The Science of Global Climate Change
The greenhouse effect is a natural component of the Earth's geophysical balance without which the biological sphere of Earth could not exist. This effect results from the process by which certain gases of the earth's atmosphere, which appear almost transparent to solar radiation, allow this energy to pass through, but absorb the heat as it radiates back from the earth's surface--- much like the windows of a greenhouse. This 'shield' of gases (principally water vapor and carbon dioxide) retains heat just long enough to keep the temperature at a comfortable level on Earth, and then allows about 80 percent of it to escape into space (Whyte 67). Without this essential mechanism, the Earth would be about 63 degrees cooler, with an average surface temperature comparable to that of Mars (Roleff 12).
The natural concentrations of these 'greenhouse gases' have trapped a comfortable amount of heat near the surface of the earthfor thousands of millennia. However, during the 1970s, some scientists began to worry about the greenhouse effect when they realized that the average surface temperature of Earth had risen nearly 1 degree Fahrenheit during the twentieth century (Roleff 12). This change, compared to the natural rate of less than one degree per millennium, signals the presence of an enhanced greenhouse effect on, or "global warming" of the Earth (Lyman et al. 18). Moreover, the enhanced effect points to an increase in the concentrations of greenhouse gases, the buildup of which causes the atmosphere to retain heat that would otherwise escape into space. Over the last 200 years, according to Whyte in Climatic Change and Human Society, the contribution of carbon dioxide to the increased effect has been 61 percent, methane 17 percent, nitrous oxide 4 percent, and the recently developed, man-made chloroflourocarbons (CFCs) 12 percent. (60) Clearly carbon dioxide has done the most damage, which leads one to wonder what natural--or human-induced-- processes could cause such a change.
Global temperature changes "are becoming large enough that scientists can look at them and judge that something other than natural causes is driving [the changes]," writes Bernard, former president of Boston-area chapters of the National Weather Association and American Meteorological Society (12). Many researchers cite the pollutants that stem from a wide range of human activities-- including energy use, industrial production, agriculture, forestry, and land use changes-- as the primary sources of this global warming, due to their direct links to the increase in atmospheric concentrations of heat-trapping gases (Bernard 2). Some of these gases that are added by man occur naturally (CO2, methane), but others, such as the highly radiation-absorbent CFCs, are completely man-made (Whyte 66). First of all, the burning of wood in deforestation and of fossil fuels such as natural gas, oil, and coal has released large amounts of carbon dioxide into the atmosphere (Roleff 12). David E. Newton, of the University of San Francisco, reports that changes in the atmosphere in the last 200 years "are reflected in the amounts of fossil fuels produced and consumed by humans" (7). For instance, the worldwide production of coal increased from 150 million tons in 1860 to 4.573 billion tons in 1987 (Newton 7). The rate of coal production has doubled every twenty years between 1860 and 1920, and in the United States alone the production of coal has mushroomed from 10 million tons in 1850 to 1 billion tons in 1990 (Newton 7). There are similar patterns in the production of oil and natural gas.
In addition, man-made chloroflourocarbons have become a factor in the enhanced greenhouse effect because they absorb a band of radiation that is not absorbed by any other gas in the atmosphere and normally escapes into space. As CFCs accumulate in the atmosphere, the 'atmospheric window' through which most bands of radiation escape begins to "close," causing more heat to be retained, and leading to temperature increase. This effect is alarming because the rate at which some CFCs are being released into the atmosphere is greater than that of any other greenhouse gas. By some estimates, "methane, nitrous oxide, ozone, and CFCs together contribute [only] a little more than half of the warming potential produced by carbon dioxide alone," but if current trends continue, reports the United Nations Intergovernmental Panel on Climate Change, "the warming potential contributed by these four gases will be equal to that of CO2 by 2030" (Newton 11).
Deforestation, particularly the destruction of tropical rainforests, which supply the Earth with large amounts of oxygen, has also had a significant effect on carbon dioxide levels. Because the extraordinary ability of these areas to take in large amounts of CO2 is lost, greater amounts remain in the atmosphere. According to some estimates, this activity has contributed 10 to 30 percent of the increase in C02 levels observed over the past four decades (Newton 9). Deforestation and other human activities have always affected the composition of the atmosphere and continue to do so today.
Yet "even though the [enhanced] greenhouse effect is the least controversial theory in atmospheric science," remarks Stephen Schneider, a former climate modeler at the National Center for Atmospheric Research (Bernard 79), there is indeed controversy over the logistics and extent of the phenomenon. "Will the rising concentrations of greenhouse gases raise the earth's temperature by one, five, or eight degrees Celsius? Will the increase take 50, 100, or 150 years?" (Steven Schneider 119) By how much will atmospheric levels of these gases increase in response to emissions? What climatic effect will the resulting buildups have, after natural and human factors that mitigate or amplify these effects are taken into account? At this time, these and many other such questions can only be answered in the form of estimates derived from studies of atmospheric and climatic computer models.
A change in global temperature by human activities of one, two, or even five degrees does not sound like it would make much of a difference, especially since this small addition would be superimposed on a natural process that has already heated the Earth's surface by more than thirty Celsius degrees (60 F). Those thirty degrees may "represent the difference between our warm, hospitable planet and a lifeless ball of ice," writes Mintzer, but "an additional increase of one degree Celsius is all that separates today's equable climate from that of the Little Ice Age,...a cold period which lasted from the fourteenth to the seventeenth century" (21). A warming of five Celsius degrees from the present level would make the average global temperature hotter than at any time in the last three million years (Mintzer 21).
