Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 33

from 280 to 560 ppm would increase T G by roughly 1.2 C. 2 This is represented by
curve 1 in Figure 5.2 .
As the Earth warms due to increasing CO 2 , various secondary processes take
place. Additional evaporation from bodies of water will tend to increase the
humidity of the atmosphere, ice sheets (with their high reflectivity of sunlight) will
shrink, cloudiness might increase or decrease in some regions, aerosol concentra-
tions might vary in unknown ways, precipitation patterns will vary, ocean currents
may change somewhat, and other changes may take place in the Earth's biota.
These so-called feedback processes will also affect T G . There is considerable
uncertainty in the magnitude (and, in the case of cloudiness, even the sign) of
such feedback effects and, as a result, there have been many diverse estimates
of T G due to a doubling of CO 2 when feedbacks are included. Some of these
are shown in Figure 2.5 as curves 2, 3, and 4. Curve 3 represents a rough
estimate that has been adopted as a consensus by climatologists of the alarmist
persuasion. Some skeptical climatologists believe that the truth lies nearer to
curves 1 and 2.
It is not our purpose herein to review the many estimates of warming due to
an increase in CO 2 concentration from 280 to 560 ppm. Rapp (2008) reviews much
of this literature. Instead, we are concerned here with the broader picture as
shown in Figure 2.4 . Can we somehow estimate the shape of the broad curve of
T G vs. CO 2 concentration over a very wide range of CO 2 concentration and,
thereby, find where this curve passes through the gray region in Figure 2.4 ?
Therefore, our intent is to review the literature on the broad dependence of T G on
CO 2 concentration over a very wide range of CO 2 concentration over geologic
time spans.
A few comments of caution need to be made at this point. One comment is
that if feedbacks prove to be as amplifying as the consensus would advocate, this
would indicate that the effect of CO 2 on climate is greatly changed by secondary
factors, and the use of data and models from hundreds of millions of years ago
may produce misleading results since we may currently be on a very different
curve in Figure 2.4 than they were a few hundred million years ago. Of particular
importance in this regard is the arrangement of the continents. In addition, the
Sun was some 6% reduced in intensity 500 million years ago. These, and other
changes, add uncertainty as to whether such data can properly be extrapolated to
the 21st century. Another comment is that proxies for T G and CO 2 concentration
over geologic time spans are not likely to be very accurate, and these should be
critically reviewed before relying on them. As Zeebe (2011) said:
''By studying the relationship between greenhouse gas forcings and global
2 This estimate derives from two factors. The forcing due to the change in CO 2 concentration
from 280 to 560 ppm is estimated to be about 4W/m 2 based on spectroscopic absorption of
outgoing IR emitted by the Earth (see Figure 2.8 ) . If the Earth acts as a blackbody with
feedbacks neglected, climate sensitivity is calculated to be about 0.3 C/(W/m 2 ). The product of
these two numbers is 1.2 C.