Photo Credit: Robert Jon Brack/Antarctic Photo Library

A glacier afloat in the Southern Ocean near the Antarctic Peninsula. A new study by scientists at Columbia University’s Lamont-Doherty Earth Observatory says shifting winds appear to pull CO2 out of the ocean and into the atmosphere

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.”

The National Science Foundation , NOAA, The Research Council of Norway  and Norwegian Polar Institute  funded the study.