A SIMULATION OF MIDCRETACEOUS CLIMATE

A series of general circulation model experiments utilizing GENESIS ha ve been completed for the mid-Cretaceous based on geography, variable atmospheric carbon dioxide concentrations (2 to 6 times present-day co ncentrations), and variable poleward oceanic heat flux (.6 to 1.2 x 10 (15) W increased from present day). By combining all three major varia bles (CO2, geography, and oceanic heat flux), the distribution of mid- Cretaceous temperatures can be achieved. In the simulations, increased CO2 is required to promote global warmth, and increased oceanic heat flux is required to prevent the tropics from overheating with higher l evels of CO2. Four times present-day CO2 with 1.2 x 10(15) W provided the best match to the distribution of mid-Cretaceous data. The best ma tch to the Cretaceous observations was achieved with a globally averag ed surface temperature increase of 6.2 degrees C, at the lower end of past estimates of mid-Cretaceous warmth. This value may be a better es timate of mid-Cretaceous global warming. Finally, the model experiment s can be used to provide a ''paleocalibration'' of the global warming expected for a doubling of atmospheric carbon dioxide. The best estima tes for the mid-Cretaceous appear to be a 2.5 to 4.0 degrees C sensiti vity, in the mid to upper range of the sensitivity of current climate models used to assess future global change.