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From The Earth Institute at Columbia University
Natural releases of carbon dioxide from the Southern Ocean
due to shifting wind patterns could have amplified global
warming at the end of the last ice age — and could
be repeated as manmade warming proceeds, a new paper in
the journal Science suggests.
Many scientists think a change in Earth’s orbit
triggered the end of the last ice age and caused the northern
part of the planet to warm. This partial climate shift
was accompanied by rising levels of the greenhouse gas
carbon dioxide (CO2), ice core records show, which could
have intensified the warming around the globe.
A team of scientists at Columbia University’s Lamont-Doherty
Earth Observatory now offers one explanation for the mysterious
rise in CO2: the orbital shift forced a southward displacement
in westerly winds, which caused heavy mixing in the Southern
Ocean around Antarctica, pumping dissolved carbon dioxide
from the water into the air.
“The faster the ocean turns over, the more deep
water rises to the surface to release CO2,” said
lead author Robert Anderson , a geochemist at Lamont-Doherty.
“It’s this rate of overturning that regulates
CO2 in the atmosphere.”
In the last 40 years, the winds have shifted south much
as they did 17,000 years ago, Anderson said. If they end
up venting more CO2 into the air, manmade warming under
way now could be intensified. Ice cores show that the
ends of other ice ages also included increases in CO2.
Two years ago, J.R. Toggweiler, a scientist at the National
Oceanic and Atmospheric Administration (NOAA) , proposed
that westerly winds in the Southern Ocean around Antarctica
may have undergone a major shift at the end of the last
ice age. This shift would have raised more CO2-rich deep
water to the surface, ampligying warming already taking
place due to the Earth's new orbital position.
Anderson and his colleagues are the first to test that
theory by studying sediments from the bottom of the Southern
Ocean to measure the rate of overturning.
The scientists say that changes in the westerlies may
have been triggered by two competing events in the northern
hemisphere about 17,000 years ago. The Earth’s orbit
shifted, causing more sunlight to fall in the north, partially
melting the ice sheets that then covered parts of the
United States, Canada and Europe.
Paradoxically, the melting may also have spurred sea-ice
formation in the North Atlantic Ocean, creating a cooling
effect there. Both events would have caused the westerly
winds to shift south, toward the Southern Ocean. The winds
simultaneously warmed Antarctica and stirred the waters
around it. The resulting upwelling of CO2 would have caused
the entire globe to heat.
Anderson and his colleagues measured the rate of upwelling
by analyzing sediment cores from the Southern Ocean. When
deep water is vented, it brings not only CO2 to the surface
but nutrients. Phytoplankton consume the extra nutrients
and multiply.
In the cores, Anderson and his colleagues say spikes
in plankton growth between roughly 17,000 years ago and
10,000 years ago indicate added upwelling. By comparing
those spikes with ice core records, the scientists realized
the added upwelling coincided with hotter temperatures
in Antarctica as well as rising CO2 levels.
At least one model supports the evidence. Richard Matear,
a researcher at Australia’s Commonwealth Scientific
and Industrial Research Organization , describes a scenario
in which winds shift south and produce an increase in
CO2 venting in the Southern Ocean. Plants, which incorporate
CO2 during photosynthesis, are unable to absorb all the
added nutrients, causing atmospheric CO2 to rise.
Some other climate models disagree. In those used by
the United Nation's Intergovernmental Panel on Climate
Change , the westerly winds do not simply shift north-south.
“It’s more complicated than this,” said
Axel Timmermann, a climate modeler at the University of
Hawaii. Even if the winds did shift south, Timmermann
argued, upwelling in the Southern Ocean would not have
raised CO2 levels in the air.
Instead, he said, the intensification of the westerlies
would have increased upwelling and plant growth in the
Southeastern Pacific, and this would have absorbed enough
atmospheric CO2 to compensate for the added upwelling
in the Southern Ocean.
“Differences among model results illustrate a critical
need for further research,” Anderson said. Future
research, he added, should include “measurements
that document the ongoing physical and biogeochemical
changes in the Southern Ocean, and improvements in the
models used to simulate these processes and project their
impact on atmospheric CO2 levels over the next century.”
Anderson said that if his theory is correct, the impact
of upwelling “will be dwarfed by the accelerating
rate at which humans are burning fossil fuels.” But,
he said, “It could well be large enough to offset
some of the mitigation strategies that are being proposed
to counteract rising CO2, so it should not be neglected.”
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Antarctic
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