Fascinating article in the June 23 issue of Science. A major puzzle of paleoclimatology is why after tens of thousands of years of glacial conditions, recent ice ages have ended with relatively sudden warm ups. Six authors have devised a comprehensive hypothesis as to why. Here is my attempt to summarize the process:
- First, you need very large ice sheets around the Northern Hemisphere. These ice sheets are so large that they depress the continents, pushing them down into the Earth’s mantle.
- Next, the Milankovitch Cycle plays a role by increasing the amount of solar energy reaching the ice sheets during summer, resulting in large amount of melt water entering the North Atlantic. Sea levels rise worldwide and encourage extensive calving of icebergs from the continental glaciers lining the North Atlantic.
- The freshwater and icebergs result in the formation of vast areas of winter sea ice in the North Atlantic, abruptly returning the Northern Hemisphere to glacial conditions with severe winters.
- The return of glacial conditions to the Northern Hemisphere affects the atmospheric circulation of the entire planet, in particular causing a southward shift of westerly winds in the Southern Hemisphere that warm Antarctica and encourage the upwelling of carbon dioxide-rich deep water in the Southern Ocean.
- The glacial conditions in the Northern Hemisphere last long enough to encourage prolonged off gassing of carbon dioxide from upwelling Southern Ocean deep water, resulting in the crossing of a threshold where there is enough carbon dioxide in the atmosphere to tip the whole planet into a new interglacial warm period.
If this research holds up to scientific scrutiny, it will bear on the current global warming debates. Some have interpreted the rise of carbon dioxide in the atmosphere at the end of the last ice age not as a cause of deglaciation but rather as an effect of deglaciation. These six authors see carbon dioxide as playing a key role in finally bringing to an end the last ice age because carbon dioxide is a greenhouse gas.