and surface, then that energy would travel unobstructed to outer space. However, so-called radi-atively active gases (also known as greenhouse gases) in the atmosphere are efficient absorbers within this range of infrared wavelengths, and these substances thereby slow the radiative cooling of the planet. When these atmospheric gases absorb infrared radiation, they develop a larger content of thermal energy, which is then dissipated by a reradiation (again, of a longer wavelength than the electromagnetic energy that was absorbed). Some of the secondarily reradiated energy is directed back to Earths surface, so the net effect is to slow the rate of cooling of the planet.

This process has been called the greenhouse effect because its mechanism is analogous to that by which a glass-enclosed space is heated by solar energy. That is, a greenhouses glass and humid atmosphere are transparent to incoming solar radiation, but absorb much of the re-radiated, long-wave infrared energy, slowing down the rate of cooling of the structure.

Water vapor (H2O) and CO2 are the most important radiatively active constituents of Earths atmosphere. Methane (CH4), nitrous oxide (N2O), ozone (O3), and chlorofluorocarbons (CFCs) play lesser roles.

Other than water vapor, the atmospheric concentrations of all of these gases have increased in the past century because of human activities. Prior to 1850, the concentration of CO2 in the atmosphere was about 280 parts per million (ppm), while 2002 it was over 360 ppm. During the same period, CH4 increased from 0.7 ppm to 1.7 ppm, N2O from 0.285 ppm to 0.304 ppm, and CFCs from nothing to 0.7 parts per billion. These increased concentrations are believed by climatologists to contribute to a significant increase in the greenhouse effect. Overall, CO2 is estimated to account for about 60% of this enhancement of the greenhouse effect, CH4 for 15%, N2O for 5%, O3 for 8%, and CFCs for 12%.

The greenhouse effect and climate change

The physical mechanism of the greenhouse effect is conceptually simple, and this phenomenon is acknowledged by scientists as helping to keep Earths temperature within the comfort zone for organisms. It is also known that the concentrations of CO2 and other greenhouse relevant gases have increased in Earths atmosphere, and will continue to do so. However, it has proven difficult to demonstrate that the observed warming of Earths surface or lower atmosphere has been caused significantly by a stronger greenhouse effect rather than by some still-unknown process of natural climate change.

Since the beginning of instrumental recordings of surface temperatures around 1880, almost all of the warmest years on record have occurred since the late 1980s. Typically, these warm years have averaged about 1.5-2.0°F (0.8-1.0°C) warmer than occurred during the decade of the 1880s. Overall, Earths surface air temperature has increased by about 0.9°F (0.5°C) since 1850.

However, the temperature data on which these apparent changes are based suffer from some important deficiencies. Firstly, air temperature is variable in time and space, making it difficult to determine statistically significant, longer-term trends. Secondly, older data are generally less accurate than modern records. Thirdly, many weather stations are in urban areas, and are influenced by heat island effects. Finally, climate can change for reasons other than a greenhouse response to increased concentrations of CO2 and other greenhouse relevant gases, including albedo-related influences of volcanic emissions of sulfur dioxide, sulfate, and fine particulates into the upper atmosphere. Moreover, it has long been thought that the interval 1350 to 1850, known as the Little Ice Age, was relatively cool, and that global climate has been generally warming since that time period. (The data one which this claim was based, however, have recently been called into question; no instrumental or global data at all are available from the period in question.)

However, some studies have provided evidence for linkages between historical variations of atmospheric CO2 and surface temperature. Important evidence comes, for example, from a core of Antarctic glacial ice that represents a 160,000-year period. Concentrations of CO2 in the ice are determined by analysis of air bubbles in ice layers of known age (determined by counting annual snowfall layers back from the present), while changes in air temperature are inferred from ratios of oxygen isotopes in the ancient ice. (Because atoms of various isotopes differ in weight, their rates of diffusion are affected by temperature differently; differences in diffusion rate, in turn, affect their relative abundance in the ice). Because changes in CO2 and surface temperature are positively correlated, a greenhouse mechanism is suggested. However, this study could not determine causal directionthat is, whether increased CO2 might have resulted in warming through an intensified greenhouse effect, or whether, conversely, warming (caused by something unknown) could have accelerated CO2 release from ecosystems by increasing the rate of decomposition of biomass, especially in cold regions.

Because of the difficulties in measurement and interpretation of climatic change using real-world data, computer models have been used to predict potential climatic changes caused by increases in atmospheric RAGs. The most sophisticated simulations are the so-called three-dimensional general circulation models (GCMs), which are run on supercomputers. GCM models simulate the extremely complex mass-transport processes involved in atmospheric circulation and the interaction of these processes with other variables that contribute to climate. To perform a simulation experiment with a GCM model, components are adjusted to reflect the probable physical influence of increased concentrations of CO2 and other greenhouse relevant gases.

Many simulation experiments have been performed using a variety of GCM models. Their results have varied according to the specifics of the experiment. However, a central tendency of experiments using a common CO2 scenario (i.e., a doubling of CO2 from its recent concentration of 360 ppm) is an increase in average surface temperature of 1.8-7.2°F (1-4°C). This warming is predicted to be especially great in polar regions, where temperature increases could be two or three times greater than in the tropics. CO2 can cause global warming, whether or not the reverse process may also occur.

Effects of climatic change

It is likely that the direct effects of climate change caused by an intensification of the greenhouse effect would be substantially restricted to plants. The temperature changes might cause large changes in the quantities, distribution, or timing of precipitation, and this would have a large effect on vegetation. There is, however, even more uncertainty about the potential changes in rainfall patterns than of temperature, and effects on soil moisture and vegetation are also uncertain. Still, it is reasonable to predict that any large changes in patterns of precipitation would result in fundamental reorganizations of vegetation on the terrestrial landscape.

Studies of changes in vegetation during the warming climate that followed the most recent, Pleistocene, glaciation, suggest that plant species responded in unique, individualistic ways. This results from the differing tolerances of species to changes in climate and other aspects of the environment, and their different abilities to colonize newly available habitat. In any event, the species composition of plant communities was different then from what occurs at the present time. Of course, the vegetation was, and is, dynamic, because plant species have not completed their postglacial movements into suitable habitats.

In any region where the climate becomes drier (for example, because of decreased precipitation), a result could be a decreased area of forest, and an expansion of savanna or prairie. A landscape change of this character is believed to have occurred in the New World tropics during the Pleistocene glaciations. Because of the relatively dry climate at that time, presently continuous rainforest may have been constricted into relatively small refugia (that is, isolated patches). These forest remnants may have existed within a landscape matrix of savanna and grassland. Such an enormous restructuring of the character of the tropical landscape must have had a tremendous effect on the multitude of rare species that live in that region. Likewise, climate change potentially associated with an intensification of the greenhouse effect would have a devastating effect on Earths natural ecosystems and the species that they sustain.

There would also be important changes in the ability of the land to support crop plants. This would be particularly true of lands cultivated in regions that are marginal in terms of rainfall, and are vulnerable to drought and desertification. For example, important crops such as wheat are grown in regions of the western interior of North America that formerly supported natural shortgrass prairie. It has been estimated that about 40% of this semiarid region, measuring 988 million acres (400 million hectares), has already been desertified by agricultural activities, and crop-limiting droughts occur there sporadically. This climatic handicap can be partially managed by irrigation. However, there is a shortage of water for irrigation, and this practice can cause its own environmental problems, such as salinization. Clearly, in many areas substantial changes in climate would place the present agricultural systems at great risk.

Patterns of wildfire would also be influenced by changes in precipitation regimes. Based on the predictions of climate models, it has been suggested that there could be a 50% increase in the area of forest annually burned in Canada, presently about 2.5-4.9 million acres (1-2 million hectares) in typical years.

Some shallow marine ecosystems might be affected by increases in seawater temperature. Corals are vulnerable to large increases in water temperature, which may deprive them of their symbiotic algae (called zooxanthellae), sometimes resulting in death of the colony. Widespread coral bleachings were apparently caused by warm water associated with an El Nino event in 1982-83.

Another probable effect of warming could be an increase in sea level. This would be caused by the combination of a thermal expansion of the volume of warmed seawater and melting of polar glaciers. The IPCC models predicted that sea level in 2100 could be 10.5-21 in (27-50 cm) higher than today. Depending on the rate of change in sea level, there could be substantial problems for low-lying, coastal agricultural areas and cities.

Most GCM models predict that high latitudes will experience the greatest intensity of climatic warming. Ecologists have suggested that the warming of northern ecosystems could induce a positive feedback to climate change. This could be caused by a change of great expanses of boreal forest and arctic tundra from sinks for atmospheric CO2, into sources of that greenhouse gas. In this scenario, the climate warming caused by increases in RAGs would increase the depth of annual thawing of frozen soils, exposing large quantities of carbon-rich organic materials in the permafrost to microbial decomposition, and thereby increasing the emission of CO2 to the atmosphere.

It is likely that an intensification of Earths greenhouse effect would have large climatic and ecological consequences. Clearly, any sensible strategy for managing the causes and consequences of changes in the greenhouse effect will require substantial reductions in the emissions of CO2 and other greenhouse relevant gases.

It is important to recognize that any strategy to reduce these emissions will require great adjustments by society and economies. Because such large quantities of CO2 are emitted through the burning of fossil fuels, there will be a need to use different, possibly

KEY TERMS

Albedo The reflectivity of a surface.

Carbon reserve An ecosystem, such as a forest, that is managed primarily for its ability to store large quantities of organic carbon, and to thereby offset or prevent an emission of carbon dioxide to the atmosphere.

Desertification A climatic change involving decreased precipitation, causing a decreased or destroyed biological productivity on the landscape, ultimately leading to desert-like conditions.

Electromagnetic energy A type of energy involving photons, which have physical properties of both particles and waves. Electromagnetic energy is divided into spectral components, which (ordered from long to short wavelength) include radio, infrared, visible light, ultraviolet, and cosmic.

Energy budget A physical accounting of the various inputs and outputs of energy for some system, as well as the quantities and locations where energy is internally stored.

Radiatively active gases (RAGs) Within the context of the greenhouse effect, these gases absorb long-wave infrared energy emitted by Earths surface and atmosphere, and thereby slow the rate of radiative cooling by the planet.

new, technologies to generate energy, and there may be a need for large decreases in total energy use. The bottom line will be a requirement to add considerably smaller quantities of greenhouse relevant gases to the atmosphere. Such a strategy of mitigation will be difficult, especially in industrialized countries, because of the changes required in economic systems, resource use, investments in technology, and levels of living standards. The implementation of those changes will require enlightened and forceful leadership.

Under the auspices of the United Nations Environment Program, various international negotiations have been undertaken to try to get nations to agree to decisive actions to reduce their emissions of RAGs. The most recent major agreement came out of a large meeting held in Kyoto, Japan, in 1997. There, most of the worlds industrial countries agreed to reduce their CO2 emissions to 5.2% below 1990 levels by the year 2012. The United States, which has about 5% of the worlds population but produces 24% of its CO2 emissions, signed the Kyoto protocol in 1998 (that is to say, its ambassador to the United Nations signed the plan) but never ratified it as a binding treaty; shortly after taking office in 2000, President George W. Bush repudiated the protocol entirely. (China, with about 23% of the worlds population, is the second-biggest CO2 producer, at 14% of total emissions.)

