A SIMULATION OF MIDCRETACEOUS CLIMATE

A series of general circulation model experiments utilizing GENESIS have been completed for the mid-Cretaceous based on geography, variable atmospheric carbon dioxide concentrations (2 to 6 times present-day concentrations), and variable poleward oceanic heat flux (.6 to 1.2 x 10(15) W increased from present day). By combining all three major variables (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 levels 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 match to the Cretaceous observations was achieved with a globally averaged 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 estimate of mid-Cretaceous global warming. Finally, the model experiments can be used to provide a ''paleocalibration'' of the global warming expected for a doubling of atmospheric carbon dioxide. The best estimates for the mid-Cretaceous appear to be a 2.5 to 4.0 degrees C sensitivity, in the mid to upper range of the sensitivity of current climate models used to assess future global change.