Over the last 50 million years, the earth's climate has varied but has drifted toward cooler conditions (Dwyer, 2000), as measured by an increase in overall ice volume and decrease in deep-sea temperature. Lear et al. (2000) have advanced the traditional method of using isotopes of oxygen for such paleoclimatic studies by looking at the ratio of magnesium to calcium concentrations in microfossil shells. This new technique will help changes in deep-sea temperatures driven by changes in deep-water production and circulation. This, in turn, may help reveal the prospects for "climate surprises" that presently are beyond the capability of climate models to simulate.
References
Dwyer, Gary S., 2000: Unraveling the signals of global climate change. Science, 287, 246-247.
Lear, C. H. Elderfield, and P. A. Wilson, 2000: Conozoic deep-sea temperatures and global ice volumes from Mg/Ca in benthic foraminiferal calcite. Science, 287, 269-272.