New research by Bornemann et al. in Science this week indicates that warming periods might actually promote ice growth. Using forams from core samples taken on the deep ocean floor off South America, the researchers reconstructed temperature records for the Turonian (93.5 to 89.3 millions years ago). Despite a relatively balmy 35 degrees C tropical sea surface temperature, the glaciation events took place (evidenced by the a positive d18 O excursion in the marine limestone). Absolutely love this quote from the article (emphasis mine)
We are left with the apparent paradox that the prevailing extraordinarily high tropical temperatures during the Turonian were not a barrier to the initiation and growth of large continental ice sheets. The development of these ice sheets could be attributed to an increase in the activity of the hydrological cycle, which must have initiated more humid conditions and enhanced precipitation in the high latitudes.
Andre Bornemann, Richard D. Norris, Oliver Friedrich, Britta Beckmann, Stefan Schouten, Jaap S. Sinninghe Damste, Jennifer Vogel, Peter Hofmann, and Thomas Wagner (2008) Isotopic Evidence for Glaciation During the Cretaceous Supergreenhouse. Science, 319:189-192.