Against The Current
Pouring cold water on a climate myth
Climatologist Richard Seager blames one of the first oceanographers, Matthew Fontaine Maury of the U.S. Navy, for putting about the story that the Gulf Stream is what keeps Europe warm. "It is the influence of this stream upon climate that makes Erin the `Emerald Isle of the Sea,' and that clothes the shores of Albion in evergreen robes," Maury wrote in 1855, "while in the same latitude, on this side, the coasts of Labrador are fast bound in fetters of ice." For "Erin" read Ireland, for "Albion" England--and to this day guidebooks credit the Gulf Stream for the relatively mild winters that permit, say, tropical plants to thrive in Cornwall. And not just guidebooks; scientists do too. The Intergovernmental Panel on Climate Change attributes "the relatively mild climate in western Europe" to ocean currents, including the Gulf Stream.
It's all an old oceanographer's tale, says Seager--"the climate equivalent of an urban myth." Labrador is indeed 30 to 40 degrees Fahrenheit colder in January than England, which is at the same latitude. But that difference, he and his colleagues at Lamont-Doherty Earth Observatory argue in a recent and much talked about paper, has little to do with the Gulf Stream. Instead, it has everything to do with the prevailing winds and, more surprising, the Rocky Mountains. That would mean one widely publicized fear about global warming--that it might actually freeze western Europe by shutting off currents including the Gulf Stream--is unfounded.
It's long been clear that the ocean causes part of the climate difference between western Europe and eastern North America. Because winds generally blow from the west in both places, eastern North America in winter gets air from a frigid continent, whereas Europe gets air from the relatively warm North Atlantic. The conventional wisdom has been, though, that a lot of that maritime heat comes from the Gulf of Mexico and other southern climes via the Gulf Stream and other northward-flowing currents.
To test that theory, Seager and his colleagues experimented with a computer model of climate that let them examine the consequences of changes in winds and ocean currents. When they turned off the currents, converting the ocean into a stagnant pond, Europe remained fairly warm. "The water was just sitting there taking up heat in summer and releasing it in winter, and not moving from one point to another," says Seager. "And we found that the temperature contrast across the North Atlantic was the same."
On the other hand, when they took out the Rocky Mountains, the contrast was cut in half, as eastern America warmed and Europe chilled. The Rockies, Seager explains, put an enormous kink in the westerly winds. Because of the physics of the spinning Earth, air that rises or sinks also veers north or south. So air rushing up the western slopes of the Rockies is diverted northward, only to swing way southward again as it plunges down the east side. In winter, when the winds are strongest, the kink creates icy northwesterlies that howl down from Canada onto the eastern seaboard and on southward over the Atlantic.
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