Future warming projected by the Intergovernmental Panel on Climate Change (IPCC) has the potential to affect every person on Earth. Extreme weather events, rising sea level, and migrating ecosystems and resources may result in socioeconomic stresses. Although we can plan and prepare for what is expected, the most dangerous aspect of our changing climate is the uncertainty in climate sensitivity. To reduce the uncertainties of climate change, paleoclimatologists are focusing on a possible yet imperfect analog to a future warmer climate. The middle part of the Pliocene epoch, approximately 3.3-3.0 million years ago, is the most recent period in Earth's history in which global warmth reached temperatures similar to those projected for the end of this century, about 2°-3°C warmer globally on average than today [IPCC, 2007]. Unlike earlier warm periods such as the Late Cretaceous (approximately 100-65 million years ago), the mid-Pliocene was similar to today in terms of the positions of the continents and oceans and atmospheric carbon dioxide (CO2) concentrations, but global warmth was distributed differently. During the mid-Pliocene, temperatures at high northern latitudes, above 70°N, were as much as 10°-20°C higher than today, but tropical temperatures were near the same. Understanding the much warmer mid-Pliocene climate, then, has the potential to unlock the secret to climate sensitivity. The past may indeed be the key to the present.