Not much of a difference? One must remember that climate is the result of the balance of complex sets of interactions first within the atmosphere, then between the atmosphere and the oceans, land surfaces, and vegetation, and finally, due to snow and ice cover (Whyte 63). Any factor in altering these interactions (forcing factors) subsequently causes the climate to readjust or change. The elements of the climate system have different response times to change, with the atmosphere changing rapidly, in the space of days and weeks, and the oceans much more slowly, in centuries. Examples of forcing factors that affect the degree of reflectivity of the Earth's surface to solar radiation include volcanic eruptions of dust and gas, snow and ice cover, and the clearance of forests. Greenhouse gas emissions are to be considered as a new forcing factor. However, because "weather and climate are not entities that can be studied in a laboratory," notes Bernard, "we are inadvertantly performing an experiment" that in time will indeed show the extent of this factor (79). Already, today's human induced changes in the climate system represent stresses equal in magnitude to those associated with the major glacier-interglacial transitions of the past (Mintzer 5).
In reference to climate change, the consensus is that the climate of the future will depend primarily on three factors: the levels of future greenhouse gas emissions, the fate of these gases in the atmosphere, and the responses (feedbacks) of the total climate system and biosphere to the gas increase (Mintzer 19). Although there is a close link between the concentrations of greenhouse gases and average global temperatures, predicting the temperatures which are likely to result from specific rises in greenhouse gas concentration is much more difficult. Moreover, projecting future emissions themselves becomes "an intricate exercise in social science" (Steven Schneider 121). It is one thing to calculate the radiative effect of temperature for particular greenhouse gases, as their radiative properties are well understood. It is harder, however, "to assess the indirect effects resulting from chemical reactions within the atmosphere and feedback processes which may amplify or reduce the amount of warming" (Whyte 67).
Difficulty in projecting future climate is brought about in part by a lack of knowledge concerning important factors of climate change, such as the oceans and its potential feedback mechanisms. Researchers know how much carbon dioxide is consumed annually, and have a very good idea of how much is being released into the atmosphere. However, since 1958 only about half of the CO2 known to have been produced has shown up in the atmosphere. Most of the remainder is absorbed by the oceans, making them a tremendous "storage locker" of CO2
In the attempt to predict future climate, models have enabled researchers to simulate the major features of global atmospheric circulation and of present world climate, "at least as far as the averaged conditions are concerned," says John Mason, Director General of the Meteorological Office in Bracknell, England (qtd. in Bernard 83). Underlying the structures of all of the models are the fundamental laws of physics that govern the behavior of the atmosphere, so in addition they also provide good simulations of the earth's seasonal changes and various historical climates (Bernard 83). Even so, critics have been quick to deem the models unreliable and an inaccurate guide to future climatic change, citing the omissions of the interactions between the atmosphere and the clouds and oceans. In the face of such criticism, climate researchers themselves are first to admit that their models are not perfect, and they are continually refining them, though their current results should not be dismissed just yet. The next major step to improving climate models, according to Schneider, will be to take a better look at what the oceans might do in response to warming (Bernard 81). Nevertheless, models have greatly improved in recent decades, and scientists now agree that their predictions have increasing validity.
The new NASA climate model represents a significant advance in climate simulation which naturally employs all laws of physics, cloud amounts and heights, and models evaporation (Bernard 83). Even though the models have become more sophisticated, "the answers that they are giving have not changed much" (Bernard 79). When Syukaro Manabe and Richard Wetherald ran their computer model in 1967, they calculated that a doubling of atmospheric CO2 would cause a global warming of 3.6 Fahrenheit (F) degrees (Bernard 79). The same calculation done with a model in 1992 also predicted this 3.6 F increase (Newton 13). Comparatively, the current generally accepted range for greenhouse warming resulting from doubled CO2 is 3 to 8 degrees F (Bernard 79). With correlations such as these, linking different years and different models with the same results, one can not wholly dismiss the validity of these climate models.
All of the five major modelling programs that are in existence today make similar broad predictions about global changes in the next century. Climate experts on the Intergovernmental Panel on Climate Change, a United Nations-sponsored organization of 2,500 scientists, have relied heavily on these models and have forecasted the climate and weather of a global-warming world, linking these to occurrences already present in the world today. Among this group is the consensus that global warming could seriously disrupt the weather by spurning higher temperatures, altering rain and snow patterns, melting the glaciers, causing the gradual thermal expansion of sea water, and disturbing the air and ocean currents enough to produce more intense and frequent hurricanes. While it should not affect day-to-day weather, global warming is forecasted to increase the chances for extreme weather, including hurricanes, tornadoes, typhoons, droughts, heavy rains, heavy snow, and heat waves.
Skeptics and believers alike affirm that the earth has been gradually warming and cooling and has been hotter than it is now. However, there are two elements that distinguish the current warming trend: First, it now seems evident that the most recent warming is caused by human activity, rather than by geological processes that were occurring before. Second, the current warming is happening much faster than it ever has.
