No land intersects the 60° circle of latitude south of Earth's equator. Instead, that parallel marks the northern limit of the Southern Ocean surrounding Antarctica. At this latitude, swift, prevailing westerly winds continually churn the waters as they circumnavigate the continent, earning the region the nickname "the screaming '60s".
But the Southern Ocean plays a more benign role in the global carbon budget: Its waters now take up about 50% of the atmospheric carbon dioxide emitted by human activities, thanks in large part to the so-called "biological pump." Phytoplankton, tiny photosynthesizing organisms that bloom in the nutrient-rich waters of the Southern Ocean, suck up carbon dioxide from the atmosphere. When the creatures die, they sink to the ocean floor, effectively sequestering that carbon for hundreds or even thousands of years. It also helps that carbon dioxide is more soluble in colder waters, and that the churning winds mix the waters at the surface, allowing the gases to penetrate the waters more easily.
There are signs, however, that the ocean's capacity to sequester atmospheric carbon dioxide has been decreasing over the past few decades, says climate scientist Samuel Jaccard of ETH Zurich in Switzerland. For one thing, the carbon doesn't stay sunk. Even as phytoplankton blooms sequester new carbon, the upwelling of deep, subsurface water currents in the region bring old, once-sequestered carbon back to the surface waters, allowing for exchange with the atmosphere. Meanwhile, the ozone hole has strengthened winds in the region, which may be hindering the carbon storage.
For clues to the future, climate scientists look to past glacial-interglacial cycles. Researchers have a record of atmospheric carbon dioxide stretching back millions of years thanks to ice cores from Antarctica, which contain trapped gas bubbles, snapshots of ancient air. But for the other half of the picture—what happened in the oceans during that time—there is only a relatively short record extending back about 20,000 years to the last glacial cycle. Ocean sediment records, which contain evidence of carbon and nutrients, are one way to reconstruct that history.
Previous ocean sediment records suggest that, as the world slipped into the last glacial period, less carbon overall reached the sediments of the Southern Ocean, coinciding with declining atmospheric carbon dioxide. During cold periods, increased sea-ice cover can keep gases trapped in the ocean—and the drier, dustier conditions bring much-needed iron to phytoplankton in the sub-Antarctic portion of the Southern Ocean, feeding blooms that gobble down carbon dioxide from the atmosphere.
What happens when the world moves into a warm, interglacial period isn't certain, but in 2009, a paper published in Science by researchers found that upwelling in the Southern Ocean increased as the last ice age waned, correlated to a rapid rise in atmospheric carbon dioxide.
Now, using two deep cores collected at two Ocean Drilling Program sites in the Southern Ocean, Jaccard and colleagues have reconstructed ocean records of productivity and vertical overturning reaching back a million years, through multiple glacial-interglacial cycles. This rapid increase in carbon dioxide as the world transitions from glacial to interglacial seems to be a pretty regular thing, they've found.
"There was relatively more carbon dioxide emitted from the deep ocean and released to the atmosphere as the climate warmed," Jaccard says. "The Southern Ocean sink was less effective."
As the world transitioned to glacial periods, on the other hand, atmospheric carbon dioxide decreased. This happened in two steps: First, in the Antarctic zone of the Southern Ocean, a reduction in wind-driven upwelling and vertical mixing brought less deep carbon to the surface. Then, about 50,000 years later, atmospheric carbon dioxide decreased again, the team reports online today in Science. This decrease, Jaccard says, is linked to blooms of phytoplankton in the sub-Antarctic Zone, slightly farther north, driven by an influx of iron carried by dusty winds.
The regularity of the glacial-interglacial signal is intriguing, and "it's a valid point to be making," says Robert Toggweiler of the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey. But he questions how to apply it to the future, because modelers have trouble making models sophisticated enough to reproduce such a signal.
It's known that when ice sheets start to melt, cooling the air in that region, the winds over the Southern Ocean strengthen, Toggweiler says. "The question is how does that signal get to the Southern Ocean?" The ozone hole plays a role in the stronger winds, but so does increasing temperature. So far, no one has been successful at taking the cooling in the north and generating winds in the south that produce much of a carbon dioxide response. "In general, models have been spectacularly unsuccessful in replicating this sort of response we're seeing here," he says.