Definition (Encyclopedia of Global Warming)
The greenhouse effect warms the lower portion of a planet’s atmosphere when heat is trapped there by gases—such as water vapor, carbon dioxide (CO2), methane, and nitrous oxides—that prevent it from escaping into space. As a result of their molecular structure, these gases are dominant absorbers and emitters of infrared radiation. They absorb infrared energy from a planet’s surface and reemit it in all directions. A significant fraction of this reradiated energy is directed back to the planet’s surface, resulting in an increase in average temperatures. This effect is somewhat analogous to the trapping of heat by a greenhouse, but the retention of heat in a greenhouse is due mostly to reduced cooling caused by the prevention of convection: Only a small amount is due to trapped infrared radiation.
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Significance for Climate Change (Encyclopedia of Global Warming)
The greenhouse effect is a natural phenomenon that has been occurring on Earth and other planets for millions of years. It allows Earth to support life. If heat were not trapped in Earth’s atmosphere, the planet would be approximately 33° Celsius cooler than it is now. A large percentage of Earth’s natural greenhouse warming is caused by water vapor. If the greenhouse effect were enhanced, the Earth would become warmer, which could cause problems for humans, plants, and animals.
In the mid-1950’s, an enhanced greenhouse effect was recognized as a concern. As a result of anthropogenic (human-induced) activities, atmospheric concentrations of the greenhouse gases (GHGs) were on the rise. This trend was associated with an increasing global atmospheric temperature. Industrialization resulted in an increase in the use of fossil fuels, which increased GHG emissions. The global mean annual temperature rose by approximately 0.5° Celsius between 1890 and 2000. Most of that increase occurred after 1970.
In addition to rising global temperature, observations of glaciers indicate that more of them are retreating than are growing. For example, eight glaciers that were advancing on Mount Baker in the northern Cascades in 1976 were all melting back at their termini by 1990. Observed climate changes associated with rising global temperatures, retreating glaciers, and increased polar ice melting are consistent...
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Overview (The Solar System)
The Earth and the other planets in the Solar system receive almost all their energy in the form of electromagnetic radiation from the Sun. While the Sun emits radiation over the entire electromagnetic spectrum, from shortwave X rays to longwave radio waves, the bulk of the Solar radiation is in the ultraviolet, visible, and infrared parts of the electromagnetic spectrum, from about 0.15 to about 4 microns, or about 150 to about 4,000 nanometers. The peak of the Sun’s radiated energy is in the visible part of the spectrum, at about 0.5 micron, or 500 nanometers, as that is the Wavelength of maximum emission for an object at a temperature of about 6,000 kelvins, which is the temperature of the Sun’s “visible surface,” the photosphere. The intensity falls off toward shorter (bluer) and longer (redder) wavelengths, and the human eye perceives this distribution of intensity versus wavelength as a yellowish-white color. Hence, the Sun appears as a yellowish-white object in the sky.
The amount of solar radiation intercepted by a planet and potentially available for heating it depends on the Sun’s luminosity, the planet’s distance from the Sun, and the planet’s cross-sectional area. For example, at Earth’s distance from the Sun, about 150 million kilometers, the flux of solar radiation, also called the solar constant, is 1,368 joules per second per square meter multiplied by Earth’s cross-sectional area of 1.28 1014...
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Methods of Study (The Solar System)
Studies of the greenhouse effect and its impact on the Earth’s climate are multidisciplinary and involve theoretical computer modeling, laboratory studies of the spectroscopic parameters and properties of GHGs, atmospheric measurements of GHGs, and aircraft and satellite measurements of parameters that control climate. Theoretical computer models of climate include zero-dimensional, one-dimensional, two-dimensional, and three-dimensional models. Zero-dimensional models give climate parameters that represent an average for the entire system, such as the mean temperature of the Earth’s surface. One-dimensional models are used to study climate in either a horizontal (latitude) or vertical (altitude) direction. In these models, a latitude-dependent surface temperature is the climate parameter of major interest. The vertical one-dimensional model is known as a radiative-convective model and is used to study the effects of changes in concentrations of GHGs on the surface temperature.
