The Sun plays a vital role in the Earth's climate system, providing the energy which drives both atmospheric and oceanic circulation, and ultimately driving the climate system. This solar energy reaches the earth's atmosphere in the form of electromagnetic radiation (infrared radiation, radio waves, visible light, and ultraviolet rays) (Figure 9) (Streete, 1991).

Although some incoming short wave electromagnetic solar radiation is reflected back into space (by clouds or dust in the Earth's atmosphere) and some is absorbed (by dust and water vapor), the earth's atmosphere is generally transparent to short-wave radiation. Hence most of this energy passes through the atmosphere and strikes the earth's surface. The portion of incoming solar radiation that reaches the Earth's surface, is either absorbed by the land and the oceans or is reflected back toward space by water, snow, ice, and other reflective surfaces (the measure of an object's reflectivity is called its albedo). Energy absorbed at the Earth's surface is then re-radiated into the atmosphere as infrared radiation. Some of this infrared radiation is in turn absorbed and re-emitted by "greenhouse gases" (gases that absorb infrared radiation), resulting in a warming of the earth's surface and lower atmosphere (Figure 10).

The major components of the Earth's atmosphere are listed in Table 1. Note that the most abundant gases are not the greenhouse gases -- carbon dioxide, methane, nitrous oxides and water vapor -- but are instead nitrogen and oxygen. Nitrogen and oxygen, however, do not absorb energy in the infrared region, as do the greenhouse gases. The greenhouse gases, with the exception of water vapor, are present in the atmosphere only in trace amounts, on the order of parts per billion (ppb) to parts per million (ppm). Despite their low concentrations, even a slight change in the greenhouse gas concentrations will produce a large change in the amount of radiation that is absorbed (Streete, 1991).

Greenhouse gases are naturally occurring chemicals whose presence in trace amounts in the atmosphere is vital to life on earth. These gases behave as one-way filters - they permit incoming solar radiation to enter the earth's atmosphere and reach the earth's surface, but prevent re-radiated infrared energy from leaving our atmosphere (Figure 9) (Streete, 1991). It has been calculated that without greenhouse gases, the mean global temperature of Earth would be approximately -17°C, significantly lower than the observed mean global temperature of approximately 15°C. At temperatures that cold, liquid water would not exist, and life could not be sustained. So the greenhouse effect, often depicted as a terrible phenomenon, is in fact a process that makes the Earth habitable. What makes the greenhouse effect a modern concern is the potential human impact on atmospheric chemistry. The anthropogenic addition of greenhouse gases may upset the balance of the atmosphere, such that increased trapping of re-radiated energy will result directly in increased surface temperatures.

Carbon Dioxide as a Greenhouse Gas

It is through the carbon cycle that the biosphere interacts with the atmosphere, the hydrosphere, and the lithosphere (Figure 11). For humans, other animals, and plants, carbon is important in the form of CO 2 for the basic physiological process of respiration. In respiration, oxygen is consumed while CO2 is released through a series of chemical reactions which break down food molecules into energy. CO2 is also used by plants in photosynthesis, a process by which plants, in the presence of sunlight, convert CO2 and water into simple sugars.

Carbon is cycled through the biosphere during photosynthesis to form organic compounds such as carbohydrates. The cycling of CO 2 through the biosphere is very rapid, such that the entire reservoir of CO2 in the atmosphere is completely cycled through the biosphere every 4.5 years. Carbon is returned to the atmosphere partly through respiration, and partly through the decay and oxidation of organic matter. When a living thing dies, the carbon stored in it combines with oxygen to again produce CO2. Some of the carbon in the decaying organic matter is not returned to the atmosphere as CO2 , but is deposited as sediment and ultimately becomes incorporated into sedimentary rock. This carbon will eventually be returned to the atmosphere as a consequence of tectonic uplift and weathering of the sedimentary rock.

Carbon is also exchanged directly with the oceans. CO 2 in the atmosphere is dissolved in the surface waters of the oceans due to a pressure gradient across the ocean-air interface. The rate at which the ocean can take up CO2 from the atmosphere in part is determined by biological processes at the surface and by the ocean's deep water circulation patterns. Algae in the oceans use CO2 in the same way as land plants, through photosynthesis and respiration. Additionally, many groups of organisms in the oceans combine dissolved inorganic carbon from CO2 and calcium to form calcium carbonate (CaCO3) shells. These biological processes remove CO2 from the surface waters, allowing for continued exchange between the atmosphere and ocean. When calcium carbonate skeletons settle to the seafloor, they are cemented and lithified into sedimentary rock, which may eventually be uplifted and eroded, allowing for the release of this carbon back to the atmosphere.

