40th Agenda 

40th Session Issues

Global Warming/Greenhouse Effect

by
Isabel Gutierrez

More than a century ago it was discovered that certain gases in the atmosphere, mainly carbon dioxide C02, had the ability to absorb infrared radiation. This enabled the Earth to retain a greater portion of the sun's energy, thus increasing its overall temperature. Since then, the continuing buildup of C02 in the atmosphere, coupled with deforestation and other factors, is contributing to a forecasted gradual warming of the planet (greenhouse effect). Scientists now predict that some warming is inevitable, even if carbon dioxide emissions end today.

Background

The term "greenhouse effect" is actually incorrect to describe global warming, since a greenhouse works on a different principle. Global warming due to C02 buildup is attributed to the reduction of infrared radiation lost to outer space, whereas a greenhouse conserves beat by lirniting the amount of energy that is lost through convection.

Basically, the greenhouse effect operates in the following manner. Visible light from the sun comes through the atmosphere and strikes the surface of the earth. About a third of this solar energy is reflected directly back to space, and the remaining energy increases the temperature of the absorbing gases, liquids, and solids. These surfaces radiate mainly in the infrared part of the spectrum. A kind of equilibrium is then established so that the amount of sunlight coming in from the sun and absorbed by the planet is precisely balanced by the amount of infrared radiation emitted by the planet back to space. In the case of a greenhouse effect, the visible light comes in just as it would if there were no atmosphere, but the atmosphere gases, transparent in the visible part of the spectrum, tend to be opaque in the infrared part of the spectrum. The thermal radiation in the infrared is impeded from getting out. As a result, the surface temperature goes up until the radiation which is leaking out in the infrared, where there isn't a lot of opacity, just balances the visible radiation coming in. This can be considered as a kind of blanketing of the Earth in the infrared and not in the visible part of the spectrum.

The greenhouse effect operating on the Earth's atmosphere is mainly the result of carbon dioxide and water vapor. If the Earth had no atmosphere, the average temperature would be well below the freezing point of water. As a result of the greenhouse effect, the Earth's surface temperature is 35 degrees centigrade higher than it would be if there were no trace gases in the atmosphere. It is precisely the greenhouse effect that makes the planet habitable. It is, in effect, the reason that the earth is hospitable to life.

Carbon Dioxide and Trace Gases

Carbon dioxide in the atmosphere is the greatest contributor to global warming. Current analysis shows that carbon dioxide levels have risen 25% since 1958 and are the highest they have been in 160,000 years. This rise can be contributed mainly to an increase in burning of fossil fuels and continued deforestation.

This increase, however, represents only about 50% of total carbon dioxide emissions. The National Oceanic & Atmospheric Administration (NOAA) measured the amount of carbon that is released to the atmosphere. They found that approximately five billion metric tons of carbon (as fossil fuel) are burned each year and about half of this carbon ends up as atmospheric carbon dioxide. What they don't understand is where the remaining 50% goes. It may be absorbed by the oceans, either by being dissolved in seawater, or absorbed by aquatic plants. Or it may be used by land-based biological systems, mainly plants and soils. Scientists do not currently have an accurate way of monitoring carbon dioxide levels in either of these systems.

Some scientists are now concerned because the percentage of carbon dioxide emitted into the atmosphere is increasing. Scientists theorize that perhaps oceans are not absorbing as much carbon dioxide as before, or that a change is taking place in land-based terrestrial ecosystems. They warn that if this is true, the kinds of global climate changes that modelers are forecasting for the future will occur more rapidly than is now predicted'

Concentrations of the trace gases methane, nitrous oxide, tropospheric ozone, and chlorofluorocarbons are much lower than carbon dioxide (green- house gases), but are rising rapidly. As a group, they are able to absorb 60% more long- wave radiation than carbon dioxide. This is due to the fact that they are able to absorb radiation in a window of the electromagnetic spectrum where carbon dioxide and water vapor are relatively transparent. Water vapor traps radiation with wavelengths below 7um, and carbon dioxide absorbs in the region above 13um. These trace gases are able to absorb the radiation between 7um and 13um.

Methane has a lifetime of approximately nine years. Alone, it is 30 times more effective than a carbon dioxide molecule in greenhouse warming and leads to the production of another greenhouse gas, ozone. More than 50% of the methane released to the atmosphere is derived from the action of anaerobic bacteria on plant material, primarily in the stomachs of ruminants such as cows and sheep and in the guts of termites, and in rice paddies and wetlands of all types in all latitudes. Methane is also released from coal mines, leaks in natural gas pipelines, leaks of natural gas associated with oil production and decomposition of organic matter in landfills. It is also emitted from incomplete combustion of vegetation, in forest or range fires, or when land is cleared for agriculture. As a result, increased agricultural production will contribute to the rise of methane levels in our atmospheres

Nitrous oxide is detrimental to the atmosphere because it contributes to greenhouse warming as well as ozone depletion. In the past, it was believed that the majority of atmospheric nitrous oxide was produced through the burning of fossil fuels. Now it is known that it is produced mainly through biomass burning and through artificial fertilization of soils."

