A “simulation” of Mid‐Cretaceous climate

Abstract

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 × 10 15 W increased from present day). By combining all three major variables (CO 2 , geography, and oceanic heat flux), the distribution of mid‐Cretaceous temperatures can be achieved. In the simulations, increased CO 2 is required to promote global warmth, and increased oceanic heat flux is required to prevent the tropics from overheating with higher levels of CO 2 . Four times present‐day CO 2 with 1.2 × 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°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°C sensitivity, in the mid to upper range of the sensitivity of current climate models used to assess future global change.