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.