Two-dimensional models involve characterizing the temperature variation as a function of both Latitude and altitude. The most complex climate model is the three-dimensional model, or the general circulation model (GCM). This model gives a complete description of climate as a function of latitude, longitude, and altitude.
Major uncertainties exist in the understanding of several key parameters in the theoretical modeling of climate. A major...
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Context (The Solar System)
The buildup in the atmosphere of GHGs such as carbon dioxide, methane, nitrous oxide, tropospheric ozone, CFC-11, and CFC-12, could result in a global warming of the Earth. Atmospheric GHGs are increasing with time. These GHGs result from a variety of human activities, including the burning of fossil fuels (carbon dioxide), the burning of living and dead biomass (carbon dioxide, methane, and nitrous oxide), and the application of nitrogen fertilizers and burning of agricultural and grasslands (nitrous oxide). GHGs are also produced from rice paddies, cattle, and sheep (methane), and from several industrial applications (CFC-11 and CFC-12).
A global warming of the Earth from the buildup of GHGs in the atmosphere would have a significant impact on people’s daily lives. Most areas would experience more days per year when temperatures exceed 32° Celsius, and the growing seasons and patterns of rainfall would change. One of the most important effects would be a predicted increase in the height of the world’s oceans. The increased height of the oceans would result from the thermal expansion of seawater because of the Earth’s high temperature (water is a compressible fluid and expands in volume when heated) and because of the melting of polar ice and snow as the Earth becomes warmer. It has been estimated that a global temperature increase of about 4 kelvins might result in a 2-meter increase in the height of the world’s oceans....
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Further Reading (The Solar System)
Ahrens, C. Donald. Essentials of Meteorology. 5th ed. Florence, Ky.: Brooks/Cole, 2007. A well-written, student-friendly introductory meteorology textbook. Includes a CD-ROM to aid self-instruction.
Andrews, David. An Introduction to Atmospheric Physics. Cambridge, England: Cambridge University Press, 2000. A well written college textbook. Includes information on the enhanced greenhouse effect, ozone depletion, and development of weather systems. Can be technical.
Cook, Alex. The Greenhouse Effect: A Legacy. Indianapolis: Dog Ear, 2007. A good introductory work for the general audience. Facts are presented through an easy-to-read narrative.
Environmental Protection Agency. “The Greenhouse Effect: How It Can Change Our Lives.” EPA Journal 15 (January/February, 1989). Popular, nontechnical accounts of the impact of climate change on agriculture, forests, energy demand, and other areas in a special issue devoted to the greenhouse effect. The principles that control and regulate global climate, including the greenhouse effect, are presented simply for a general audience. Well illustrated with photographs and charts.
Frederick, John. Principles of Atmospheric Science. Sudbury, Mass.: Jones and Bartlett, 2008. A comprehensive work covering all areas of atmospheric physics, including the greenhouse effect. Geared toward undergraduate students.
Goody, R. M.,...
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Greenhouse effect (Encyclopedia of Environmental Issues, Revised Edition)
Since 1880, at which point historical measurement records become reliable enough and of sufficient spatial distribution to provide an overview of recent global climate trends, the earth’s surface atmospheric temperatures have on average become warmer. During this period—and since the mid-eighteenth century, when the Industrial Revolution began—human activity has released increasing quantities of what are known as greenhouse gases into the atmosphere. These gases include naturally occurring substances such as carbon dioxide, methane, nitrous oxide, and ozone, as well as synthetic chemicals such as chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride. (Water vapor is the most abundant of the naturally occurring greenhouse gases, but human activity has an insignificant influence on its atmospheric concentrations.) Anthropogenic (human-caused) greenhouse gases have been identified as likely contributors to the rise in global surface temperature.
The temperature increase may lead to drastic changes in climate and food production, as well as widespread coastal flooding. As a result, many scientists, organizations, and governments have called for curbs on greenhouse gas emissions. Because their predictions are not definite, however, debate continues about the financial and societal costs of reducing the production of these gases given the lack of certainty regarding the benefits....
