Gulf Stream is slowing down faster than ever, scientists say

via The Independent

The Gulf Stream that helps to keep Britain from freezing over in winter is slowing down faster now than at any time in the past millennium according to a study suggesting that major changes are taking place to the ocean currents of the North Atlantic.

Scientists believe that the huge volumes of freshwater flowing into the North Atlantic from the rapidly melting ice cap of Greenland have slowed down the ocean “engine” that drives the Gulf Stream from the Caribbean towards north-west Europe, bringing heat equivalent to the output of a million power stations.

However, the researchers believe that Britain is still likely to become warmer due to climate change providing the Gulf Stream does not come to a complete halt – although they remain unsure how likely this is.

Calculations suggest that over the 20th century the North Atlantic meridional overturning circulation – the northward flow of warm surface water and the southward flow of deep, cold water – has slowed by between 15 and 20 per cent, said Professor Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany.

“There is more than a 99 per cent probability that this slowdown is unique over the period we looked at since 900 AD. We conclude that the slowdown many have described is in fact already underway and it is outside of any natural variation,” Professor Rahmstorf said.

The scientists calculated that some 8,000 cubic kilometres of freshwater has flowed from Greenland into the Atlantic between 1900 and 1970, and this rose significantly to 13,000 cubic kilometres between 1970 and 2000.

Freshwater is lighter than salty water which means that it tends to float on the surface of the ocean and in doing so disturbs the normal sinking of dense, cold saltwater to the ocean floor, which is the main driver of the Atlantic circulation.

In a study published in the journal Nature Climate Change, Professor Rahmstorf and colleagues point out that maps of global surface temperatures have consistently indicated an overall warming trend around the world, except for the region of the North Atlantic south of Greenland.

“It is conspicuous that one specific area of the North Atlantic has been cooling in the past hundred years while the rest of the world heats up,” said Professor Rahmstorf, who added that previous research had indicated that a slowdown in ocean currents may be the explanation.

“Now we have detected strong evidence that the global conveyor has indeed been weakening in the past hundred years, particularly since 1970,” he said.

The study used proxy measurements of the Atlantic currents, using ice cores, tree rings, coral growth and ocean and lake sediments, to estimate regional temperature variations and so assess how the Gulf Stream has changed over the past 1,000 years.

Jason Box of the Geological Survey of Denmark and Greenland, who helped to calculate the amount of freshwater flowing into the Atlantic from melting ice caps, said that the slowdown can be linked to man-made climate change.

“Now freshwater coming off the Greenland ice sheet is likely disturbing the circulation. So the human-caused mass loss of the Greenland ice sheet appears to be slowing down the Atlantic overturning, and this effect might increase if temperatures are allowed to rise further,” Dr Box said.

Michael Mann of Pennsylvania State University said: “Common climate models are underestimating the change we’re facing, wither because the Atlantic overturning is too stable in the models or because they don’t properly account for Greenland ice melt, or both.”