Causes of the Greenhouse Effect

Since preindustrial times, ambient concentrations of the greenhouse gases have exhibited substantial increases, inter alia CO2 by 30% to about 360 parts per million (ppm), CH 4 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 CO2 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 CO2 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 CO2, 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 re radiation of the absorbed energy. An example of the change in atmospheric concentration of CO2 at one measuring site is shown. 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 CO2 emissions (Keeling et al., 1995). The distribution and a few properties of selected atmospheric gases that have infrared absorption in the atmospheric window (7 to despite the cooling effect of the volcanic eruption of Mt. Pinatubo in 1991 13/~m). Carbon dioxide is by far the most abundant and is indicated 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 CO2 on the earth’s surface temperature is estimated to be 0.25, 0.6 to 0.9, 0.95, and 2 to 3~ respectively.

Methane is present at much lower concentrations than CO2, but is estimated to increase the surface temperature by almost 1~ 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 CO2. 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 can be expected. For CO2, 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 CO2 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” (cf. 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.