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42 ontemporary political summits and policy decisions, like those at Johannesburg and Rio de Janeiro and the Montreal and Kyoto Protocols, are clear evidence that world leaders and their communities are increasingly concerned about the Earth’s climate and its future. Policy makers frequently turn to scientists for definitive answers and pragmatic solutions to climate problems and too often focus on only anthropomorphic influences. Although human impact on the environment certainly deserves the attention it receives, all of us must recognize that the Earth’s climate is a complicated and multi-faceted system that responds to inputs on both human and geologic time scales. The correct interface of science and policy should consider not only anthropogenic stimulants to Earth’s climate, but also the natural responses to and controls of this system. Scientists who try to explain, model, and predict future climate change examine Earth’s former climates in the geologic record to test their models and predictions. Past climates let us not only gauge our current understanding of Earth’s dynamic climate, but also evaluate how climate varies over time periods that span innumerable human generations. Most geologists regard the Cretaceous (a period of geologic time 145 to 65 Ma, where Ma denotes million years ago) as the model “greenhouse” Earth. The planet was ice-free, sea levels were very high, and carbon dioxide levels were fourto-eight times the present level. Cretaceous climate was characterized by warm temperatures and low global temperature gradients. Fossil plant and animal data are also suggestive of a very equable climate: there were forests and ectothermic reptiles (e.g., “cold blooded” champsosaurs and turtles) at polar latitudes. Recent research, however, suggests that this anomalously high, globally distributed warmth did not characterize the entire Cretaceous. Several authors report evidence compatible with the presence of polar ice and strong global temperature gradients at the beginning of the Early Cretaceous. For example, Abreu et al. correlated Cretaceous and Cenozoic (the last 65 Myr) stable oxygen isotope data and eustasy (global sea level) and concluded that there is a positive correlation between the two data sets. The ratio of O to O in ocean and meteoric water is temperature dependant, which means that it can be used to estimate paleotemperatures. The positive correlation between stable oxygen isotopes and sea level suggest that there may have been continental ice in the Albian (late Early Cretaceous, 110 Ma). These results suggest that climate may have been dramatically different during short (< a few million years) intervals when compared to the elevated mean temperature that characterized most of Cretaceous time. Stable carbon isotope data from low latitude Tethyan (the Tethys was an ancient global, subtropical sea) sites also suggest that there were short but significant perturbations in the Earth’s carbon cycle during Valanginian (137-132 Ma) and Albian-Aptian (121-99 Ma) time and CenomanianTuronian (99-89 Ma) time. Organisms prefer to utilize C in their metabolic reactions, so organic carbon is enriched in C; conversely, inorganic carbon, such as carbonate, is often enriched in C. Dramatic changes in the C/C ratio (usually displayed as δC values, where positive δC values indicate an increase in inorganic carbon relative to organic carbon) indicate a change in how carbon is partitioned in the global carbon budget; these changes are referred to as carbon isotope events. There are many characteristic C-isotope events during the Cretaceous, and they may be related to widespread volcanic outgassing that elevated atmospheric CO 2 . CO 2 is an effective greenhouse gas, so more CO 2 would amplify greenhouse conditions and spur climatically induced ecological changes, which in turn would have significant effects on the global carbon cycle. One possible feedback follows: as the Earth became more humid, increased chemical and mechanical weathering and increased continental runoff added significant quantities of nutrients, particularly phosphate, to the oceans. High nutrient levels could have stimulated biologic activity and indirectly created an ecological crisis in which increased marine productivity and subsequent eutrophication led to widespread anoxia in intermediate and deep waters. These ecological crises are referred to as oceanic anoxic events (OAEs) and are often coeval with extensive reduced C A cold start to the Cretaceous