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Gulf Stream's role in warming questioned

Recent research and climate models raise a tantalizing possibility: Can the slight weakening of the Gulf Stream expected over the next century actually help to offset the effects of global warming in northern Europe?
Plumbing The Gulf Stream
Scientist Bogi Hansen retrieves sonar devices aboard his ship in Torshavn, Faeroe Islands, on May 22. For the past year, Hansen's sonar devices fitted to underwater buoys sent pings into the murky depths of the North Atlantic, measuring a crucial ocean current that has kept this subpolar archipelago unfrozen for centuries.John Mcconnico / AP
/ Source: The Associated Press

From the deck of a research ship moored in these gusty north Atlantic islands, workers are offloading three bright orange buoys whose sonar devices will help Bogi Hansen fill more gaps in an intriguing twist on climate change forecasts.

For the past year, the Faeroese scientist’s sonar has been pinging the Gulf Stream, the warm ocean current that has kept this subpolar archipelago unfrozen for centuries. His findings are of big interest because they contradict one of the most catastrophic predictions linked to global warming: that Arctic melting will strangle the Gulf Stream, thrusting Europe into a new Ice Age.

In fact, Hansen’s research and recent climate models raise a tantalizing possibility: Can the slight weakening of the Gulf Stream expected over the next century actually help to offset the effects of global warming in northern Europe?

Some scientists think so.

“We will benefit a little bit from this,” said Helge Drange, of the Nansen Environmental and Remote Sensing Center in Bergen, Norway, a researcher who builds climate models. “Instead of warming of 3-4 degrees C (5-7 F), we may get 2-3 degrees C (4-5 F) in northwestern Europe.”

The U.N.-sponsored Intergovernmental Panel on Climate Change said this year that the global ocean circulation powering the Gulf Stream is likely to slow, but is “very unlikely” to suffer an abrupt change.

No climate models project a complete shutdown of the Gulf Stream, which feeds warm water up the east coast of North America and across the Atlantic Ocean to Europe.

“It’s one of the good news things in the climate story,” said Andrew Weaver, a Canadian researcher and lead author of a chapter dealing with ocean currents in the IPCC report. “To be perfectly honest, it’s difficult to fathom a mechanism that could cause its collapse.”

Hansen said his latest measurements on the underwater Greenland-Scotland ridge show no weakening in the North Atlantic Drift, the crucial northward branch of the Gulf Stream.

Fresh vs. salt water
Scientists expect the flow to taper off in coming decades by up to 50 percent as Greenland’s melting ice sheet releases freshwater into the north Atlantic, slowing the main pump that drives what is known as the ocean conveyor belt — the global circulation of currents. It is high salinity that causes Arctic water to sink and generate the energy for the Gulf Stream.

Hansen said current projections show that this process “would mitigate the global warming” rather than cause an abrupt and cataclysmic cooling.

Still, there are plenty of uncertainties.

While northwestern Europe, from Britain to Scandinavia, can expect continued gradual warming, the net effect of climate change and a slower Gulf Stream is harder to predict for north Atlantic islands such as Iceland or the Faeroes, a semiautonomous Danish territory with 50,000 inhabitants.

Here, right in the middle of the North Atlantic Drift, is where the warming effect is most pronounced. The average winter temperature in Torshavn is 37 F — about 22 F higher than in Anchorage, Alaska, which is on the same latitude.

“The Faeroes would be very much colder but also large areas of this region and the whole Arctic would be very much affected if this flow of heat would weaken considerably,” Hansen said.

Even a slight cooling could mean the difference between green and white winters for places like the Faeroes where average winter temperatures are just above freezing.

A slowdown in the circulation could also affect marine life, because it transports oxygen and other substances to the deep ocean.

Natural cycle to salinity?
Researchers also are reconsidering the commonly held view that a drop in north Atlantic salinity was caused by melting Arctic sea ice. The salt level has started recovering since 2000 and scientists now say the fluctuations reflect a natural cycle.

“We now realize that the observed decline in ocean salinity that occurred from 1965-2000 had more to do with the wind patterns and storm tracks than with global warming,” said Ruth Curry, an oceanographer at the Woods Hole Oceanographic Institution in Massachusetts.

Nevertheless, climate change is expected to play a bigger role in the next cycle of freshening expected around 2020, because the Greenland ice cap is melting faster, Curry said.

“Will it slow the ocean conveyor? It’s possible,” said Curry, who is not connected to Hansen’s research. “Will it cause the same sort of complete alteration that we know happened 12,000 years ago? No, that’s very unlikely.”

Even the long-established tenet that Europe owes its mild winters to the Gulf Stream is under scrutiny, most vocally by Richard Seager, a scientist at the Lamont-Doherty Earth Observatory of Columbia University in Palisades, N.Y.

He calls the Gulf Stream effect a myth, and claims the prevailing wind patterns have a much bigger role in explaining why Europe is several degrees warmer in winter than the equivalent latitudes in North America.

“The amount of warming that the current gives — only about 2-3 degrees over land on either side — is really small compared to the temperature difference between those regions, which is more like 15 to 20 centigrade in winter,” he said. “So no one should ever confuse that temperature difference between the two regions as being in any way caused by the movement of heat by the Gulf Stream.”

Uncertainty also surrounds future climate predictions, primarily because little is known about how fast the Greenland ice cap will melt, and exactly how that will affect oceanic circulation.

Drange played down the possibility of a massive influx of freshwater disrupting the mechanism that drives ocean circulation.

“There is no indication that this is happening now and we don’t expect it will happen in this century,” he said.

Faeroese fishermen are less sure. Jogvan Trondarson, a veteran shrimper, said he’s seeing more and bigger icebergs off Greenland’s eastern coast than 20 years ago, and believes they could be a sign the ice sheet is melting faster.

Off the west coast of the giant island, the sea ice has crept back by 120 miles, and storms have gotten stronger and more persistent on both sides, he said.

“For me it’s facts,” said Trondarson, tapping his pen on a rough map of Greenland.