[Brad from Tennessee]

From Wikipedia:

Carbon dioxide

Carbon dioxide concentration during the mid Pliocene has been estimated at around 400 ppmv from 13C/12C ratio in organic marine matter[11] and stomatal density of fossilized leaves,[12] decreasing carbon dioxide levels during late Pliocene may have contributed substantially to global cooling and the onset of northern hemisphere glaciation.

http://en.wikipedia.org/wiki/Pliocene_climate

[palmer]

decreasing carbon dioxide levels during late Pliocene may have contributed substantially to global cooling and the onset of northern hemisphere glaciation.

They are pretty much wrong. The isthmus of Panama closed and triggered glaciation. Decreasing CO2 was a bit player, just a minor amplifier of the cooling.