Relative contributions of greenhouse gas emissions to global warming

Abstract

IN the past few years, many workers have noted that the combined effect on climate of increases in the concentrations of a large number of trace gases could rival or even exceed that of the increasing concentration of carbon dioxide^1–3. These trace gases, principally methane, nitrous oxide and chlorofluorocarbons, are present at concentrations that are two to six orders of magnitude lower than that of carbon dioxide, but are important because, per molecule, they absorb infrared radiation much more strongly than carbon dioxide. Indeed a recent study⁴ shows that trace gases are responsible for 43% of the increase in radiative forcing from 1980 to 1990 (Fig. 1). An index to compare the contribution of various 'greenhouse' gas emissions to global warming is needed to develop cost-effective strategies for limiting this warming. Estimates of relative contributions to additional greenhouse forcing during particular periods do not fully take into account differences in atmospheric residence times among the important greenhouse gases. Here we extend recent work on halocarbons^5,6 by proposing an index of global warming potential for methane, carbon monoxide, nitrous oxide and CFCs relative to that of carbon dioxide. We find, for example, that methane has, per mole, a global warming potential 3.7 times that of carbon dioxide. On this basis, carbon dioxide emissions account for 80% of the contribution to global warming of current greenhouse gas emissions, as compared with 57% of the increase in radiative forcing for the 1980s.