Ozone is somewhat paradoxical in nature. At high altitudes, it safeguards the earth through the absorption of ultraviolet radiation. At lower altitudes, however, it is a pollutant and a greenhouse gas. Scientists believe that ozone depletion in the lower stratosphere may cause some global cooling, while depletion in the upper stratosphere may cause warming. In addition, an ozone molecule in the upper troposphere produces much more surface warming than does a molecule at the surface or in the mid or upper stratosphere. However, since there is very little data on ozone concentrations in the upper troposphere, scientists are unable to make concrete projections.

Chlorofluourcarbons (CFC's) are a greenhouse gas and contributes to the destruction of stratospheric ozone. CFCs catalyze the breakdown of ozone molecules in the upper atmosphere into oxygen molecules. Their production and use has risen drastically in recent years, but the 1987 Montreal Protocol may curtail their production.

Reducing emissions of greenhouse gases may not be an easy task. Perhaps the biggest obstacle is the difference in in the amounts of greenhouse gas emissions of developed and underdeveloped nations. As an example, estimates show that per capita emissions of carbon dioxide are five tons for every man, woman, and child in the U.S., compared with only about 0.1 to 0.4 tons in China and India. To continue planned development, China and India will find it inevitable to drastically increase their current greenhouse emissions.

Deforestation

Plants are able to use carbon dioxide in photosynthesis, transforming it into oxygen. In this way, they are able to rid the planet of a great deal of carbon dioxide. In addition, carbon dioxide is emitted when trees are burned, or decomposed by bacteria. Because of this, the felling of trees and clearing of lands for agriculture is greatly exacerbating the problem of carbon dioxide in our atmosphere. A rapidly growing rain forest can fix annually between one and two kilograms of carbon per square meter, whereas cultivated fields fix only 0.2 to 0.4 kilograms per square meter. It is now estimated that deforestation is responsible for about 10 to 30% (as much as fossil fuel emissions) of the C02 buildup in our atmosphere.

Unfortunately, countries continue to cut down rain forests in an effort to develop economically. If deforestation continues at its current rate, this will influence global warming drastically.

Consequences of Global Warming

A rise in global temperatures of even 10 to f C will have drastic consequences, affecting all of earth's ecosystems. At the peak of the last ice age, 18,000 years ago, the planet was only about 5" C cooler than it is today. It is also known that during the past million years, the sustained temperature has never been more than a few degrees higher than it is today.

The rising of sea levels will probably be the most severe consequence of global warming. Scientists estimate that sea levels may rise 0.5 to 1.5 meters in the next 50 to 100 years, assuming there is no breakup of the West Antarctic ice sheet. The rise will be due to the melting of mountain glaciers and the thermal expansion of ocean water.

If sea levels rise as estimated, the chance of flooding will increase. Flooding will damage coastal structures, destroy wetlands, and cause saltwater intrusion into rivers and water supplies near the ocean. Since 1/3 of the world's population lives in low lying coastal areas, flooding would present a great risk to much of the world. Among the big cities in immediate danger if flooding occurs, are Bangkok, Galveston, Miami, New Orleans, New York, Rotterdam, Taipei, and Venice.

The melting of ice caps will also contribute to rising sea levels, causing them to rise more than currently predicted. It is now unclear whether this can be expected to occur. Scientists predict that the ice cap on East Antarctica will not melt in the foreseeable future. The West Antarctic ice sheet, however, since it is in contact with ocean waters, has a higher probability of melting. There is data, however, suggesting that it has been intact for several million years.

The regional effects of global warming on agriculture are still unclear. It is known that global warming will affect the length of the growing season and the intensity of heat waves and rainfall. Perhaps the most determinant of these factors will be the amount of precipitation in these regions. Model predictions have disagreed in their projections for regional rainfall. The general consensus seems to be that global warming will have deleterious effects on water supplies because more water will be required for irrigation to combat the effects of heat stress.

Global warming in certain regions may have a positive effect on agriculture. Rising global temperatures will shift the crop growing region from south to north in the Northern Hemisphere. If food growing acreage remains the same, but moves north, productivity would not necessarily remain the same. This is because there is no guarantee that soil quality will be the same in the South.

Another uncertain factor is how plants will respond to a warmer world and increased levels of C02. Certain plants will actually benefit from higher levels of carbon dioxide and will grow at a quicker rate. High carbon dioxide levels also cause some plants to use water more efficiently. Unfortunately, crops fertilized with excess carbon dioxide may be more susceptible to pests and diseases, and weeds will generally benefit more than crop plants.