Figure 11 illustrates the carbon cycle before and after the industrial revolution. Human activities have changed both the size of the different reservoirs (systems that hold carbon), and the magnitude of the fluxes of carbon between the reservoirs. For example, the current atmospheric reservoir of carbon dioxide is 750 Gt C, as contrasted with the pre-industrial level of 600 Gt C (Houghton et al., 1994). Human activities, such as deforestation and fossil fuel burning are increasing the concentration of carbon dioxide in the atmosphere by releasing CO2 that was previously stored in these reservoirs. Natural processes that remove CO2 from the atmosphere cannot keep pace with the rate at which anthropogenic processes add it, resulting in a significant increase in atmospheric CO2.

The increase of the CO2 concentration in the atmosphere since the late 1950's can be seen in figure 12. Atmospheric concentrations were computed monthly over the past several decades at Mauna Loa Observatory, Hawaii, a fairly remote and non-polluted area. Figure 12 shows an overall exponential increase in CO2, with superimposed peaks and troughs occurring yearly. During the primary growing season in the Northern Hemisphere spring and summer, the biosphere actively removes CO2 from the atmosphere through photosynthesis, creating the troughs in the diagram. During winter months, when less plant growth and photosynthesis occurs, higher atmospheric CO2 concentrations cause the peaks. The longer-term trend, however, is one of an increase in atmospheric CO2 concentrations. Studies predict that due to anthropogenic emissions, atmospheric carbon dioxide concentrations will double its 1950 level sometime in the next century (Shaw, 1992).

Other Greenhouse Gases

Methane concentrations in the atmosphere, like atmospheric carbon dioxide concentrations, are exponentially increasing (Figure 13). Methane is released by human activities such as rice cultivation, leaks in domestic and industrial gas lines, and as a waste product from the digestive systems of animals. Although methane is considerably less abundant in the atmosphere than CO2, it is 25% more effective at absorbing infrared radiation (Table 1).

Other gases that are present in the atmosphere in trace amounts and which act as greenhouse gases include water vapor, nitrous oxides and chlorofluorocarbons. Water vapor is the most efficient greenhouse gas, and it is added to the atmosphere due to evaporation of water on Earth's surface. However, the atmosphere's ability to hold water vapor is itself determined by temperature. At present temperatures, the atmosphere is saturated or nearly saturated with respect to water vapor and there is little variation in water vapor content. However if temperatures should rise due to other greenhouse gases, the amount of atmospheric water vapor could also increase, creating a positive feedback loop and additional warming. Nitrous oxides are anthropogenically added to the atmosphere through combustion and the use of nitrogen-rich fertilizers. Chlorofluorocarbons (CFCs) present in the atmosphere are entirely due to human activities. CFCs were manufactured as propellants in aerosol spray cans, in the manufacture of plastic foams, and as refrigerants. When first produced in 1928, these chemicals were thought to be perfect for these purposes, because they are non-toxic and non-flammable. However, CFCs are about 30,000 times more effective at absorbing radiation than CO2 , and atmospheric CFCs are increasing at a rate of approximately 4% each year. CFCs are such a stable compound that they are neither removed from the atmosphere by rain, or by absorption into the ground. CFCs are a double evil; they contribute to both the greenhouse effect and to the destruction of the ozone layer.

Effects of Increased Greenhouse Gases

Global climate models indicate that the increases in greenhouse gases are likely to change the yearly average values and annual patterns of temperature and rainfall. Both increases and decreases in overall temperature and rainfall are possible, depending on the part of the globe considered (Ennis, 1993). 'Global warming' is a household term which describes the potential consequence of increased greenhouse gases. Historical records demonstrate that overall global mean temperatures have increased by 0.5 °C since the late 19th century and, as described previously, studies at the Mauna Loa Observatory have shown that global CO2 concentrations have also been increasing (Figure 12).

