Can humans force a return to a ‘Cretaceous’ climate?

Abstract

The modern pole-to-equator sea-level temperature difference is about 50 °C; that of the mid-Cretaceous ranged from 30 °C to as little as 24 °C, implying a much more equable climate. This may have been caused by 1) reduction of the ice-forced albedo of the polar regions, 2) more efficient meridional energy transport by the atmosphere and ocean, and 3) increased atmospheric greenhouse gas concentrations. Earth's icy polar regions stabilize its present ‘inequable’ climate through the ice-albedo feedback effect. The polar ice results in permanent atmospheric highs that stabilize Earth's wind systems. In turn the stable winds drive the ocean currents and determine the location of the frontal systems that separate the low- and high-latitude oceanic gyre systems and bound the region where water sinks into the ocean interior as thermocline and intermediate water masses. Increased ocean heat transport can assist in making a more equable climate, but unrealistic volume transports would be required to warm the polar regions to Cretaceous levels. The major factor forcing the equable climate of the Cretaceous is now thought to be increased greenhouse gas concentrations, dominated by CO 2. The modern rate of change in atmospheric concentration is greater than 200 ppmv per century and increasing. This compares with 1 ppmv per century during the last deglaciation. At current rates of fossil fuel burning, atmospheric CO 2 levels will reach Cretaceous levels of 2 times the pre-industrial level about 2070 and 8 times the pre-industrial level shortly after 2300. It is likely that Cretaceous atmospheric CO 2 concentrations will last for many thousands to tens of thousands of years. In addition to increased atmospheric greenhouse gas concentrations, a return to climatic conditions resembling those of the Cretaceous would require ice-free poles and large changes in atmospheric and oceanic circulation. Arctic sea-ice is melting much more rapidly than had been expected, and the Arctic Ocean will soon be free of sea-ice in summer. The Greenland ice sheet is melting more rapidly than expected because of greenhouse warming. Surface meltwater forms lakes, and then flows down through crevasses and holes in the ice to lubricate the base, allowing ice steams to flow much more rapidly. The lifetime of the Greenland ice sheet may be only a few hundred years. The West Antarctic ice sheet is inherently unstable, being grounded on rock well below sea level. The ice shelves blocking ice streams off West Antarctica have begun to melt from beneath and break up as the southern ocean warms. The East Antarctic ice sheet has been regarded as highly stable but discovery of lakes beneath the ice and fast-flowing ice streams raises questions about whether the ice sheet will ultimately succumb to global warming and disintegrate. I conclude that a return to climatic conditions resembling those of the mid-Cretaceous is not only possible but also likely unless humanity can organize an effective campaign to stop CO 2 emissions to the atmosphere and remove some of the excess CO 2 already introduced.