The development and maintenance of ecosystems that store large quantities of carbon to offset industrial emissions would require very large areas of land. These carbon reserves would preclude other types of economically important uses of the land. This strategy would therefore require a substantial commitment by society; however, so would any other possible means of decreasing greenhouse gases, and so would a decision to do nothing at all (or to keep researching the problem indefinitely, which amounts to much the same thing). There are no easy solutions to problems of this type and magnitude.

Resources

BOOKS

Avery, Dennis T. and S. Fred Singer. Unstoppable Global Warming: Every 1500 Years. New York: Rowman & Littlefield Publishers, 2006.

Hocking, Colin. Global Warming & the Greenhouse Effect. Berkeley, CA: GEMS, 2002.

Houghton, John. Global Warming: The Complete Briefing. Cambridge: Cambridge University Press, 2004.

Morganstein, Stanley. The Greenhouse Effect. Cheshire, UK: Trafford Publishing, 2003.

Bill Freedman
Larry Gilman

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Greenhouse Effect

Greenhouse effect


The greenhouse effect is a natural phenomenon that traps radiation within the earth's atmosphere . Natural greenhouse gases include water vapor, carbon dioxide , nitrous oxide , methane , and ozone , all essential to support life. The enhanced greenhouse effect, the direct result of human activities, increases concentrations of these gases in the atmosphere, and leads to pollution of the lower atmosphere and contributes to global warming. These gases let in sunlight but tend to insulate Earth against the loss of heat, as do the glass walls of a greenhouse. A higher concentration of the greenhouse gases means a warmer climate . For example, the twentieth century was been 1° warmer on worldwide average than the nineteenth centurywarming at a rate 20 times faster than average.

Carbon dioxide (CO2) is considered the predominant greenhouse gas and has the greatest impact on global heat. From April 1958, when monthly measurements of CO2 from atop the Mauna Loa volcano began, through June 1991, the CO2 concentration in parts per million went from 316 ppm to almost 360 ppm. The peak concentration is due to the destruction of tropical rain forests and the burning of fossil fuels , which accounts for half of the greenhouse gases added to the atmosphere. CO2 is dumped into the atmosphere at a much faster rate than it can be withdrawn or absorbed by the oceans or living things in the biosphere ; since 1765, its presence in the atmosphere has increased by over 27%. CO2 buildup in the next few decades to centuries could be one of the principal controlling factors of the near-future climate.

Methane, another greenhouse gas, is produced when oxygen is not freely available and bacteria have access to organic matter, such as in swamps, bogs, rice paddies and moist soils. Methane also is produced in the guts of termites and cows, in garbage dumps, landfills, emissions from coal mining, natural gas production and distribution, and changing land use . Methane concentrations have increased over 100% since 1765.

Nitrous oxide concentrations have increased in recent years due to fertilizer use and chemical production, such as in the manufacture of nylon. Nitrous oxide is also dispersed during fossil fuel combustion , biomass burning and changing land use.

CFCs (chlorofluorocarbons ), also implicated in ozone layer depletion , act as greenhouse gases. While useful and widely used as refrigerants, their total effect is significant because compared to a molecule of carbon dioxide, each molecule of CFC absorbs much more radiation, thereby trapping heat in the atmosphere.

Other greenhouse gases are ground-level ozone (sunlight reacting with automobile emissions ). and water vapor. Water vapor represents about two% of total atmospheric composition, and is the most abundant greenhouse gas. With methane and carbon dioxide, it plays an important role in regulating the temperature of the planet through the production of clouds.

Rain forest destruction also contributes to global warming. When the canopy of leaves is removed through clear-cutting or burning, the sudden warming of the forest floor releases methane and CO2, in a kind of biochemical burning. The massive increase in the number of dead tree trunks and branches leads to a population explosion of termites, which themselves produce methane. Dead trees can no longer store CO2 or convert it to oxygen.

Two factors which appear to mitigate the effect of enhanced greenhouse gases are aerosols and dust. Aerosols, minute solid particles, are finely dispersed in the atmosphere and have become an influence on the greenhouse effect. Aerosols are produced by combustion, but they also come from natural sources, primarily volcanoes. By blocking light, aerosols and dust can offset warming from greenhouse gasses. For example, a significant cooling trend in the spring and summer of 1992 seemed to correlate with the eruption of Mount Pinatubo in the Philippines. The fall and winter of 1992 were fairly mild on worldwide average. As all the particulate matter from the Mt. Pinatubo eruption settled out of the atmosphere, the surface cooling effects abated and the global warming trend resumed.

Anthropogenic (human-caused) greenhouse gases now appear responsible for increasing the global average temperature. According to current projections, global temperatures may rise as much as 35.637.4° F (23° C) above the pre-industrial temperatures by the year 2100. To place this change in perspective, the temperature rise that brought the planet out of the most recent ice age was only about 37.439.2° F (34° C).

The top 10 warmest years of average global recorded temperatures were in the last 15 years of the twentieth centry and saw devastating fires in Yellowstone National Park , flooding in Bangladesh, record number of hurricanes and tornados, and a deadly heat wave and drought in the southeastern United States. It is probable, based on computer models, that a resumption in warming will accompany changes in regional weather. A 40-year trend of increased precipitation in Europe and decreased precipitation in the African Sahel (Ethiopia, the Sudan, Somalia) may be an early consequence of global warming due to the greenhouse effect. Longer and more frequent heat waves would result in public health threats as well as inconveniences such as road buckling, electrical brownouts, or blackouts.

Precipitation is likely to increase regionally because as the temperature increases, more evaporation takes place, leading to more precipitation. The average precipitation event is likely to be heavier: wetter monsoons in coastal subtropics; more frequent and heavier winter snows at high altitudes and high latitudes; an earlier snowmelt, and a wetter spring.

Increases in rain- or snowfall are not expected to offset the effects of higher temperatures on soil , however. Higher temperatures are expected to dry the soil in North America and southern Europe, among other places, by boosting the rates of evaporation and transpiration through plants. More favorable agricultural conditions in high latitudes could move the center of agriculture farther north into Canada and Siberia and out of the United States.

Other consequences of global warming from the enhanced greenhouse effect include the reduction of sea ice, coastal sea level rises of several feet per century, more frequent and powerful hurricanes, and more frequent and severe forest fires. In the United States, the frequency of tornadoes is near or above record levels for the years 19901994.

The rise of sea level is the most easily predicted consequence. The one-degree increase in temperature over the past century contributed to a 4-in (1020-cm) rise in mean sea level. This could lead to severe and frequent storm damage, flooding and disappearance of wetlands and lowlands, coastal erosion , loss of beaches and low islands, wildlife extinctions, and increased salinity of rivers, bays and aquifers.
However, because the global atmosphere operates as a complex system, it is difficult, even with today's sophisticated computer models, to predict the exact nature of the changes we are likely to cause with increased greenhouse gases. Scientists have predicted that low-lying areas and islands, including the Seychelles, the Maldives, the Marshall Islands, and large areas of Bangladesh, Egypt, Florida, Louisiana, and North Carolina will disappear over the next few decades.

The earth's natural atmospheric cleanser rain may wash excess greenhouse gases out of the atmosphere. But until rates of greenhouse gases slow their rapid increases or actually begin to decrease, the planet will get warmer. In response to climate projections, the United Nations Framework Convention on Climate Change (UNFCC), adopted and signed by 162 countries in 1992 at the Rio Earth Summit, sets country-by-country standards to reduce the emissions of greenhouse gases, particularly carbon dioxide.

Policy-makers in the United States, including Vice President Albert Gore Jr., propose stricter requirements for more fuel-efficient cars, "environment taxes" that penalize heavy polluters and help pay for cleansing the atmosphere, and trading technological advances for rain forest protection in Third World countries. However, because global warming often is made a political ping-pong ball, changes in political administrations worldwide can extend to policy makers and climate researchers, who depend on government assistance for research.

The greatest controversy over slowing the rate of greenhouse gases injected into the atmosphere seems to be how to do it. Some scientists advocate increased use of nuclear power to reduce dependence on fossil fuels, but that carries its own controversies. Nuclear power plants are so energy-intensive just to build, the trade-off is negligible. Conservation and a switch from a dependence on fossil fuels to dependence on renewable resources such as wind and solar energy , slows the rate of increase of carbon dioxide emissions into the atmosphere.

See also Environmental economics; Environmental policy; The Global 2000 Report

[Linda Rehkopf ]


RESOURCES

BOOKS


Bates, A. X. Climate in Crisis. Summertown, TN: The Book Publishing Co., 1990.

Houghton, J. T., and L. G. Meira Filho, ed. Climate Change 1995: The Science of Climate Change. Contribution of Working Group I to the Second Assessment Report of the Intergovernmental Panel on Climate Change. CambridgeUniversityPress,1996.


PERIODICALS

"Indices of Climate for the United States." Bulletin of the American Meteorological Society 77, no. 2 (February 1996): 279292.

OTHER

Changing by Begrees: Steps to Reduce Greenhouse Gases. U.S. Congress, Office of Technology Asessment, 1991.

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Greenhouse Effect

Greenhouse Effect

Introduction

The greenhouse effect occurs when Earth, or some other planet, retains more heat because of the blanketing effect of its atmosphere. When the sun warms any planet, solar energy arrives at the planet's surface as light (electromagnetic waves) and is retained as heat (random molecular motion). A greenhouse effect causes more heat to be retained than lost. This occurs because certain gases, including carbon dioxide (CO2) and methane (CH4) are transparent to most of the wavelengths of light from the sun but are relatively opaque to infrared or heat radiation, which is radiated by Earth's surface and atmosphere. Light arriving from the sun passes through Earth's atmosphere, is converted to heat by absorption in the atmosphere at the surface, and is re-radiated into space more slowly because of the greenhouse effect.

The same process is used to heat a solar greenhouse, only using glass, rather than greenhouse gas, as the heat-trapping material. (In an actual greenhouse, the glass traps heat by preventing warm interior air from rising and mixing with the environment, rather than by blocking re-radiation of infrared light; the effect, however, is similar.) The greenhouse effect maintains Earth's surface temperature within a range appropriate for living things; without it, all of Earth's surface would be below the freezing point of water.

The greenhouse effect is a natural phenomenon. However, its intensity has increased because human activities since the beginning of the Industrial Revolution around 1750—especially the burning of fossil fuels and the clearing of forests (which remove CO2 from the atmosphere, store the carbon in cellulose, and release the oxygen back to the atmosphere)—have raised atmospheric concentrations of CO2 and other greenhouse gases. An observed consequence of Earth's artificially intensified greenhouse effect is a significant warming of the atmosphere.