Supporting the correlation between increased atmospheric concentrations of carbon dioxide and global warmth are observations from ice cores that atmospheric CO2 levels are lower during glacial periods and higher during the warmer interglacial periods. Note for example, high carbon dioxide concentrations during the present and previous interglacial periods (Figure 14). However, details of the cause and effect relationship remain uncertain - does the CO2 increase cause an increase in temperature, or do changes in temperature affect the level of CO2?

In discussions of global warming and the greenhouse effect, carbon dioxide is the gas most often mentioned because CO 2 is both effective at absorbing long-wave radiation and relatively abundant in the atmosphere (compared to the other greenhouse gases). Water vapor, however, is the most effective greenhouse gas, and changes in its atmospheric concentration are important. A possible feedback between global warming and water vapor results from the increase in the saturation vapor pressure of water with increased temperature. As global temperatures increase, additional water may evaporate. Consequently, the water vapor content of the atmosphere would rise. Since water vapor is a greenhouse gas, the net result would be amplification of rising temperatures (Shaw, 1992). The overall influence of increased water vapor in the atmosphere is ambiguous however, as clouds may also reflect incoming solar radiation away from the Earth, resulting in cooling.

Global warming has the potential to affect the geosphere and biosphere through several inter-related mechanisms. Not only is global warming capable of upsetting basic biological functions by changing the optimal living environments of plants and animals (including humans), but warming may result in increased sea level, changes in oceanic and atmospheric circulation patterns, and increased strength and frequency of tropical storms.

BIOSPHERE

Ennis and Marcus (1993) discussed the potential effects of global warming on the biosphere: "Living organisms are found everywhere on this planet, even in areas that humans may consider hostile, such as hot springs, the deep sea, and under Antarctic sea ice. However, a given species does not occur everywhere. Species have very specific requirements that govern their distributions. These physical and chemical limits of tolerance, together with availability of food and biological interactions such as predation and competition, determine the global distribution patterns of species. The climate plays a key role in establishing these distribution patterns, insofar as it determines the physical and chemical attributes of the abiotic environment.

Land plants (terrestrial vegetation) offer a good illustration of the connection between climate and the biota...The broad terms for vegetation zones (such as boreal forest, tropical forest, temperate forest) in fact carry climatic connotations. This offers the first clue that climate variables influence plant distribution...

Temperature is the most important climatic factor that governs the biology of animals and plants. It has a direct effect on the rate of most biological processes (example: photosynthesis, respiration, digestion, excretion). If proper internal temperatures are not maintained, these processes cannot proceed normally and an individual will be stressed or possibly die...The mechanisms for controlling internal temperature, such as shivering, sweating, and panting, work by altering the organism's metabolic rate. Because warm-blooded animals are able to regulate their body temperature, they are less affected by external temperature variation...If environmental temperatures rise, those species living near the upper thermal limits of their existence will be stressed. Although death may not be the immediate response, sub-lethal effects such as a decrease in the number of offspring may contribute to the eventual decline or demise of the species ... A 1 °C increase in average temperature may seem inconsequential. In terms of climate, this change corresponds to a 100-150 kilometer distance in latitude! How will plants and animals adjust as their optimal distribution ranges literally shift out from under them?

Moisture is also a critical variable limiting the distribution of organisms. Water, which accounts for 85 to 90% of the weight of most living organisms, is essential for proper plant and animal physiology. This is because the chemistry of physiological processes is carried out in the water of living tissues. Water is the equivalent of an organism's transportation system, permitting the transport of biological molecules to and from the sites of biochemical reactions such as photosynthesis and respiration. Nevertheless, organisms differ greatly in their water requirements and their abilities to withstand dryness and flooding. Some lower plants, such as algae and fungi, adjust their activity according to the humidity of the surrounding air. In periods of low humidity, they are able to enter a resting state. Some water-dwelling microorganisms have a similar ability to withstand periods of total dryness.

This dependence on water links the biota to several climate-related variables, such as relative humidity, rainfall amount, and the distribution of rainfall through the year...The frequency of extreme events such as droughts and floods is also critical. Reproduction is especially vulnerable to such extremes, with the survival of eggs and seeds often tied to the presence or absence of moisture. For example, amphibians such as toads and salamanders lay eggs in the fringes of ponds, placing them at risk in periods of drought. Larger animals can sometimes move (migrate) to minimize the impacts of extreme events, but plants and smaller animals don't have this option."

