Download Global warming could halt ocean circulation, with harmful result

Global warming could halt ocean circulation, with harmful result
Absent any climate policy, scientists have found a 70 percent chance of shutting down the
thermohaline circulation in the North Atlantic Ocean over the next 200 years, with a 45 percent
probability of this occurring in this century. The likelihood decreases with mitigation, but even the
most rigorous immediate climate policy would still leave a 25 percent chance of a thermohaline
collapse.
"This is a dangerous, human-induced climate change," said Michael Schlesinger, a professor of
atmospheric sciences at the University of Illinois at Urbana-Champaign. "The shutdown of the
thermohaline circulation has been characterized as a high-consequence, low-probability event. Our
analysis, including the uncertainties in the problem, indicates it is a high-consequence, highprobability event."
The thermohaline circulation is driven by differences in seawater density, caused by temperature and
salinity. Like a great conveyor belt, the circulation pattern moves warm surface water from the
southern hemisphere toward the North Pole. Between Greenland and Norway, the water cools, sinks
into the deep ocean, and begins flowing back to the south.
"This movement carries a tremendous amount of heat northward, and plays a vital role in
maintaining the current climate," Schlesinger said. "If the thermohaline circulation shut down, the
southern hemisphere would become warmer and the northern hemisphere would become colder.
The heavily populated regions of eastern North America and western Europe would experience a
significant shift in climate."
Higher temperatures caused by global warming could add fresh water to the northern North Atlantic
by increasing the precipitation and by melting nearby sea ice, mountain glaciers and the Greenland
ice sheet. This influx of fresh water could reduce the surface salinity and density, leading to a
shutdown of the thermohaline circulation.
"We already have evidence dating back to 1965 that shows a drop in salinity around the North
Atlantic," Schlesinger said. "The change is small, compared to what our model needs to shut down
the thermohaline, but we could be standing at the brink of an abrupt and irreversible climate
change."
"We found that there is a 70 percent likelihood of a thermohaline collapse, absent any climate
policy," Schlesinger said. "Although this likelihood can be reduced by the policy intervention, it still
exceeds 25 percent even with maximal policy intervention."
Because the risk of a thermohaline collapse is unacceptably large, Schlesinger said, "measures over
and above the policy intervention of a carbon tax -- such as carbon capture and sequestration -should be given serious consideration."
What is the Atlantic thermohaline circulation?
The Atlantic thermohaline circulation, which includes the Gulf Stream, acts like an oceanic conveyer
belt that carries heat from the tropics to the North Atlantic region. Warm surface water from the
tropics travels northward by the Gulf Stream. As the warm water cools in the North Atlantic, it sinks
to the ocean floor, and then slowly moves southward until it returns once again to the tropics. This
ocean circulation pattern is caused by differences in water temperature and salinity in the ocean.
Could climate change shut down the thermohaline circulation?
Global warming is expected to increase ocean temperatures and to increase the flow of freshwater
into the ocean through precipitation, run-off, and melting of glaciers. Many climate models have
projected that increased surface ocean temperatures and reduced salinity could slow the
thermohaline circulation. A few models have projected a complete shutdown of the thermohaline
circulation in the case of severe global warming, but this is being debated by the scientific
community.
What are the chances of the thermohaline circulation shutting down?
We don’t yet know the probability of the thermohaline circulation shutting down. It depends on how
much and how quickly the atmosphere warms. In general, it is considered possible but not very likely.
If it were to occur, it would probably not happen within the next 100 years, and circulation would
eventually recover, after decades or centuries.
How can global warming cause cold weather?
Without the thermohaline circulation, not as much heat would be transported from the tropics to the
North Atlantic region. We don’t know how much of this cooling would be balanced by the
simultaneous warming in the atmosphere. While it is possible there would be cooling in the North
Atlantic region, it is considered more likely that it would continue to warm, but more slowly than the
rest of the world.
If “The Day After Tomorrow” is fiction, what is the truth about global warming?