All but a few scientists agree that this anthropogenic (human-caused) atmospheric warming is already occurring and is accelerating. This, in turn, is causing warmer seas, rising sea levels, and changes in patterns of precipitation and weather such as less rain in some places, more in others; more droughts and floods; more violent storms; melting of glaciers; shifts in ranges of plants and animals; extinctions; and more.

Historical Background and Scientific Foundations

How the Greenhouse Effect Works

The habitability of Earth depends both on its distance from the sun—neither too close nor too far—and on the balance between energy arriving from the sun and being radiated away into space. In the absence of the greenhouse effect, Earth's surface temperature would average about 0°F (–18°C), which is well below the freezing point of water and colder than life could tolerate over the long term, except for organisms deriving their energy from Earth's interior heat via hot deep-sea vents. The greenhouse effect maintains Earth's surface at an average of about 59°F (15°C).

On average, 35% of incoming solar radiation is reflected back to space by Earth's atmosphere (7%), clouds (24%), and surface (4%). The albedo (reflectivity) of any given area of Earth's surface is dependent on cloud cover, the density of tiny particulates in the atmosphere (aerosols), and the brightness of the surface, which is high for snow and ice, moderate for water, and relatively low for soil and vegetation.

Another 18% of incoming solar radiation is absorbed by gases (16%) and clouds (2%) in Earth's atmosphere. Forty-seven percent of incoming solar radiation reaches Earth's surface and is absorbed. Upon absorption, its energy is transformed into thermal kinetic energy (i.e., heat, the energy of random molecular motion). The warmed surface and atmosphere reradiate most of this energy in all directions as longer-wavelength (7–14 æm) infrared radiation (a small fraction drives winds, currents, and chemical processes). Much of this re-radiated energy escapes to

space, and some is re-absorbed by the atmosphere—where the greenhouse effect occurs.

The amount of energy arriving as short-wavelength sunlight is approximately the amount being reflected as short-wavelength light or radiated as long-wavelength infrared radiation. While Earth is warming, as is happening now, it is radiating slightly less energy than it is absorbing.

Infrared radiation emitted by the land and sea are absorbed by a number of gases in Earth's atmosphere. The two main constituents of the atmosphere, molecular oxygen (O2, 21% of the atmosphere by volume) and nitrogen (N2, 78%), do not absorb infrared light: they are transparent to it, as they are to visible light. Water vapor (H2O) and CO2, though present in relatively small fractions—0.25% and 0.038%, respectively—are the most important absorbers of infrared light in Earth's atmosphere. Methane (CH4), nitrous oxide (N2O), ozone (O3), and chlorofluorocarbons (CFCs) play lesser but still significant roles. Some of these gases are much more effective absorbers of infrared than either water or CO2, molecule for molecule, but they contribute less to the greenhouse effect because they are present in much lower concentrations.

Absorbing infrared light warms the greenhouse gases. All warm matter radiates infrared light, so the warmed gases radiate infrared light. Some of this infrared is radiated into space while shining down on Earth's surface, warming it.

This process has been called the “greenhouse effect” because its mechanism is analogous to that by which a glass-enclosed space is heated by sunlight. A greenhouse's glass and humid atmosphere are transparent to incoming solar radiation, but absorb much of the long-wave infrared light that is radiated from plants and other objects inside the greenhouse. This slows down the rate of cooling of the structure, which reaches equilibrium (gets into balance) with its environment at a higher temperature than it would otherwise.

Other than water vapor, the atmospheric concentrations of all of the greenhouse gases have increased in the past century because of human activities. Prior to 1850, the concentration of CO2 in the atmosphere was about 280 parts per million (ppm); by 2005 it had gone up to 379 ppm, a 36% increase. During the same period, CH4 increased from 0.7 ppm to 1.8 ppm, N2O from 0.285 ppm to 0.319 ppm, and CFCs from nothing to 0.7 parts per billion. These increased concentrations of greenhouses gases have caused a significant increase in the greenhouse effect. Overall, CO2 is estimated to account for about 60% of this enhancement of the greenhouse effect, CH4 for 15%, N2O for 5%, O3 for 8%, and CFCs for 12%.

Water vapor is a special case because although most of the greenhouse effect is caused by water vapor, it enters and leaves the atmosphere very rapidly compared to the other gases. The concentration of water vapor is thus controlled by the amount of other greenhouse gases in the atmosphere. The other greenhouse gases are said to contribute forcing to global warming, while water vapor contributes feedback.

Absorption of solar high-frequency radiation heats air, sea, and land in Earth's blanketing atmosphere. By slowing Earth's loss of this energy to outer space through the increased density of the greenhouse gases, the heat increases.

The extra greenhouse effect caused by anthropogenic (human-caused) greenhouse gases is often called global warming. Scientists usually prefer the term “global climate change,” however, because not all parts of Earth's surface are actually warming (only the great majority of them) and because warming is causing many other changes in climate, some of them superficially contradicted by warming. For example, melting of glaciers around the southern coasts of Greenland have increased, but so has snowfall in the center of the island: both effects are predicted by scientists studying global climate change.

WORDS TO KNOW

AEROSOL: Particles of liquid or solid dispersed as a suspension in gas.

ALBEDO: A numerical expression describing the ability of an object or planet to reflect light.

BIOMASS: The sum total of living and once-living matter contained within a given geographic area. Plant and animal materials that are used as fuel sources.

CHLOROFLUOROCARBONS: Members of the larger group of compounds termed halocarbons. All halocarbons contain carbon and halons (chlorine, fluorine, or bromine). When released into the atmosphere, CFCs and other halocarbons deplete the ozone layer and have high global warming potential.

CORAL BLEACHING: Decoloration or whitening of coral from the loss, temporary or permanent, of symbiotic algae (zooxanthellae) living in the coral. The algae give corals their living color and, through photosynthesis, supply most of their food needs. High sea surface temperatures can cause coral bleaching.

EL NIÑO/SOUTHERN OSCILLATION: Global climate cycle that arises from interaction of ocean and atmospheric circulations. Every 2 to 7 years, westward-blowing winds over the Pacific subside, allowing warm water to migrate across the Pacific from west to east. This suppresses normal upwelling of cold, nutrient-rich waters in the eastern Pacific, shrinking fish populations and changing weather patterns around the world.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

HABITABILITY: The degree to which a given environment can be lived in by human beings. Highly habitable environments can support higher population densities, that is, more people per square mile or kilometer.

INFRARED RADIATION: Electromagnetic radiation of a wavelength shorter than radio waves but longer than visible light that takes the form of heat.

OZONE: An almost colorless, gaseous form of oxygen with an odor similar to weak chlorine. A relatively unstable compound of three atoms of oxygen, ozone constitutes, on average, less than one part per million (ppm) of the gases in the atmosphere. (Peak ozone concentration in the stratosphere can get as high as 10 ppm.) Yet ozone in the stratosphere absorbs nearly all of the biologically damaging solar ultraviolet radiation before it reaches Earth's surface, where it can cause skin cancer, cataracts, and immune deficiencies, and can harm crops and aquatic ecosystems.

PLEISTOCENE EPOCH: Geologic period characterized by ice ages in the Northern Hemisphere, from 1.8 million to 10,000 years ago.

WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. Although humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.

The Greenhouse Effect and Climate Change

The mechanism of the greenhouse effect is simple, but the details of global warming are complicated by atmospheric and oceanic circulations and by various feedback mechanisms. Although less than 1% of the solar radiation absorbed by Earth drives mass-transport processes in the oceans and lower atmosphere (currents and winds), these spread some of Earth's unevenly distributed thermal energy and are a crucial part of Earth's weather and climate system. Feedback occurs whenever some outcome of a process affects an earlier part of that process. There are many feedbacks involved with the greenhouse effect and global warming. For example, some of the heat at Earth's surface causes water to evaporate from plants and open water. Water vapor is a greenhouse gas, so this increased evaporation tends to increase global warming, which tends to increase evaporation further, and so on.

Scientists have long understood that the greenhouse effect keeps Earth's temperature within a livable range for most life. They have also known that concentrations of CO2 and other greenhouse relevant gases have increased in Earth's atmosphere and will continue to increase for many decades to come. However, until the 1990s it was difficult to show that the observed warming of Earth's surface and lower atmosphere were being caused mostly by anthropogenic enhancement of the greenhouse effect rather than by some still-unknown process of natural climate change, such as the increased brightness of the sun. Earth's climate has changed thousands of times over hundreds of millions of years, and it was not scientifically out of the question that such natural processes might be at work today.

Since about 1990, however, many types of independent evidence have shown not only that the climate is changing rapidly but that human activities, especially emissions of greenhouse gases and deforestation, are the cause of most of this change. Since the beginning of instrumental recordings of surface temperatures around 1850, almost all of the warmest years on record have occurred since the late 1980s. Typically, these warm years have averaged about 1.4–1.8°F (0.8–1.0°C) warmer than occurred during the decade of the 1880s. Overall, as of 2007, the temperature of Earth's atmosphere near the surface had risen about 1.33°F (0.74°C) since 1906. More recent years have seen more rapid increases.

The temperature data are not simple to interpret. First, air temperature is variable in time and space, making it necessary to collect data over wide areas and long time periods to detect significant long-term trends. Second, older data are generally less accurate than modern records. Third, many weather stations are in urban areas, which warm the air around them independently of global climate: this effect must be accounted for when analyzing the data. Finally, climate can change for reasons other than a greenhouse response to human-caused increases in greenhouse gases, including volcanic emissions of sulfur dioxide, sulfate, and fine particulates into the upper atmosphere and naturally shifting ocean and atmospheric circulations.

However, there are observations linking prehistoric variations of atmospheric CO2(the most important greenhouse gas) and climate warming. Important evidence comes, for example, from cores of ancient ice from Antarctica and Greenland—glacial ice that contained layered ice from annual snowfalls dating back hundreds of thousands of years. Concentrations of CO2 in the ice are determined by analysis of air bubbles in ice layers of known age (determined by counting snowfall layers back from the present, much like tree-rings). Changes in air temperature are inferred from ratios of oxygen isotopes in the ancient ice, so the temperature record can be compared to the CO2 record over long periods of time. Ice cores show that CO2 levels are higher today than at any time in at least 650,000 years.

The fact that changes in CO2 and surface temperature are positively correlated—that is, increased CO2 almost always accompanies warming of climate—suggests a greenhouse mechanism. However, correlation alone, the fact that two things happen together, does not show which thing caused which, or whether both were caused by some other phenomenon. In this case, scientists have argued that increased CO2 might have resulted in warming through an intensified greenhouse effect; others have argued that warming (caused by some unknown process) could have accelerated CO2 release from ecosystems by increasing the rate of decomposition of biomass, especially in cold regions. It is also possible that both effects may occur, either separately or together, one triggering or accelerating the other.

Such questions are resolved using computer models based on the laws of physics that predict climatic changes caused by increases in greenhouse gases or other factors. The most sophisticated simulations are three-dimensional general circulation models (GCMs), which are run on supercomputers. GCMs simulate global atmospheric and ocean circulations and the interaction of these with other variables that contribute to climate. To perform a simulation experiment with a GCM, numbers representing concentrations of CO2 and other greenhouse relevant gases, along with other factors (e.g., solar brightness, vegetation cover, ice melting, and many more), are fed into equations that represent the laws of physics governing circulation, heat, absorption, life processes, atmospheric chemistry, and the like. The goal is to produce the most realistic possible mathematical description of Earth's climate machine and its inputs, and to use that description or model to understand the behavior of Earth's past and future climate.

