http://eces.org/articles/000346.phpGrowing body of evidence suggests that the rapidly accelerating warming the world is now experiencing could trigger dramatic climatic changes across the entire planet, including, paradoxically, the onset of a new ice age in the U.K. and western Europe.
According to an excellent article in the U.K. Guardian by Bill McGuire, Benfield Professor of Geophysical Hazards and director of the Benfield Hazard Research Centre at University College London, a growing body of evidence is indicating that there is a serious risk that the rapidly accelerating warming the world is now experiencing could trigger dramatic climatic changes across the entire planet, including, paradoxically, the onset of a new ice age in the U.K. and western Europe.
The problem lies with the ocean current known as the Gulf Stream, which bathes the U.K. and north-west Europe in warm water carried northwards from the Caribbean. It is the Gulf Stream, and associated currents, that allow strawberries to thrive along the Norwegian coast, while at comparable latitudes in Greenland glaciers wind their way right down to sea level. The same currents permit palms to flourish in Cornwall and the Hebrides, whereas across the ocean in Labrador, even temperate vegetation struggles to survive. Without the Gulf Stream, temperatures in the U.K. and north-west Europe would be five degrees centigrade or so cooler, with bitter winters at least as fierce as those of the so-called Little Ice Age in the 17th to 19th centuries.
The Gulf Stream is part of a more complex system of currents known by a number of different names, of which the rather cumbersome North Atlantic Meridional Overturning Circulation (NAMOC) is probably the most apt. The NAMOC incorporates not only the Gulf Stream but also the cold return currents that convey water southwards again. As it approaches the Arctic, the Gulf Stream loses heat and part of it heads back to warmer climes along the coast of Greenland and eastern Canada in the form of the cold, iceberg-laden current responsible for the loss of the Titanic. Much of the cold return current, however, overturns - cooling and sinking beneath the Nordic seas between Norway and Greenland, before heading south again deep below the surface.
In the past, the slowing of the Gulf Stream has been intimately linked with dramatic regional cooling. Just 10,000 years ago, during a climatic cold snap known as the Younger Dryas, the current was severely weakened, causing northern European temperatures to fall by as much as 10 degrees. Ten thousand years before that, at the height of the last ice age, when most of the U.K. was reduced to a frozen wasteland, the Gulf Stream had just two-thirds of the strength it has now.
What's worrying is that for some years now, global climate models have been predicting a future weakening of the Gulf Stream as a consequence of global warming. Such models visualize the disruption of the NAMOC, including the Gulf Stream, as a result of large-scale melting of Arctic ice and the consequent pouring of huge volumes of fresh water into the North Atlantic, in a century or two. New data suggest, however, that we may not have to wait centuries, and in fact the whole process may be happening already.
In order for the warm, saline surface waters of the Gulf Stream to continue to push northwards, there must be a comparable, deep return current of cold, dense water from the Nordic seas. Disturbingly, this return current seems to have been slowing since the middle of the last century.
Bogi Hansen at the Faroese fisheries laboratory, and colleagues in Scotland and Norway, have been monitoring the deep outflow of cold water from the Nordic seas as it passes over the submarine Greenland-Scotland ridge that straddles the North Atlantic at this point. Their results show that the outflow has fallen by 20% since 1950, which suggests a comparable reduced inflow from the Gulf Stream. And although there is as yet no direct substantiation of this, Hansen and his colleagues point to reports of the cooling and freshening of the Norwegian Sea and to temperatures that are already falling in parts of the region as possible evidence of contemporary Gulf Stream weakening.It also seems that it is not only the intensity of the outflow of cold water that is changing. Bob Dickson of the Centre for Environment, Fisheries, and Aquaculture Science at Lowestoft, and colleagues, have reported a sustained and widespread freshening of returning deep waters south of the Greenland-Scotland ridge, which appears to have been going on for the past three or four decades. Already the freshening is extending along the North American eastern seaboard towards the equator, in the so-called Deep Western Boundary current.
One of the scariest aspects of the current dramatic changes occurring in the system of North Atlantic currents is that the deep, southward-flowing limb of the NAMOC can be thought of as representing the headwaters of the worldwide system of ocean currents known as the Global Thermohaline Circulation. The possibility exists, therefore, that a disruption of the Atlantic currents might have implications far beyond a colder U.K. and north-west Europe, perhaps bringing dramatic climatic changes to the entire planet.
Yet again, this highlights the fact that global warming, for which we have only ourselves to thank, is nothing more nor less than a great planetary experiment, many of the outcomes of which we cannot predict. Wallace Broecker, an ocean circulation researcher at New York's Lamont-Doherty Earth observatory, described the situation perfectly when he pointed out that "climate is an angry beast and we are poking at it with sticks". Let's hope that when it truly turns on us, its teeth don't match its outrage.
* * * * *
The above U.K. Guardian article seems to have been prompted by the announcement by Australian scientists on the same day that Antarctic sea ice has declined 20% since the 1950s (see related ECES weblog entry ). Interestingly, although the BBC hasn't yet covered the Australian scientists' announcement, it also ran an article the same day as the Guardian about the increasing worry among scientists that global warming could trigger another ice age in Britain and Europe.
According to the BBC , Dr. Terry Joyce, an oceanographer from Woods Hole Oceanographic Institute in the U.S., believes there is a 50% chance of a sudden climate change happening in the next 100 years. "It will be quick," he says. "Suddenly one decade we're warm, and the next decade we're in the coldest winter we've experienced in the last 100 years, but we're in it for a 100 years."
Dr. Bill Turrell, from the Marine Laboratory, Aberdeen, has measured a drop in the salinity, the first warning sign that the current might collapse. "These changes are fundamental. They are substantial. They are going to impact our climate and the climate our children have to live in," Turrell says.
In addition, the U.S. space agency NASA has measured large increases in the speed of some of Greenland's largest glaciers, and melt water on the Greenland ice sheet in 2001 was twice that recorded 10 years ago.
Scientists also predict that with an increase in global temperatures will come an increase in rain at northern latitudes. Indeed, huge Siberian rivers are discharging more water into the North Atlantic than ever before, and are predicted to increase their discharge by up to 50% in the next 100 years.
All of these factors combined could lead to a large amount of fresh water making its way into the geographical region of the North Atlantic ithat is the point at which the Gulf Stream current sinks and overturns to join the Atlantic Conveyer, a vast rotating belt that takes cold water back to the tropics on the floor of the ocean. That sinking of the Gulf Stream is vital for powering the Atlantic Conveyer and relies on a change in the density of water. As sea-ice forms at high northern latitudes, it leads to an increase in the salinity of the cold, dense salty water underneath, which sinks down into the depths.
The one thing that can stop that sinking is fresh water, which effectively dilutes the salty seawater to the point at which it cannot sink - thereby shutting down the Atlantic Conveyer. With no "conveyer belt", there is no Gulf Stream and western Europe's mild winters come to an end.
Most ocean scientists believe the conveyer has a crucial freshwater threshold level, at which it will shut off - like a light bulb. The trouble is no one really knows where that threshold level is. Joyce says: "The likelihood of having an abrupt change is increasing - global warming is moving us closer and closer to the brink. We don't know where it is, but we know one thing: we're moving closer to the edge."
And once the light bulb is turned off, no one knows how to turn it back on.
Printer-friendly format
Email this thread to a friend
Bookmark this thread
(1000+ posts)
Send PM |
Profile |
Ignore
