5. Carbon Isotopes in Atmospheric 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).

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