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The Source of Europe's Mild Climate

The notion that the Gulf Stream is responsible for keeping Europe anomalously warm turns out to be a myth

Richard Seager

A Sea Change in Climate?

Evidence from ocean sediments suggests that at times during the last Ice Age the North Atlantic thermohaline circulation was considerably weaker than it is today, or perhaps it even shut down entirely. One such event took place about 12,900 years ago, during the last deglaciation, and is called the Younger Dryas (after a European cold-dwelling flower that marks it in some terrestrial records). The Younger Dryas began with a dramatic reversal in what was a general warming trend, bringing near-glacial cold to the North Atlantic region. This episode ended with an even more dramatic warming about 1,000 years later. In Greenland and western Europe, the beginning and end of the Younger Dryas involved changes in winter temperature as large as 20 degrees taking place in little more than a decade. But the Younger Dryas was not a purely North Atlantic phenomenon: Manifestations of it also appeared in the tropical and southern Atlantic, in South America and in Asia.

For many years, the leading theory for what caused the Younger Dryas was a release of water from glacial Lake Agassiz, a huge, ice-dammed lake that was once situated near Lake Superior. This sudden outwash of glacial meltwater flooded into the North Atlantic, it was said, lowering the salinity and density of surface waters enough to prevent them from sinking, thus switching off the conveyor. The North Atlantic Drift then ceased flowing north, and, consequently, the northward transport of heat in the ocean diminished. The North Atlantic region was then plunged back into near-glacial conditions. Or so the prevailing reasoning went.

Recently, however, evidence has emerged that the Younger Dryas began long before the breach that allowed freshwater to flood the North Atlantic. What is more, the temperature changes induced by a shutdown in the conveyor are too small to explain what went on during the Younger Dryas.  Some climatologists appeal to a large expansion in sea ice to explain the severe winter cooling.  I agree that something of this sort probably happened, but it's not at all clear to me how stopping the Atlantic conveyor could cause a sufficient redistribution of heat to bring on this vast a change.

In any event, the still-tentative connections investigators have made between thermohaline circulation and abrupt climate change during glacial times have combined with the popular perception that it is the Gulf Stream that keeps European climate mild to create a doomsday scenario: Global warming might shut down the Gulf Stream, which could "plunge western Europe into a mini ice age," making winters "as harsh as those in Newfoundland," or so claims, for example, a recent article in New Scientist. This general idea been rehashed in hundreds of sensational news stories.

The germ of truth on which such hype is based is that most atmosphere-ocean models show a slowdown of thermohaline circulation in simulations of the 21st century with the expected rise in greenhouse gases. The conveyer slows because the surface waters of the subpolar North Atlantic warm and because the increased transport of water vapor from the subtropics to the subpolar regions (where it falls as rain and snow) freshens the subpolar North Atlantic and reduces the density of surface waters, which makes it harder for them to sink. These processes could be augmented by the melting of freshwater reserves (glaciers, permafrost and sea ice) around the North Atlantic and Arctic.

But from what specialists have long known, I would expect that any slowdown in thermohaline circulation would have a noticeable but not catastrophic effect on climate. The temperature difference between Europe and Labrador should remain. Temperatures will not drop to ice-age levels, not even to the levels of the Little Ice Age, the relatively cold period that Europe suffered a few centuries ago. The North Atlantic will not freeze over, and English Channel ferries will not have to plow their way through sea ice. A slowdown in thermohaline circulation should bring on a cooling tendency of at most a few degrees across the North Atlantic—one that would most likely be overwhelmed by the warming caused by rising concentrations of greenhouse gases. This moderating influence is indeed what the climate models show for the 21st century and what has been stated in reports of the Intergovernmental Panel on Climate Change. Instead of creating catastrophe in the North Atlantic region, a slowdown in thermohaline circulation would serve to mitigate the expected anthropogenic warming!

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