rather than allowing it to pass undeterred through the atmosphere to space. Because the atmos- 

 phere traps heat and warms the earth in a manner somewhat analogous to the glass panels of a 

 greenhouse, this phenomenon is generally known as the "greenhouse effect." Without the green- 

 house effect of the gases that occur in the atmosphere naturally, the earth would be 

 approximately 33 °C (60 °F) colder than it is currently (Hansen et al. 1984). 



In recent decades, the concentrations of "greenhouse gases" have been increasing. Since the 

 industrial revolution, the combustion of fossil fuels, deforestation, and cement manufacture have 

 released enough C0 2 into the atmosphere to raise the atmospheric concentration of carbon 

 dioxide by 20 percent. As Figure 1-2 shows, the concentration has increased 8 percent since 1958 

 (Keeling, Bacastow, and Whorf 1982). 3 Recently, the concentrations of methane, nitrous oxide, 

 chlorofluorocarbons. and a few dozen other trace gases that also absorb infrared radiation have 

 also been increasing (Lacis et al. 1981). Ramanathan et al. (1985) estimate that in the next fifty 

 years, these gases will warm the earth as much as the increase in C0 2 alone. 



Although there is no doubt that the concentration of greenhouse gases is increasing, the 

 future rate of that increase is uncertain. A recent report by the National Academy of Sciences 

 (NAS) examined numerous uncertainties regarding future energy use patterns, economic growth, 

 and the extent to which C0 2 emissions remain in the atmosphere (Nordhaus and Yohe 1983). 

 The Academy estimated a 98 percent probability that C0 2 concentrations will be at least 450 

 parts per million (1.5 times the year-1900 level) and a 55 percent chance that the concentration 

 will be 550 parts per million by 2050. The Academy estimated that the probability of a doubling 

 of C0 2 concentrations by 2100 is 75 percent. Other investigators had estimated that a doubling is 

 likely by 2050 (Wuebbles, MacCracken, and Luther 1984). 



If the impact of the trace gases continues to be equal to the impact of C0 2 , the NAS analysis 

 implies that the "effective doubling" of all greenhouse gases has a 98 percent chance of occurring 

 by 2050. 4 An international conference of scientists recently estimated that an effective doubling 

 by 2030 is likely (UNEP, WMO, ICSU 1985). However, uncertainties regarding the emissions of 

 many trace gases are greater than those for C0 2 . Although the sources of chlorofluorocarbons 

 (CFCs) are well known, future emissions involve regulatory uncertainties. Because these gases 

 can cause deterioration of stratospheric ozone, forty nations have tentatively agreed to cut emis- 

 sions of the most important CFCs by 50 percent. However, additional cutbacks may be imple- 

 mented, and other nations may sign the treaty; on the other hand, emissions of gases not 

 covered by the treaty may increase. 



Considerable uncertainty also exists regarding the impact of a doubling of greenhouse gases. 

 Physicists and climatologists generally agree that a doubling would directly raise the earth's 

 average temperature by about 1 °C if nothing else changed. However, if the earth warmed, many 

 other aspects of climate would be likely to change, probably amplifying the direct effect of the 

 greenhouse gases. These indirect impacts are known as "climatic feedbacks." 



Figure 1-3 shows estimates by Hansen et al. (1984) of the most important known feedbacks. 

 A warmer atmosphere would retain more water vapor, which is also a greenhouse gas, and would 

 warm the earth more. Snow and floating ice would melt, decreasing the amount of sunlight 

 reflected to space, causing additional warming. Although the estimates of other researchers differ 

 slightly from those of Hansen et al., climatologists agree that these two feedbacks would amplify 

 the global warming from the other greenhouse gases. However, the impact of clouds is far less 

 certain. Although recent investigations have estimated that changes in cloud height and cloud 

 cover would add to the warming, the possibility that changes in cloud cover would offset part of 

 the warming cannot be ruled out. After evaluating the evidence, two panels of the National 

 Academy of Sciences concluded that the eventual warming from a doubling of greenhouse gases 

 would be between 1.5° and 4.5 °C (3°-8°F) (Charney et al. 1979; Smagorinsky 1982). 