Many experiments have been performed using a variety of GCM models. Their results have varied according to the specifics of the experiment, but a central tendency of experiments using a common CO2 scenario (i.e., a doubling of CO2 from its recent concentration of 379 ppm) is an increase in average surface temperature of 1.8–7.2°F (1–4°C). This warming is predicted to be especially great in polar regions, where temperature increases could be two or three times greater than in the tropics. Such accelerated warming of the Arctic (north polar region) and the West Antarctic Peninsula has already been observed; central Antarctica is not predicted to warm in this way. Increased CO2 can, therefore, cause global warming, whether or not the reverse process (warming causing increased CO2) may also occur.

Impacts and Issues

Increased warming of global climate is causing significant changes in the quantities, distribution, and timing of precipitation. This, in turn, has a large effect on vegetation, on which all animal life ultimately depends. There is, however, more uncertainty about potential changes in rainfall patterns than about changes in global average temperature. Regional effects of changed precipitation on soil moisture and vegetation are also difficult to predict precisely. Studies of changes in vegetation during the warming climate that followed the most recent (Pleistocene) glaciation suggest that plant species respond to climate change according to the differing tolerances of various species to ranges of temperature, rainfall, and seasonal timing, and their different abilities to colonize newly available habitat.

In a region where the climate becomes drier because of increased evaporation, decreased precipitation, or both, one result can be decreased forest and expanded savanna or prairie. A landscape change of this character is believed to have occurred in the New World tropics during the Pleistocene glaciations. Because of the relatively dry climate at that time, presently continuous rainforest may have been constricted into relatively small, isolated patches. These forest remnants would probably have existed within a landscape matrix of savanna and grassland. Such an enormous restructuring of the character of the tropical landscape must have had a tremendous effect on the multitude of rare species that live in that region. Likewise, climate change potentially associated with an intensification of the greenhouse effect would have a profound effect on Earth's natural ecosystems and the species that they sustain.

There will also be important changes in the ability of some areas to support crop plants. This would be particularly true of regions that are marginal in terms of rainfall or temperature, and are vulnerable to drought and desertification. For example, important crops such as wheat are grown in regions of the western interior of North America that formerly supported natural shortgrass prairie. Ecologists estimate that about 40% of this semiarid region, measuring 988 million acres (400 million hectares), has already been desertified by agricultural activities, and crop-limiting droughts occur there sporadically. This climatic handicap can be partially offset by irrigation. However, there is a shortage of water for irrigation (which may be enhanced by climate change). The practice of irrigation can cause its own environmental problems, such as salinization of (salt buildup in) soils. Clearly, in many areas substantial changes in climate would place the present agricultural systems at great risk. In a few areas, agriculture will probably be benefited by climate change, but the global balance is likely to be negative.

Patterns of wildfire are also influenced by changes in precipitation patterns. For example, based on the predictions of climate models, there could be a 50% increase in the area of forest annually burned in Canada, presently about 2.5–4.9 million acres (1–2 million hectares) in typical years. These wildfires would be due, primarily, to the dryness of the forests, with the change in precipitation levels.

Some shallow marine ecosystems are being affected by increases in seawater temperature and by increased ocean acidity caused by higher CO2 concentrations. Corals are especially vulnerable to increases in water temperature, which can deprive them of their symbiotic algae and may kill them. Widespread coral bleaching has been observed, partly due to natural weather cycles such as the El Niño/Southern Oscillation and partly due to anthropogenic climate change.

Another observed effect of warming is rising sea level. This is caused by a combination of a thermal expansion of warmed seawater—water, like most materials, expands slightly when warmed—and the melting of alpine and polar glaciers. GCMs consulted by the United Nations' Intergovernmental Panel on Climate Change (IPCC) predicted in 2005 that sea level in 2100 could be 7–23 in (0.18–59 m) higher than today, with greater rises to follow over centuries or millennia. Since 2005, many scientists have argued that the upper possible range of sea-level rise by 2100 should be much higher than the IPCC's estimate, perhaps as high as 6.5 ft (2 m), because data have since shown surprisingly fast melting of parts of Greenland's ice cap. Depending on the rate and magnitude of change in sea level, it could be problematic or even disastrous for lowlying islands and coastal populations.

Most GCMs predict that high latitudes—regions closer to the poles—will experience the greatest intensity of climatic warming, and such areas have, in fact, been observed already to be warming at about twice the average rate of the rest of the world. The warming of northern ecosystems adds positive feedbacks to climate change. These are caused partly by a change of great expanses of arctic tundra from sinks for atmospheric CO2 into sources of CO2 and of methane. The climate warming caused by the rise in greenhouse gases is already increasing the depth of annual thawing of frozen soils, exposing large quantities of carbon-rich organic materials in the permafrost to microbial decomposition, and thereby increasing the emission of CO2 and CH4 to the atmosphere. Melting of Arctic snow and sea ice also decreases albedo, which speeds warming. Arctic sea-ice melting in the summer of 2007 was the greatest ever observed.

It is likely that a further intensification of Earth's greenhouse effect will occur and have large climatic and ecological consequences. Strategies for managing the causes and consequences of the anthropogenic greenhouse effect include reducing emissions of CO2 and other greenhouse gases. It will also be necessary, at the very least, to adapt human society to whatever amount of climate change can no longer be avoided because of past emissions.

Any strategy to reduce greenhouse emissions will require adjustments by societies and economies. Because large quantities of CO2 are emitted through the burning of fossil fuels, to mitigate (reduce the severity of) global climate change would require societies to use different, possibly new, technologies to generate energy, to increase the efficiency of energy use, and possibly to decrease total energy use. Such a strategy of mitigation will be difficult, especially in poorer countries, because of the changes required in economic systems, resource use, technologies, and lifestyles.

Various international negotiations have been undertaken to try to get nations to agree to decisive actions to reduce their emissions of greenhouse gases. The most recent major agreement came out of a large meeting held in Kyoto, Japan, in 1997. There, most of the world's industrial countries agreed to reduce their CO2 emissions to 5.2% below 1990 levels by the year 2012. The United States, which has about 5% of the world's population but produces about 24% of its greenhouse emissions, signed the Kyoto Protocol in 1998 (that is to say, its ambassador to the United Nations signed the plan) but never ratified it as a binding treaty, which would have needed approval from both the president and Congress. International negotiations for a new, more effective treaty to replace the Kyoto Protocol began in late 2007.

As of January 2007, the warmest year in recorded weather history in terms of global average temperature was 2005, which was 0.94°F (0.52°C) above the average for 1961–1990. The warmest year on record so far for the United States, prior to 2007, was actually 1934. So far, the United States has experienced less warming than many other parts of the world.

Increased extreme weather is another predicted consequence of global climate change. In August 2007, scientists at the World Meteorological Organization, an agency of the United Nations, announced that during the first half of 2007, Earth showed significant increases above long-term global averages in both high temperatures (e.g., heat waves) and frequency of extreme weather events, including heavy rainfalls, cyclones, and wind storms. Global average land temperatures for January and April of 2007 were the warmest recorded for those two months since recordkeeping began in the 1880s.

See Also Global Warming; Greenhouse Gases.

BIBLIOGRAPHY

Books

Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. Emmaus, PA: Rodale Press, 2006.

Hocking, Colin. Global Warming & the Greenhouse Effect. Berkeley, CA: GEMS, 2002.

Houghton, John. Global Warming: The Complete Briefing Cambridge: Cambridge University Press, 2004.

Morganstein, Stanley. The Greenhouse Effect. Cheshire, UK: Trafford, 2003.

Solomon, S., et al, eds. Climate Change 2007:The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Periodicals

Karl, Thomas R., and Kevin R. Trenberth. “Modern Global Climate Change.” Science 302 (2003): 1,719–1,723.

Web Sites

“Climate Change.” U.S. Environmental Protection Agency (EPA), November 19, 2007. < www.epa.gov/climatechange/> (accessed November 25, 2007).

Bill Freedman

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Greenhouse Effect

Greenhouse Effect

Introduction

The greenhouse effect occurs when Earth or some other planet retains more heat because of the blanketing effect of its atmosphere. Solar energy arrives at the planet’s surface as light (electromagnetic waves) and is retained as heat (random molecular motion). A greenhouse effect causes more heat to be retained than lost. This occurs because certain gases, including carbon dioxide (CO2) and methane (CH4), are transparent to most of the wavelengths of light from the sun but are relatively opaque to infrared or heat radiation, which is radiated by all warm objects, including Earth’s surface and atmosphere. Light arriving from the sun passes through Earth’s atmosphere. Its energy is converted to heat by absorption in the atmosphere and at the surface; this energy is eventually re-radiated into space in the form infrared radiation, but is lost more slowly because of the greenhouse effect.

A similar process is used to heat a solar greenhouse, only using glass, rather than greenhouse gas, as the heat-trapping material. (In an actual greenhouse, the glass traps heat by preventing warm interior air from rising and mixing with the environment, rather than by blocking re-radiation of infrared light; the overall effect, however, is similar.) The greenhouse effect maintains Earth’s surface temperature within a range appropriate for living things. Without it, all of Earth’s surface would be below the freezing point of water.

The greenhouse effect is, to a large extent, a natural phenomenon. However, its intensity has increased because human activities since the beginning of the Industrial Revolution around 1750—especially the burning of fossil fuels and the clearing of forests (which remove CO2 from the atmosphere, store the carbon in cellulose, and release the oxygen back to the atmosphere)—have raised atmospheric concentrations of CO2 and other greenhouse gases. An observed consequence of Earth’s artificially intensified greenhouse effect is a significant warming of the atmosphere.

All but a few scientists agree that this anthropogenic (human-caused) global warming is already occurring and is accelerating. This, in turn, is causing warmer seas, rising sea levels, and changes in patterns of precipitation and weather such as less rain in some places, more in others; more droughts and floods; more violent storms; melting of glaciers; shifts in ranges of plants and animals; and extinctions.

Historical Background and Scientific Foundations

How the Greenhouse Effect Works

The habitability of Earth depends both on its distance from the sun—neither too close nor too far—and on the balance between energy arriving from the sun and being radiated away into space. In the absence of the greenhouse effect, Earth’s surface temperature would average about 0°F (-18°C), which is well below the freezing point of water and colder than life could tolerate over the long term, except for organisms deriving their energy from Earth’s interior heat via hot deep-sea vents. The greenhouse effect maintains Earth’s surface at an average of about 59°F (15°C).

On average, 35% of incoming solar radiation is reflected back to space by Earth’s atmosphere (7%), clouds (24%), and surface (4%). The albedo (reflectivity) of any given area of Earth’s surface is dependent on cloud cover, the density of tiny particulates in the atmosphere (aerosols), and the brightness of the surface, which is high for snow and ice, moderate for water, and relatively low for soil and vegetation.

