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
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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