The Associated Press, June 14, 2007
For the past year, the Faeroese scientist's sonar has been pinging the
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
Some scientists think so.
"We will benefit a little bit from this," said Helge Drange, of the Nansen Environmental and
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
"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
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
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
Here, right in the middle of the North Atlantic Drift, is where the warming effect is most pronounced. The average winter temperature in
"The Faeroes would be very much colder but also large areas of this region and the whole
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.
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
Nevertheless, climate change is expected to play a bigger role in the next cycle of freshening expected around 2020, because the
"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
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
"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
Uncertainty also surrounds future climate predictions, primarily because little is known about how fast the
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
Off the west coast of the giant island, the sea ice has crept back by 200 kilometers (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
Scientists Back Off Theory of a Colder
The New York Times, May 15, 2007
OSLO -- Mainstream climatologists who have feared that global warming could have the paradoxical effect of cooling northwestern Europe or even plunging it into a small ice age have stopped worrying about that particular disaster, although it retains a vivid hold on the public imagination.
The idea, which held climate theorists in its icy grip for years, was that the North Atlantic Current, an extension of the Gulf Stream that cuts northeast across the Atlantic Ocean to bathe the high latitudes of Europe with warmish equatorial water, could shut down in a greenhouse world.
Without that warm-water current, Americans on the Eastern Seaboard would most likely feel a chill, but the suffering would be greater in
All that has now been removed from the forecast. Not only is northern
The concern had previously been that we were close to a threshold where the Atlantic circulation system would stop, said Susan Solomon, a senior scientist at the National Oceanic and Atmospheric Administration. We now believe we are much farther from that threshold, thanks to improved modeling and ocean measurements. The
After consulting 23 climate models, the United Nations Intergovernmental Panel on Climate Change said in February it was very unlikely that the crucial flow of warm water to Europe would stall in this century. The panel did say that the gradual melting of the Greenland ice sheet along with increased precipitation in the far north were likely to weaken the North Atlantic Current by 25 percent through 2100. But the panel added that any cooling effect in
The bottom line is that the atmosphere is warming up so much that a slowdown of the North Atlantic Current will never be able to cool Europe, said Helge Drange, a professor at the Nansen Environmental and
But in recent years, climatologists have said prevailing winds and other factors independent of the current are responsible for at least half of the temperature anomaly.
For the European warm-water current to stop altogether, the Greenland ice sheet would have to melt fast enough to create a vast freshwater pool in the
The ocean circulation is a robust feature, and you really need to hit it hard to make it stop, said Eystein Jansen, a paleoclimatologist who directs the
The worst imaginable collapse would likely take centuries to play out, he said. Any disruption to the North Atlantic Current whose volume is 30 times greater than all the rivers in the world combined would thus occur beyond the time horizon of the United Nations climate panel.
The last big freshwater dilution is thought to have occurred 8,200 years ago, when a huge lake atop the retreating North American ice sheet burst through to the
If the North Atlantic Current weakened 25 percent this century, fractionally offsetting the effect of global warming,
When climate modelers simulate a 50 percent slackening of the North Atlantic Current, they still see a net warming in those countries. It is when they completely switch off the current, as they say nature is disinclined to do, that the European climate cools to a level below that of today.
Scientists at the Hadley Center for Climate Prediction and Research near London found that a shutdown of the North Atlantic Current in 2049 would cause temperatures in most of Britain and Norway to fall from a level several degrees warmer than today to a level 4 or 5 degrees chillier than today. That would be enough to curtail agriculture sharply.
In a 1998 cover article for The Atlantic Monthly titled The Great Climate Flip-flop, William H. Calvin spelled out a worst-case scenario for Atlantic Ocean dynamics and concluded, I hope never to see a failure of the northernmost loop of the North Atlantic Current, because the result would be a population crash that would take much of civilization with it, all within a decade.
In 2004, the makers of the Hollywood blockbuster The Day After Tomorrow imagined the sudden icing over of
Preparing for a cold future has never been high on the political agenda. Perhaps understandably, European leaders have been more preoccupied with responding to the 2003 summer heat wave that killed 15,000 people across
Richard Seager, a senior research scientist at the Lamont-Doherty Earth Observatory of Columbia University in