Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 76

Figure 2.34. Royer (2006)'s estimate of the net forcing due to CO 2 variability and a gradually
strengthening Sun compared with that of Crowley and Berner (2001). The vertical gray bars are
Royer's estimates of glacial periods, as compared with the curve of temperature given by
Crowley and Berner (2001).

CO 2 levels below 1000 ppm. A CO 2 threshold of below 500 ppm is suggested for
the initiation of widespread, continental glaciations, although this threshold was
likely higher during the Paleozoic due to a lower solar luminosity at that time.
Also,

aCO 2 threshold of below 1000 ppm is proposed for the initiation of
cool non-glacial conditions. A pervasive, tight correlation between CO 2 and
temperature is found both at coarse (10 million-year timescales) and fine
resolutions up to the temporal limits of the data set (million-year timescales),
indicating that CO 2 , operating in combination with many other factors such as
solar luminosity and paleogeography, has imparted strong control over global
temperatures for much of the Phanerozoic.''

...

With the passage of time since about 2004, the belief that CO 2 controls global
temperature has become more widespread. Vaughn (2007) reviewed the field and
concluded that CO 2 concentration is an important factor in major long-term
changes in climate.
Came et al. (2007) claimed that they had developed an improved method for
estimating paleotemperatures. They claimed that their results show a much better
correlation of CO 2 variability and temperature change. However, they only
estimated the temperature at two specific times during the Phanerozoic, and their
results are not very convincing to this writer.
Fletcher et al. (2008) contributed new estimates of CO 2 concentrations from
200 million years ago to 60 million years ago. They concluded that there is a
coupling of CO 2 and temperature.