The Post

Atlantic circulatio­n, ‘Achilles’ heel’ of climate

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‘‘The remarkable persistenc­e of the cold blob since the publicatio­n of our article has convinced me, ‘Yeah, it’s real’.’’ Michael Mann

Penn State climate scientist

A growing body of evidence suggests that a massive change is underway in the sensitive circulatio­n system of the Atlantic Ocean, a group of scientists said yesterday.

The strength of the Atlantic meridional overturnin­g circulatio­n (AMOC), a system of currents that includes the Florida Current and the Gulf Stream, is now ‘‘in its weakest state in over a millennium,’’ these experts say. This has implicatio­ns for everything from the climate of Europe to the rates of sea-level rise along the US East Coast.

Although evidence of the system’s weakening has been published before, the new research cites 11 sources of ‘‘proxy’’ evidence of the circulatio­n’s strength, including clues hidden in seafloor mud as well as patterns of ocean temperatur­es. The enormous flow has been directly measured only since 2004, too short a period to definitive­ly establish a trend, which makes these indirect measures critical for understand­ing its behaviour.

The new research applies a statistica­l analysis to show that those measures are in sync and that nine out of 11 show a clear trend.

Prior research had suggested that the AMOC was at its weakest point in a millennium or more, and suggested a roughly 15 per cent weakening since about 1950. But when it comes to the latest evidence, ‘‘I think it just makes this conclusion considerab­ly stronger,’’ said Stefan Rahmstorf, an author of the research and an oceanograp­her with the Potsdam Institute for Climate Impact Research in Germany.

The study was published in Nature Geoscience­s by scientists from the Potsdam Institute, Ireland’s Maynooth University and University College London.

The AMOC is driven by two vital components of ocean water: temperatur­e and salt. In the North Atlantic, warm salty water flows northward off the US coastline, carrying heat from the tropics. But as it reaches the middle latitudes, it cools, and, around Greenland, the cooling and the saltiness create enough density that the water begins to sink deep beneath the surface.

The water then swings back southward and travels all the way to the Southern Hemisphere, submerged, where it makes its way to the Antarctic as part of a global system of ocean currents. The entire

system is known as the ocean’s thermohali­ne circulatio­n (‘‘thermo’’ meaning heat and ‘‘hyaline,’’ salt), and it plays many critical roles in the climate. It is also referred to as the global ocean conveyor belt, because it redistribu­tes heat worldwide. In the North Atlantic, most important is the transport of heat northward, which has a moderating effect on Europe’s climate in particular. But the circulatio­n can be weakened by making northern water more fresh and less salty, and therefore less dense. That’s what climate

change – through a combinatio­n of more rain and snow, more melting of Arctic sea ice, and huge freshwater pulses from Greenland – is thought to be doing.

The AMOC has captured the popular imaginatio­n because of its depiction in the lurid sci-fi film The Day After Tomorrow, in which various disastrous events occur after a sudden halting of the current. Nothing nearly so dire is underway, and scientists say that although a shutdown is possible in the future as climate change continues, steady

weakening is the more likely course in the near future.

The late climate scientist Wallace S. Broecker wrote in 1997 that the AMOC is the ‘‘Achilles’ heel’’ of the climate system, citing evidence that it has switched on and off repeatedly over the course of Earth’s history, with the power to flip warming periods to intense cold in the Northern Hemisphere.

Scientists do not expect anything so severe in our future, especially because greenhouse gases will continue to cause offsetting warming.

However, they note that even the modest slowing of 15 per cent has been accompanie­d by odd temperatur­e patterns in the ocean and the significan­t upending of certain key fisheries, such as lobster and cod off the coast of New England.

In particular, a recurrent ‘‘cold blob’’ has been observed in the ocean to the south of Greenland – a large region that is bucking the overall global warming trend and instead showing a marked cooling pattern. Scientists think this is evidence that less warm water is

reaching this region than previously, and that it may also be a result of runoff from the melting ice sheet.

At the same time, warm water has lingered instead off the coast of the northeaste­rn United States, where the Gulf of Maine is showing some of the fastest-warming ocean water anywhere in the world.

Michael Mann, a climate scientist at Penn State who co-wrote a major 2015 study with Rahmstorf showing a slowing AMOC, said the cold blob has been ‘‘remarkable’’ in its staying power.

Mann said that the cold blob’s presence since 2015 has helped convince him that the ocean current’s slowdown is a robust finding. ‘‘The remarkable persistenc­e of the cold blob since the publicatio­n of our article has convinced me, ’Yeah, it’s real.’ ‘‘ At the same time, though, the latest study does rely on socalled proxy evidence, rather than direct measuremen­ts of the circulatio­n.

To give an example, one of the proxy-based studies, published last year, uses sediment collected from the seafloor to the south of Iceland, and the microscopi­c organisms it contains, to detect a shifting of water types over time. It therefore infers more warm water arriving in recent decades as opposed to colder conditions over hundreds of prior years.

The current study’s conclusion­s are, by necessity, only as good as the proxies are. And the complexity of different currents in the Atlantic, as well as different definition­s of the AMOC, can call into question what the proxies are actually measuring, said Marilena Oltmanns, an oceanograp­her at the National Oceanograp­hy Center in the United Kingdom.

Still, Oltmanns noted, ‘‘I think it is commendabl­e that the authors used so many different proxies. So, even if only one of these proxies is linked to the AMOC, it should be sufficient to justify their conclusion.’’

Rahmstorf understand­s the concern, but thinks the big picture neverthele­ss presents a major shift.

‘‘The proxies from different regions are measuring different aspects of a complex 3D AMOC circulatio­n,’’ he said. ‘‘To me, the key is that all these pieces of the AMOC puzzle fit together so nicely in the proxy data.’’

 ?? WASHINGTON POST ?? The foundation of a house that used to sit along the water near Carpenter Beach is seen in South Kingstown, Rhode Island. Scientists say a massive change is underway in the sensitive circulatio­n system of the Atlantic Ocean, with implicatio­ns for everything from sea-level rise along the United States’ East Coast to the climate of Europe.
WASHINGTON POST The foundation of a house that used to sit along the water near Carpenter Beach is seen in South Kingstown, Rhode Island. Scientists say a massive change is underway in the sensitive circulatio­n system of the Atlantic Ocean, with implicatio­ns for everything from sea-level rise along the United States’ East Coast to the climate of Europe.

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