Abstract: Records of past climate variability preserved in ice and marine cores indicate that the Earth's climate can switch modes, from "cold" to "warm", in just a few decades. Because these abrupt changes are also large, and may occur faster than the response time of our society, we need to understand how, why, and when these fluctuations occur. Despite their importance, there is no widely accepted hypothesis that adequately explains the origin of millennial-scale climate change (MSCC). To address this problem, and to test specific hypotheses, the Ocean Drilling Program (ODP) has recovered cores where sediments accumulated rapidly in thick sequences. North Atlantic drift sites (Leg162) records suggest that ice-sheet size exerts a primary influence on the amplitude of MSCC. Dr. Oppo will discuss this and other insights gleaned from study of North Atlantic sediment records.