If our average
temperature fell about 5-8 degrees C. below the average
temperature of our time it would suffice; and it is further
calculated that if the quantity of carbon-dioxide in our
atmosphere were reduced by half, we should have this required
fall in temperature. So great a reduction would not be necessary
in view of the other refrigerating agencies. Now it is quite
certain that the proportion of carbon-dioxide was greatly reduced
in the Pleistocene. The forests of the Tertiary Era would
steadily reduce it, but the extensive upheaval of the land at its
close would be even more important. The newly exposed surfaces
would absorb great quantities of carbon. The ocean, also, as it
became colder, would absorb larger and larger quantities of
carbon-dioxide. Thus the Pleistocene atmosphere, gradually
relieved of its vapours and carbon-dioxide, would no longer
retain the heat at the surface. We may add that the growth of
reflective surfaces--ice, snow, cloud, etc.--would further lessen
the amount of heat received from the sun.
Here, then, we have a series of closely related causes and
effects which would go far toward explaining, if they do not
wholly suffice to explain, the general fall of the earth's
temperature.
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