The oceans absorb nearly a third of the carbon dioxide humans put into the air, almost certainly slowing the pace of global warming. The Southern Ocean plays an outsize role; the waters ringing Antarctica comprise a quarter of the marine surface, but account for some 40 percent of oceanic CO2 uptake. However, scientists have noticed an alarming trend; since the late 1980s, the Southern Ocean’s rate of uptake appears to have stagnated, failing to grow in proportion to rising CO2 levels in the air. Is the ocean approaching a saturation point, where it will no longer help buffer the effects of carbon emissions?
A new study says no: newly updated ship and satellite data show that uptake started growing again in 2002, and that the Southern Ocean is now absorbing proportionately as much CO2 as ever. Changes in wind patterns probably caused the reinvigoration, say the authors. The study suggests that large-scale weather variations may play a big role in the oceans’ ability to absorb carbon, but these are not well understood, and thus the future is hard to predict. The study appears this week in the leading journal Science.
“In the last 10 years, we were afraid that the Southern Ocean was going to quit giving us a break from climate change,” said coauthor Taro Takahashi of Columbia University’s Lamont-Doherty Earth Observatory. “This study shows that it’s recovered its ability to take up carbon dioxide, and that’s good news.” Takahashi has been working for decades to understand the cycling of CO2 between air and oceans, and is regarded as one of the world’s leading authorities on the subject.
The stormy, frigid Southern Ocean behaves like a giant lung, breathing in vast amounts of carbon dioxide from the air, and later releasing some of it. Much of the gas is stored in the depths for hundreds of years, and on the whole, more is absorbed than released. This has slowed the increase of atmospheric carbon dioxide due to human use of fossil fuels, and thus arguably the pace of human-induced climate change.
Many researchers had previously assumed that more CO2 in the air would mean correspondingly more absorption by the ocean, moving in a straight line. But by the early 2000s, scientists showed that the Southern Ocean was taking up about the same amount of carbon as it was in the late 1980s, despite fast-rising levels in the air. Some thought that this could indicate the waters were slowly becoming saturated.
The new study finds that the ocean has been catching up. The researchers combined measurements taken by research and commercial vessels, and satellite observations of seawater temperature, salinity and chlorophyll. They then used powerful new computing methods to fill in gaps in the data. This demonstrated that carbon uptake started regrowing around 2002. By 2012, it had moved up to match to the increased CO2 in the atmosphere.
What caused the decline, and renewal? The study says that during the 1990s, chronically strong, unidirectional winds stirred the much of the Southern Ocean, causing deeper waters to well up and release large amounts of carbon dioxide. Since then, those winds have abated, allowing the waters to retain more of the gas, say the authors. At the same time, wind patterns have shifted to drive more extremely cold air from the Antarctic continent to parts of the Southern Ocean. Colder water holds more carbon dioxide, so this also would boost uptake and retention.
Physicist Nicolas Gruber of ETH Zurich, one of the study’s lead authors, said that the study suggests the ocean’s ability to absorb carbon dioxide may vary in periodic cycles, rather than increasing in lockstep with the amount of gas in the air. “We were surprised to see such large variations,” he said.
Takahashi said no one can predict what the ocean will do next. “Some areas might get weaker, and then the next decade they might get stronger,” he said. “That’s why it’s important to keep measuring, so we can understand what’s happening.”