13 May 2013

AGU Video: Prospects dim for ice-free Arctic Ocean helping slow global warming

Posted by mcadams


By Sarah Charley

The surface waters of a major portion of the Arctic Ocean are becoming saturated with carbon dioxide sooner than many scientists expected, all but halting the watery region’s ability to sop up more of the greenhouse gas from Earth’s atmosphere, new research finds.

Previously, scientists theorized that the Arctic Ocean would act like a CO2-absorbing sponge—once its protective skin of sea ice melted—and join the rest of the world’s unfrozen oceans as they draw in and impound this greenhouse gas. Deposition of carbon dioxide in the oceans slows the rise in average global surface temperature that greenhouse gases in Earth’s atmosphere are causing.

However, the new findings from a marine zone called the Canada Basin, which accounts for about 20 percent of the Arctic Ocean, add to prior indications that that ocean can’t take in copious amounts of carbon dioxide as expected.

Melting Sea Ice near Resolute, Canada

Randall Scharien surveys melting sea ice near Resolute, Canada. Photo by Brent Else.

“Scientists thought that this region Arctic Ocean would be able to absorb a lot of carbon dioxide—the equivalent of shutting down one hundred coal-fired electrical plants,” said Brent Else from the University of Manitoba in Winnipeg, Canada, who led the new study. “However, in reality, it will only absorb the equivalent of shutting down two plants.”

In 2008, Wei-Jun Cai from the University of Georgia in Athens, Georgia, and his colleagues measured the concentration of carbon dioxide in the Canada Basin and calculated that this region of the Arctic had a limited ability to absorb carbon dioxide and predicted that this ability would become even more limited over time—a prediction this new study confirms.

Still, some anticipated changes accompanying the loss of summer sea ice in the Arctic Ocean, such as cycles of freezing and unfreezing, could to some degree promote CO2  uptake by Arctic waters, Else said.

He and his colleagues visited the Canada Basin in 2009 and measured the concentration of carbon dioxide gas in the surface water. It was “surprisingly high,” the team notes in a paper recently published in Geophysical Research Letters, a journal of the American Geophysical Union.

During this 2009 visit to the Arctic, Else and his team took detailed measurements of the depth, salinity, pH, and ice extent, as well as the concentrations of not just CO2, but also of other compounds, such as nitrate and phosphate, at four different stations. After plugging these values into a simple model, Else and his team determined the rate of air-sea gas exchange and found that the CO2 capacity of the Canada Basin will actually decrease as the sea becomes more and more ice free.

Ocean waters sequester carbon dioxide in two main ways: Phytoplankton convert carbon dioxide into organic matter and ocean currents slowly cycle carbon-dioxide-rich surface water deep below the surface. But, in the Canada Basin, neither of these two processes is very active.

Else explains that as the sea-ice melts in the Arctic, it turns into a layer of fresh water that floats on top of the less buoyant salt water. This thick skin of fresh water doesn’t mix with salty ocean water below. Without that mixing, the newly ice-free sea doesn’t get nutrients cycling up from the deep ocean to stimulate biological productivity, nor the churning of currents what would normally carry the carbon dioxide-rich surface water down.

“The top layer of fresh water doesn’t communicate very well with the deep ocean,” Else said. “Once this surface layer gets filled with CO2, that’s it.”

Sarah Charley is AGU’s science writing intern