Study: Global warming is weakening key ocean circulation

WASHINGTON (AP) — 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 Europe, and could increase sea level rise along the U.S. East Coast, they said.

The slowdown also raises the prospect of a complete circulation shutdown, which would be a dangerous "tipping point," according to a study in Wednesday's 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 but 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. "We still don't know how far away or close to this tipping point we might be. ... This is uncharted territory."

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.

"This overturning circulation redistributes heat on our planet," said study lead author Levke Caesar, a physicist at the Potsdam Institute. "It brings heat from the tropics to the high latitudes."

The Caesar 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's been in about 1,500 years. And the slowdown is intensifying.