Another 18% of incoming solar radiation is absorbed by gases (16%) and clouds (2%) in Earth’s atmosphere.

WORDS TO KNOW

AEROSOL: Liquid droplets or minute particles suspended in air.

ALBEDO: A numerical expression describing the ability of an object or planet to reflect light.

BIOMASS: The sum total of living and once-living matter contained within a given geographic area; or, organic matter that can be converted to fuel and is regarded as a potential energy source.

CHLOROFLUOROCARBONS: Chemical compounds containing chlorine, fluorine, carbon, and oxygen. They are widely used in refrigeration and air-conditioning systems and are destructive of the ozone layer in Earth’s stratosphere.

CORAL BLEACHING: Decoloration or whitening of coral from the loss, temporary or permanent, of symbiotic algae (zooxan-thellae) living in the coral.

ECOSYSTEMS: According to the Intergovernmental Panel on Climate Change (and as published in IPCC reports): A system of living organisms interacting with each other and their physical environment. The boundaries of what could be called an ecosystem are somewhat arbitrary, depending on the focus of interest or study. Thus, the extent of an ecosystem may range from very small spatial scales to the entire planet Earth ultimately.

EL NIÑO/SOUTHERN OSCILLATION: A global climate cycle that arises from interaction of ocean and atmospheric circulations. Every 2-7 years, westward-blowing winds over the Pacific subside, allowing warm water to migrate across the Pacific from West to East. This suppresses normal upwelling of cold, nutrient-rich waters in the eastern Pacific, shrinking fish populations and changing weather patterns around the world.

HABITABILITY: The degree to which a given environment can be lived in by human beings. Highly habitable environments can support higher population densities, that is, more people per square mile or kilometer.

INFRARED RADIATION: Electromagnetic radiation of a wavelength shorter than radio waves but longer than visible light that takes the form of heat.

OZONE: An almost colorless, gaseous form of oxygen, with an odor similar to weak chlorine, that is produced when an electric spark or ultraviolet light is passed through air or oxygen.

PLEISTOCENE EPOCH: The geologic period characterized by ice ages in the Northern Hemisphere, from 1.8 million to 10,000 years ago.

TREE RINGS: Marks left in the trunks of woody plants by the annual growth of a new coat or sheath of material. Tree rings provide a straightforward way of dating organic material stored in a tree trunk.

WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. While humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.

Forty-seven percent of incoming solar radiation reaches Earth’s surface and is absorbed. Upon absorption, its energy is transformed into thermal kinetic energy (i.e., heat, the energy of random molecular motion). The warmed surface and atmosphere reradiate most of this energy in all directions as longer-wavelength (7-14 æm) infrared radiation (a small fraction drives winds, currents, and chemical processes). Much of this re-radiated energy escapes to space, and some is re-absorbed by the atmos-phere—where the greenhouse effect occurs.

The amount of energy arriving as short-wavelength sunlight is approximately the amount being reflected as short-wavelength light or radiated as long-wavelength infrared radiation. While Earth is warming, as is happening now, it is radiating slightly less energy than it is absorbing.

Infrared radiation emitted by the land and sea are absorbed by a number of gases in Earth’s atmosphere. The two main constituents of the atmosphere, molecular oxygen (O2, 21% of the atmosphere by volume) and nitrogen (N2, 78%), do not absorb infrared light: they are transparent to it, as they are to visible light. Water vapor (H2O) and CO2, though present in relatively small fractions—0.25% and 0.038%, respectively—are the most important absorbers of infrared light in Earth’s atmosphere. Methane (CH4), nitrous oxide (N2O), ozone (O3), and chlorofluorocarbons (CFCs) play lesser but still significant roles. Some of these gases are much more effective absorbers of infrared than either water or CO2, molecule for molecule, but they contribute less to the greenhouse effect because they are present in much lower concentrations.

Absorbing infrared light warms the greenhouse gases. All warm matter radiates infrared light, so the warmed gases radiate infrared light. Some of this infrared is radiated into space; some is re-absorbed in the atmosphere yet again; some shines down on Earth’s surface, warming it.

This process has been called the “greenhouse effect”because its mechanism is analogous to that by which a glass-enclosed space is heated by sunlight. A greenhouse’s glass and humid atmosphere are transparent to incoming solar radiation, but absorb much of the long-wave infrared light that is radiated from plants and other objects inside the greenhouse. This slows down the rate of cooling of the structure, which reaches equilibrium (gets into balance) with its environment at a higher temperature than it would otherwise.

Other than water vapor, the atmospheric concentrations of all of the greenhouse gases have increased in the past century because of human activities. Prior to 1850, the concentration of CO2 in the atmosphere was about 280 parts per million (ppm); by 2005 it had gone up to 379 ppm, a 36% increase, and was continuing to rise. During the same period, CH4 increased from 0.7 ppm to 1.8 ppm, N2O from 0.285 ppm to 0.319 ppm, and CFCs from nothing to 0.7 parts per billion. These increased concentrations of greenhouses gases have caused a significant increase in the greenhouse effect. Overall, CO2 is estimated to account for about 60% of this enhancement of the greenhouse effect, CH4 for 15%, N2O for 5%, O3 for 8%, and CFCs for 12%.

Water vapor is a special case because although most of the greenhouse effect is caused by water vapor, it enters and leaves the atmosphere very rapidly compared to the other gases. The concentration of water vapor is thus controlled by the amount of other greenhouse gases in the atmosphere. The other greenhouse gases are said to contribute forcing to global warming, while water vapor contributes feedback.

Absorption of solar high-frequency radiation heats air, sea, and land in Earth’s blanketing atmosphere. By slowing Earth’s loss of this energy to outer space through the increased density of the greenhouse gases, the heat increases.

The extra greenhouse effect caused by anthropogenic (human-caused) greenhouse gases is often called global warming. Scientists usually prefer the term “global climate change,” however, because only most, not all parts of Earth’s surface are warming, and because warming is causing many other changes in climates, some apparently in contradiction to warming. For example, melting of glaciers around the southern coasts of Greenland has increased, but so has snowfall in the center of the island: Both effects are predicted by scientists studying global climate change.

The Greenhouse Effect and Climate Change

The mechanism of the greenhouse effect is simple, but the details of global warming are complicated by atmospheric and oceanic circulations and by various feedback mechanisms. Although less than 1% of the solar radiatio absorbed by Earth drives mass-transport processes in the oceans and lower atmosphere (currents and winds), these spread some of Earth’s unevenly distributed thermal energy and are a crucial part of Earth’s weather and climate system. Feedback occurs whenever some outcome of a process affects an earlier part of that process. There are many feedbacks involved with the greenhouse effect and global warming. For example, some of the heat at Earth’s surface causes water to evaporate from plants and open water. Water vapor is a greenhouse gas, so this increased evaporation tends to increase global warming, which tends to increase evaporation further, and so on.

Scientists have long known that the greenhouse effect keeps Earth’s temperature within a livable range for most life. They have also known that concentrations of CO2 and other greenhouse relevant gases have increased in Earth’s atmosphere and will continue to increase for many decades to come. However, until the 1990s it was difficult to show that the observed warming of Earth’s surface and lower atmosphere were being caused mostly by anthropogenic enhancement of the greenhouse effect rather than by some still-unknown process of natural climate change, such as the increased brightness of the sun. Earth’s climate has changed thousands of times over hundreds of millions of years, and it was not scientifically out of the question that such natural processes might be at work today.

Since about 1990, however, many independent lines of evidence have shown not only that the climate is changing rapidly but that human activities, especially emissions of greenhouse gases and deforestation, are the cause of most of this change. Since the beginning of instrumental recordings of surface temperatures around 1850, almost all of the warmest years on record have occurred since the late 1980s. Typically, these warm years have averaged about 1.4 to 1.8°F (0.8 to 1.0°C) warmer than occurred during the decade of the 1880s. Overall, as of 2007, the temperature of Earth’s atmosphere near the surface had risen about 1.33°F (0.74°C) since 1906. More recent years have seen more rapid increases.

These temperature data are not simple to interpret. First, air temperature is variable in time and space, making it necessary to collect data over wide areas and long time periods to detect significant long-term trends. Second, older data are generally less accurate than more recent data. Third, many weather stations are in urban areas, which warm the air around them independently of global climate: This effect must be accounted for when analyzing temperature records. Finally, climate can change for reasons other than a greenhouse response to human-caused increases in greenhouse gases, including naturally shifting ocean and atmospheric circulations and volcanic emissions of sulfur dioxide, sulfate, and fine particulates.

Nevertheless, there are data linking prehistoric variations of atmospheric CO2 (the most important greenhouse gas) and climate warming. Important evidence comes, for example, from cores of ancient ice from Antarctica and Greenland—glacial ice that contained layered ice from annual snowfalls dating back hundreds of thousands of years. Concentrations of CO2 in the ice are determined by analysis of air bubbles in ice layers of known age (determined by counting snowfall layers back from the present, much like tree-rings). Changes in air temperature are inferred from ratios of oxygen isotopes in the ancient ice, so the temperature record can be compared to the CO2 record over long periods of time. Ice cores show that CO2 levels are higher today than at any time in at least 650,000 years.

The fact that changes in CO2 and surface temperature are positively correlated—that is, increased CO2 almost always accompanies warming of climate—suggests a greenhouse mechanism. However, correlation alone, the fact that two things happen together, does not show which thing caused which, or whether both were caused by some other phenomenon. In this case, scientists have argued that increased CO2 might have resulted in warming through an intensified greenhouse effect; others have argued that warming (caused by some unknown process) could have accelerated CO2 release from ecosystems by increasing the rate of decomposition of biomass, especially in cold regions. It is also possible that both effects may occur, either separately or together, one triggering or accelerating the other.

Such questions are resolved using computer models based on the laws of physics that predict climatic changes caused by increases in greenhouse gases or other factors. The most sophisticated simulations are three-dimensional general circulation models (GCMs), which are run on supercomputers. GCMs simulate global atmospheric and ocean circulations and the interaction of these with other variables that contribute to climate. To perform a simulation experiment with a GCM, numbers representing concentrations of CO2 and other greenhouse relevant gases, along with other factors (e.g., solar brightness, vegetation cover, ice melting, and many more), are fed into equations that represent the laws of physics governing circulation, heat, absorption, life processes, atmospheric chemistry, and the like. The goal is to produce the most realistic possible mathematical description of Earth’s climate machine and its inputs, and to use that description or model to understand the behavior of Earth’s past and future climate.

Many experiments have been performed using a variety of GCM models. Their results have varied according to the specifics of the experiment, but a central tendency of experiments using a common CO2 scenario (i.e., a doubling of CO2 from its recent concentration of 379 ppm) is an increase in average surface temperature of 1.8 to 7.2°F (1 to 4°C). This warming is predicted to be especially great in polar regions, where temperature increases could be two or three times greater than in the tropics. Such accelerated warming of the Arctic (north polar region) and the West Antarctic Peninsula has already been observed; central Antarctica is not predicted to warm in this way. Increased CO2 can, therefore, cause global warming, whether or not the reverse process (warming causing increased CO2) may also occur.

