Earth's climate has changed considerably in the past, and is predicted to change in the future. By studying past climates we gain a broader understanding of what climates are possible and likely in the future. In this proposal we focus on the very warm climates of the past and their relationship to global warming. In the far past, some 60 million years ago, the planet was very warm. However, the warming was not distributed uniformly over the globe. Rather, the high latitudes warmed much more than low latitudes, to the extent that palm trees grew in Wyoming and crocodile-like animals roamed Northern Canada. The evidence for this is very robust, since fossil remains are unambiguous. Crocodilians are intolerant to cold, meaning there were no long periods of very cold weather, even in winter, in northern North America. This is a complete mystery that current climate models cannot explain. We will study this problem using a novel suite of models, and apply what we learn to better understand the global warming ahead of us.