Against The Current

[bluespirit]

Against The Current


by Robert Kunzig | 6Ԡ02 '03

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.

But on the other side of the Atlantic, the air meanders northward again, so that--thanks to the Rockies--Europe is blessed with southwesterlies in winter. Not only are those winds passing over an ocean that, unlike the North American continent, has held on to the heat of summer; they are also coming from latitudes that are hotter to begin with. Those two effects, the group reports in the Quarterly Journal of the Royal Meteorological Society, account for all of the winter temperature difference between western Europe and eastern North America: "The movement of heat by the ocean currents doesn't account for any," says Seager.

Climate switch. Other scientists are surprisingly willing to concede much of Seager's challenge to the Gulf Stream myth. Michael Vellinga of the British Meteorology Office's Hadley Centre agrees that the currents do not warm Europe preferentially. But he and others emphasize that they do warm both sides of the Atlantic in winter, by roughly 5 degrees. Although Seager calls that a "modest" effect, it's more than the difference between today's climate and that of the Little Ice Age, from the 16th century to the 19th century, when both Europe and the United States endured many a rude winter. "If you switch [the currents] off, you get massive cooling," says Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany.

Global warming could trip that switch. The Gulf Stream is part of a larger "conveyor belt" of currents in the North Atlantic, powered mostly by the sinking of cold, salty water off Labrador and north of Iceland. A warmer planet would be a rainier one, with more fresh water flowing into the North Atlantic; there are signs of that already. The fresh water could jam the belt by diluting the salty water, making it more buoyant and less prone to sinking. Clues in ocean sediments and Greenland ice suggest the conveyor belt suddenly shut down several times during the last Ice Age--and that North Atlantic climate cooled abruptly at the same times. Hence the fears about a new ice age, or at least a little one.

But any future cooling would most likely be lost in the general global warming, says Seager, and there's now no reason to think that western Europe would cool disproportionately. A couple of regions actually do draw substantial benefits from the Gulf Stream, however. They might have more reason to worry about the greenhouse cooling paradox. One is northern Norway, whose ice-free harbors could freeze over. And the other is . . . the eastern United States, where winter storms now carry some of the Gulf Stream's heat back over the land--and which just might, in a brave new greenhouse world, see more hard winters like the last one.

Wind and water

The Gulf Stream and other warm currents have long been credited with keeping western Europe's winters mild. A new study instead traces the warming influence to the Rocky Mountains. The mountains steer prevailing winds southward over warm regions of the Atlantic, where the air picks up heat before veering northward toward Europe.

[Map is not available.]

[labels]

Prevailing winds

Warm air

Rocky Mountains

Gulf Stream

Source: La Recherche

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