The ocean current that gives western and northern Europe a relatively mild climate might be at greater risk of shutdown than we thought. If the North Atlantic current – the northern segment of the Gulf Stream – did grind to a halt the effects could be severe, from greater sea-level rise on Atlantic coasts to more intense droughts in Africa.
Surface seawater in the subpolar region chills during the winter months, which makes it so cold and dense that it sinks. This process, known as ocean convection, is an important part of the large-scale ocean circulation.
But the process is threatened by the rapid warming in polar regions. To investigate, Marilena Oltmanns and her colleagues at the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, studied seawater salinity and temperature data collected in the Irminger Sea to the south of Greenland between 2002 and 2014.
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In some summers, the seawater at the surface had an unusually high temperature and low salinity – particularly in 2010. This is a sign that more fresh water was flooding into the region, perhaps from melting ice in Greenland or the Arctic Ocean.
The fresh water poses a threat to convection, because, being less dense than seawater, it has to be cooled to a greater degree before it will sink.
Not chilled out
To make matters worse, Oltmanns’s team also found evidence that the summers featuring the largest bodies of fresh water are followed by winters that are too mild to provide adequate chilling.
Measurements taken during the northern hemisphere winter of 2010-2011 confirmed the significance of the problem. Conditions were mild, and so much fresh water had accumulated during the previous summer that 40 per cent of it still remained in the upper 200 metres of the water column when spring arrived.
“We were very surprised that so much remained after winter,” says Oltmanns. “It shows that the fresh water clearly impeded convection.”
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It was a similar story in other years. For seven of the 12 winters examined, more than 25 per cent of the fresh water that pooled in the summer remained in place at the end of winter.
Oltmanns says that if several unusually warm years occur in succession, so much fresh water could build up that it would become impossible for convection to begin at all in winter. In effect, part of the North Atlantic Ocean convection might shutdown (see diagram, above).
Floods and droughts
No one knows for sure what would happen in the event of such a shutdown. Oltmanns says some people think it might spell the end of the North Atlantic’s relatively mild climate.
Eirik Galaasen at the University of Bergen, Norway, says we could expect other impacts too. that a breakdown in ocean circulation would trigger a sea level rise of 40 centimetres or more around Europe and eastern North America. that a shutdown would worsen the severity of droughts in heavily populated areas of West Africa. And some climate scientists argue that effects could be seen even further afield: , which could be bad news for the region’s rainforests.
Some scientists argue that a shutdown could be permanent, says Oltmanns, representing a tipping point that leaves the climate fundamentally and irreversibly changed. But she is cautious of making such a prediction.
Galaasen also thinks permanent shutdown is by no means a certainty. He says there is evidence that convection has stopped a number of times in the past. “The Atlantic recovered every time,” he says.
For instance, about 8400 years ago a vast glacial lake in North America burst, dumping at least 150,000 km3 of freshwater into the North Atlantic. Circulation halted, but Galaasen says it restarted within roughly a century – barely anything in geological terms but quite a long period of time in human terms.
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Valuable data
Not everyone is convinced the new research suggests that a shutdown – even a temporary one – is imminent.
“The implications are physically conceivable,” says Carl Wunsch at Harvard University – but he adds that because the team studied a relatively small region of the Atlantic we can only speculate how the entire ocean convection system would respond. “If North Atlantic convection slows down or stops because of local freshening, will there be an increased – or decreased – import of much saltier water from the south?”he says.
Michael Alexander at the NOAA Earth System Research Laboratory in Colorado says making broad conclusions on the basis of limited data is “speculative at best”. But he thinks that the connections Oltmanns’s team makes – between summer/winter conditions and the downward mixing of ocean water – are interesting and important.
Oltmanns agrees that it’s wise not to infer too much about an approaching shutdown from her team’s data, particularly since the research covered relatively few years of observation. But she thinks it gives us valuable information about the risk of ocean convection shutdown.
“Until now, models have predicted that fresh water will threaten convection in the future,” says Oltmanns. “It is already affecting convection to a greater extent than we thought.”
Nature Climate Change
