Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 26

global average temperature decrease of roughly 4.5 C. Hansen and Sato (2011),
Chylek and Lohmann (2008), and Kohler et al. (2009) all independently estimated
the forcing at the LGM (see Table 2.2 on p. 33). The contribution of the diminu-
tion of CO 2 at the LGM to total cooling was estimated by these studies to be in
the range 16% to 33%. While it seems likely that solar input to higher latitudes
triggered the cycles, the variability of CO 2 concentration is believed to have played
a part in determining the extremity of the temperature cycle that resulted from
this trigger. The changes in CO 2 concentration between glacial maxima and inter-
glacials ( 180 to 280 ppm) are well documented in ice core records, although no
one seems to have a satisfactory explanation for why the CO 2 concentration
changed this much (simple solubility in the oceans does not suce). However, the
estimates of forcings, particularly due to dust, vary considerably from investigator
to investigator and it is dicult to pin down climate sensitivity to CO 2 change.
There are reasonably good estimates available of the global average temperature
and the CO 2 concentration at the LGM 20,000 years ago, and if these data are
compared with values in the pre-industrial era (a few hundred years ago) one can
thereby estimate the sensitivity of the climate to CO 2 concentration over the range
180 to 280 ppm. Using this estimated climate sensitivity, one can then estimate
the global average temperature rise in going from 280 to 560 ppm. Various investi-
gators have come up with a range of projections. It is noteworthy that the
estimates for the real world D T G due to doubling CO 2 from 280 to 560 ppm range
from 1to 3 C. However, these estimates do not take into account possible
differences in humidity and cloudiness.
Over much longer time periods (up to 540 million years ago) the evidence
from benthic sediments (and other geological evidence as well) indicates that there
have been periods of great warmth with no glaciation at all on Earth, with
occasional periods when the Earth was heavily glaciated. The widely held view
amongst geologists and climatologists alike is that the primary cause of these
climate changes was variability of CO 2 concentration due to long-term imbalances
between CO 2 degassing at spreading centers and the conversion of atmospheric
CO 2 to mineral carbon through long-term silicate weathering and oceanic carbon-
ate formation. The argument goes (more or less): ''If it wasn't CO 2 , what else
could it have been?'' Foster et al. (2009) described this as the ''accepted paradigm''
that requires global temperature to vary in unison with CO 2 . So, paleoclimatolo-
gists have been trying for decades to establish a relationship between climate and
CO 2 concentration over many millions of years. The more audacious of these
have attempted to establish a quantitative relationship between climate and CO 2
concentration in order to try to estimate Earth system climate sensitivity. Unfortu-
nately, the proxy data for CO 2 over many millions of years are very widely
scattered and the results are equivocal. There is a general tendency for warmer
climates to be associated with higher CO 2 concentrations, but this mainly relates
to very large temperature excursions, and even then there are many exceptions.
The supposed one-to-one correspondence between CO 2 and climate is fuzzy and
sometimes contrary. Evidently, other factors than CO 2 must also influence the
climate. In his ''Perspective'' article, Ruddiman (2010) emphasized that there is no