SEA LEVEL

Model predictions of the hydrosphere's response to a global temperature increase demonstrate that sea level may rise due to thermal expansion of the waters upon heating and due to the melting of polar ice caps. The predicted amount of sea level rise varies from a couple of feet to many meters. For example, studies show that if the West Antarctic Ice Sheet, the last remaining marine-based ice sheet, collapsed, the 3.2 million cubic kilometers of ice contained within would raise global sea level by six meters (West Antarctic Ice Sheet Initiative).

However, even a small rise in sea level could be disastrous for coastal cities that are barely above sea level. Today, nearly half of the U.S. population lives in a coastal zone. Miami, for example, is home to about three million people and it lies at or slightly above sea level. Studies have indicated that just a one-foot rise in sea level would cause Florida's high tide mark to move inland by 200 to 1,000 feet, and would cause Louisiana's shorelines to move in by several miles (Revkin, 1992). Other U.S. coastal cities such as New Orleans, New York, and Houston would face similar problems. Outside of the U.S., Bangladesh, built on the world's largest delta plain, would face disaster as ocean surges flooded the city, and the small island of the Maldives, sticking out of the ocean by a mere six feet, is likely to disappear completely. The effect of coastal encroachment due to rising sea level would be compounded by cities already undergoing subsidence, such as Venice, where excessive pumping of water has caused the city to subside. Saltwater encroachment into drinking supplies would be most probable in areas like Miami, where the drinking water is pumped from shallow aquifers lying just a few feet below the city.

OCEANIC CIRCULATION PATTERNS

Melting ice caps would not only raise sea level but may also influence the ocean's heat transport processes. Currently, warm waters at the equator move along the surface of the ocean toward the poles, where they cool, increase in salinity, and sink. The heat released by this cooling is responsible for Northern Europe's relatively mild summers. The cool, dense waters then return to the equator along the bottom of the ocean, where they surface to once again make the journey to the poles. A complete circuit takes about 1,000 years. An influx of freshwater to the high-latitude ocean surface (due to melting of both glacial and sea ice) has the potential to slow down or even shut down this thermohaline circulation system by decreasing the salinity (and hence density) of the surface water. Any change in this oceanic circulation pattern would alter the distribution of heat and moisture, as well as severely affecting oxygen levels and nutrient cycling in the ocean system.

TROPICAL STORMS

Studies indicate that global warming may cause an increase in both the number and intensity of hurricanes each year. It is believed that increased temperatures will result in an increase in heavy, equatorial monsoon rains, which permit the development of strong low pressure areas offshore. The heavy rains, low pressure areas, and warm waters are excellent conditions for hurricane development. The most devastating hurricanes, that historically have occurred every century or so, may soon begin to occur on a time scale of decades (Revkin, 1992).

PRODUCTIVITY

It is possible that increasing atmospheric CO 2 may actually increase global primary productivity (Ennis and Marcus, 1993). For example, "CO2 fertilization" of plants is practiced by some horticulturists, who spike the air in greenhouses with higher CO2. It is questionable whether increased carbon dioxide concentrations in the natural world would also increase the globally-averaged primary productivity. Plant growth is often limited by other factors, such as light availability, and concentration of the nutrients nitrogen and phosphate. Without similar increases in these substances, response to CO 2 fertilization may remain a minor feedback.

The issue of CO2 fertilization is important to the climate system because of the possibility of positive and/or negative feedbacks to the atmosphere. For example, increased atmospheric concentration of CO2 may enhance plant growth and photosynthesis (and hence CO2 consumption by plants), which could lower the atmospheric concentration of CO2 (a negative feedback system). On the other hand, increased atmospheric CO 2 could result in increased global temperatures, which could cause the rate of decomposition of organic matter to increase, which would return more CO2 to the atmosphere and contribute to global warming (positive feedback). The complexities of the carbon and climate systems make it difficult, if not impossible, to predict the ultimate response to increased carbon dioxide input to the atmosphere.

