Further evidence has emerged that the rise in worldwide average temperatures will have some counterintuitive effects, notably that northern European winters are likely to get colder. Moreover, the latest paper points to a new mechanism adding to the previously recognized forces driving the shift.
The idea that ice melt from Greenland could weaken the Gulf Stream to such an extent that temperatures fall against the worldwide trend is not new. Something similar is thought to have caused the Younger Dryas event when temperatures around the North Atlantic plunged 11,500 years ago, temporarily interrupting the climb out of the Ice Age.
This time, however, circumstances are different, and there has been an ongoing debate among climate scientists as to whether something similar could occur while the world warms. The idea, exaggerated a hundredfold, was made into a major disaster film, but for decades it has been unclear whether any cooling effects would be large enough to outweigh the buildup of warmth from blanketing gases.
Now, however, the evidence seems to be tilting toward future white Christmases. No doubt this will lead to future incidents of snowballs being presented as evidence that the world is not warming. In March, evidence was published that the current 15–20% weakening of the northward flow of warm waters in the North Atlantic is the largest drop for at least 1000 years.
A paper in Nature Climate Change reinforces this evidence, showing that the consequences are already being felt in heat exchange between the air and sea off Greenland and Iceland.
When the Gulf Stream's warm waters reach subpolar latitudes they lose heat and moisture to the air. The cold, dense water produced in this process sinks to great depths, where it flows south, far beneath the mighty "river in the sea".
The greatest heat exchange occurs at the edge of the polar sea ice. However, Professor G.W.K Moore of the University of Toronto, Mississauga, wondered what effect the retreat in polar sea ice was having on this process. The region of maximum heat exchange occurs further north, which Moore found is less suited to large-scale convection.
"The heat exchange is weaker - it's like turning the stove down 20%," Moore said. While data goes back to 1958, the fall has occurred since 1979, when polar sea ice was roughly twice as extensive as it is today. "A warm Western Europe requires a cold North Atlantic Ocean, and the warming that the North Atlantic is now experiencing has the potential to result in a cooling over Western Europe."
Moore predicts, "The weakening will continue and eventually cause changes in the Atlantic Meridional Overturning Circulation and the Gulf Stream, which can impact the climate of Europe." Negative effects are also anticipated for the Northeastern United States.