Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 30

Figure 2.2. Hypothetical single curve relating T G to CO 2 concentration.
For example, we have good data on CO 2 concentration during the Last Glacial
Maximum, some 20,000 years ago, so that is one important historical point for
further study. In addition, there are also a variety of estimates of CO 2 concentra-
tion that go back as far as 500 million years, but unfortunately such data are very
scattered and do not appear to be very reliable. But, climatologists are a sturdy
lot, and they are willing to derive a dollar's worth of conclusions from a penny's
worth of data.
What we seek is a relationship between CO 2 concentration and the Earth's
climate over long geological periods during which the CO 2 concentration varied
over a wide range. There is evidence that the CO 2 concentration may have been
well over 20,000 ppm in the distant past, and it has been as low as 180 ppm only
20,000 years ago. It would be very nice if there were a single curve relating T G to
CO 2 concentration such as that shown in Figure 2.2 . In that case, if we could find
several points on the curve, we could attempt to map out a good portion of the
curve.
However, over long time periods, the variation of T G with CO 2 concentration
depends on various factors such as the placement of the continents on Earth, the
functionality of ocean currents, the past history of the climate, the orientation of
the Earth's orbit relative to the Sun, the luminosity of the Sun, the presence of
aerosols in the atmosphere, volcanic action, land clearing, biological evolution,
etc. Hence, there is probably no single curve relating T G to CO 2 concentration
but, rather, a set of curves that depend on the above factors (see Figure 2.3 ) .
Over the past few million years, the Earth has vacillated between alternating
ice ages and interglacial periods, probably driven by variations in the Earth's orbit
about the Sun, accompanied by variations in CO 2 concentration and other global
and regional changes. Over the past 10,000 years (the Holocene), the Earth has
been in a relatively quiescent interglacial period with only moderate variations in