Study: Main circulation of Atlantic Ocean weakening
WASHINGTON — Global warming is likely slowing the main Atlantic Ocean circulation, which has plunged to its weakest level on record, according to a new study.
The slowdown in the circulation — a crucial part of Earth’s climate — had been predicted by computer models, but researchers said they can now observe it. It could make for more extreme weather across the Northern Hemisphere, especially in Europe, and could increase sea level rise along the U.S. East Coast, they said.
The slowdown raises the prospect of a complete circulation shutdown, which would be a dangerous “tipping point,” according to a study published in the journal Nature.
Such a shutdown was the premise of the scientifically inaccurate 2004 disaster movie The Day After Tomorrow. Study authors said a collapse is at least decades away and would be a catastrophe.
“We know somewhere out there is a tipping point where this current system is likely to break down,” said study co-author Stefan Rahmstorf, a climate scientist at the Potsdam Institute for Climate Impact Research in Germany.
Some other scientists are skeptical, citing a scarcity of data.
The Atlantic meridional overturning circulation, called AMOC, is a key conveyor belt for ocean water and air, creating weather. Warm salty water moves north from the tropics along the Gulf Stream off the U.S. East Coast to the North Atlantic, where it cools, sinks and heads south. The faster it moves, the more water is turned over from warm surface to cool depths.
The study and another one published in the same issue of Nature by a different team indicate that the Atlantic’s circulation is the weakest it has been in about 1,500 years. And the slowdown is intensifying.
Since the middle of the 20th century, the speed at which the ocean moves water in the AMOC has dropped 15 per cent, the study found, using cold subpolar water temperatures as an indirect measurement. And it has plummeted in recent years, the study concluded.
Scientists blame global warming in a couple of ways.
Warmer water lessens the amount of cooling and makes it harder for the water to sink and turn over. Ice sheets and glaciers in Greenland are melting and the fresh water is pouring into the area where the water turns over, making it less salty, less dense and therefore less likely to sink.
There’s also more rain and snow in northern areas and more evaporation in southern areas, altering the flow, Rahmstorf said.