Impacts and Issues

Increased warming of global climate is causing significant changes in the quantities, distribution, and timing of precipitation. This, in turn, has a large effect on vegetation, on which all animal life ultimately depends. There is, however, more uncertainty about potential changes in rainfall patterns than about changes in global average temperature. Regional effects of changed precipitation on soil moisture and vegetation are also difficult to predict precisely. Studies of changes in vegetation during the warming climate that followed the most recent (Pleistocene) glaciation suggest that plant species respond to climate change according to the differing tolerances of various species to ranges of temperature, rainfall, and seasonal timing, and their different abilities to colonize newly available habitat.

In a region where the climate becomes drier because of increased evaporation, decreased precipitation, or both, one result can be decreased forest and expanded savanna or prairie. A landscape change of this character is argued to have occurred in the New World tropics during the Pleistocene glaciations. Because of the relatively dry climate at that time, presently continuous rain forest may have been constricted into relatively small, isolated patches. These forest remnants would probably have existed within a landscape matrix of savanna and grassland. Such an enormous restructuring of the character of the tropical landscape must have had a tremendous effect on the multitude of rare species that live in that region. Likewise, climate change potentially associated with an intensification of the greenhouse effect would have a profound effect on Earth’s natural ecosystems and the species that they sustain.

There will also be important changes in the ability of some areas to support crop plants. This would be particularly true of regions that are marginal in terms of rainfall or temperature, and are vulnerable to drought and

desertification. For example, important crops such as wheat are grown in regions of the western interior of North America that formerly supported natural short-grass prairie. Ecologists estimate that about 40% of this semiarid region, measuring 988 million acres (400 million hectares), has already been desertified by agricultural activities, and crop-limiting droughts occur there sporadically. This climatic handicap can be partially offset by irrigation. However, there is a shortage of water for irrigation (which may be enhanced by climate change). The practice of irrigation can cause its own environmental problems, such as salinization of (salt buildup in) soils. Clearly, in many areas substantial changes in climate would place the present agricultural systems at great risk. In a few areas, agriculture will probably be benefited by climate change, but the global balance is likely to be negative.

Patterns of wildfire are also influenced by changes in precipitation patterns. For example, based on the predictions of climate models, there could be a 50% increase in the area of forest annually burned in Canada, presently about 2.5 to 4.9 million acres (1 to 2 million hectares) in typical years. These wildfires would be due, primarily, to the dryness of the forests, with the change in precipitation levels.

Some shallow marine ecosystems are being affected by increases in seawater temperature and by increased ocean acidity caused by higher CO2 concentrations. Corals are especially vulnerable to increases in water temperature, which can deprive them of their symbiotic algae and may kill them. Widespread coral bleaching has been observed, partly due to natural weather cycles such as the El Niño/Southern Oscillation and partly due to anthropogenic climate change.

Another observed effect of warming is rising sea level. This is caused by a combination of a thermal expansion of warmed seawater—water, like most materials, expands slightly when warmed—and the melting of alpine and polar glaciers. GCMs consulted by the United NationsIntergovernmental Panel on Climate Change (IPCC) predicted in 2005 that sea level in 2100 could be 7 to 23 in (0.18 to 59 m) higher than today, with greater rises to follow over centuries or millennia. Since 2005, many scientists have argued that the upper possible range of sea-level rise by 2100 should be much higher than the IPCC’s estimate, perhaps as high as 6.5 ft (2 m), because data have since shown surprisingly fast melting of parts of Greenland’s ice cap. Depending on the rate and magnitude of change in sea level, it could be problematic or even disastrous for low-lying islands and coastal populations.

Most GCMs predict that high latitudes—regions closer to the poles—will experience the greatest intensity of climatic warming, and such areas have, in fact, been observed already to be warming at about twice the average rate of the rest of the world. The warming of northern ecosystems adds positive feedbacks to climate change. These are caused partly by a change of great expanses of arctic tundra from sinks for atmospheric CO2 into sources of CO2 and of methane. The climate warming caused by the rise in greenhouse gases is already increasing the depth of annual thawing of frozen soils, exposing large quantities of carbon-rich organic materials in the permafrost to microbial decomposition, and thereby increasing the emission of CO2 and CH4 to the atmosphere. Melting of Arctic snow and sea ice also decreases albedo, which speeds warming. Arctic sea-ice melting in the summer of 2007 was the greatest ever observed.

It is likely that a further intensification of Earth’s greenhouse effect will occur and have large climatic and ecological consequences. Strategies for managing the causes and consequences of the anthropogenic greenhouse effect include reducing emissions of CO2 and other greenhouse gases. It will also be necessary, at the very least, to adapt human society to whatever amount of climate change can no longer be avoided because of past emissions.

Any strategy to reduce greenhouse emissions will require adjustments by societies and economies. Because large quantities of CO2 are emitted through the burning of fossil fuels, to mitigate (reduce the severity of) global climate change would require societies to use different, possibly new, technologies to generate energy, to increase the efficiency of energy use, and possibly to decrease total energy use. Such a strategy of mitigation will be difficult, especially in poorer countries, because of the changes required in economic systems, resource use, technologies, and lifestyles.

Various international negotiations have been undertaken to try to get nations to agree to decisive actions to reduce their emissions of greenhouse gases. The most recent major agreement came out of a large meeting held in Kyoto, Japan, in 1997. There, most of the world’s industrial countries agreed to reduce their CO2 emissions to 5.2% below 1990 levels by the year 2012. The United States, which has about 5% of the world’s population but produces about 24% of its greenhouse emissions, signed the Kyoto Protocol in 1998 (that is to say, its ambassador to the United Nations signed the plan) but never ratified it as a binding treaty, which would have needed approval from both the president and Congress. International negotiations for a new, more effective treaty to replace the Kyoto Protocol began in late 2007.

As of early 2008, the warmest year in recorded weather history in terms of global average temperature had been 2005, which was 0.94°F (0.52°C) above the average for 1961–1990. (By some calculations, 1998 was slightly warmer than 2005.) So far, the United States has experienced less warming than many other parts of the world; the warmest year on record so far for the United States prior to 2007 was 1934.

Increased extreme weather is another predicted consequence of global climate change. In August 2007, scientists at the World Meteorological Organization, an agency of the United Nations, announced that during the first half of 2007, Earth showed significant increases above long-term global averages in both high temperatures (e.g., heat waves) and frequency of extreme weather events, including heavy rainfalls, cyclones, and wind storms. Global average land temperatures for January and April of 2007 were the warmest recorded for those two months since records began in 1880.

See Also Carbon Dioxide (CO2); Carbon Dioxide (CO2) Emissions; Climate Change; Global Warming; Greenhouse Gases; IPCC 2007 Report; Kyoto Protocol

BIBLIOGRAPHY

Books

Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. Emmaus, PA: Rodale Press, 2006.

Hocking, Colin. Global Warming and the Greenhouse Effect. Berkeley, CA: GEMS, 2002.

Houghton, John. Global Warming: The Complete Briefing. Cambridge, MA: Cambridge University Press, 2004.

Morganstein, Stanley. The Greenhouse Effect. Cheshire, UK: Trafford, 2003.

Periodicals

Karl, Thomas R., and Kevin R. Trenberth. “Modern Global Climate Change.” Science. 302 (2003): 1719–1723.

Web Sites

U.S. Environmental Protection Agency. “Climate Change.” November 19, 2007. http://www.epa.gov/climatechange/ (accessed March 21, 2008).

Bill Freedman

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Greenhouse Effect

Greenhouse effect

The greenhouse effect is the retention by the Earth's atmosphere in the form of heat some of the energy that arrives from the Sun as light . Certain gases, including carbon dioxide (CO2) and methane (CH4), are transparent to most of the wavelengths of light arriving from the Sun but are relatively opaque to infrared or heat radiation ; thus, energy passes through the Earth's atmosphere on arrival, is converted to heat by absorption at the surface and in the atmosphere, and is not easily re-radiated into space. The same process is used to heat a solar greenhouse, only with glass , rather than gas, as the heat-trapping material. The greenhouse effects happens to maintain the Earth's surface temperature within a range comfortable for living things; without it, the Earth's surface would be much colder.

The greenhouse effect is mostly a natural phenomenon, but its intensity, according to a majority of climatologists, may be increasing because of increasing atmospheric concentrations of CO2 and other greenhouse gases. These increased concentrations are occurring because of human activities, especially the burning of fossil fuels and the clearing of forests (which remove CO2 from the atmosphere and store its carbon in cellulose , [C6H10O5]n). A probable consequence of an intensification of Earth's greenhouse effect will be a significant warming of the atmosphere. This in turn would result in important secondary changes, such as a rise in sea level (already occurring), variations in the patterns of precipitation . These, in turn, might accelerate the rate at which species are already being to extinction by human activity, and impose profound adjustments on human society.


The greenhouse effect

The Earth's greenhouse effect is a reasonably well-understood physical phenomenon. Scientists believe that in the absence of the greenhouse effect, Earth's surface temperature would average about -0.4°F (-18°C), which is below the freezing point of water and more frigid than life on the surface of the Earth could tolerate over the longer term—except, perhaps, organisms deriving their energy from hot deep-sea vents. The greenhouse effect maintains Earth's surface at an average temperature of about 59°F (15°C). This is about 59.5°F (33°C) warmer than it would otherwise be.

The energy budget

To understand the greenhouse effect, Earth's energy budget must be known. An energy budget is an account of all of the energy coming into and leaving a system and of any energy that is stored in (or produced by) the system itself. Almost all of the energy coming to Earth from space has been radiated by the closest star , the Sun. The Sun emits electromagnetic energy at a rate and spectral quality determined by its surface temperature. In this it resembles all bodies having a temperature greater than absolute zero (i.e., -459°F or -273°C). Fusion reactions occurring in the core the Sun give it a high surface temperature, about 10,800°F (6,000°C). As a consequence, about one-half of the Sun's emitted energy is visible radiation with wavelengths between 0.4 and 0.7 æm, so called because this is the range of electromagnetic wavelengths that the human eye can perceive. Most of the remainder is in the near-infrared wavelength range, between about 0.7 and 2.0 æm. The Sun also emits radiation in other parts of the electromagnetic spectrum , such as ultraviolet and x rays ; however, these wavelengths convey relatively insignificant amounts of energy away from the Sun.

At the average distance of Earth from the Sun, the rate of input of solar energy to the Earth's surface is about 2 calories per minute per square centimeter, a value termed the solar constant. There is a nearly perfect energetic balance between this quantity of energy incoming to Earth and the amount that is eventually dissipated to outer space. The myriad ways in which the incoming energy is reflected, dispersed, transformed, and stored make up Earth's energy budget.

reflection. On average, one-third of incident solar radiation is reflected back to space by the Earth's atmosphere or its surface. Earth's local reflectivity (albedo ) is strongly dependent on cloud cover, the density of tiny particulates in the atmosphere, and the nature of the surface, especially vegetation and ice and snow.


