Oceans Could Absorb Much More CO2

Sept. 1, 2009 -- Earth's oceans are vast reservoirs of carbon dioxide (CO2) with the potential to control the pace of global warming.

It all hinges on the fate of marine "snow" -- a constant sprinkle of carbon-rich bits that flutter down from the sea surface to the cold depths below. And according to a new study, the flurries could suck much more of the greenhouse gas out of the atmosphere than previously thought.

Each year, phytoplankton floating in the seas' big blue expanse drink in 10 billion tons of carbon from the air (humans emit about 8 billion tons). Their shells and excretions rain down from the surface, providing a feast for creatures that recycle up to 90 percent of the carbon back into the water as CO2. Only a light dusting lands on the ocean floor.

But small changes in this carbon system have big implications for climate.

Today, most of the recycling happens in the first 210 meters (689 feet) below the ocean surface. According to a new study published in the journal Nature Geoscience, if that depth sank by just 24 meters, it could remove up to 27 parts per million more of CO2 from the atmosphere.

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This is because the deeper the snow falls into the ocean without being eaten, the more carbon-rich snow reaches the ocean floor. Once it is eaten, it becomes dissolved CO2, and it's just a matter of a short time (months to years instead of tens of thousands of years for the snow) before it makes its way back into the atmosphere.

"People are going to be scratching their heads and saying, 'Wow, that's really sensitive,'" Ken Buesseler of the Woods Hole Oceanographic Institute in Massachusetts, who was not involved in the study said. "That's not very big -- natural variability of that depth is several hundred meters."

By comparison, scientists estimate the ocean helped usher in the most recent ice age tens of thousands of years ago when it drained between 30 and 77 parts per million of CO2 from the air.

That won't happen any time soon. Humans have added well over 100 parts per million of CO2 as well as other greenhouse gases to the atmosphere in the last two centuries, and many scientists predict that ocean warming will raise the depth at which most carbon cycles back into the water. If that happens, the seas will hasten global warming as they spew CO2 back out into the air.

"On the other hand, a decrease in oxygen concentration in the ocean, which might be caused by enhanced stratification of the global ocean, might slowdown bacterial metabolic rates," and increase the amount of snow that reaches the deep ocean, Eun Young Kwon of Princeton University, the study's lead author said.

"The answer is I don't know. This is the major gap to be filled in our research community in the future."

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