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Global Warming and Human Interference (Encyclopedia of Environmental Issues, Revised Edition)
The naturally occurring greenhouse effect takes place because the gases that make up the atmosphere are able to absorb only particular wavelengths of energy. The atmosphere is largely transparent to shortwave solar radiation, so sunlight basically passes through the atmosphere to the earth’s surface. Some is reflected or absorbed by clouds, some is reflected from the earth’s surface, and some is absorbed by dust or the earth’s surface. Only small amounts are actually absorbed by the atmosphere. Sunlight therefore contributes very little to the direct heating of the atmosphere. On the other hand, the greenhouse gases are able to absorb longwave, or infrared, radiation from the earth, thereby heating the earth’s atmosphere.
Discussion of the greenhouse effect has been confused by terms that are imprecise and even inaccurate. For example, when the term “greenhouse effect” was coined during the early nineteenth century, the atmosphere was believed to operate in a manner similar to that of a greenhouse, in which glass lets visible solar energy in but is also a barrier preventing the heat energy from leaving. In actuality, the reason that the air remains warmer inside a greenhouse is probably because the glass prevents the warm air from mixing with the cooler outside air. Therefore the greenhouse effect could more accurately be called the “atmospheric effect,” but the term “greenhouse...
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Problems of Prediction (Encyclopedia of Environmental Issues, Revised Edition)
How much the temperature of the earth might rise as a result of an intensified greenhouse effect is not clear. So far, the temperature increase of around half a degree Celsius (a single degree Fahrenheit) since the late nineteenth century is within the range of normal (historical) trends. The possibility of global warming became a serious concern during the late twentieth century because the decades of the 1980’s and the 1990’s included some of the hottest years recorded for more than one hundred years. This trend continued into the twenty-first century, with the decade from 2000 to 2009 establishing itself as the warmest on record. On the other hand, warming has not been consistent since 1880. For example, there was a slight global cooling from the 1940’s to the 1970’s, possibly a result of the increase of another product of fossil-fuel combustion, sulfur dioxide aerosols, which reflect sunlight and thus lessen the amount of solar energy entering the atmosphere. The reduction in sulfur dioxide emissions following the implementation of pollution controls after 1970 may account for the subsequent observed rise in temperature. Similarly, during the early 1990’s temperatures declined, most likely because of the ash and sulfur dioxide ejected into the atmosphere during the 1991 eruption of Mount Pinatubo (the second-largest terrestrial volcanic eruption of the twentieth century). By the late 1990’s temperatures were...
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Mitigation Attempts (Encyclopedia of Environmental Issues, Revised Edition)
International conferences have been held and international organizations have been established to research and minimize the potential detriments of global warming. In 1988 the United Nations Environment Programme and the World Meteorological Organization established the IPCC to assess and compile climate change information for use by policy makers. The IPCC issued assessment reports in 1990, 1995, 2001, and 2007; its fifth assessment report is due in 2013 or 2014. The first of the IPCC’s assessment reports concluded that global warming was sure to result if human emissions of greenhouse gases were not brought under control.
In June, 1992, the United Nations Conference on Environment and Development, also known as the Earth Summit, was held in Brazil. Participants devised the United Nations Framework Convention on Climate Change, considered a landmark international treaty. It required signatories to reduce and monitor their greenhouse gas emissions. Developed nations agreed on a voluntary year 2000 target of stabilizing their emissions at 1990 levels, a goal that many ratifying governments failed to meet.
A binding agreement, the Kyoto Protocol, was adopted in December, 1997, and entered into force in February, 2005. It set compulsory targets for reducing emissions of carbon dioxide, methane, nitrous oxide, sulfur hexafluoride, HFCs, and PFCs over the five-year period of 2008 through 2012 for thirty-seven...
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Further Reading (Encyclopedia of Environmental Issues, Revised Edition)
Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. New York: Rodale Books, 2006.
Houghton, John Theodore. Global Warming: The Complete Briefing. 4th ed. New York: Cambridge University Press, 2010.
Lankford, Ronald D., ed. Greenhouse Gases. Detroit: Greenhaven Press, 2009.
Schneider, Stephen Henry, et al., eds. Climate Change Science and Policy. Washington, D.C.: Island Press, 2010.
Shulk, Bernard F., ed. Greenhouse Gases and Their Impact. New York: Nova Science, 2007.
Weart, Spencer W. The Discovery of Global Warming. Rev. ed. Cambridge, Mass.: Harvard University Press, 2008.
