Author: Bruce

Source: ft.com

Shutdown of ocean flows would likely lead to more stormy winters and drier summers in Europe Other signs of global warming include high sea surface temperatures in temperate regions of the northern hemisphere, while sea ice around Antarctica is at an all-time low ©

Henrik Egede-Lassen

The ocean circulation in the north Atlantic is likely to collapse sooner than expected as a result of climate change, causing further upheaval in weather patterns around the globe, new peer-reviewed scientific analysis finds.

The latest study of the currents or “conveyor belt” that carry warmer water upwards from the tropics concludes the Atlantic Meridional Overturning Circulation (Amoc) will shut down at some point between 2025 and 2095, with the 2050s most likely.

The University of Copenhagen researchers predicted the outcome with 95 per cent confidence in the paper published in the journal Nature Communications.

The findings by Copenhagen professors Peter Ditlevsen and Susanne Ditlevsen contrast with the view of the UN Intergovernmental Panel on Climate Change that Amoc is unlikely to collapse this century, and some scientists remain wary of departing from the IPCC’s predictions.

A collapse of Amoc, which includes the Gulf Stream stretching from Florida to north-western Europe, would produce pronounced cooling across the northern hemisphere, leading to stormier winters and drier summers in Europe.

Conversely, heat would intensify further south, as less warmth is transferred to temperate and polar latitudes, and there would be large changes in tropical rainfall and monsoons.

It is one of the most feared of the “tipping points” for the planet, or irreversible changes, that are threatened by global warming.

“I was surprised we found that the tipping point would come so soon and that we could constrain its timing so strongly to the next 70 years,” said Peter Ditlevsen. He said the IPCC models were “too conservative” and did not take into account early warning signals of instability reported more recently.

Stefan Rahmstorf, professor of ocean physics at Potsdam University and one of Europe’s leading climate scientists, said the growing body of science around the world’s ocean current systems showed a marked shift.

“The findings are in line with a couple of other studies in recent years suggesting that the Amoc tipping point is perhaps much closer than we previously thought. The evidence is mounting and is in my view alarming.”

Tim Lenton, one of the world’s foremost experts on tipping points and professor of climate science at Exeter university, noted the study had “made important improvements to the methods of providing early warning of a climate tipping point directly from data”.

“Once past the tipping point, the collapse of the Amoc would be irreversible,” Lenton said. “The collapse and its impacts will take time to unfold, but how long is uncertain,” he added.

Other climate scientists were more doubtful about the data and analytical methods used by the Copenhagen researchers.

“It is an interesting paper and emphasises Amoc collapse as a reason for concern,” said Richard Wood, head of the climate and oceans group at the UK Met Office Hadley Centre. “But I’m not abandoning the IPCC view, expressed with medium confidence, that it won’t collapse this century, though we do expect a weakening of the Amoc.”

Geological evidence suggests that during the last ice ages drastic changes in Atlantic circulation took place within a decade or two, but some climate models predict that it might take a century or so for the Amoc to halt completely under 21st-century circumstances. Even a partial shutdown would exacerbate the disruption caused by global warming.

Other worrying manifestations of global warming in the oceans include exceptionally high sea surface temperatures now being recorded around temperate regions of the northern hemisphere — as much as 5C above average off the east coast of Canada — while sea ice around Antarctica is at an all-time winter low. These are not directly related to changes in the Amoc.

The Ditlevsens — a brother and sister research partnership — said their results added to the urgency of global action to cut greenhouse gases. But Susanne Ditlevsen was not optimistic about the chances of avoiding an Amoc collapse.

“From what I see in the data it doesn’t look as though we can reverse it, unless there is a huge change in political views everywhere in the world, including China and the United States,” she said.