North Atlantic Conveyor Comes Back to Bite Again

During the past two decades—as concern about climate change, and especially abrupt climate change, has mounted—Exhibit A has been a compelling scenario that explains a sharp cold snap that occurred in the Northern Hemisphere during what's called the Younger Dryas period, about 12,000 years ago. In 1987 Wallace S. Broecker postulated that fresh waters from the southern rim of the North American ice sheet spontaneously spilled into the North Atlantic through what’s now the St. Lawrence River. Such a deluge would have shut down the salt-and-temperature driven currents that draw warm waters into the ocean there and keep Europe temperate.

The scenario has been repeatedly tested in the last decade, using the most sophisticated ocean-atmosphere climate models and the world’s most power supercomputers, to see whether melting of Artic and Greenland ice could produce a similar repeat catastrophe. Generally the results have been reassuring. Then, two and a half years ago, an international team found evidence that Broecker’s "big chill" was induced not by a spontaneous spilling of water from the St. Lawrence River but rather by the explosion of a large comet or asteroid over what's now Canada. Further geological evidence for such an explosion was found last year, as reported here.

But disaster scenarios involving the North Atlantic currents keep coming back to haunt us. Now a study published  in Geophysical Research Letters finds that if Greenland’s ice were to melt at moderate or high rates, the effect could be to shift sea currents and cause sea levels off the northeast coast of North America to rise by 50 to 50 centimeters more than the average sea level rise induced by global warning. The latest estimate by the Intergovernmental Panel on Climate Change, in 2007, found that the average global  sea level rise by the end of this century would be 18 to 59 cm.

Using the Community Climate System Model at the National Center for Atmospheric Research, in Boulder, the NCAR-led team assessed the probable effects of three ice melt scenarios: one in which Greenland’s ice melt rate continues to increase by 7 percent per year, and ones in which the rate slows to 3 or 1 percent. The 3 percent scenario yields a Northeast sea level rise of 54 cm above average, and the 1 percent scenario a rise of 20 cm above average. Counter-intuitively, the 7 percent scenario might lead to some recovery of Arctic sea ice by the end of the century, mitigating sea level rises.

Independently of its specific findings, the study draws attention to the fact that the effects of climate change are uneven in terms not only of global temperatures but sea levels as well. This is because, contrary to what one might suppose, sea levels are not the same around the world to begin with. Levels can vary by as much as a meter from one region to another, depending on factors like ocean circulation and compression of water at lower depths.

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