NSF PR 01-92 - November 19, 2001
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Melting Glaciers Diminished Gulf Stream, Cooled
Western Europe, During Last Ice Age
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North Atlantic Ocean Circulation System
Credit: Deborah McLean,
NASA Goddard Space Flight Center Multimedia
Design Studio
A larger
version is here.
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At the end of the last Ice Age --11.5 to 13 thousand
years ago -- the north Atlantic deep water circulation
system that drives the Gulf Stream may have shut down
because of melting glaciers that added freshwater
into the north Atlantic Ocean over several hundred
years, confirm researchers funded by the National
Science Foundation (NSF)'s paleoclimate program.
"For the first time, we have shown that realistic additions
of glacial meltwater into the north Atlantic would
have shut down north Atlantic deep water production
over a period of a few hundred years, if the initial
ocean circulation was somewhat weaker than that of
today," said David Rind, lead author of the study
and a senior climate researcher at the NASA Goddard
Institute for Space Studies. The study appears in
the current issue of Journal of Geophysical Research
- Atmospheres.
While the study finds that freshwater input could slow
the Gulf Stream, it would not stop it entirely. That's
because the stream is partially pushed by winds. As
a result, the model shows the reduced Gulf Stream
would transport only about half as much heat northward,
thereby cooling western Europe.
"This discovery illustrates the importance of a systems
approach to research-studying the interactions among
land, ocean, and atmospheric processes-to understand
the complex behavior of Earth's climate," said James
Yoder, director of NSF's division of ocean sciences.
"But we still have much to learn before we can explain
the rapid climate changes that have apparently occurred
over the past few thousand years, and what those changes
can tell us about what may happen in the future."
The computer model simulations of ocean and atmosphere
processes used in this study imply that a similar
phenomenon has the potential to occur in the future
due to freshwater additions from increased rain and
snow caused by global climate change.
When Rind and his colleagues entered into their model
realistic estimates of freshwater from melting glaciers,
they found the north Atlantic circulation stopped
completely after some 300 years. When the model was
adjusted to make the circulation weaker than it is
today, cessation of the Gulf Stream took only 150-200
years, matching current estimates based on paleo-climate
records.
When the Gulf Stream moves warm surface water from
the equator north through the Atlantic, the water
cools, gets saltier due to evaporation and becomes
very dense. By the time it approaches the coast of
Newfoundland, or further northeast in the Norwegian
Sea, it becomes dense enough to sink. This process
is called overturning. The dense water then slowly
travels through the deep water southward into the
Southern Hemisphere, with the return flow to the north
occurring at the surface.
But when freshwater gets mixed with the salty water
in the north Atlantic, it makes the water less dense
and slows the overturning process and the ocean circulation.
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