Some of the AMOC's disruption may be driven by the melting ice sheet of Greenland, another consequence of climate change that is altering the region's water composition and interrupts the natural processes. The slowdown also raises the prospect of a complete circulation shutdown, which would be a dangerous "tipping point", the study says. Such a shutdown was the premise of the scientifically inaccurate 2004 disaster movie The Day After Tomorrow. Climate change catastrophe movies such as The Day After Tomorrow have stoked public fear about the "tipping point". 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." Studies from the planet's history suggest that such a sudden change in the North Atlantic has occurred many times in earth's past, perhaps as recently as about 13,000 years ago. But it's not clear how close the tipping point might be. This is "something that climate models have predicted for a long time, but we weren't sure it was really happening. I think it is happening," Rahmstorf said. "And I think it's bad news." "I think in the long run ... Greenland will start melting even faster, so I think the long-term prospect for that ocean circulation system is that it will weaken further," Rahmstorf said.

"And I think that's going to affect all of us basically, in a negative way." But the full role of climate change in the slowing ocean current is not fully understood, and another study released on Wednesday drew somewhat different conclusions. This study, which was also published in the journal Nature, found that the AMOC has slowed over the past 150 years, and similarly found that it is now weaker than at any time in more than a millennium. "The last 100 years has been its lowest point for the last few thousand years," said Jon Robson, a researcher at the University of Reading and one of the study's authors. The study's lead author was David Thornalley of the University College London. The two studies have their differences: the second suggests the slowdown probably began for natural reasons about the time of the industrial revolution in 1850, rather than being spurred by human-caused climate change, which fully kicked in later.

But like the first study, the second too finds that the circulation has remained weak, or even weakened further, through the present era of warming. "These two new papers do point strongly to the fact that the overturning has probably weakened over the last 150 years," Robson said. "There's uncertainty about when, but the analogy between what happened 150 years ago and today is quite strong." The AMOC circulation is just one part of a far larger global system of ocean currents, driven by differences in the temperature and salinity of ocean water. Warm surface waters flow northwards in the Atlantic, eventually cooling and - because cold, salty water is very dense - sink and travel back southwards at great depths. The circulation has thus been likened to a conveyor belt. A fisherman sails past melting icebergs in Greenland, Credit:New York Times But the melting of Arctic sea ice and Greenland's ice sheet can freshen northern waters and interfere with sinking. Recent research has in fact confirmed that meltwater from Greenland is lingering on the ocean surface, where it could be interrupting the circulation.

Direct measurements of the circulation are only a little over a decade old. And while those have shown a downturn, that's too short a time period to detect a definitive trend. So the new studies sought to infer the state of the circulation from more indirect evidence. In the first, the authors highlight a curious pattern of ocean temperatures that match what one would expect from a weakening AMOC - namely, a strong warming off the coast of the eastern US, paired with a cooling south of Greenland, which has sometimes been called the cold "blob". The research finds that the odd alignment, which has produced regions of both record cold and also record warmth right next to one another, has been developing since the 1950s and closely matches what a very high resolution climate model predicted would occur. The study was led by the Potsdam Institute's Levke Caesar along with co-authors at institutions in Germany, Greece, and Spain, as well as from the US National Oceanic and Atmospheric Administration.

The second study, meanwhile, draws on sediment samples from the deep ocean off Cape Hatteras, North Carolina, to infer the strength of the current going back well over 1000 years. Because a stronger current can carry thicker sand grains, the study was able to detect a weakening beginning about 160 or 170 years ago when the "Little Ice Age" in the northern hemisphere ended. That trend has then continued through the present. "In terms of this initial drop in the AMOC, it's very likely that's a kind of natural process," Robson said. "It's very likely, based on other evidence, that human activities may have continued to suppress the AMOC, or maybe led to further weakening." Meric Srokosz, an oceanographer at the National Oceanography Centre in Britain, noted that the two studies have "somewhat different messages" - but emphasised that neither makes a direct measurement of the circulation. "Essentially, what view you take of the results depends on how good you believe the models used are and likewise how well the chosen proxies represent the AMOC over the time scales of interest," he said. Marilena Oltmanns, an oceanographer at the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, went further, saying that the two studies may not be measuring the same thing.

"I think, by applying different methods and looking at different time scales, the two studies focused on different components of the ocean circulation," she said. "Both of them had to use some kind of approximation or proxy which inevitably results in limitations and cannot give a complete picture." But Rahmstorf argued in an email that, given the difficulties and limitations involved in such work, "I think the overall agreement of the various independent estimates is very good!" The authors of the first study believe the shift in the circulation may already be having a big impact along the US coastline. "Of all the US waters, this region has definitely warmed the fastest in the last decade," said Vincent Saba, a marine biologist at the National Oceanic and Atmospheric Administration and one of its co-authors. And that has had major impacts on fisheries. The Gulf of Maine, for instance, has seen a giant boom in the local lobster industry and crash of the cod fishery.

"A lot of these changes are happening relatively fast, and our fisheries management is unable to keep up," Saba said. "We're trying to figure out how to deal with some of these species shifts that we're seeing." It's not just fisheries: if the slowdown trend continues, it is expected to drive strong sea-level rise against the eastern seaboard. Previous research has already shown that from 2009 to 2010, sea level in the region suddenly shot up 13 centimetres, thanks in part to a brief slowdown of the circulation. This occurs, Rahmstorf said, because the northward flow of the Gulf Stream pushes waters to its right - which means that the ocean piles up against the coast of Europe. But as the current weakens, some of the water flows back toward the US east coast instead. The Washington Post, with AP