Earth's Climate System: Paleoclimate
Paleoclimatology studies climate conditions and climate changes in the geologic past.
Paleoclimatology pages at NASA's Earth Observatory (links to additional pages at bottom)
Paleoclimatology: Reconstructing past climate (Page Paleontological Science Center)
Paleoclimatology Visualizations (SERC at Carleton College)
Royer et al.: CO2 as driver of past climate change (GSA Today, 2004)
Staniford: Living in the Eemian (climate change 125,000 years ago; long, detailed, but really excellent post at The Oil Drum).
The Paleocene-Eocene Thermal Maximum: Discussion of Zeebe et al (2009) at RealClimate.