Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 75

This was counteracted by greenhouse forcing from CO 2 with R(CO 2 ) reaching
values higher than 20. On balance, the forcing due to these two factors is shown
as the middle curve in Figure 2.33 . If CO 2 was the main driver of climate change,
then tropical sea temperature should follow the forcing curve. For gray area B in
Figure 2.33 , there is a perfect consonance between low CO 2 concentration and low
forcing. However, for gray areas A and C, the forcings were strongly positive but
temperatures were low.
Crowley and Berner (2001) pointed out:

''There is a major discrepancy during the period 120 to 220Ma between
cold low-latitude temperatures and high levels of CO 2 . The overall low

correspondence between low-latitude d 18 O and net [CO 2 levels] begs for an
explanation, especially because of the striking correspondence between low net
[CO 2 levels] and major continental glaciation from 256 to 338Ma''.

...

They suggested that in the case of the relatively short-lived glaciation of about
440Myr, which occurred at a time of high R(CO 2 ), ''climate models suggest that
the unusual continental configuration of a large landmass tangent to the South

Pole could result in conditions where high CO 2 and glaciation can co-exist.''
Nevertheless, they insisted that ''the persistent Phanerozoic de-correlation between
tropical d 18 O and R(CO 2 ) demands a more comprehensive explanation.'' They
suggested that one possibility is that the temperature estimates are erroneous.
Another is that ''climate change in the tropics can be largely decoupled from
mid-high-latitude ice volume changes.'' While they insisted that ''the first-order
agreement between the CO 2 record and continental glaciation continues to
support the conclusion that CO 2 has played an important role in long-term
climate change,'' they nevertheless concluded:

...

''Given the need for better confidence in some of the paleoclimate data and
unanticipated complications arising from altered tectonic boundary conditions,
it may be hazardous to infer that existing discrepancies between models and data
cloud interpretations of future anthropogenic greenhouse gas projections.''

Royer (2006) compared 490 published proxy records of CO 2 over the
Phanerozoic with records of global cool events to evaluate the strength of CO 2 -
temperature coupling over the Phanerozoic. Figure 2.34 shows Royer's result in
comparison with that of Crowley and Berner (2001). Royer found that the
predominant glacial periods were between 350 and 290 million years ago, and the
past 30 million years when CO 2 concentrations were lower. The glacial period at
445 million years ago seems to be a contradiction of the CO 2 -climate connection,
but Royer (2006) argued that the glacial period was brief and the CO 2 level at that
time is uncertain.
Royer (2006) concluded:

''For periods with sucient CO 2 coverage, all cool events are associated with