Polar Regions: Does sea ice affect the exchange of CO2?

December 06, 2015

Narragansett (Bernd F. Laeschke – 14.11.2012): By creating an artificial frosty environment with temperatures hovering around 20 degrees Fahrenheit, researchers from the University of Rhode Island (URI) tried to reproduce the surface of the frozen Arctic Ocean and study what factors influence the exchange of gases between the water and atmosphere. They used a 262,000-gallon tank filled with salt water to replicate Arctic conditions, creating channels and blowing strong winds to manipulate the movement of the ice.

“The polar regions are very important areas for gas uptake by the ocean because the surface water there gets pulled down to the deep ocean,” said Brice Loose, an assistant professor at the URI Graduate School of Oceanography. “But sea ice can act as a barrier to the exchange of gases between ocean and atmosphere, so we’re trying to figure out exactly how much exchange you get. The ice creates a huge wake behind it, and that turbulence leads to gas exchange.”

Inside the U.S. Army Corps of Engineers’ Cold Regions Research and Engineering Laboratory, the team used a 262,000-gallon tank filled with salt water to replicate Arctic conditions, creating channels and blowing strong winds to manipulate the movement of the ice.

“We want to come up with a predictive model that says, when you have this much sea ice cover and these kinds of winds and currents and temperatures, you can figure out the gas exchange,” Loose said. “If we can prove such a relationship, then climate modelers can factor it into their models so we have a better understanding of ocean circulation and the distribution of carbon dioxide.”

Wind speed is most commonly used to estimate the rate of gas exchange, because wind is the primary mechanism for forcing Carbon Dioxide (CO2) out of the atmosphere and into the ocean. But the presence of sea ice is believed to influence the exchange rate, and the changing amount of sea ice due to climate change adds uncertainty to that calculus.

The researchers grew eight inches of ice on top of the 6-foot deep water in the tank, added CO2 and other gases to the water, and used a wind tunnel to create currents and to move the ice. They measured the flow of water beneath the ice, the percentage of ice cover on the water surface, and the effect of convection driven by cooling at the surface to come up with a rate of gas exchange, using a variety of sensors and other equipment.

“We’re finding that all of these mechanisms play a role in gas exchange,” Loose said. “But it’s extremely difficult to predict what the ice is going to do - sometimes it flows like crazy, sometimes it jams up, and when the ice is in motion, freezing happens later - so all that will take more study.”

The next step will be to freeze the entire surface of the tank and make an opening in the ice and blow strong winds to compare the currents beneath the ice with the wind over the top and see how the gas exchange rates change.