Atmospheric absorption and radiation

Another one-third of incoming solar radiation is absorbed by certain gases and vapors in Earth's atmosphere, especially water vapor and carbon dioxide. Upon absorption, the solar electromagnetic energy is transformed into thermal kinetic energy (i.e., heat or energy of molecular vibration). The warmed atmosphere then reradiates energy in all directions as longer-wavelength (7–14 æm) infrared radiation. Much of this reradiated energy escapes to outer space.

absorption and radiation at the surface. Much of the solar radiation that penetrates to Earth's surface is absorbed by living and nonliving materials. This results in a transformation to thermal energy, which increases the temperature of the absorbing surfaces and of air in contact with those surfaces. Over the medium term (days) and longer term (years) there is little net storage of energy as heat; almost all of the thermal energy is re-radiated by the surface as electromagnetic radiation of a longer wavelength than that of the original, incident radiation. The wavelength spectrum of typical, reradiated electromagnetic energy from Earth's surface peaks is within the long-wave infrared range.

evaporation and melting of water. Some of the electromagnetic energy that penetrates to Earth's surface is absorbed and transformed to heat. Much of this thermal energy subsequently causes water to evaporate from plant and open-water surfaces, or melts ice and snow.

winds, waves, and currents. A small amount (less than 1%) of the absorbed solar radiation causes mass-transport processes to occur in the oceans and lower atmosphere, which disperses of some of Earth's unevenly distributed thermal energy. The most important of these physical processes are winds and storms, water currents , and waves on the surface of the oceans and lakes.

photosynthesis. Although small, an ecologically critical quantity of solar energy, averaging less than 1% of the total, is absorbed by plant pigments, especially chlorophyll . This absorbed energy is used to drive photosynthesis , the energetic result of which is a temporary storage of energy in the interatomic bonds of certain biochemical compounds. This energy is released when plant material is digested or burned.

Now we are ready to explain the greenhouse effect. If the atmosphere was transparent to the long-wave infrared energy that is reradiated by Earth's atmosphere and surface, then that energy would travel unobstructed to outer space. However, so-called radiatively active gases (or RAGs; also known as "greenhouse gases") in the atmosphere are efficient absorbers within this range of infrared wavelengths, and these substances thereby slow the radiative cooling of the planet . When these atmospheric gases absorb infrared radiation, they develop a larger content of thermal energy, which is then dissipated by a reradiation (again, of a longer wavelength than the electromagnetic energy that was absorbed). Some of the secondarily reradiated energy is directed back to Earth's surface, so the net effect of the RAGs is to slow the rate of cooling of the planet.

This process has been called the "greenhouse effect" because its mechanism is analogous to that by which a glass-enclosed space is heated by solar energy. That is, a greenhouse's glass and humid atmosphere are transparent to incoming solar radiation, but absorb much of the re-radiated, long-wave infrared energy, slowing down the rate of cooling of the structure.

Water vapor (H2O) and CO2 are the most important radiatively active constituents of Earth's atmosphere. Methane (CH4), nitrous oxide (N2O), ozone (O3), and chlorofluorocarbons (CFCs) play lesser roles. On a per-molecule basis, all these gases differ in their ability to absorb infrared wavelengths. Compared with CO2, methane is 11–25 times more effective at absorbing infrared, nitrous oxide is 200–270 times, ozone 2,000 times, and CFCs 3,000–15,000 times.

Other than water vapor, the atmospheric concentrations of all of these gases have increased in the past century because of human activities. Prior to 1850, the concentration of CO2 in the atmosphere was about 280 parts per million (ppm), while 2002 it was over 360 ppm. During the same period, CH4 increased from 0.7 ppm to 1.7 ppm, N2O from 0.285 ppm to 0.304 ppm, and CFCs from nothing to 0.7 parts per billion. These increased concentrations are believed by climatologists to contribute to a significant increase in the greenhouse effect. Overall, CO2 is estimated to account for about 60% of this enhancement of the greenhouse effect, CH4 for 15%, N2O for 5%, O3 for 8%, and CFCs for 12%.


The greenhouse effect and climate change

The physical mechanism of the greenhouse effect is conceptually simple, and this phenomenon is acknowledged by scientists as helping to keep Earth's temperature within the comfort zone for organisms. It is also known that the concentrations of CO2 and other RAGs have increased in Earth's atmosphere, and will continue to do so. However, it has proven difficult to demonstrate that the observed warming of Earth's surface or lower atmosphere has been caused significantly by a stronger greenhouse effect rather than by some still-unknown process of natural climate change.

Since the beginning of instrumental recordings of surface temperatures around 1880, almost all of the warmest years on record have occurred since the late 1980s. Typically, these warm years have averaged about 1.5–2.0°F (0.8-1.0°C) warmer than occurred during the decade of the 1880s. Overall, Earth's surface air temperature has increased by about 0.9°F (0.5°C) since 1850.

However, the temperature data on which these apparent changes are based suffer from some important deficiencies, including: (1) air temperature is variable in time and space, making it difficult to determine statistically significant, longer-term trends; (2) older data are
generally less accurate than modern records; (3) many weather stations are in urban areas, and are influenced by "heat island" effects; and (4) climate can change for reasons other than a greenhouse response to increased concentrations of CO2 and other RAGs, including albedo-related influences of volcanic emissions of sulfur dioxide , sulfate, and fine particulates into the upper atmosphere. Moreover, it has long been thought that the interval 1350 to 1850, known as the Little Ice Age, was relatively cool, and that global climate has been generally warming since that time period. (The data one which this claim was based, however, have recently been called into question; no instrumental or global data at all are available from the period in question.)

However, some studies have provided evidence for linkages between historical variations of atmospheric CO2 and surface temperature. Important evidence comes, for example, from a core of Antarctic glacial ice that represents a 160,000–year period. Concentrations of CO2 in the ice are determined by analysis of air bubbles in ice layers of known age (determined by counting annual snowfall layers back from the present), while changes in air temperature are inferred from ratios of oxygen isotopes in the ancient ice. (Because atoms of various isotopes differ in weight, their rates of diffusion are affected by temperature differently; differences in diffusion rate, in turn, affect their relative abundance in the ice). Because changes in CO2 and surface temperature are positively correlated, a greenhouse mechanism is suggested. However, this study could not determine causal direction—that is, whether increased CO2 might have resulted in warming through an intensified greenhouse effect, or whether, conversely, warming (caused by something unknown) could have accelerated CO2 release from ecosystems by increasing the rate of decomposition of biomass , especially in cold regions.

Because of the difficulties in measurement and interpretation of climatic change using real-world data, computer models have been used to predict potential climatic changes caused by increases in atmospheric RAGs. The most sophisticated simulations are the socalled "three-dimensional general circulation models" (GCMs), which are run on supercomputers. GCM models simulate the extremely complex mass-transport processes involved in atmospheric circulation and the interaction of these processes with other variables that contribute to climate. To perform a simulation "experiment" with a GCM model, components are adjusted to reflect the probable physical influence of increased concentrations of CO2 and other RAGs.

Many simulation experiments have been performed using a variety of GCM models. Their results have, of course, varied according to the specifics of the experiment. However, a central tendency of experiments using a common CO2 scenario (i.e., a doubling of CO2 from its recent concentration of 360 ppm) is an increase in average surface temperature of 1.8–7.2°F (1–4°C). This warming is predicted to be especially great in polar regions, where temperature increases could be two or three times greater than in the tropics.

One of the best-known models was designed by the International Panel on Climate Change (IPCC). This GCM model makes assumptions about population and economic growth, resource availability, and management options that result in increases or decreases of RAGs in the atmosphere. Scenarios were developed for emissions of CO2, other RAGs, and sulfate aerosols , which may cool the atmosphere by increasing its albedo and by affecting cloud formation. For a simple doubling of atmospheric CO2, the IPCC estimate was a 4.5°F (2.5°C) increase in average surface temperature. The estimates of more advanced IPCC scenarios (with adjustments for other RAGs and sulfate) were similar, and predicted a 2.7–5.4°F (1.5–3°C) increase in temperature by the year 2100, compared with 1990. Thus, theoretical studies tend to back the claim that CO2 can cause global warming , whether or not the reverse process may also occur.


Effects of climatic change

It is likely that the direct effects of climate change caused by an intensification of the greenhouse effect would be substantially restricted to plants. The temperature changes might cause large changes in the quantities, distribution, or timing of precipitation, and this would have a large effect on vegetation. There is, however, even more uncertainty about the potential changes in rainfall patterns than of temperature, and effects on soil moisture and vegetation are also uncertain. Still, it is reasonable to predict that any large changes in patterns of precipitation would result in fundamental reorganizations of vegetation on the terrestrial landscape.

Studies of changes in vegetation during the warming climate that followed the most recent, Pleistocene, glaciation, suggest that plant species responded in unique, individualistic ways. This results from the differing tolerances of species to changes in climate and other aspects of the environment, and their different abilities to colonize newly available habitat . In any event, the species composition of plant communities was different then from what occurs at the present time. Of course, the vegetation was, and is, dynamic, because plant species have not completed their post-glacial movements into suitable habitats.

In any region where the climate becomes drier (for example, because of decreased precipitation), a result could be a decreased area of forest, and an expansion of savanna or prairie . A landscape change of this character is believed to have occurred in the New World tropics during the Pleistocene glaciations. Because of the relatively dry climate at that time, presently continuous rainforest may have been constricted into relatively small refugia (that is, isolated patches). These forest remnants may have existed within a landscape matrix of savanna and grassland. Such an enormous restructuring of the character of the tropical landscape must have had a tremendous effect on the multitude of rare species that live in that region. Likewise, climate change potentially associated with an intensification of the greenhouse effect would have a devastating effect on Earth's natural ecosystems and the species that they sustain.

There would also be important changes in the ability of the land to support crop plants. This would be particularly true of lands cultivated in regions that are marginal in terms of rainfall, and are vulnerable to drought and desertification . For example, important crops such as wheat are grown in regions of the western interior of North America that formerly supported natural short-grass prairie. It has been estimated that about 40% of this semiarid region, measuring 988 million acres (400 million hectares), has already been desertified by agricultural activities, and crop-limiting droughts occur there sporadically. This climatic handicap can be partially managed by irrigation . However, there is a shortage of water for irrigation, and this practice can cause its own environmental problems, such as salinization. Clearly, in many areas substantial changes in climate would place the present agricultural systems at great risk.

Patterns of wildfire would also be influenced by changes in precipitation regimes. Based on the predictions of climate models, it has been suggested that there could be a 50% increase in the area of forest annually burned in Canada, presently about 2.5-4.9 million acres (1-2 million hectares) in typical years.

Some shallow marine ecosystems might be affected by increases in seawater temperature. Corals are vulnerable to large increases in water temperature, which may deprive them of their symbiotic algae (called zooxanthellae), sometimes resulting in death of the colony. Widespread coral "bleachings" were apparently caused by warm water associated with an El Niño event in 1982-83.

