Snowball Earth

Could global warming ultimately lead to a snowball Earth? Amy Seif reports on new research, published in the journal Science, which found �the average annual discharge of freshwater from the six largest Eurasian rivers into the Arctic Ocean to have increased 7% since 1936. The authors have correlated these changes to historic patterns of climate change, and find, that if these patterns hold, there will likely be an 18 � 70% increase in river discharge over the next 100. An increase of such magnitude may have large-scale impacts on the ocean circulation pattern that brings heat to the northern latitudes.

"Too much freshwater 'leaking' from the land into the Arctic Ocean could reduce or shift the patterns of Atlantic deep water formation and stall the ocean 'conveyor belt' that helps to bring heat to the northern latitudes," explains Charles V�r�smarty, co-author and professor of Earth Sciences at the University of New Hampshire's Institute for the Study of Earth, Oceans, and Space. "England, for example, without the beneficial effect of this ocean circulation, would plunge into a deep freeze due to its high latitude."

V�r�smarty is referring to the "Atlantic thermohaline circulation" pattern of the ocean, which is sensitive to changes in freshwater loading. As both common sense and science indicate, as temperatures warm globally, evaporation of surface water will increase and more moisture will be held in the atmosphere. This moisture will lead to more precipitation at high latitudes, such as the Arctic, and more river runoff. If global surface temperatures have increased 0.6 degrees Celsius over the past century, then moisture transport and precipitation to the Arctic would likely have increased as well.

This research finds 128 cubic km per year more river discharge today entering the Arctic Ocean from its six largest Eurasian rivers � which include three of the largest rivers on Earth � then in 1936. The study also finds that the discharge correlates with increases in global surface air temperature and the North Atlantic Oscillation (NAO), both indicators of global climate change. The NAO is often described as "a seesaw of atmospheric mass" which dominates winter climate variability; a positive NAO is associated with more and stronger winter storms crossing the Atlantic and warm and wetter winters in Europe and the Eastern U.S�.

Canadian geologist Paul F. Hoffman and Professor of geology Daniel P. Schrag ask the question, �Could we end up with a snowball Earth in the future?" For the last million years, the Earth has been in its coldest state since the Neoproterozoic. We are now living in a relatively warm episode, but some evidence suggests that each successive glaciation over the last several cycles has been getting stronger and stronger. During the most recent glacial event, 20,000 years ago, the deep ocean cooled to near its freezing point, and sea ice reached latitudes as low as 40 to 45 degrees north and south, still below the critical threshold needed to plunge us into the snowball Earth state, although enough to make life for humans extremely unpleasant.

But could such a state be in our future? Certainly we are more concerned with anthropogenic effects on climate, as the Earth heats up in response to emissions of carbon dioxide. But only time will tell where the Earth�s climate will drift over the oncoming years. There a a number of pointers suggesting the polar continental "safety switch" could fail, and we may again experience a global ice catastrophe which would inevitably jolt life in some new direction.� This would be the beginning of "Snowball Earth" scenario, and would change forever the way we live on this planet.

The IPCC AR4 Report, commenting on Abrupt Climate Change, takes a much more conservative approach.

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