The truth is that global warming is happening and that it is already too late to avoid some of the
effects. Even under the most optimistic circumstances, atmospheric scientists expect global climate
change to result in increased flooding and droughts, more severe storms, and a loss of plant and
animal species. These events will occur, even if climate change is gradual.
Our report, “A Synthesis of Potential Climate Change Impacts on the U.S.," summarizes the possible
effects of global climate change on the natural resources and economy of the United States.
Climate change: The great Atlantic shutdown
Is Europe's central-heating system about to break down, causing climate chaos around the world?
Late last year, oceanographers reported a sudden and shocking slowdown in the currents of the
North Atlantic, a critical part of the vast system of ocean circulation that influences temperatures and
weather around the world. A shutdown could cause famine in south Asia, kill off the Amazon
rainforest and plunge western Europe into a mini ice age.
However, if you live in Europe, don't order that snowcat just yet. The conclusions reported last year
have been dismissed by many climate scientists, who say their models show the current will keep
going for at least another hundred years or so. So what is really going on? Are changes in ocean
circulation about to turn our lives upside down, or is this something only our grandchildren will have
to cope with?
This vital question is in doubt because the behaviour of ocean currents is still remarkably obscure. On
a crude level, the oceans of the world are linked together by a network of currents sometimes called
the global conveyor, with warm surface flows connecting to cold deep currents. The conveyor is
driven by winds and by a more complicated process called thermohaline circulation - and this is the
process that has climatologists worried.
As its name implies, thermohaline circulation depends on heat and salt. An offshoot of the Gulf
Stream called the North Atlantic Drift flows all the way to the seas off Greenland and Norway.
Evaporation makes the water saltier, so as it is chilled by Arctic winds it becomes denser than the
waters underneath it and sinks. It then spills back southward over the undersea ledges between
Greenland and Scotland to form a slow, cold, undersea river called the North Atlantic Deep Water.
This flows all the way to the Southern Ocean, with some water going as far the Indian Ocean, where
it gradually wells up again, perhaps a millennium after it sank.
The weak link is the sinking process. Climate change is injecting ever more fresh water into the Arctic
by increasing river flows and accelerating the calving of icebergs from Greenland. This fresh water
dilutes the North Atlantic Drift, reducing its density and making it more buoyant. If the fresh water
input reaches a critical rate, around 100,000 tonnes per second, sinking could stop entirely. The
northern branch of the conveyor would stop, and warm tropical waters would no longer flow past
the west coast of Europe. With that million-gigawatt heat supply switched off, climate models
suggest that air temperatures in the region could fall by between 5 and 10 °C, and parts of the US
and Canada would suffer too. A switch-off like this is blamed for a cold snap 12,000 years ago called
the Younger Dryas, which turned the forests of Scandinavia into tundra.
Could it be happening again? That spectre was raised in December by Harry Bryden of the National
Oceanography Centre in Southampton, UK. His team took a ship from Florida to the coast of North
Africa, stopping at 120 points en route to lower a bundle of instruments all the way to the sea floor.
The researchers compared their results with similar measurements made at irregular intervals since
1957. According to their analysis, the deep, cold return leg of the circulation has weakened by 30 per
cent (New Scientist, 3 December 2005, p 6). If that has slowed, they reasoned, then the northward
branch of warm water must have slowed too.
In fact, the slowdown seems to have started nearly a decade ago. When the US National Oceanic &
Atmospheric Administration made a similar survey of the Atlantic in 1998 it was interested in carbon
dioxide levels and did not calculate the flow rate. When Bryden's team did the sums, they found the
flow had been relatively steady between 1957 and 1992, dropped off by 1998 and remained low.
One question mark is whether his team has simply seen short-term fluctuations in the ocean. "The
ocean is a very turbulent beast. We tend to assume that at great depth it is quiet, but that's not
necessarily so," says Stefan Rahmstorf of the University of Potsdam in Germany. Wunsch likens it to
the vagaries of the weather: "It might get colder for a few days in England, but you don't necessarily
say we're entering a new ice age." Bryden, however, thinks his team has found more than a stutter.
"If we just had the 1998 data we'd be nervous, but 2004 is similar to 1998," he says.
Are the models wrong?
