MODELLING CRETACEOUS CLIMATE

The discovery of tropical floras at 70 N in Cretaceous strata in western Greenland in 1870 made it apparent that Earth’s climate had been very different from that of today. Efforts to simulate a warm Earth by numerical modelling started in the late 1970's. Early studies indicated that plate tectonic movements, higher sea levels, mountain ranges, or changes in ocean heat transport could not account for the warm polar regions. Earth’s major greenhouse gas. water vapor, is controlled by the availability of water and by the temperature of the planet’s surface; it is ineffective in the polar regions. Other greenhouse gases, carbon dioxide and methane are more evenly distributed and could preferentially warm the polar regions and continental interiors.

The atmospheric circulation of today’s Earth depends on a hot equatorial region with rising air forming a permanent atmospheric low pressure system at the surface, and frigid polar regions with sinking air forcing a permanent atmospheric high pressure system at the surface. The rotation speed of the Earth dictates that between these fixed features there must be three atmospheric cells in each hemisphere. Removing the polar ice should allow seasonal alternation of the atmospheric pressure system, resulting in alternation between two and three cell circulation with the seasons. The breakdown in steady atmospheric forcing has a major effect on ocean circulation. The trade winds remain constant and the westward-flowing equatorial ocean currents they drive also remain constant. However, today’s eastward-flowing return currents are forced by the mid-latitude westerly winds. Removing polar ice destabilizes them resulting in breakdown of the low-latitude gyres. The subtropical and polar ocean fronts also depend on steady westerly winds. They are the sources of the thermocline and intermediate waters of the ocean. Without steady westerly winds this system would break down changing the structure of the ocean.

Atmosphere-ocean general circulation models are just now reaching the stage of development where they can effectively simulate the very different atmospheric and oceanic circulation system that existed in the Cretaceous.