ity, resulting from changes in the Earth's orbital cycles, pioneered by the CLIMAP project and described by Imbrie et al. (1992, 1993), has been verified and further elucidated by the SPECMAP project. Orbitally induced variations in insolation at the Milankovitch periods (primarily 100,000, 41,000 and 23,000 years) explain much of the change in global ice volume throughout the late Pleistocene and have been identified in a variety of paleoclimate records (e.g., marine and ice cores and loess sequences). CO2 and CH4 figure prominently in climate change over the last glacial/interglacial cycle, as demonstrated by the close association between Vostok (Antarctica) ice core CO2 and temperature (see Figure 6.1).3 This dramatic demonstration of the long-term association between temperature and CO2 has had a profound effect on the implications of anthropogenically induced greenhouse gas warming. However, the fact that CO2 lags temperature at major climate transitions (e.g., the end of the last interglacial) suggests that the system response may be complex.