November 24, 2008

Southern Ocean Resilient Against Global Warming

A recent study has found that the Southern Ocean has proved more resilient to global warming than previously thought and remains a major store of mankind's planet-warming carbon dioxide.

Oceans act as a brake on climate change by absorbing large portions of the extra CO2 released by mankind through burning fossil fuels or deforestation and experts say the Southern Ocean is the largest of these "carbon sinks."

Researchers in the past have suggested the vast ocean between Australia and Antarctica was losing its potency because climate change had affected its currents and increased powerful westerly winds.

The analysis between ship-based measurements of the ocean since the 1960s and more recent data from hundreds of robotic floats shows the Southern Ocean has maintained its ability to soak up excess carbon despite changes to currents and wind speeds.

"It's a positive thing. It's one thing it looks like we don't have to worry about as much as we thought," said Steve Rintoul of the Center for Australian Weather and Climate Research, part of a team researchers that also included scientists from the Institute for Marine Research at the University of Kiel in Germany.

The new data as well as previous studies showed the Southern Ocean was becoming warmer, and also fresher, Rintoul said. The study was published this week in Nature Geoscience.

The data on salinity and temperature allowed the team to measure the density of seawater and how that density changed from one place to another in relation to how fast water was moving between two places.

"By looking at the density we could say something about the way the major currents were or were not changing.

"And this was the surprise. We found that the currents had not changed. They had shifted their position, they'd shifted closer to Antarctica but not become stronger or weaker."

The Southern Ocean is now being monitored for any changes in the Antarctic Circumpolar Current, a vast body of water that runs west to east around the continent from about 40 degrees south and driven in part by powerful westerly winds blowing on the ocean's surface.

Rintoul said the current is closer to Antarctica now than it was in the previous decade but it's carrying about the same amount of water.

Carbon dioxide is absorbed by the Southern Ocean's turbulent surface layer and then carried to the depths by circulation patterns. Billions of tiny phytoplankton and other organisms, which fall to the ocean bottom when they die, also absorb it.

Near Antarctica, some of the carbon-rich water from the depths rises releasing CO2, while further away from the continent, it sinks again because it is less dense. Overall, though, the ocean absorbs much more than it releases.

"Our results suggest that that part of the circulation, the upwelling near Antarctica and the sinking further north, has also not changed."

Past studies suggested the faster winds blowing on the surface increased the upwelling of the deep carbon-rich water.

"It was hard to tell what would happen to the ocean in the future largely because computer climate models weren't powerful enough to take into account the impacts of small-scale turbulence or eddies," said Rintoul.  "These help shift the circumpolar current to the south but not change its strength."

The same computer models suggested the circumpolar current should have sped up because of the stronger winds and caused more CO2-rich water to upwell from the depths, he said.

"The point of this study is that we don't see that."


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