If this is really happening, it’s a big deal. The circulation, sometimes called the Atlantic meridional overturning circulation or AMOC, transports enormous amounts of warmth northward from lower latitudes. If it slows down, there will be less heat transport to Europe and higher latitudes, as well as key consequences for warming and sea level rise along the U.S. east coast.

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However, this is a vast, enormously complicated and inadequately studied ocean system, and there are well-known patterns of natural variability in the Atlantic that could also drive things. Indeed, the circulation had already been observed to be slowing over the past decade, but it is less clear whether this slowdown is the result of climate change or, simply, part of that variability (or both).

And now, in the journal Nature Geoscience, a new paper has emerged suggesting that while Greenland has already contributed enough water to the ocean to begin to freshen seas (specifically, the Labrador Sea between Greenland and Baffin Island), there hasn’t been enough freshwater yet to slow the AMOC, which is driven by differences in the density of cold and warm salt water.

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The researchers, led by Claus Böning of the Helmholtz Centre for Ocean Research in Kiel, Germany, used an estimate of 3,200 billion tons of freshwater that has poured out of Greenland between 1995 and 2010, and a prediction that due to increasing melt, that will total 7,500 billion tons by 2020. Their study then uses a high-resolution ocean circulation model of the region to determine where that freshwater has gone, or will go, and what it has done to the nature of the ocean.

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In particular, the model was able to capture the behavior of ocean eddies, small-scale swirls or vortices, in the Labrador Sea. This sea features a so-called “boundary current” that leads waters entering from the east to circle around the outer rim of the sea before heading southward on the west side as part of the cold Labrador Current that flows off Newfoundland. However, the overturning circulation occurs more in the center of the Labrador Sea — so this is where a freshening trend really matters.

“So the real question is, what makes it into the interior Labrador Sea, where it can affect the deep convection?” Böning said.

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“There’s an instability region in a very limited area off southwest Greenland, where eddies transport the water from the shelf into the interior Labrador Sea,” he added. “They take with them whatever the water properties of the shelf are.”

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And it was by including these eddies that the model found that most Greenland meltwater doesn’t stay around — it exits and flows farther southward off the U.S. and Canadian coasts.

So while the remaining freshwater is probably having an effect, the researchers say, it’s not big enough yet to cause major changes. “The buildup of freshwater in the Labrador Sea is relatively slow, and according to our calculation, it cannot have any significant effect, up to now,” Böning said.

However, the authors add in the new paper that because melt from Greenland is “ongoing, and perhaps, accelerating,” a change here could happen before “clear signals” of it become recognizable.

Certainly this year, the subpolar North Atlantic Ocean, south of Greenland, has been showing an intriguing temperature pattern, characterized by a large cold spot that some scientists think is an indicator that less warm water may be traveling northward as part of the AMOC’s flow:

But this only captures temperatures over a relatively short time period, and the interpretation of all of this remains contested. In an accompanying essay to the new study in Nature Geoscience, Johns Hopkins University’s Thomas Haine cites other recent studies, along with the current one, and remarks that they have found “an absence of anthropogenic effects on the AMOC, at least so far.” For instance, another scientific study published last month in Nature Geoscience suggested that a slowdown in the circulation since the year 2004 could be explained by natural variations.

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The new paper appears to represent an advance, scientifically, in understanding how the critical Labrador Sea works. “One of the most interesting things is the use of the high-resolution ocean model to capture eddies,” said Marco Tedesco, a Greenland expert with the Lamont-Doherty Earth Observatory at Columbia University who was not involved with the research. “These are extremely important for understanding the fate of freshwater in the sea around Greenland, and the paper shows that including those processes can provide more insights in the physical processes governing the fate of freshwater.”

But not everyone is convinced. Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Potsdam, Germany, is the lead author of the aforementioned paper that did suggest that an “exceptional” slowdown of the AMOC has already been observed – whether caused by climate change’s effect on Greenland’s melting or other processes, such as increased rainfall, in the rapidly changing Arctic region.

Remarking on the debate engendered by the recent studies, Rahmstorf said that “it is the classic mix-up between ‘absence of evidence’ and ‘evidence for absence’ of anthropogenic effects.”

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His study looked at a far longer period of human influence on Greenland’s melt and the Atlantic circulation, Rahmstorf said, going back to the beginning of the 20th century, and found 13 trillion tons of freshwater contribution over that entire timespan. It also found that Greenland’s melt was only one possible contributor to an overall slowdown of the circulation over time.

The study concluded that “the AMOC weakness after 1975 is an unprecedented event in the past millennium” and that continuing melting of Greenland would make it worse. Rahmstorf has also cited the much discussed “cold blob” in the North Atlantic Ocean (pictured above), which has drawn increasing attention from scientists lately, as an indicator of this trend.

“Despite their claims these studies provide no evidence against our conclusion of a long-term AMOC decline since the early 20th century in response to global warming,” Rahmstorf said in an email.

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Regarding the Rahmstorf study, Böning said that while he agrees that there have been changes to the circulation, he thinks they’re naturally driven fluctuations. “You can explain these multi-decadal changes in the AMOC by the atmospheric forcing,” he said. “So there is no mystery around that. You don’t need to invoke any additional meltwater in order to explain these trends.”

Rahmstorf notes that there is agreement over one fundamental and pretty important thing — what the circulation has been doing lately (whatever the cause). Namely, it strengthened from 1990 through to about the middle of the 2000s, and then has weakened since. The key issue then becomes precisely how much of this is natural variability, and how much is….something else.

The bottom line is that there is an emerging debate in the science world over precisely why the AMOC has shown a slowdown in recent years and how much human-caused climate change, by melting Greenland and causing other Arctic changes, is contributing to that. But in this debate, the key question is not whether humans can be responsible for changing the Atlantic circulation — something that has long been expected if we let climate change get bad enough. Rather, it’s over when we can say for sure that that has already happened.

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