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Greenhouse Effect (Encyclopedia of Science)
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...
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Greenhouse Effect (Science Experiments)
We are all affected by the greenhouse effect
Fossil Fuels: What happens when fossil fuels burn?
In 1827, a French mathematician named Jean-Baptiste-Joseph Fourier came up with an interesting theory. He said Earth's atmosphereLayers of air that surround Earth. protected its inhabitants against the freezing temperatures of space. Fourier pointed out that Earth's atmosphere acted as an , an effect similar to what happens when heat is trapped within the glass walls and roof of a greenhouse. He called his theory the greenhouse effectThe warming of Earth's atmosphere due to water vapor, carbon dioxide, and other gases in the atmosphere that trap heat radiated from Earth's surface..
Today we know that the greenhouse effect takes place when sunlight passes through the atmosphere and is absorbed by land and water. The energy in the sunlight is converted to heat energy to warm the surface of Earth. Some of this heat energy is re-radiated out into the atmosphere in the form of . The infrared radiation has a longer wavelength than the sunlight and is absorbed by certain gases in the atmosphere, such as carbon dioxide. This traps the heat, keeping Earth's surface warm....
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Greenhouse Effect (Encyclopedia of Science and Religion)
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
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
Greenhouse Gases and Greenhouse Effect (World of Earth Science)
The greenhouse effect is the physical mechanism by which the atmosphere helps to maintain Earth's surface temperature within a range comfortable for organisms and ecological processes. The greenhouse effect is largely a natural phenomenon, but its intensity may be changing because of increasing concentrations of carbon dioxide and some other gases in the atmosphere. These increased concentrations are occurring as a result of human activities, especially the burning of fossil fuels and the clearing of forests. A probable consequence of an intensification of Earth's greenhouse effect will be a significant warming of the atmosphere. This could likely result in important secondary changes, such as a rise in sea level, variations in the patterns of precipitation, and large and difficult ecological and socio-economic adjustments.
Earth's greenhouse effect is a well-understood physical phenomenon. Scientists believe that in the absence of the greenhouse effect, Earth's surface temperature would average about .4°F (8°C), which is colder than the freezing point of water, and more frigid than life could tolerate long term. By slowing the rate at which the planet cools itself, the greenhouse effect helps to maintain 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, and is within the range of temperature that life can tolerate.
An energy budget is a physical analysis of all of the energy coming into a system, all the energy going out, and any difference that might be internally transformed or stored. Almost all of the energy coming to Earth from outer 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 temperaturell bodies do this, as long as they have a temperature greater than absolute zero, or 59°F (73°C). Fusion reactions occurring within the Sun maintain an extremely hot surface temperature, about 10,800°F (6,000°C). As a direct consequence of this surface temperature, about one-half of the Sun's emitted energy is so-called "visible" radiation with wavelengths between 0.4 and 0.7 µm (this is called visible radiation because it is the range of electromagnetic energy that the human eye can perceive), and about one-half is in the near-infrared wavelength range between about 0.7 and 2.0 µm. The Sun also emits radiation in other parts of the electromagnetic spectrum, such as ultraviolet and cosmic radiation. However, these are relatively insignificant amounts of energy (although even small doses can cause biological damage).
At the average distance of Earth from the Sun, the rate of input of solar energy is about 2 cal cm min, a value referred to as the solar constant. There is a nearly perfect energetic balance between this quantity of electromagnetic energy incoming to Earth, and the amount that is eventually dissipated back to outer space. The myriad ways in which the incoming energy is dispersed, transformed, and stored make up Earth's energy budget.
On average, one-third of incident solar radiation is reflected back to space by the earth's atmosphere or its surface. The planet's reflectivity (or albedo) is strongly dependent on cloud cover, the density of tiny particulates in the atmosphere, and the nature of the surface, especially the cover of vegetation and water, including ice and snow.
Another one-third of the incoming 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 (that is, heat, or energy of molecular vibration). The warmed atmosphere then re-radiates energy in all directions as longer-wavelength (74 µm) infrared radiation. Much of this re-radiated energy escapes to outer space.
