Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 58

Figure 2.18. Comparison of CO 2 and a temperature proxy across the Eocene-Oligocene
boundary (adapted from Pearson et al., 2009).
They also suggested that the threshold for initiation of Antarctic glaciation is
in the range 700-850 ppm.
These conclusions seem to be influenced by adherence to the ''accepted
paradigm'' that CO 2 is the main factor in climate change. While there was indeed
a moderate decrease in CO 2 as the Earth approached the Eocene-Oligocene
boundary, the so-called ''hysteresis effect'' cannot be brushed away so easily.
CO 2 levels popped up to well above the threshold while temperatures remained
low. While CO 2 is clearly a factor in climate change, once again we must adopt
the comment by Foster et al. (2009) that CO 2 variations are ''not exactly
sympathetically with climate as the paradigm suggests.''
Peters et al. (2010) used ''an unusually well exposed coastal incised river-valley
complex in the Western Desert of Egypt to show that eustatic sea level fell and
then rose by 40m, 2 million years prior to establishment of a permanent
Antarctic Ice Sheet.''
They concluded:
''This fall in sea level is associated with a positive oxygen isotope excursion
that records buildup of an Antarctic Ice Sheet with a volume 70% of the
present-day East Antarctic Ice Sheet. Both the sea-level fall and subsequent rise
were coincident with a transient oscillation in atmospheric CO 2 concentration
down to 750 ppm, which climate models indicate may be a threshold for South-
ern Hemisphere glaciation. Because many of the carbon emission scenarios for
the coming century predict that atmospheric CO 2 will rise above this same
750 ppm threshold, our results suggest that global climate could transition to
a state not unlike the Late Eocene, when a large permanent Antarctic Ice Sheet
was not sustainable.''