We study the mechanisms that cause climate to change on time-scales from decades to tens-of-thousands of years and how those changes are propagated through the ocean-atmosphere-ice system. To accomplish this they develop paleoclimate records from sediment cores from throughout the world's oceans and lakes. Molecular fossils and their hydrogen, carbon and nitrogen isotopic ratios provide estimates of past temperature, precipitation and biological productivity. Those observations are then used to test mechanisms of climate change with computer simulations. Reconstructing natural climate changes in the past provides the means to determine when the modern climate is outside the range of natural variability and will improve predictions of how it will change in the future.