Deep Sea Sediments: locations of sediment showing Antarctic upwelling
Credit: Robert Anderson, Lamont-Doherty Earth Observatory

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 that the end of the last ice age was triggered by a change in Earth’s orbit that caused the northern part of the planet to warm. This partial climate shift was accompanied by rising levels of the greenhouse gas 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 triggered 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, said Anderson. If they end up venting more CO2 into the air, manmade warming underway now could be intensified.

Scientists have been studying the oceans for more than 25 years to understand their influence on CO2 levels and the glacial cycles that have periodically heated and chilled the planet for more than 600,000 years. Ice cores show that the ends of other ice ages also were marked by rises 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, and thus amplified 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.

In the same issue of Science, Toggweiler writes a column commenting on the work.  “Now I think this really starts to lock up how the CO2 changed globally,” he said in an interview. “Here’s a mechanism that can explain the warming of Antarctica and the rise in CO2. It’s being forced by the north, via this change in the winds.”

At least one model supports the evidence. Richard Matear, a researcher at Australia’s Commonwealth Scientific and Industrial Research Organisation, 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 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 argues, upwelling in the Southern Ocean would not have raised CO2 levels in the air. Instead, he says, 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,” said Anderson. These, 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 says 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.”

In addition to Anderson, the paper was coauthored by Simon Nielsen of Florida State University, and five Lamont-Doherty researchers: Shahla Ali, Louisa Bradtmiller, Martin Fleisher, Brenton Anderson and Lloyd Burckle. The study was funded by NOAA, the National Science Foundation, Norwegian Research Council and Norwegian Polar Institute.

Ice Age Ocean Temperature Swings Suggest Future Global Warming May Extend Farther South

By Lucas Held

Barnard Professor Julian Sachs

Wide swings in ocean temperature during the last ice age extended well south of the polar and subpolar Atlantic region and all the way into the warm, subtropical ocean, a new study demonstrates, suggesting that the effect of future global warming may extend farther south than some previous predictions.

The study, published in the Oct. 22 issue of Science, was undertaken by Julian Sachs, assistant professor of environmental science at Barnard College, and Scott J. Lehman, associate research professor of geological sciences at the Institute of Arctic and Alpine Research at the University of Colorado, Boulder. The two examined sediments recovered northeast of Bermuda from under more than two miles of water.

Measurements in Greenland ice cores by Pieter Grootes and coworkers at the University of Washington over the last decade documented large, rapid swings in air temperature during the last glacial period. Similar swings in polar and subpolar sea temperatures have been inferred from the distribution of micro-fossil shells in those sediments. But Sachs and Lehman are the first to demonstrate that dramatic temperature changes of up to 5 degrees Celsius (or 9 degrees Fahrenheit) occurred not only in the north, but well into the warm ocean, during the period 60,000 to 30,000 years ago.

“It has been known for the better part of a decade that Greenland and the polar Atlantic region experienced ocean-driven flip flops in temperature every few thousand years during the last glacial period approximately 80,000 to 100,000 years ago,” said Sachs and Lehman. “What is new here is the clear evidence that, like the polar Atlantic, the warm Atlantic was also undergoing related, very large, and very rapid – in terms of degree per decade-temperature changes.”

Where climate models predict subtropical sea temperature change of up to 5 degrees Celsius between maximum glacial and warm interglacial (i.e., modern) conditions, a period of 10,000 years, the large, rapid temperature swings the two scientists observed over a scant 250 years can probably only be caused by disruption or even halting of the North Atlantic conveyor-like circulation, a concept pioneered by Columbia’s Wallace Broecker, Newberry Professor of Earth and Environmental Sciences.

The circulation of the North Atlantic conveyor transports warm, tropical water north to the polar areas, via the Gulfstream and North Atlantic Drift currents. Once north, the salty warm water cools and then sinks to the bottom of the ocean, a process that draws more warm surface water from the south.

“This north-south conveyor is what keeps northern Europe far warmer than the Canadian provinces at the same latitude-in short, what keeps London from having a climate like Newfoundland,” said Sachs.

But if the salty water does not become cold enough to sink, due to global warming, or is diluted with too much freshwater, the North Atlantic conveyor halts. This appears to have happened repeatedly throughout the time period studied by Sachs and Lehman, since no other mechanism appears capable of producing the large, sudden temperature swings they document.

Most numerical models used to predict the climate response to increased greenhouse gas concentration do not predict the large temperature changes of the warm ocean documented by the two researchers, suggesting that those models may have to be altered.

“Most climate models developed over the past 10-15 years suggest the effects of a shutdown of the North Atlantic’s conveyor-like circulation, such as that due to global warming, will be localized in the far northeast Atlantic-Iceland and Scandinavia,” said Sachs. “Our data suggests the footprint may be much larger.”

“The fact that we observed such large temperature fluctuations in connection with changes in ocean circulation documented by Lloyd Keigwin, senior scientist at the Woods Hole Oceanographic Institute, and Ed Boyle, professor of chemical oceanography, at MIT, suggests future climate changes may not only be severe for Northern Europe but could affect more southerly latitudes,” Sachs added.

