Ocean
Circulation Shut Down by Melting Glaciers After Last Ice Age At
the end of the last Ice Age 13 to 11.5 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, NASA and university researchers confirm. Since the Gulf Stream
brings warm tropical waters north, Western Europe cooled. The
National Science Foundation (NSF) funded study also finds that if a shutdown persisted
for a long enough time, the entire Northern Hemisphere would eventually cool. The
computer model simulations of ocean and atmosphere processes used in this study
imply 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. "For
the first time, it is shown that realistic additions of glacial meltwater into
the North Atlantic would have shutdown 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 NASA's Goddard Institute for Space Studies in New
York, NY. The study appears in the November 16 issue of Journal of Geophysical
Research - Atmospheres. When
Rind and his colleagues entered realistic estimates of freshwater from melting
glaciers into their model, 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 . Freshwater
additions into the ocean through the St. Lawrence River have a profound effect
on the ocean circulation. "The
more freshwater you add, and the longer you add it, the greater reduction in the
North Atlantic circulation," Rind said. "According to our model, this
is a linear response." 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.
While
the study finds that freshwater input could slow and stop overturning, this would
not stop the Gulf Stream entirely. That's because the stream is partially pushed
by winds. As a result, the model shows the reduced Gulf Stream would only transport
about half as much heat northward, thereby cooling Western Europe. Were this to
occur in a global warming scenario, it would act to partly counter the effects
of projected greenhouse warming in parts of Western Europe. Many
scientists suspect more rainfall in parts of the Northern Hemisphere during this
century as a result of greenhouse warming. That's because warmer temperatures
increase the atmosphere's capacity to carry water. "The North Atlantic circulation
may already be weakening due to freshwater rainfall additions associated with
global warming," Rind said. But
the model shows a number of inconsistencies with previous studies on the last
ice age. Those studies speculate that once freshwater stopped flowing, the ocean
circulation would return within only a few decades, matching a rapid warming seen
in the climate record. The model finds that deepwater circulation does not return
for at least hundreds of years when the freshwater additions end. Also contrary
to observations, the model showed cooling throughout the Northern Hemisphere;
during the last ice age, the majority of the United States land mass did not appear
to cool. "It's
hard to understand how parts of the Northern Hemisphere might have cooled to the
magnitude suggested, but not North America," Rind said. "That seems
to imply that either the paleo-records are being misinterpreted, or something
else went on, something major that is not being accounted for. This isn't necessarily
the end of the story." Back
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