Golfstrømmens altafgørende betydning for klimaet er en myte

Why doesn’t the ocean exert a greater influence on North Atlantic climate?

According to scientists’ best estimates, the ocean and atmosphere move about an equal amount of heat in the deep tropics. But at mid-latitudes, the atmosphere carries several times more heat. Thus, if one considers the region north of, say, 35 degrees North, the atmosphere is much more effective than the ocean in warming winter climates. Also, the winter release of the heat absorbed during the summer is several times greater than the amount of heat that the ocean transports from low to high latitudes in a year. Hence it is the combined effect of atmospheric heat transport and seasonal heat storage and release that keep the winters outside the tropics warmer than they otherwise would be—by several tens of degrees

One subtle but important effect stems from a fundamental principle in physics: the conservation of angular momentum.

The conservation of angular momentum, it turns out, causes the mountains of North America to contribute substantially to the dramatic difference in temperatures across the Atlantic.

Such waves are of massive scale. The southward flow takes place over all of central and eastern North America, bringing Arctic air south and dramatically cooling winters on the East Coast. The return northward flow occurs over the eastern Atlantic Ocean and western Europe, bringing mild subtropical air north and pleasantly warming winters on the far side of ocean.

Topographically forced atmospheric waves contribute significantly to the large difference in winter temperature across the Atlantic. When Battisti and I removed mountains from our climate models, the temperature difference was cut in half. Our conclusion was that the large difference in winter temperature between western Europe and eastern North America was caused about equally by the contrast between the maritime climate on one side and the continental climate on the other, and by the large-scale waviness set up by air flow over the Rocky Mountains.

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.

When Battisti and I had finished our study of the influence of the Gulf Stream, we were left with a certain sense of deflation: Pretty much everything we had found could have been concluded on the basis of results that were already available.

All Battisti and I did was put these pieces of evidence together and add in a few more illustrative numerical experiments. Why hadn’t anyone done that before? Why had these collective studies not already led to the demise of claims in the media and scientific papers alike that the Gulf Stream keeps Europe’s climate just this side of glaciation? It seems this particular myth has grown to such a massive size that it exerts a great deal of pull on the minds of otherwise discerning people.

This is not just an academic issue. The play that the doomsday scenario has gotten in the media—even from seemingly reputable outlets such as the British Broadcasting Corporation—could be dismissed as attention-
grabbing sensationalism. But at root, it is the ignorance of how regional climates are determined that allows this misinformation to gain such traction. Maury should not be faulted; he could hardly have known better. The blame lies with modern-day climate scientists who either continue to promulgate the Gulf Stream-climate myth or who decline to clarify the relative roles of atmosphere and ocean in determining European climate. This abdication of responsibility leaves decades of folk wisdom unchallenged, still dominating the front pages, airwaves and Internet, ensuring that a well-worn piece of climatological nonsense will be passed down to yet another generation.

Hele artiklen: American Scientist

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