During the history of Earth, there have been several ice ages. What caused the climate to cool so much to create these ice ages?
Wikipedia sometimes misrepresent the orginal sources like Pearson & Palmer 2000. See fig 4 where such a reduction is dated just before 51Ma, hence some 2 Ma before the Azolla event:....
Most scientists mark the current icehouse earth as beginning about 49 million years ago when CO2 levels plummeted from about 3400 ppm to 650 ppm. ...http://en.wikipedia.org/wiki/Azolla_event.
See also how pCO2 increases from about 600ppm to about 2600ppm 'shortly' after 49Ma, in the grey band. The authors note:"We note that the termination of North Atlantic volcanism at about 54±53 Myr ago corresponds approximately to the initial drop that we record in pCO2"
Note that the more recent Zachos et al 2008 (fig 2) is consistent with those spikes.Our data do not support a precise covariation of pCO2 and temperature; indeed we record a pCO2 peak during the cooling phase at approximately 45.5 Myr ago.
This post requires further discussion and rectification when it turns out that the references are doubtful.Wikipedia...
Paleotemperature records (multiple refs) demonstrate that the inception of the current icehouse climate mode was not strictly coincident with the Eocene-Oligocene boundary (33.9Ma); instead the Earth "staggered"into the boundary, punctuated by short lived (<1 m.y.) warm and cool phases (fig 1)