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TABLE 3.1 Key Greenhouse Gases Influenced by Human Activity







Preindustrial atmospheric concentration

280 ppmv

0.8 ppmv



288 ppbv

Current atmospheric concentration (1990)a

353 ppmv

1.72 ppmv

280 pptv

484 pptv

310 ppbv

Current rate of annual atmospheric accumulationb

1.8 ppmv (0.5%)

0.015 ppmv (0.9%)

9.5 pptv (4%)

17 pptv (4%)

0.8 ppbv (0.25%)

Atmospheric lifetime (years>c






NOTE: Ozone has not been included in the table because of lack of precise data. Here ppmv = parts per million by volume, ppbv = parts per billion by volume, and pptv = parts per trillion by volume.

aThe 1990 concentrations have been estimated on the basis of an extrapolation of measurements reported for earlier years, assuming that the recent trends remained approximately constant.

bNet annual emissions of CO2 from the biosphere not affected by human activity, such as volcanic emissions, are assumed to be small. Estimates of human-induced emissions from the biosphere are controversial.

cFor each gas in the table, except CO2, the ''lifetime" is defined as the ratio of the atmospheric concentration to the total rate of removal. This time scale also characterizes the rate of adjustment of the atmospheric concentrations if the emission rates are changed abruptly. CO2 is a special case because it is merely circulated among various reservoirs (atmosphere, ocean, biota). The "lifetime" of CO2 given in the table is a rough indication of the time it would take for the CO2 concentration to adjust to changes in the emissions.

SOURCE: Intergovernmental Panel on Climate Change. 1990. Climate Change: The IPCC Scientific Assessment, J. T. Houghton, G. J. Jenkins, and J. J. Ephraums, eds. New York: Cambridge University Press. Reprinted by permission of Cambridge University Press.

of CH4 absorbs radiative energy 25 times more effectively than each molecule of CO2, and CFC-12 is 15,800 times more effective than CO2 on a per molecule basis and, since molecules of the two gases have different mass, 5,750 times more effective on a per mass basis. Figure 3.2 incorporates a simple extrapolation of current atmospheric transformation rates. It displays the incremental energy absorption rates that would accompany various emission scenarios. The energy absorption is given in watts per square meter (W/m2) and, in accord with the vocabulary of this subject, changes in the absorption

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