Known causes of global climate change, like cyclical eccentricities in Earth's rotation and orbit, as well as variations in the sun's energy output, are the primary causes of climate cycles measured over the last half million years. However, secondary greenhouse effects stemming from changes in the ability of a warming atmosphere to support greater concentrations of gases like water vapor and carbon dioxide also appear to play a significant role. As demonstrated in the data above, of all Earth's greenhouse gases, water vapor is by far the dominant player.
http://www.geocraft.com/WVFossils/greenhouse_data.html
Good research there supported by some good numbers.