Concern over global warming has resulted in some proposals to artificially manipulate the carbon cycle to reduce atmospheric CO 2. The possibility that increased marine productivity and subsequent uptake of CO2 could partially control the level of atmospheric carbon dioxide has led to some intriguing hypotheses and experiments. In most of the world's oceans, primary production is limited by the major nutrients nitrate and phosphate. In some areas however (such as the North Pacific, the Equatorial Pacific and the Southern Oceans), biomass is low despite high concentrations of major nutrients. In these locations, measurements of extremely low levels of iron, a trace element essential for photosynthesis, has led some researchers to postulate that iron limits production in these waters (for example, Martin et al., 1990). This theory generated the controversial idea that iron fertilization of specific regions of the ocean could lead to increased primary production and consequent "drawdown" of atmospheric carbon dioxide. Such an experiment, in part, has already taken place in the equatorial Pacific Ocean, where the artificial addition of iron to a 64 km 2 area resulted in a significant increase in plant biomass and production (Martin et al., 1994).
 
 

The atmosphere is composed of several layers, distinguished by temperature changes at different altitudes. The stratosphere is a shell in the upper atmosphere, between about 11 to 50 km altitude. This section of the atmosphere has received increasing attention during the past decade, due to reports of ozone depletion. Ozone (O3) is essential for life on earth, as it absorbs a portion of the incoming ultraviolet radiation with wavelengths between 210-310 nm. Excessive exposure to ultraviolet radiation can result in damage to the immune system, generation of genetic mutations, and increased frequency of skin cancers. Studies have shown that a 1% decrease in ozone would lead to a 2% increase in malignant melanomas ( the most dangerous form of skin cancer), and a 0.3-2.0% increase in deaths due to this skin cancer (Ennis and Marcus, 1993).

Animals aren't the only organisms that are harmed by overexposure to UV-B radiation. Studies have shown that plants exposed to increased amounts of UV-B have stunted growth, smaller leaf area, and are less capable of photosynthesis. Reduced growth rate and photosynthetic capabilities have been observed in surface dwelling marine algae, phytoplankton, due to increased exposure to UV-B. Since phytoplankton are the base of the oceanic food chain, ozone depletion has the capability ultimately to affect all marine life (Ennis and Marcus, 1993).

A delicate balance exists between ozone and oxygen in the stratosphere. Ultraviolet radiation from the sun breaks down molecules of O2 into oxygen atoms which then are free to combine with other O2 molecules to form ozone (O 3). Ozone absorbs ultraviolet radiation and is in turn broken down itself into oxygen atoms and O2. Humans have disrupted this natural balance by releasing certain compounds into the atmosphere that destroy ozone in the stratosphere. The main compounds responsible for the destruction of the ozone layer are chlorofluorocarbons (CFCs). CFCs, which are discharged into the lower atmosphere, eventually rise to the stratosphere where they are broken down by ultraviolet radiation. Destruction of the CFC compound releases chlorine (Cl), which attacks and breaks down ozone molecules (Figure 15). The chlorine atoms are so effective at destroying ozone molecules, that just one chlorine atom can destroy up to 100,000 ozone molecules before it is removed from the stratosphere by a series of chemical reactions.

In 1985, British scientists discovered a large hole, about the size of Canada , in the stratospheric ozone layer above Antarctica. By the late 1980's, studies had shown that ozone concentrations above the Antarctica region had dropped by approximately 50%, except for the region between altitudes of 15 and 20 km, where ozone concentrations dropped by approximately 95% (Jones and Shanklin, 1995). Atmospheric conditions in Antarctica are particularly conducive to ozone depletion. During the polar winter, strong winds, termed the circumpolar vortex, essentially isolate the southern polar regions from warmer air masses to the north. The intense cold causes the formation of stratospheric clouds containing ice crystals, which act as sites of adherence for ozone depleting chemicals. In the polar spring to summer, the strength of the circumpolar vortex decreases, permitting the mixture of warmer air masses with the cold polar air. Conditions become less favorable for ozone depletion. But, the existing ozone-depleted air masses become less isolated, spreading northward and lowering ozone levels over New Zealand, Australia and South America. Although ozone depletion was first observed over Antarctica, and conditions for ozone depletion are optimal there, satellite measurements now suggest that ozone concentration over populated regions of the Northern Hemisphere is also decreasing at a rate of 1% every two to three years (Ennis and Marcus, 1993).