Much of the carbon dioxide in Earth's atmosphere goes south, literally. Researchers using new supercomputer models have described for the first time how the Southern Ocean sucks the greenhouse gas out of the air and then shuttles it into the deep sea far from the Antarctic. The findings should give climate scientists a better understanding of this critical component of Earth's carbon cycle.
The carbon dioxide (CO2) emitted into the atmosphere ends up several places. Some is absorbed by green plants, which turn it into food. Some CO2 remains in the atmosphere for thousands of years, where it absorbs solar radiation. And some eventually works its way into the oceans and exits the carbon cycle. Oceans capture CO2 because wind-driven waves churn the surface, mixing the gas into the water. Then ocean currents carry the carbon into the deep sea and away from the atmosphere.
Scientists have known about this process for decades. They've also known that the Southern Ocean, which surrounds Antarctica and stretches to the tips of South America, Africa, and Australia, pulls in far more than its share of CO2. The strong winds there blow almost continuously, making the region an excellent absorber of the gas. The Southern Ocean accounts for only about 6% of the world's ocean area, but scientists estimate that it absorbs as much as 40% of the CO2 taken in by the seas. There's a mystery, however: The Southern Ocean retains only about 9% of the CO2 it absorbs. What happens to the rest of it?
The answer has proven elusive because the Southern Ocean's remoteness and severe weather have made it extremely difficult to collect the necessary data. Satellite sensors have helped, and now researchers have plugged satellite measurements--along with available surface readings on CO2 absorption, wind patterns, and water currents in the Southern Ocean--into a new high-resolution supercomputer model. Using data for 2005 and 2006, the first years that included these satellite observations, the team was able to pinpoint the wind patterns and currents that distribute ocean CO2. As the team reports in today's issue of Nature, instead of sinking in place, almost all of the CO2 is carried away from the Antarctic and toward the subtropics--hence the missing CO2 in the Southern Ocean.
The researchers also found that "the atmosphere overlying the Southern Ocean is undergoing significant climate change," says oceanographer and lead author Takamitsu Ito, of Colorado State University, Fort Collins. It's possible, he says, that "changing the atmospheric wind pattern in the region could alter the rate at which the Southern Ocean absorbs CO2," possibly reducing the ocean's ability to consume the gas and leading to further global warming.
Oceanographer Michael Follows of the Massachusetts Institute of Technology in Cambridge, says the finding is significant because it demonstrates the dominant role that wind-driven currents play in transporting and storing carbon in the Southern Ocean. And it "reinforces how important changes in Southern Hemisphere winds can be for Earth's carbon cycle and its climate," he says.