A recent modeling study has also indicated that gateway changes could be important for the global carbon cycle. Using an Earth system climate model, Fyke et al. showed that the opening of Drake Passage can lead to an increase in Atlantic overturning circulation and a decrease in Pacific overturning circulation. As a result, the characteristic residence times in the two basins change, such that the Atlantic reservoir of dissolved inorganic carbon becomes smaller and that of the Pacific grows. The net effect is a global increase in the ocean carbon reservoir, and hence an increase in CO2 drawdown from the atmosphere.
Galeotti et al. now present sedimentological evidence from Antarctica that calls for a reinterpretation of the ice sheet history across the Eocene-Oligocene transition. Geochemical proxy records previously suggested that the early ice sheet was large and transient, perhaps reflecting an “overshoot” of the climate system in response to a rapid forcing. The ice sheet was thought to have retreated after this “Early Oligocene Glacial Maximum,” as climate feedback processes resulted in a new steady state with a smaller ice sheet. Instead, Galeotti et al. suggest that the early ice sheet did not reach the coast at the Ross Sea. Rather than retreating, it subsequently advanced, reaching the Ross Sea continental margin 32.8 million years ago. Galeotti et al. propose that the initial, smaller ice sheet was able to respond dynamically to local variations in insolation on the comparatively short time scales of orbital precession and obliquity changes (tens of thousands of years). Once it reached the continental margin, it became relatively insensitive to local insolation forcing, instead fluctuating in size on the longer eccentricity time scale (hundreds of thousands of years), in conjunction with other components of the global climate system...."