CO2

CH4

CFC-11

CFC-12

N2O

Preindustrial atmospheric concentration

280 ppmv

0.8 ppmv

0

0

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

(50–200)

10

65

130

150

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.