The remaining one-third of the incoming energy from the Sun is transformed or dissipated by the following processes:
Absorption and radiation at the surface
Much of the solar radiation that penetrates to Earth's surface is absorbed by living and non-living materials. This results in a transformation to thermal energy, which increases the temperature of the absorbing surfaces. Over the medium term (days) and longer term (years) there is little net storage of energy as heat. This occurs because 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, re-radiated electromagnetic energy from Earth's surface peaks at about 10 µm, which 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 inorganic surfaces, or it causes ice and snow to melt.
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.
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 inter-atomic bonds of biochemical compounds.
If the atmosphere was transparent to the long-wave infrared energy that is re-radiated 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 re-radiation (again, of a longer wavelength than the electromagnetic energy that was absorbed). Some of the secondarily re-radiated 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 green-house'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 carbon dioxide (CO2) are the most important radiatively active constituents of Earth's atmosphere. Methane (CH4), nitrous oxide (N2O), ozone (O3), and chlorofluorocarbons (CFCs) play a more minor role. On a per-molecule basis, these gases differ in their ability to absorb infrared wavelengths. Compared with carbon dioxide, a molecule of methane is 115 times more effective at absorbing infrared, nitrous oxide is 20070 times, ozone 2,000 times, and CFCs 3,0005,000 times.
Other than water vapor, the atmospheric concentrations of all of these gases have increased in the past century because of emissions associated with human activities. Prior to 1850, the concentration of CO2 in the atmosphere was about 280 ppm, while in 1994 it was 355 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 zero to 0.7 ppb. These increased concentrations are believed to contribute to a hypothesized increase in the intensity of Earth's greenhouse effect, an increase attributable to human activities. Overall, CO2 is estimated to account for about 60% of this enhancement of the greenhouse effect, CH4 15%, N2O 5%, O3 8%, and CFCs 12%.
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 a warming of Earth's surface or lower atmosphere has been caused by a stronger greenhouse effect.
Since the beginning of instrumental recordings of surface temperature around 1880, it appears that almost all of the warmest years have occurred during the late 1980s and 1990s. Typically, these warm years have averaged about 1.5.0°F (0.8.0°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 is well known 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.
Some studies have provided evidence for linkages between historical variations of atmospheric CO2 and surface temperature. Important evidence comes from a core of Antarctic glacial ice that represents a 160,000-year time period. Concentrations of CO2 in the ice were determined by analysis of air bubbles in layers of known age, while changes in air temperature were inferred from ratios of oxygen isotopes (because isotopes differ in weight, their rates of diffusion are affected by temperature in predictably different ways, and this affects their relative concentrations in the glacial ice). Because changes in CO2 and surface temperature were positively correlated, a potential greenhouse mechanism is suggested. However, this study could not determine whether increased CO2 might have resulted in warming through an intensified greenhouse effect, or whether warming could have increased 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 with 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. Of course, the results vary according to the specifics of the experiment. However, a central tendency of experiments using a common CO2 scenario (a doubling of CO2 from its recent concentration of 360 ppm) is for an increase in average surface temperature of 1.8.2°F (1°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 and used by the International Panel on Climate Change (IPCC). This GCM model made assumptions about population and economic growth, resource availability, and management options that resulted 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 for 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 2.7.4°F (1.5°C) increase in temperature by the year 2100, compared with 1990.
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 shortgrass prairie. It has been estimated that about 40% of this semiarid region, measuring 988 million acres (400 million ha), 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 2.5.9 million acres (1 million ha) 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 19823.
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 10.51 in (270 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 Earth's greenhouse effect would have large climatic and ecological consequences.
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. One recent major agreement came out of a large meeting held in Kyoto, Japan, in 1997. There, industrial countries, such as those of North America and Western Europe, agreed to reduce their CO2 by as much as 5% of their 1990 levels by the year 2012. These reductions will be a huge challenge for those countries to achieve.
One possible 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 CO2emissions through the conversion of high-carbon forests into low-carbon agro-ecosystems.
See also Atmospheric chemistry; Atmospheric circulation; Atmospheric composition and structure; Desert and desertification; Earth (planet); El Niño and La Niña phenomena; Forests and deforestation; Fuels and fuel chemistry; Global warming; Ozone layer and hole dynamics; Ozone layer depletion; Petroleum, economic uses of