Will we return to Cretaceous climatic conditions?

Category Climate system
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Hay, William
Holding Date 21 September 2008

The Cretaceous was a time of global warmth, with global average temperatures 5 to 8°C warmer than during the past century. The poles were ice free. Sea-ice formed only during the winter if at all. Sea level was about 300 meters higher than today, flooding about 30% of the present land surface. The Atlantic was only half as wide as today, and the Panthalassic Ocean, from which the modern Pacific is descended, occupied more than half of the surface of the planet. There was a circumglobal seaway, the Tethys, at a low northern latitude. This was in contrast to the modern Circum-Antarctic seaway at high southern latitudes. Atmospheric CO2 concentrations are thought to have been 4 to 8 times the modern pre-industrial values. The ocean basins were sometimes anoxic, sometimes highly oxic.
Cretaceous winds were much more variable than those of today. There were seasonal reversals of atmospheric pressure at the poles. Because the North Pole was covered by water and the South Pole by land, low and high pressure systems developed simultaneously over both poles, forcing the entire Earth to alternate between two-cell and three-cell per hemisphere circulation with the seasons. Transport of water into the continental interiors was more efficient not only because the actual land areas were smaller, but because water-conserving C4 plants were not present in large numbers.
The large ocean gyres of the modern world were replaced by a disorganized circulation with a myriad of mesoscale eddies generated by storms. There were no subtropical and polar frontal systems to limit poleward heat transport. At present the deep water of the ocean interior is formed by sinking of cold saline waters in the polar regions. In the Early Cretaceous the Arctic Basin with its broad shelves, was open to the Panthalassic Ocean, making it an ideal site for rapid deep water formation and ocean ventilation. Yet the Early Cretaceous ocean is characterized by black shale deposition. The Arctic became isolated from Panthalassa in the middle Cretaceous, eliminating it as a deep water source, but ocean red beds are most common in the Late Cretaceous.
Much of the geologic record reflects circulation generated in marginal seas. Today, warm marginal seas, particularly the Mediterranean, make a major contribution to interior waters only in the Atlantic. During the Cretaceous the northern margin of the Tethys, with its shelf seas in North America and Eurasia, lay beneath the descending limb of the northern Hadley cell, and provided large volumes of warm saline water to the ocean interior. Depending on changing climatic conditions associated with Milankovitch forcing, these dense waters could either promote or restrict ventilation of the adjacent basins.
The massive melt-back of Arctic sea ice in 2007 presages a return to seasonal and then perhaps perennial ice-free conditions in the Arctic in the foreseeable future. Earth will be well along on a return to Cretaceous conditions.