Utilization of paleoclimate results to validate projections of a future greenhouse warming

Paleoclimate data provide a rich source of information for testing projections of future greenhouse trends. This paper summarizes the present state-of-the-art as to assessments of two important climate problems. (1) Validation of climate models - The same climate models that have been used to make greenhouse forecasts have also been used for paleoclimate simulations. Comparisons of model results and observations indicate some impressive successes but also some cases where there are significant divergences between models and observations. However, special conditions associated with the impressive successes could lead to a false confidence in the models; disagreements are a topic of greater concern. It remains to be determined whether the disagreements are due to model limitations or uncertainties in geologic data. (2) Role of CO2 as a significant climate feedback: Paleoclimate studies indicate that the climate system is generally more sensitive than our ability to model it. Addition or subtraction of CO2 leads to a closer agreement between models and observations. In this respect paleoclimate results in general support the conclusion that CO2 is an important climate feedback, with the magnitude of the feedback approximately comparable to the sensitivity of present climate models. If the CO2 projections are correct, comparison of the future warming with past warm periods indicate that there may be no geologic analogs for a future warming; the future greenhouse climate may represent a unique climate realization in earth history