Tons of Carbon Dioxide Stored in Subtropical Oceans

by Steve Wampler

When scientists first started using computer models to see what happens to carbon dioxide added to the atmosphere by humans, the models showed that lots of the greenhouse gas was sponged up from the atmosphere and stored in the cold Southern Ocean, said Kenneth Caldeira, a climate scientist from Lawrence Livermore National Laboratory.

But when scientists tested the water in the Southern Ocean, they didn't find the massive stockpile that would have accumulated if the ocean was storing man-made carbon dioxide. "They found that there was very little anthropogenic carbon in the Southern Ocean, so it seemed that some of the early model results were wrong," said Caldeira.

The ocean absorbs about one-third of all man-made carbon dioxide, said Caldeira, and does it mainly in the cold regions because carbon dioxide dissolves easily into cold water, just like a soda in the refrigerator will stay bubbly a lot longer than a soda sitting in the sun. If the ocean didn't soak up carbon dioxide, the amount in the atmosphere would increase a lot faster.

Caldeira and Philip Duffy, also from Lawrence Livermore, added factors to their computer model that made the global oceans more realistic. "Water in the ocean is layered," said Caldeira. "Warm water sits on the top with colder, dense water below." When the water is very cold, like it is in the Southern Ocean in wintertime, the cold layer of water is very close to the surface and it grabs carbon dioxide out of the atmosphere.

There is a boundary dividing warm surface water and colder waters below. That boundary is very close to the surface in the Southern Ocean, but it becomes deeper and deeper as it runs north into the tropics, where the interface between warm and cold water is as much as a mile deep.

Using their new model, Caldeira and Duffy found that the carbon dioxide that gets absorbed by the Southern Ocean actually ends up in the subtropical latitudes as it slides along base of the cold, dense water layer and sinks into the deep subtropical ocean.

Some scientists expect that global warming will make the Southern Ocean less able to take carbon dioxide out of the air. "The fear is that if you warm things up too much, more precipitation will make the surface of the Southern Ocean less dense," said Caldeira. "You may start shutting off the entrance of carbon dioxide into the ocean, and things would warm up a lot faster," he said. But Caldeira warns that studies also show that if things warm up, more microscopic plants that use carbon dioxide could compensate for a Southern Ocean shutdown.

Although there is plenty of room in the deep tropical ocean to store carbon dioxide, the ocean may not be taking up as much of the greenhouse gas in the future, said Caldeira. "As the ocean absorbs more and more carbon dioxide, it becomes less able to absorb additional carbon dioxide because the water becomes acidic, so the oceans may become less efficient at carbon uptake," said Caldeira. A problem, he said, that could make the climate change more quickly.

The findings were reported in the Jan. 28 issue of the journal Science.