5. Carbon Isotopes in Atmospheric CO2


Houghton et al. (1990) assumed for the IPCC model 21% of our present-day atmospheric CO2has been contributed from burning of fossil fuel. This has been made possible by CO2 having a "rough indication" (sic!) lifetime of 50 - 200 years. It is possible to test this assumption by inspecting the stable 13C/12C isotope ratio (expressed as delta13CPDB*) of atmospheric CO2. It is important to note that this value is the net value of mixing all different CO2 components, and would show the results of all natural and non-natural (i.e. anthropogenic) processes involving CO2.

The natural atmospheric CO2 reservoir has delta13C close to -7 permil when in isotopic equilibrium with marine HCO3- and CaCO3 (Ohmoto, 1986). CO2 from burning of fossil-fuel and biogenic materials has delta13C of about -26 permil (Hoefs, 1980). Mixing these two CO2 components with the ratio 21% CO2 from fossil fuel burning + 79% "natural" CO2 should give a delta13C of the present atmospheric CO2 of approximately -11 permil.

Keeling et al. (1989) have reported delta13C of atmospheric CO2 over the last decades. The delta13C reported for atmospheric CO2 was -7.489 permil in December 1978, decreasing to -7.807 permil in December 1988, values close to that of the natural atmospheric CO2 reservoir, far from the delta13C value of -11 permil expected from the IPCC model. Hence the IPCC model is not supported by 13C/12C evidence.

Segalstad (1992, 1993) has by isotope mass balance considerations calculated the atmospheric CO2 lifetime and the amount of fossil fuel CO2 in the atmosphere. The December 1988 atmospheric CO2 composition was computed for its 748 GT C total mass and delta13C = -7.807 permil for 3 components: (1) natural fraction remaining from the pre-industrial atmosphere; (2)cumulative fraction remaining from all annual fossil-fuel CO2 emissions (from production data); (3) carbon isotope mass-balanced natural fraction. The masses of the components were computed for different atmospheric lifetimes of CO2.

The calculations show how the IPCC's (Houghton et al., 1990) atmospheric CO2 lifetime of 50-200 years only accounts for half the mass of atmospheric CO2. However, the unique result fits an atmospheric CO2 lifetime of approximately 5 (5.4) years, in agreement with numerous 14C studies compiled by Sundquist (1985) and chemical kinetics (Stumm & Morgan, 1970). The mass of all past fossil-fuel and biogenic emissions remaining in the current atmosphere was in December 1988 calculated to be approximately 30 GT C or less, i.e. maximum about 4%, corresponding to an atmospheric CO2 concentration of about 14 ppmv.

This small amount of anthropogenic atmospheric CO2 probably contributes less than half a Watt/m2 of the 146 W/m2 "Greenhouse Effect" of a cloudless atmosphere, contributing to less than half a degree C of radiative heating of the lower atmosphere.

The implication of the approximately 5 year lifetime is that about 135 GT C (18%) of the atmospheric CO2 pool is exchanged each year. This is far more than the about 6 GT C in fossil fuel CO2 now contributed annually to the atmosphere.

The isotopic mass balance calculations show that at least 96% of the current atmospheric CO2 is isotopically indistinguishable from non-fossil-fuel sources, i.e. natural marine and juvenile sources from the Earth's interior. Hence, for the atmospheric CO2 budget, marine equilibration and degassing, and juvenile degassing from e.g. volcanic sources, must be much more important, and burning of fossil-fuel and biogenic materials much less important, than assumed by the authors of the IPCC model (Houghton et al., 1990).

Return to Table of Contents


Last Updated June 20, 1997