Southern Ocean May Be Losing Ability to Absorb Greenhouse Gas
This has got to be a critical tipping point.
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.