There are other reasons to be cautious. Climate models do not predict any substantial slowdown in
Atlantic currents until near the end of this century. "It would mean all our models are wrong," says
Rahmstorf. Bryden thinks they might well be: "I think if we measure a slowdown, the models will
follow." The trouble for Bryden is that not all observations fit in with his conclusions. If less warm
water is flowing north, the seas off western Europe ought to be cooler than normal. They are not. In
fact, these waters are slightly warmer than a decade ago. And direct measurements of the cold, deep
currents that spill southwards over the ledges joining Scotland, Iceland and Greenland do not show a
downward trend. Although these currents did slow between 1995 and 2000, they have picked up
again. "We are faced with conflicting evidence," says Rahmstorf.
How to resolve this conflict? It is possible that Bryden's group has got its physics wrong. Like other
groups, the team did not measure flow rates directly, but instead calculated them from
measurements of temperature and salinity. Rahmstorf and Wunsch both point out that these
calculations rely on assumptions that are far from proven.
Or it could be that the currents are changing in ways that no one has anticipated. There is a vast
stretch of ocean between Bryden's measurements at 25° north and the overspill at around 65°. In
between, the warm surface current becomes meandering and unstable, and difficult to measure,
says oceanographer Tore Furevik of the University of Bergen in Norway. "There are certainly large
changes going on beneath the surface of the North Atlantic, but we are still missing too many pieces
in the puzzle to get the picture clear," he says.
For the moment, this rather unsatisfactory answer is the best we have. Oceanographers and climate
scientists agree that thermohaline circulation will slow as the world warms, but most think it will
happen later rather than sooner. In its report due next year, the Intergovernmental Panel on Climate
Change is likely to predict a slowdown of at most 50 per cent by the end of this century.
This prediction, however, relies on estimates of the freshwater input. While the discharge from
Siberian rivers is being monitored, that from Canadian rivers is not. The input from the Greenland ice
cap could change too. The glaciers that drain the ice cap are accelerating, and in the past decade the
amount of ice they spit into the ocean has doubled. Nobody can predict with confidence what they
will do in the coming decades. "At the moment models don't represent the dynamics of Greenland
glaciers, which may or may not start moving faster," says Richard Wood of the Hadley Centre for
Climate Prediction and Research in Exeter, UK. What all this means is that even if Bryden is wrong
and Atlantic circulation is not yet slowing, a shutdown could still happen sooner than most models
predict.
Farming hit hard
For Europe, the timing of any slowdown or shutdown is critical. If it does happen soon, the weather
will certainly get chilly. Average temperatures would be about 5 °C lower, and winters could be as
harsh as those in Newfoundland. In bad years the Thames might freeze over, and even in good years
farming will be hit hard.
On the other hand, if currents hold fairly steady until the late 21st century, the cooling effect of a
shutdown would help to mitigate warming. There might be drastic changes in other aspects of the
climate - not to mention a relatively rapid rise in sea level around the northern Atlantic (see "Rising
waters") - but Europeans might escape much of the warming that occurs elsewhere.
Their distant descendants might need those snowcats, though. If greenhouse gases do eventually fall
to pre-industrial levels and the world cools down again, there could be a lag of a thousand years
before ocean circulation restarts. So Europe still faces the big freeze - just not for a few hundred
years. Coming back to this century, other parts of the world face even more serious consequences
than Europe. A slowdown in the thermohaline circulation would reduce the transfer of heat from the
southern to the northern hemisphere, shifting the Earth's "thermal equator" to the south. "One of
the things that really struck us is that rainfall patterns over the whole world change dramatically,"
says Wood.
When he tried artificially switching off thermohaline circulation in one climate model, he found that
monsoon rains weaken over India, and parts of central and south America lose half their rainfall. "It
would have a huge impact on the climate of those regions," says Wood. He estimates that
agricultural productivity in parts of India could fall by 30 per cent. And in the Americas? "If you lose
the rain then the rainforest tends to die out." Although all of this is based on an imminent shutdown,
which climatologists think very unlikely, even a delayed slowdown could seriously disturb rainfall
patterns.
