March 25, 2015, by Tim Radford
Meltwater on the Greenland ice sheet could affect currents as it enters the sea.
Image: James Balog/Aurora Photos via Flickr
Ocean scientists find evidence of an increasing slowdown in the Atlantic’s “invisible river” that could seriously affect weather and sea levels in the US and Europe. LONDON, 25 March, 2015 − Climate scientists have once again confirmed an alarming slowdown in the circulation of the Atlantic Ocean − the process that drives the current that warms Europe, and powers the planetary climate. And this time, they are prepared to say that the changes are recent − and may be linked to global warming. The Atlantic Conveyor is a great invisible river that flows in two directions at the same time. The equatorial surface waters − warm, and therefore less dense − flow towards the north in the form of the Gulf Stream. Around Greenland, the denser and colder Arctic waters sink to the ocean bottom and begin their progress towards the south. It is the difference in temperatures that maintains the turnover and keeps the climate engine going. As a consequence, the two-way traffic of warm and cold water redistributes heat around the planet and keeps Britain and maritime Europe in relatively mild conditions. But as global average temperatures rise, and the Greenland ice sheet melts, ocean scientists have warned that the speed of the ocean turnover could be put at risk.
Stefan Rahmstorf, an ocean physicist at the Potsdam Institute for Climate Impact Research in Germany, is lead author of a report in Nature Climate Change that says they now have evidence of a slowdown during the 20th century, and greater weakening since the first alarms 40 years ago about the possible effects of greenhouse emissions. “It is conspicuous that one specific area in the North Atlantic has been cooling in the past hundred years, while the rest of the world heats up,” Professor Rahmstorf says. “Now we have detected strong evidence that the global conveyor has indeed been weakening in the past hundred years, particularly since 1970.” The paradox of the Atlantic current is that, in a warmer world, it could slow down or halt, which would deliver uncomfortable consequences for maritime Europe. Fears of such an effect provided the scenario for the 2004 climate disaster movie, The Day After Tomorrow, which predicated a frozen Britain and a glaciated US.
“Now we have detected strong evidence that the global conveyor has indeed been weakening in the past hundred years, particularly since 1970”
No such extreme outcome was ever likely, but the Gulf Stream certainly makes a big difference to Britain. A former UK chief scientist once calculated that it delivered 27,000 times the warmth that Britain’s power stations could supply and, as a consequence, the UK is on average 5°C warmer than it might be, given its latitude.