A. The Greenhouse Effect

Since the early 1960s, climate change and air quality have become major and often controversial issues in many countries and among groups from

governments to various scientific communities. Prominent among these issues is the greenhouse effect, in which the gradually increasing tropospheric concen­trations of carbon dioxide (C02), methane (CH4), and nitrous oxide (N20) are believed to trap an excessive amount of solar radiation reflected from the earth. The trapped radiation is predicted to cause significant ambient temperature increases. Other issues include ozone (03) formation over popu­lated areas due to photochemical interactions of hydrocarbon, carbon monox­ide (CO), and nitrogen oxide (NOx) emissions, primarily from motor vehicles; natural ozone layer destruction in the stratosphere by photochemical reactions of organic chlorofluorocarbon compounds (CFCs) resulting in increased pene­tration to the earth’s surface of shorter-wavelength ultraviolet light that can cause skin cancers; and acid rain, which has harmful effects on buildings and the growth of biomass and is caused by sulfur oxide (SOx) emissions from the combustion of sulfur-containing fossil fuels. The predictions of some of the resulting environmental effects are quite dramatic. In the U. S. National Re­search Council’s first assessment of the greenhouse effect in 1979, one of the primary conclusions was that if the C02 content of the atmosphere is doubled and thermal equilibrium is achieved, a global surface warming of between 2 and 3.5°C can occur, with greater increases occurring at higher latitudes (National Research Council, 1979). Some of the earlier predictions indicated that this increase is sufficient to cause warming of the upper layers of the oceans and a substantial rise in sea level, a pronounced shift of the agricultural zones, and major but unknown changes in the polar ice caps.

There has by no means been universal acceptance among the experts of many of the predictions that have been made, and there are many who have opposing views of the causes of some of the phenomena that have been observed and experimentally measured. However, several detailed reports were issued in the 1990s in which the consensus of large groups of experts is that human activities, largely the burning of fossil fuels, are affecting global climate. At any one location, annual variations can be large, but analyses of meteorologi­cal and other data over decades for large areas provide evidence of important systemic changes.

One of the first comprehensive estimates of global mean, near-surface tem­perature over the earth’s lands and oceans was reported in 1986 (Jones et ah, 1986). The data showed a long-timescale warming trend. The three warmest years were 1980, 1981, and 1983, and five of the nine warmest years in the entire 124-year record up to 1984 were found to have occurred after 1978. It was apparent from this study that over this period, annual mean temperature increased by about 0.6 to 0.7°C, and that about 40 to 50% of this increase occurred since about 1975. According to many analysts, the warmest year on record up to 1995 is 1995, and recent years have been the warmest since 1860 despite the cooling effect of the volcanic eruption of Mt. Pinatubo in 1991

(с/. Intergovernmental Panel on Climate Change, 1991 and 1995). Nighttime temperatures over land have generally increased more than daytime tempera­tures, and regional changes are also evident. Warming has been the greatest over the mid-latitude continents in winter and spring, with a few areas of cooling such as the North Atlantic Ocean. Precipitation has increased over land in the high latitudes of the Northern Hemisphere, especially during the cold season. Global mean surface temperature has increased by between 0.3 and 0.6°C since the late nineteenth century and average global surface temperature increases of 1 to 3.5°C, somewhat lower than originally predicted, are expected to occur by the middle of the twenty-first century. Global sea level has risen by between 10 and 25 cm over the past 100 years, and much of the rise may be related to the increase in global mean temperature.

Since preindustrial times, ambient concentrations of the greenhouse gases have exhibited substantial increases, inter alia C02 by 30% to about 360 parts per million (ppm), CH4 by 145% to more than 1700 parts per billion (ppb), and N20 by 15% to more than 300 ppb. The growth rates in the concentrations of these gases in the early 1990s were lower than predicted, while subsequent data indicate that the growth rates are comparable to those averaged over the 1980s. If C02 emissions were maintained near mid-1990 levels, analysts have predicted that this would lead to a nearly constant increase in atmospheric concentrations for at least two centuries, reaching about 500 ppm by the end of the twenty-first century, and that stabilization of atmospheric C02 concentrations at 450 ppm could only be achieved if global anthropogenic emissions drop to 1990 levels by about 2035, and subsequently drop substan­tially below 1990 levels (Intergovernmental Panel on Climate Change, 1995). It is estimated that the corresponding atmospheric lifetimes of C02, CH4, and N20 are about 50 to 200, 12, and 120 years, respectively, and that together with increasing emissions to the atmosphere, they account for the steadily rising ambient concentrations of the greenhouse gases.

These gases are called greenhouse gases because they selectively allow more of the shorter wavelengths of solar radiation to reach the earth’s surface, but absorb more of the reflected longer wavelength infrared radiation than that allowed to leave the atmosphere. The result is the greenhouse effect on reradia­tion of the absorbed energy. An example of the change in atmospheric concen­tration of C02 at one measuring site is shown in Fig. 1.11 (Whorf, 1996). These data were accumulated from 1958 to 1995 by experimental measurement at Mauna Loa, Hawaii and show how the concentration increased from about 315 to 360 ppm over the measurement period and how it varies during the biomass growing season. The data show an approximate proportionality be­tween the rising atmospheric concentrations and industrial C02 emissions (Keeling et ah, 1995). The distribution and a few properties of selected atmo­spheric gases that have infrared absorption in the atmospheric window (7 to

13 fxm) are listed in Table 1.7. Carbon dioxide is by far the most abundant and is indicated in this table as the relative infrared standard. The gas-to — carbon dioxide infrared absorption ratios in the atmospheric window of CH4, N20, and the CFCs are much greater than 1.0. The effect of doubling the concentration of N20, CO, CH4, and C02 on the earth’s surface temperature is estimated to be 0.25, 0.6 to 0.9, 0.95, and 2 to 3°C, respectively.

Methane is present at much lower concentrations than C02, but is estimated to increase the surface temperature by almost 1°C on doubling of its concentra­tion. This is predicted to occur because the methane-to-carbon dioxide infrared ratio in the atmospheric infrared window is about 25, and hence CH4 is a much stronger absorber of infrared radiation than C02. Presuming the current rates of increase in ambient concentrations of the greenhouse gases continue, the doubling times can be estimated at which the surface temperature effects in Table 1.7 can be expected. For C02, various studies indicate that its concen­tration will double by the latter part of the twenty-first century. Although there is disagreement as to the exact time of doubling, there is virtually no

dispute among scientists that the concentrations of atmospheric C02 have increased about 30% since 1850.

A note of caution is necessary regarding the predictions that have been made regarding global temperature increases. The predictions made in the mid-1990s by the Intergovernmental Panel on Climate Change rely heavily on the use of computerized climate models. There is much uncertainty inherent in this technique because few models can reliably simulate even the present climate without “flux adjustments” (с/. Kerr, 1997). Consequently, there is considerable disagreement about the specific effects on global temperature of the greenhouse gases, and even clouds and pollutant hazes, and whether global warming can be correlated with human activities or is a natural phenomenon. Application of improved computer models that do not use flux adjustments indicates that global warming is occurring at the lower end of the many predictions that have been made.