Paleoclimate modeling at GISS is aimed at better understanding how the Earth’s climate system operates under conditions that are extreme compared to the environmental range of the 20th century. Spanning time periods that include the early evolution of the Earth’s atmosphere, oceans, biosphere and cryosphere to just before the development of modern meteorological instruments, this research both constrains and expands NASA’s modeling program. Paleoclimate studies allow us to examine the full range of variability throughout geologic time, the potential magnitude of climate change in Earth’s near future, and even the prospective habitability of extrasolar earth-like planets. By it’s nature, paleoclimate research brings together both data and modeling techniques to provide the broadest possible view of Earth’s climate history and it especially helps delineate the processes the operate when climates are either much warmer or colder than at present. In turn, advances in our ability to simulate high-magnitude, global-scale climate changes that actually occurred in the past strengthens our confidence in the conclusions drawn from simulations of future climate.