Greenhouse Effect

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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.