Another probable effect of warming could be an increase in sea level. This would be caused by the combination of (1) a thermal expansion of the volume of warmed seawater, and (2) melting of polar glaciers . The IPCC models predicted that sea level in 2100 could be 10.5-21 in (27-50 cm) higher than today. Depending on the rate of change in sea level, there could be substantial problems for low-lying, coastal agricultural areas and cities.

Most GCM models predict that high latitudes will experience the greatest intensity of climatic warming. Ecologists have suggested that the warming of northern ecosystems could induce a positive feedback to climate change. This could be caused by a change of great expanses of boreal forest and arctic tundra from sinks for atmospheric CO2, into sources of that greenhouse gas. In this scenario, the climate warming caused by increases in RAGs would increase the depth of annual thawing of frozen soils, exposing large quantities of carbon-rich organic materials in the permafrost to microbial decomposition, and thereby increasing the emission of CO2 to the atmosphere.


Reducing atmospheric RAGs

It is likely that an intensification of Earth's greenhouse effect would have large climatic and ecological consequences. Clearly, any sensible strategy for managing the causes and consequences of changes in the ggreenhouse effect will requir substantial reductions in the emissions of CO2 and other RAGs.

It is important to recognize that any strategy to reduce these emissions will require great adjustments by society and economies. Because such large quantities of CO2 are emitted through the burning of fossil fuels, there will be a need to use different, possibly new, technologies to generate energy, and there may be a need for large decreases in total energy use. The bottom line, of course, will be a requirement to add considerably smaller quantities of RAGs to the atmosphere. Such a strategy of mitigation will be difficult, especially in industrialized countries, because of the changes required in economic systems, resource use, investments in technology, and levels of living standards. The implementation of those changes will require enlightened and forceful leadership.

Under the auspices of the United Nations Environment Program, various international negotiations have been undertaken to try to get nations to agree to decisive actions to reduce their emissions of RAGs. The most recent major agreement came out of a large meeting held in Kyoto, Japan, in 1997. There, most of the world's industrial countries agreed to reduce their CO2 emissions to 5.2% below 1990 levels by the year 2012. The United States, which has about 5% of the world's population but produces 24% of its CO2 emissions, signed the Kyoto protocol in 1998 (that is to say, its ambassador to the United Nations signed the plan) but never ratified it as a binding treaty; shortly after taking office in 2000, President George W. Bush repudiated the protocol entirely. (China, with about 23% of the world's population, is the second-biggest CO2 producer, at 14% of total emissions.)

A complementary way to balance the emissions of RAGs would be to remove some atmospheric CO2 by increasing its fixation by growing plants, especially through the planting of forests onto agricultural land. Similarly, the prevention of deforestation will avoid large amounts of CO2 emissions through the conversion of high-carbon forests into low-carbon agro-ecosystems.

The development and maintenance of ecosystems that store large quantities of carbon to offset industrial emissions would require very large areas of land. These carbon reserves would preclude other types of economically important uses of the land. This strategy would therefore require a substantial commitment by society; however, so would any other possible means of decreasing greenhouse gases, and so would a decision to do nothing at all (or to keep researching the problem indefinitely, which amounts to much the same thing). There are no easy solutions to problems of this type and magnitude.

See also Air pollution; Energy budgets; Hydrochlorofluorocarbons; Ozone layer depletion.


Resources

books

Hamblin, W.K., and Christiansen, E.H. Earth's Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001.

Hancock P.L. and Skinner B.J., eds. The Oxford Companion to the Earth. New York: Oxford University Press, 2000.

periodicals

Kerr, R.A. American Association for the Advancement of Science (AAAS). "Clearing the Air—Global warming: Rising Global Temperature, Rising Uncertainty. Greenhouse Warming Passes One More Test. Science. 292 (2001): 267.

Schneider, S. H. "The Changing Climate." Scientific American. 261 (1989): 70-79.

other

Nebehay, Stephanie. "2002 Second Hottest as Global Warming Speeds, Says WMO." Reuters. December 18, 2002 [cited January 6, 2003]. <http://www.enn.com/news/wirestories/2002/12/121820 02/reu_49197.asp>.


Bill Freedman
Larry Gilman

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Albedo

—Refers to the reflectivity of a surface.

Carbon reserve

—An ecosystem, such as a forest, that is managed primarily for its ability to store large quantities of organic carbon, and to thereby offset or prevent an emission of carbon dioxide to the atmosphere.

Desertification

—A climatic change involving decreased precipitation, causing a decreased or destroyed biological productivity on the landscape, ultimately leading to desert-like conditions.

Electromagnetic energy

—A type of energy, involving photons, which have physical properties of both particles and waves. Electromagnetic energy is divided into spectral components, which (ordered from long to short wavelength) include radio, infrared, visible light, ultraviolet, and cosmic.

Energy budget

—A physical accounting of the various inputs and outputs of energy for some system, as well as the quantities and locations where energy is internally stored.

Radiatively active gases (RAGs)

—Within the context of the greenhouse effect, these gases absorb long-wave infrared energy emitted by Earth's surface and atmosphere, and thereby slow the rate of radiative cooling by the planet.

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Greenhouse Effect

Greenhouse Effect


The greenhouse effect is the name given to the trapping of heat in the lower atmosphere and the warming of Earth's surface that results. Although a natural phenomenon, this warming effect tends to increase as more human-produced gases are released into the atmosphere. This increased warming could result in climate changes that affect crops, as well as melting glaciers and coastal flooding.

Only in the past few decades has the term "greenhouse effect" implied something bad for the environment. In fact, the phenomenon that it describes is absolutely essential to life on Earth. Without it, Earth would be a cold and lifeless planet. What this phenomenon does is hold on to some of the heat given off by the Sun. Specifically, Earth's atmosphere is the mixture of gases and water vapor that surrounds the entire Earth. Composed mainly of oxygen and nitrogen as well as small amounts of other trace gases, the atmosphere is essential to photosynthesis (the process a plant uses light to make food). It also protects organisms from the Sun's infrared rays because it absorbs much of these. Since the atmosphere is located physically between the surface of Earth and the Sun, the molecules of gas that make it up act somewhat like a pane of glass in a greenhouse, which is where the name "greenhouse effect" originates. The glass on a greenhouse lets light in and out but holds in its heat. What the atmosphere does is allow wavelengths of visible light from the Sun to reach Earth's surface. While doing that, however, the atmosphere blocks the escape into space of the longer infrared wavelengths. That is, they trap the light's heat by reabsorbing these wavelengths, much of which get sent back down to Earth again. Overall, this makes Earth a warm place that is hospitable to life.

Human activities have begun to alter this process of capturing heat, however. The result of these activities may be the experiencing of too much of a good thing. Too much of a greenhouse effect means that things are heating up too fast. In the past few decades, human activities have begun to change, if not harm, the atmosphere, so that it is trapping more heat than it should. Specifically, our steady burning of fossil fuels (coal, oil, natural gas) has increased the amount of carbon dioxide in the atmosphere. Carbon dioxide is the most important gas in the greenhouse effect, since it is the molecules of carbon dioxide that do the actual absorbing of long-wave infrared radiation. The more carbon dioxide in the atmosphere, the more heat it keeps in. Besides all of the carbon dioxide being pumped into the air by car exhausts and factories, there is the added problem that humans

are vastly reducing the world's forests, whose trees and plants take in carbon dioxide and give off oxygen as part of photosynthesis.

Since humans are artificially warming Earth this way, many scientists feel that Earth is already beginning to experience the negative consequences. Climatologists (scientists who study Earth's climate or weather) now believe that Earth's long-term climate patterns are changing. Some project as much as a 4 or 5 degree increase by the middle of the twenty-first century. This could have disastrous effects, since such global warming could cause the polar ice caps and the mountain glaciers to melt. This could result in mass flooding, making many islands disappear, and putting coastal cities completely under water. Since it is climate that mostly determines what will grow where, crops also could be seriously affected. A global temperature rise could produce entirely new patterns and extremes of rainfall or drought in certain areas.

SVANT AUGUST ARRHENIUS

Swedish chemist Svant Arrhenius (1859–1927) is considered to be the founder of physical chemistry, a fairly new field which blends chemistry and physics. He not only contributed to the founding of a new branch of science, but added a new concept, now called the "greenhouse effect," to the study of the life sciences.

Svant Arrhenius was a child prodigy (exceptionally smart) who taught himself to read at three years of age. Born at Vik, Sweden, the youngster is said to have become interested in mathematics while watching his father, a surveyor (a person who determines boundaries), add columns of figures. Naturally brilliant in school, he earned a bachelor's degree from the University of Uppsala at the age of nineteen while studying physics, mathematics, and chemistry. For his doctoral dissertation in 1884, he offered a theory that explained what occurred when electricity passes through a solution at the atomic level. His ideas were so revolutionary, however, that his committee gave him the lowest passing grade possible. Nineteen years later, Arrhenius would receive the Nobel Prize in Chemistry for the same work that had barely earned him a degree.

All his life, Arrhenius was regularly pushing the limits of science, and in 1908 he published a book titled Worlds in the Making that marked him as one of the forerunners of molecular biology. Molecular biology is the study of the complex chemicals, like proteins and nucleic acids, that make up living things. In this work, Arrhenius offered what he called the universality of life, meaning that life was not to be found only on Earth. It was his belief that life on Earth had sprung from "spores" that had been driven though outer space by radiation pressure from other planets. This meant to him that life had been spread throughout the universe and took hold wherever conditions were favorable. There are now many physical reasons why his spore theory is not correct, and it also offers no explanation as to where or how life originated in the first place.

It was in this book, however, that Arrhenius also suggested what we now call the greenhouse effect. There he speculated that carbon dioxide gas in the atmosphere heats Earth by first allowing sunlight to reach its surface, and then trapping much of the heat radiation or preventing the heat from escaping back into space. He argued that because of this phenomenon, any rise in the amount of carbon dioxide in the atmosphere would raise Earth's temperature since it would act like a greenhouse and trap even more of the heat. He also argued that the reverse could happen and that a major decrease in carbon dioxide could result in a cooling effect that might even cause another Ice Age. Today, many scientists think that global warming is in fact occurring, and that it is caused by the carbon dioxide released when fossil fuels, like coal, are burned in factories and power plants. Arrhenius was certainly a man ahead of his time.

Steps are being taken to prevent possible disastrous effects from happening. Efforts to reduce the pumping of greenhouse gases (carbon dioxide, methane, and nitrous oxide) into the atmosphere have begun in certain countries. This involves making automobile engines more efficient and clean, or better still, encouraging the use of public transportation. Electric cars soon may become practical. The use of nitrogen-based fertilizers can be reduced, and the destruction of entire forests can be stopped. International agreements have already been made to reduce the production of such gases by industry. Many scientists point to the nearby planet Venus as an example of what could happen if a "runaway" greenhouse effect ever occurs. The atmosphere around Venus keeps its surface temperatures as high as 932°F (504°C). To date, there is no evidence of life on Venus.

[See alsoCarbon Dioxide; Forests; Ozone; Pollution; Rain Forests ]

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