The importance of studying Earth History stems from the unique insights that can be drawn. Such studies can document the natural climate and its rates of change and variability prior to human activity; can be used to estimate the sensitivity of the Earthıs climate system to changes in carbon dioxide, volcanic eruptions, and changes in the land surface; can be used to test the reliability of climate models by evaluating their simulations for conditions very different from the modern climate; and can be used to examine the integrated climate, chemical, and biological responses of the Earth to a variety of perturbations. Studies of Earth system history are best done by combining and reconciling the findings from observational and analytical studies with integrating studies using numerical models of the climate.

The geological history of the Earth provides strong evidence that climate has changed on time scales of decades to millions of years. Understanding this history can provide a basis for evaluating projections of climate change due to anthropogenic greenhouse perturbations and provide a context for human influences amongst the archive of natural perturbations of climate. Dr. Crowley will provide examples of how climate has changed on a variety of timescales and will highlight some of the lessons that can, and have, been learned by examining past records of climate change. For example, projections of warming for the next century suggest that temperatures will approach levels that have not occurred in many millions of years. The rapidity of this projected warming is greater than has occurred in the past and will lead to a very different climate state than exists today. The Earthıs geological record also suggests that changes in the atmospheric concentrations of CO2 have contributed significantly to past climate changes, underscoring the importance of the role of CO2 in determining the climate state

Two aspects of the study of Earth history with models (referred to as paleoclimatic modeling) are particularly valuable. First, comparison of climate model simulations of past climatic periods with geologic data suggest that for some variables (e.g., storm tracks) the climate models evidently yield robust predictions even for conditions very different from the present day climate. For other variables, such as regional precipitation, the model predictions are not yet representing the estimated regional patterns that have been developed from the geological record. Second, a variety of past climatic periods can be utilized as ³case studies² of climate sensitivity. For these cases, the consistency of the estimates of climate sensitivity to carbon dioxide variations described in the recent IPCC Second Assessment report are comparable to the sensitivities required to explain the geologic record. In other words, the geologic record offers the opportunity to assess some of the limitations and strengths of climate model predictions, as well as to assess climate sensitivity to changes such as increases in greenhouse gases. While uncertainties exist, in each case analyzed to date, the geologic record suggests that the mid to upper range of climate sensitivity given by the IPCC report is most reasonable.

Dr. Thomas J. Crowley has held a number of positions in paleoclimatology: assistant professor at the University of Missouri - St. Louis, program director in climate dynamics at the National Science Foundation, National Research Council Fellow at NASA/Goddard Space Flight Center, senior scientist for the Applied Research Corporation, and most recently Professor of Oceanography at Texas A&M University. Although his early work involved the study of the North Atlantic Ocean circulation during the last ice age, he has subsequently become interested in the effects of the movement of continents on climate, both from a modeling and observational viewpoint. Dr. Crowley has also been involved in several studies synthesizing paleoclimate data, especially with respect to its relevance to better understanding future climate projections due to the anthropogenic greenhouse perturbation. He is the author of a number of articles on past climates and is co-author of a recent book on the subject. Dr. Crowley received his Ph.D. in marine geology from Brown University in 1976.