Could the change in the Gulf Stream be causing the warming?




"jlrogers" wrote in message
...
The powerful ocean current that bathes Britain and northern Europe in warm
waters from the tropics has weakened dramatically in recent years, a
consequence of global warming that could trigger more severe winters and
cooler summers across the region, scientists warn today.
Researchers on a scientific expedition in the Atlantic Ocean measured the
strength of the current between Africa and the east coast of America and
found that the circulation has slowed by 30% since a previous expedition
12
years ago.

The current, which drives the Gulf Stream, delivers the equivalent of 1m
power stations-worth of energy to northern Europe, propping up
temperatures
by 10C in some regions. The researchers found that the circulation has
weakened by 6m tonnes of water a second. Previous expeditions to check the
current flow in 1957, 1981 and 1992 found only minor changes in its
strength, although a slowing was picked up in a further expedition in
1998.
The decline prompted the scientists to set up a £4.8m network of moored
instruments in the Atlantic to monitor changes in the current
continuously.

The network should also answer the pressing question of whether the
significant weakening of the current is a short-term variation, or part of
a
more devastating long-term slowing of the flow.

If the current remains as weak as it is, temperatures in Britain are
likely
to drop by an average of 1C in the next decade, according to Harry Bryden
at
the National Oceanography Centre in Southampton who led the study. "Models
show that if it shuts down completely, 20 years later, the temperature is
4C
to 6C degrees cooler over the UK and north-western Europe," Dr Bryden
said.

Although climate records suggest that the current has ground to a halt in
the distant past, the prospect of it shutting down entirely within the
century are extremely low, according to climate modellers.

The current is essentially a huge oceanic conveyor belt that transports
heat
from equatorial regions towards the Arctic circle. Warm surface water
coming
up from the tropics gives off heat as it moves north until eventually, it
cools so much in northern waters that it sinks and circulates back to the
south. There it warms again, rises and heads back north. The constant
sinking in the north and rising in the south drives the conveyor.

Global warming weakens the circulation because increased mel****er from
Greenland and the Arctic icesheets along with greater river run-off from
Russia pour into the northern Atlantic and make it less saline which in
turn
makes it harder for the cooler water to sink, in effect slowing down the
engine that drives the current.

The researchers measured the strength of the current at a latitude of 25
degrees N and found that the volume of cold, deep water returning south
had
dropped by 30%. At the same time, they measured a 30% increase in the
amount
of surface water peeling off early from the main northward current,
suggesting far less was continuing up to Britain and the rest of Europe.
The
report appears in the journal Nature today.

Disruption of the conveyor-belt current was the basis of the film The Day
After Tomorrow, which depicted a world thrown into chaos by a sudden and
dramatic drop in temperatures. That scenario was dismissed by researchers
as
fantasy, because climate models suggest that the current is unlikely to
slow
so suddenly.

Marec Srokosz of the National Oceanographic Centre said: "The most
realistic
part of the film is where the climatologists are talking to the
politicians
and the politicians are saying 'we can't do anything about it'."

Chris West, director of the UK climate impacts programme at Oxford
University's centre for the environment, said: "The only way computer
models
have managed to simulate an entire shutdown of the current is to magic
into
existence millions of tonnes of fresh water and dump it in the Atlantic.
It's not clear where that water could ever come from, even taking into
account increased Greenland melting."

Uncertainties in climate change models mean that the overall impact on
Britain of a slowing down in the current are hard to pin down. "We know
that
if the current slows down, it will lead to a drop in temperatures in
Britain
and northern Europe of a few degrees, but the effect isn't even over the
seasons. Most of the cooling would be in the winter, so the biggest impact
would be much colder winters," said Tim Osborn, of the University of East
Anglia climatic research unit.

The final impact of any cooling effect will depend on whether it outweighs
the global warming that, paradoxically, is driving it. According to
climate
modellers, the drop in temperature caused by a slowing of the Atlantic
current will, in the long term, be swamped by a more general warming of
the
atmosphere.

"If this was happening in the absence of generally increasing
temperatures,
I would be concerned," said Dr Smith. Any cooling driven by a weakening of
the Atlantic current would probably only slow warming rather than cancel
it
out all together. Even if a slowdown in the current put the brakes on
warming over Britain and parts of Europe, the impact would be felt more
extremely elsewhere, he said.

http://www.guardian.co.uk/science/st...html?gusrc=rss