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Global warming could halt ocean circulation, with harmful results
James E.
Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@illinois.edu
12/6/05
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CHAMPAIGN, Ill.
— 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, high-probability
event.”
Schlesinger will present a talk “Assessing the Risk of a Collapse
of the Atlantic Thermohaline Circulation” on Dec. 8 at the United
Nations Climate Change Conference in Montreal. He will discuss recent
work he and his colleagues performed on simulating and understanding
the thermohaline circulation in the North Atlantic Ocean.
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.”
To analyze the problem, Schlesinger and his colleagues first used an
uncoupled ocean general circulation model and a coupled atmosphere-ocean
general circulation model to simulate the present-day thermohaline circulation
and explore how it would behave in response to the addition of fresh
water.
They then used an extended, but simplified, model to represent the wide
range of behavior of the thermohaline circulation. By combining the
simple model with an economic model, they could estimate the likelihood
of a shutdown between now and 2205, both with and without the policy
intervention of a carbon tax on fossil fuels. The carbon tax started
out at $10 per ton of carbon (about five cents per gallon of gasoline)
and gradually increased.
“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.”
Collaborators on this research are U. of I. research programmer Bin
Li, Princeton University researchers Sergey Malyshev and Jianjun Yin,
University of Michigan research scientist Natasha Andronova, and Wesleyan
University economics professor Gary Yohe.
The National Science Foundation funded the work.
Editor’s note: To reach Michael Schlesinger, call 217-778-9891;
e-mail: schlesin@illinois.edu.