New studies have revealed that global warming will also aggravate air pollution. As the climate warms, fewer and weaker storms, combined with sluggish movement of air masses, will be expected. As a result, dirty air masses will be left hovering over industrial centers longer than they are today. Smog will form closer to its sources, primarily in the centers of cities and industrial areas. This same study predicts that in a warmer climate there will be more evaporation of moisture into the air, thus increasing humidity by 30% to 40%.

The greenhouse effect will also increase the possibility of ozone depletion. With greenhouse warming, the layer of atmosphere from the ground to between six and 12 miles up would warm while the stratosphere above it would cool. This cooling could be drastic enough to cause the formation of ice clouds worldwide. Through PSC reactions, these clouds could contribute to the rapid destruction of the ozone layer. This would then allow more dangerous ultraviolet rays to reach the Earth, warming it further.

Global warming may not affect everyone negatively. Some nations may actually benefit from it. Studies show that countries at higher latitudes will experience greater warming than equatorial regions. With the shift in weather patterns, East Africa may not experience its severe droughts, parts of the Soviet Union, Scandinavia and Canada may have longer growing seasons. However, current projections estimate that a larger portion of the planet would be adversely affected. Also, since scientists do not have a clear idea of how global warming will affect different regions, it is unclear to what degree regions would actually benefit.

Additional Factors: The Oceans and Clouds

It is difficult for scientists to predict exactly how much the Earth will warm, when it will take place and what regions will be affected. There are many different factors that can contribute or detract from the greenhouse effect. More must be learned about the way in which the oceans, atmosphere, ice sheets, clouds and volcanoes interact in order to predict the full effects of global warming.

Oceans are the most important of these factors and also one of the least understood. The oceans are able to absorb both excess carbon dioxide and thermal energy. At present, scientists' do not have an accurate prediction of how much carbon dioxide or heat the earth is absorbing. Because of this, it is difficult for scientists to predict the severity of global warming. Should the ocean remove more than is currently estimated, the warming will be delayed; should it absorb less, the warming will come sooner. The ocean, by delaying the climate change at Earth's surface, confounds scientists' attempts to infer global climate sensitivity to greenhouse gases from observed climate change.

Clouds are another major factor that must be considered in order to obtain a reliable prediction of greenhouse warming. In a warmer world, more water will evaporate from the oceans. What is not known is whether this additional water vapor will condense to create more clouds or whether it will tend to remain as vapor. If more clouds are created, it is not known what types of clouds will dominate.

Clouds can both warm and cool the planet. Bright, low and mid-level clouds, by reflecting short-wave radiation from the sun, tend to cool the planet, whereas high cirrus clouds warm it by re-radiating long-wave radiation. If clouds in a warmer world are like those in the tropics, they will amplify the warming. If they are more like those currently in high and mid latitudes they will reduce the warming.

Models now in use are not capable of combining the ocean and the atmosphere in their calculations. A coupled atmosphere ocean model would require a more advanced computer. Current supercomputers are not capable of handling such a large amount of data. It is likely to take 10 to 20 years before such a model can be developed.

Has Global Warming Started?

Many scientists now believe that there is enough evidence to suggest that global warming has begun. Studies done by the National Aeronautics & Space Administration's (NASA) Goddard Institute for Space Studies (GISS) and the Climatic Research Unit in East Anglia, England have shown that the past ten years have been the warmest decade on record. These studies also revealed that global temperatures have increased by 1.5 degrees centigrade in the past century.

However, other scientists counter that these increases could be a attributed to other causes. They cite the natural fluctuations of global temperatures as the possible reason for these increases. Stephen H. Schneider, head of the interdisciplinary climate systems section of the National Science Foundation's (NSF) National Center for Atmospheric Research (NCAR) says that "most climatologists do not yet proclaim that the observed temperature changes this century were caused beyond doubt by the greenhouse effect." Whether scientists believe greenhouse warming has begun or not, most climate modelers do agree that if present emission trends for greenhouse gases continue, global temperatures will rise 2 to 6 degrees centigrade over the next century.

Suggestions for Further Research

Deforestation

Ozone Depletion

Alternative Energy Sources

Consult "The Environmental Perspective for the Year 2,000," A Report by the U.S. Environmental Protection Agency (EPA) on possible solutions to delay or lessen global warming

Sources

Begley, Sharon, Mary Hager and Mark Miller "The Endless Summer", Newsweek , 7/11/88, pp. 18-20.

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Hileman, Bette "Global Warming", C&.EN, 3/13/89, pp. 2544.

McCuen, Gary E. Our Endangered Atmospitere, Wisconsin, GEM Publications 1987, pp. 932.

Monastersky, R. "Global Change: the Scientific Challenge", Science News, Vol 135, pp. 232235.

Raloff, J. "Global Smog: Newest Greenhouse Projection', Science News, Vol 135, 4/29/89, pp. 262, 263.

"The United Nations and the Global Environment", UNA-USA,, Fact sheet.

Zurer, Pamela "EPA Urges Nations to Limit Greenhouse Gases,