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Published by Cambridge University Press, the book builds upon
scientific findings presented at the "Avoiding Dangerous Climate
Change" conference in Exeter, England, in February last year. The
conference was sponsored by the United Kingdom Department of
Environment, Food and Rural Affairs. The conference brought
together more than 200 scientists and political leaders from 30
nations. Major themes included key vulnerabilities of the climate
system and critical thresholds, socioeconomic effects, and
technologies to limit greenhouse-gas emissions.
Based on his talk at the conference, Schlesinger contributed a
book chapter titled "Assessing the Risk of a Collapse of the
Atlantic Thermohaline Circulation."
Higher temperatures caused by global warming could add fresh
water to the northern North Atlantic Ocean by increasing the
precipitation and by melting nearby sea ice, mountain glaciers and
the Greenland ice sheet, Schlesinger said. This influx of fresh
water could reduce the surface salinity and density, leading to a
shutdown of the thermohaline circulation.
"We have evidence dating back to 1965 that shows a drop in
salinity around the North Atlantic," Schlesinger said. "So far, the
salinity change is small, but we could be standing at the brink of
an abrupt and irreversible climate change."
Among the talking points Schlesinger is covering at the news
conferences:
- The observed warming during 1856-1990 was predominantly
human-induced.
"Using a simple climate-ocean model, we calculated the
contributions to the observed changes in global-mean, near-surface
temperature caused by human and volcano forcing, and putative
variations in the irradiance of the sun for the years 1856-1990,"
Schlesinger said. "We found the human effect has steadily increased
and is now the dominant external factor. Variations in solar output
played only a minor role in the observed temperature change, and we
found no significant contribution from volcanoes."
- The observed melting of alpine glaciers, the Greenland and
West Antarctic ice sheets, the freshening of the North Atlantic
Ocean, and the slowdown of the Atlantic thermohaline circulation
are the "smoking gun" of global warming.
"We are seeing dangerous, human-induced climate change,"
Schlesinger said. "The melting of the Greenland ice sheet would
raise sea level by 18 feet. Melting of the Antarctic ice sheet would
raise sea level an additional 22 feet. Most coastal cities would be
inundated."
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 These observed changes in climate and ongoing research have shown
that human-induced warming is proceeding more quickly than
anticipated.
"Not only are the Greenland and Antarctica ice sheets melting
much faster than models predicted, measurements show a significant
freshening (influx of fresh water) of the North Atlantic Ocean and a
30 percent reduction of North Atlantic circulation within the past
50 years," Schlesinger said. "What we are seeing is very worrisome.
It is now clear that we have no time to spare -- we must act
immediately."
If the present course of increasing emissions continues, there is
a high likelihood that the Atlantic thermohaline circulation will
shut down during the next 200 years.
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."
- Two major factors affect the range of possible future
temperature increases: Scientists don't know precisely how
sensitive the climate system will be to future emissions; and
they don't know exactly how much humankind will emit. People can
control only one of the factors. By reducing emissions, the
amount of future warming and associated effects can be reduced.
"Recent work by five independent research teams has shown that
climate sensitivity could be larger than the 4.5 degrees Celsius
upper bound published by the Intergovernmental Panel on Climate
Change," Schlesinger said. "In fact, climate sensitivities as high
as 9 degrees Celsius are not implausible. Paralysis in near-term
action to significantly reduce emissions could make mitigation
nearly impossible to attain."
Two other authors and one of the book's editors are also
participating in the news conferences. A news conference Tuesday
morning was at JP Morgan-Chase corporate headquarters in Manhattan.
The news conference today (Wednesday) will begin at 3 p.m. in Room
485 of the Russell Senate Office Building.
[University
of Illinois news release] |
