LIVERMORE, Calif. -- The cold Southern Ocean surroundingAntarctica soaks up carbon dioxide from the atmosphere like a sponge,but scientists have discovered that the greenhouse gas doesn't staythere.
Now researchers from the Department of Energy's Lawrence LivermoreNational Laboratory have found that the carbon dioxide actually endsup deep in the subtropical ocean. They report their findings intoday's edition of the journal Science.
When scientists first started using computer models to see whathappens to carbon dioxide added to the atmosphere by humans, themodels showed that much of the greenhouse gas was sponged up from theatmosphere and stored in the cold Southern Ocean, said KennethCaldeira, a Lawrence Livermore climate scientist. But when scientiststested the water in the Southern Ocean, they didn't find the massivestockpile that would have accumulated if the ocean was storing man-made carbon dioxide.
"They found that there was very little carbon dioxide in theSouthern Ocean from the burning of fossil fuel, so it seemed thatsome of the early model results were wrong," Caldeira said.
The ocean absorbs about one-third of all man-made carbon dioxide,Caldeira said. It does so mainly in the cold regions because carbondioxide dissolves easily into cold water, just like a soda in therefrigerator will stay bubbly much longer than a soda sitting in thesun. If the ocean didn't soak up carbon dioxide, the amount in theatmosphere would increase significantly faster.
Caldeira and Philip Duffy, also from Lawrence Livermore, addedfactors to their computer model that made the global oceans morerealistic.
"Water in the ocean is layered," Caldeira said. "Warm water sitson the top with the colder, denser water in the deepest parts."
When the water is very cold, like it is in the Southern Ocean inwintertime, the cold layer of water is very close to the surface andit grabs carbon dioxide out of the atmosphere.
There is a boundary dividing warm surface water and colder watersbelow. That boundary is very close to the surface in the SouthernOcean, but it becomes deeper and deeper as it runs north into thetropics, where the interface between warm and cold water is as muchas a mile deep.
Using their new model, Caldeira and Duffy found that the carbondioxide that gets absorbed by the Southern Ocean actually ends up inthe subtropical latitudes as it slides along the cold, dense waterlayers and sinks into the deep subtropical ocean.
Some scientists expect that global warming will make the SouthernOcean less able to take carbon dioxide out of the air.
"The fear is that if you warm things up too much, moreprecipitation will make the surface of the Southern Ocean lessdense," Caldeira said. "You may start shutting off the entrance ofcarbon dioxide into the ocean, and things would warm up a lotfaster."
But Caldeira warns that studies also show that if things warm up,more microscopic plants that use carbon dioxide could compensate fora Southern Ocean shutdown.
Although there is plenty of room in the deep tropical ocean tostore carbon dioxide, the ocean may not be taking up as much of thegreenhouse gas in the future, Caldeira said.
"As the ocean absorbs more and more carbon dioxide, it becomesless able to absorb additional carbon dioxide because the waterbecomes acidic, so the oceans may become less efficient at carbonuptake," Caldeira said. This problem, he said, could make the climatechange more quickly.
This study was supported by the Department of Energy Center forResearch on Ocean Carbon Sequestration, the NASA OceanographyProgram, and Lawrence Livermore's Laboratory Directed Research andDevelopment Program.
Founded in 1952, LLNL is a national security laboratory with amission to ensure national security and apply science and technologyto the important issues of our time. LLNL is managed by theUniversity of California for the U.S. Department of Energy.