Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 81

present knowledge of climates of the geological past, that there is no simple
straightforward relation between levels of atmospheric CO 2 , as estimated by
the various modelers and changes in the global climatic gradient.''
Although the review by Boucota and Gray (2001) was written prior to several
of the papers cited herein, the conclusions seem to remain valid.
2.3.10.9 Concluding remarks
The widely held view amongst geologists and climatologists alike is that the
primary cause of long-term climate changes is 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 carbonate 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 CO 2 to vary in unison with global
temperature. Thus, paleoclimatologists have been trying for decades to establish a
relationship between climate and CO 2 concentration over many millions of years.
There is some evidence that over many millions of years, higher CO 2 concentra-
tions are often, but not always, associated with warmer climates. However, there is
a great deal of scatter in the CO 2 proxy data, and this relationship is dicult to
pin down quantitatively. Royer (2010) began his commentary with the statement:
''Global temperatures have covaried with atmospheric carbon dioxide (CO 2 )
over the last 450 million years of Earth's history. Critically, ancient greenhouse
periods provide some of the most pertinent information for anticipating how the
Earth will respond to the current anthropogenic loading of greenhouse gases.
Paleo-CO 2 can be inferred either by proxy or by the modeling of the long-term
carbon cycle. For much of the geologic past, estimates of CO 2 are consistent
across methods.''
This seems to be a rather optimistic view, considering the data from his paper
(see Figure 2.37 ) . This figure compares various estimates of tropical sea surface
temperature (SST) with estimates of CO 2 concentration over the time period 120
million years ago to 40 million years ago. Royer's point (I think) is that through-
out this period SST was at least several degrees warmer than today and, even
though there is much scatter in the CO 2 estimates, the general level of CO 2 con-
centration was much higher than today. This argument seems to make some sense
from 120 million years ago to 90 million years ago. Yet, there are diculties
from 70 million years ago to 50 million years ago when SST remained high, yet
the CO 2 concentration appears to have been much lower. In any event, the
extreme scatter in the data in Figure 2.37 does not convey confidence that any
valid conclusions can be drawn.
The best chance to use paleoclimatic data to infer climate sensitivity is
probably the Last Glacial Maximum (LGM) some 20,000 years ago, when the