Observed Keigwin, “Because the climate system can respond this quickly means it could respond this quickly to man’s influence and it may respond unpredictably.”

While no single study will send climate modelers back to the drawing board, said Keigwin, “the kinds of changes they see may be greater than what the models predicted and that may lead to some recalculation.”

One caution, according to Sachs, is that the climate system today may have a different sensitivity than it did 30-60 thousand years ago, when ice sheets, solar radiation receipts and greenhouse gas concentrations were different.

Circulation in the waters near the Antarctic coast may be one of the planet’s critical means of regulating levels of carbon dioxide in the Earth’s atmosphere, according to researchers from MIT, Princeton and the National Oceanic and Atmospheric Administration.

Though climate scientists have long debated why atmospheric levels of carbon dioxide vary over lengthy periods in Earth’s history, researchers now appear to have found a clue.

In a recent issue of the journal Nature, the team reports that computer modeling has revealed that the waters in the Southern Ocean below 60 degrees south latitude — the region that hugs the continent of Antarctica — play a far more significant role than was previously thought in regulating atmospheric carbon.

The waters north of this region do comparably little to regulate it, confuting past theories, the team found.

“Cold water that wells up regularly from the depths of the Southern Ocean spreads out on the ocean’s surface along both sides of this dividing line, and we have found that the water performs two very different functions depending on which side of the line it flows toward,” said Irina Marinov, the study’s lead author and a postdoctoral fellow in MIT’s Department of Earth, Atmospheric and Planetary Sciences.

“While the water north of the line generally spreads nutrients throughout the world’s oceans, the second, southward-flowing stream soaks up carbon dioxide, a greenhouse gas, from the air. Such a sharply defined difference in function has surprised us. It could mean that a change to one side of the cycle might not affect the other as much as we once suspected.”

The research team also includes Professor Jorge Sarmiento of Princeton as well as Anand Gnanadesikan and Robbie Toggweiler of the National Oceanic and Atmospheric Administration (NOAA).

Two years ago, Sarmiento and colleagues discovered that the nutrients in the world’s oceans were dependent on the Southern Ocean’s circulation pattern. Scientists have also been aware that cold Antarctic waters have the ability to absorb atmospheric carbon dioxide.

What has been difficult is drawing the distinctions between one effect and another.

“The new paper shows that carbon dioxide and nutrient flow are separated quite dramatically,” Sarmiento said. “What we are trying to do is understand better the balance of forces that help our planet maintain a steady environmental state, so we can anticipate what might cause that state to change. This paper helps us clarify how those forces interact.”

Changing levels of atmospheric carbon dioxide have long concerned the scientific community, as this well-known greenhouse gas could be a major influence on global warming. Marinov said the discovery could shed light on how the Earth reacted far back in history, which might offer clues to how it will behave in the future.

“In the last ice age, for example, the atmosphere experienced very low levels of carbon dioxide, and no one is completely sure why,” she said. “However, we now understand the Southern Ocean plays a large role in regulating how much of the gas gets dissolved in water, and how much remains in the atmosphere.”

The current study, she said, indicates that to better understand the Southern Ocean’s effect on atmospheric carbon, scientists should pay greater attention to the Antarctic than to the more northerly sub-Antarctic region.

“In the Antarctic, the circulation pattern moves the surface water carrying carbon dioxide deep into the ocean’s depths, where the sequestered carbon could potentially be trapped for a long time,” Marinov said. “According to the models we used, the deep Antarctic is the critical region where we need to concentrate our research.”

The team also indicated that the findings had implications for future research into carbon sequestration, a strategy for coping with increased atmospheric carbon dioxide levels. Some scientists propose that sequestration could one day capture atmospheric carbon and store it in places such as the deep ocean, thus mitigating humanity’s greenhouse gas emissions.

“An interesting idea of recent years is that we can sequester a lot of carbon if we dump iron into the ocean to encourage the growth of certain microorganisms, which incorporate carbon as they grow,” Marinov said. “These organisms would then fall to the ocean floor after they die, taking the carbon with them. The overall effect would be to lower the concentration of carbon in the surface waters, allowing more atmospheric carbon dioxide to dissolve into the sea. Our research has implications for future iron fertilization experiments, the focus of which we conclude should shift to the Antarctic.”

Marinov said that the findings were based strongly on the team’s computer models, which have limitations that they will now concentrate on eliminating.

“While we are confident about the paper’s conclusions, we are always looking for ways to clarify our understanding of the Southern Ocean,” she said. “Our model, for example, does not take into account the fact that the circulation patterns are strongest in the winter, when the Antarctic is covered in darkness and the phytoplankton cannot grow very much. It is important that we understand the impact of this process on atmospheric carbon dioxide through future research.”

This research was sponsored in part by the U.S. Department of Energy and NOAA.