GEOLOGIC HISTORY OF ATMOSHPERIC CO2
- Through molecular diffusion, the concentration
of carbon dioxide in the atmosphere is nearly constant everywhere.
However, its value has fluctuated over geologic time,
from as high as 4200 ppmv during the Lower Carboniferous period
(345-320 million years ago) (Budyko, 1986)
to as low as 190 ppmv during the
last glacial age (21,000 years ago) (IPCC, 1996a).
Long-term variations in carbon dioxide are related
to fluctuations in the intensity of volcanic activity (Budyko, 1986).
The latter appears to have subsided in the Cenozoic era,
from 65 million years ago to the present,
which has produced a substantial decrease of atmospheric carbon dioxide
concentration within this period.
- The reservoirs of fossil fuels which lie underground
developed over a long period of time,
starting at the Upper Devonian period (360-345 million years ago)
(Williamson, 1967; Magoon and Dow, 1994).
As rates of productivity exceeded rates of respiration,
effective sequestering of carbon took place,
eventually leading to the formation of fossil fuels.
- Over the past 360 million years, the atmospheric carbon dioxide
concentration has fluctuated markedly, but the tendency
has been generally downwards, from 4200 ppmv to about 200 ppmv.
This has allowed the Earth to cool,
and has set the stage for {\em Homo sapiens} to develop
in the cooler climatic conditions which have prevailed in
the past one million years (Budyko, 1978).
- In the last glacial age (21000 years ago), the concentration of
atmospheric carbon dioxide reached an all-time low value of 190 ppmv.
Since then it has increased slowly, reaching 282 ppmv
at the beginning of the industrial revolution, c. 1800 (IPCC, 1996a),
This increase represents an average preindustrial rate of
of 0.44 ppmv per century, which could be partly attributed
to the permanent indirect artificial combustion
produced by the agricultural revolution,
which began about 10,000 years ago.
- At the beginning of the twentieth century,
the carbon dioxide concentration was 299 ppmv,
i.e., an increase of 17 ppmv in the nineteen century.
The present level (1998) is 363 ppmv, i.e., an increase of
64 ppmv, which amounts to an average rate of 65 ppmv per century.
This last rate is 148 times greater than the average preindustrial rate.
Recent rates of increase are even greater;
for instance, during 1994, the increase was 1.6 ppmv,
i.e., nearly 2.5 times the average rate of the twentieth century.
These concentrations are global mean annual values.
Thus, they do not reflect seasonal and other climatic variations
(approximately $\pm$ 1 ppmv), which are partly a function of
latitude and altitude (IPCC, 1996a).
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