"One approach toward assessing the role of pCO2 in forcing climate change is to evaluate records of isotopic fractionation that occurred during marine photosynthetic carbon fixation. The isotopic composition of photosynthetic marine organic carbon is primarily a function of [CO2aq], growth rate, and cell geometry of the organism. By sampling sedimentary alkenones from oligotrophic-type settings, the effect of growth rate and cell geometry is presumably minimized, thereby leaving [CO2aq] as the major control on alkenone isotopic compositions.
Our results show that pCO2 ranged between 1000 to 1500 ppmv in the middle to late Eocene, and then decreased in several steps during the Oligocene, and reached modern levels by the latest Oligocene. The fall in pCO2 likely allowed for a critical expansion of ice sheets on Antarctica, and promoted conditions that forced the onset of terrestrial C4 photosynthesis."
Google: it's good for your brain.
Now it's up to other scientists to come up with alternative Eocene-Oligocene climate scenarios where the radiative forcing effects of CO2 are not the major cause of significant climatic cooling, because the researchers have shown a) that CO2 levels were dropping significantly when the ice sheets were established and expanding, and b) the likeliest cause of the cooling was the reduced greenhouse effect due to lower levels of atmospheric CO2 (climate modeling, dontcha know).
For Sheppard to say there's "no evidence"; I repeat the statement of "ridiculous".