Wood's model also predicts that a shutdown would warm the southern hemisphere by 0.2 °C on
average - not much, but against a background of rising global temperatures any extra warming will
hardly be welcome. In 2004, Brazil was hit by the first ever hurricane recorded in the South Atlantic,
perhaps a consequence of rising sea-surface temperatures. Could a slowdown in circulation have
contributed? "Theoretically, this is possible," says Michael Mann of Pennsylvania State University in
Philadelphia, "but I think it would be a leap to tie any observed change to thermohaline circulation. It
could just be fluke."
While the precise effects on the climate remain uncertain, there is little doubt that a shutdown will
wreak serious damage beneath the waves, since upwelling waters supply vital nutrients to the
phytoplankton that are the basis of ocean food chains. A study last year predicted that the
productivity of the world's oceans would fall by a fifth if the Atlantic thermohaline circulation shuts
down.
At the moment, the oceans are soaking up a lot of the excess carbon dioxide pouring into the
atmosphere. Without the thermohaline circulation, however, surface waters will soon become
saturated and greenhouse gases will build up faster still. Any reduction in carbon fixing as forests and
ocean ecosystems fail would only compound the effect.
A lot therefore depends on what is really happening in the Atlantic. To find out, the UK has launched
a project called RAPID, an unprecedented effort to monitor North Atlantic currents. In 2004, on the
same voyage that found the controversial signs of a slowdown, Bryden's team planted a series of 22
moorings along a line from Africa to America (see Diagram). Cables fixed to the seafloor tether
instrument packages that are constantly measuring ocean properties such temperature and salinity.
With continuous measurements now coming in from the Atlantic, it should be possible to distinguish
between short-term fluctuations and a longer-term trend. "Soon we'll find what seasonal variability
there is and know whether what we said was a 30 per cent slowdown was above the noise level. I'm
hoping we'll eventually be able to get 10 years' worth of measurements."
Then again, monitoring a single cross section of one ocean might not be enough. "People are
obsessed with the North Atlantic and it's only 10 per cent of the ocean," Wunsch says. "There is a
danger we're neglecting the rest." For example, there are areas of sinking ocean water around
Antarctica that also help to drive the global ocean circulation. "Recent papers have started to suggest
there are changes happening in the Southern Ocean," says oceanographer Steve Rintoul, based in
Hobart, Australia. Some studies show that the water in some Antarctic seas is getting less salty, for
instance, but the picture in the Southern Ocean is even less clear than it is in the Atlantic. "We have
to observe this system globally and indefinitely," says Wunsch. "But how the devil do you get
governments to do that?"
Rising waters
In the Hollywood disaster movie The Day After Tomorrow, the shutdown of the Atlantic circulation
somehow results in a giant wave crashing over New York. That is nonsense, of course, but a
shutdown of the Atlantic thermohaline current would raise sea levels in the North Atlantic.
At present, sinking waters in the Arctic produce a kind of plughole effect, lowering the sea level of
this region and slightly raising it elsewhere. If sinking stops, sea levels will fall in the Southern Ocean
and rise by up to a metre on some North Atlantic coasts.
A group led by Anders Levermann of the Potsdam Institute for Climate Impact Research in Germany
has run a simulation that shows northern Norway getting a metre rise by the time the circulation
stops altogether, while the UK, north-eastern Canada and the US see rises of up to 80 centimetres
(see Map). With global sea levels also rising as the oceans warm up and glaciers melt, these regions
face total rises of well over a metre by the end of the century.
One NASA study shows that if the sea level was just a metre higher on the New York coast, storm
surges that now occur just once in a century would happen almost every year, flooding some lowlying suburbs. In the Thames estuary, planners believe they can handle total rises of up to two
metres, but if there is also a large increase in the frequency and ferocity of storms, as some models
predict, then an extra half-metre rise from a thermohaline shutdown could mean abandoning some
areas to the sea.
Summary of Ideas
Arguments supporting the theory
Evidence
Arguments against the theory
Evidence
Summary of Ideas
Possible Effects
Impact on Humans and Built
Environment
Impact on Natural Environment