A new study—the first of its kind—has completed an annual accounting of the oceans’ intake of carbon over the past 250 years, and the news is troubling. According to the study, published in Nature, the oceans’ ability to sequester carbon is struggling to keep-up with mankind’s ever-growing emissions. Since 2000 researchers estimate that while every year the oceans continue to sequester more anthropogenic carbon emission, the overall proportion of carbon taken in by the oceans is declining.
The globe’s oceans are massive carbon sinks: more than a quarter of carbon emissions from humans have been sequestered by the ocean, according to the study.
Beginning with the year 1768, researchers found that carbon intake remained steady until the 1950s when rising anthropogenic carbon emissions forced the oceans to take in more carbon. By 2000 the oceans began to lose the race: the overall percentage of carbon sequestered by the ocean began to decline. In 2008, the oceans sequestered a record 2.3 billion tons of CO2 from the burning of fossil-fuels, yet the percentage of overall emissions had dropped 10 percent since 2000.
 The Great Blue Hole off the coast of Belize. Photo by: Rhett A. Butler. |
“The more carbon dioxide you put in, the more acidic the ocean becomes, reducing its ability to hold CO2,” the study’s lead author, Samar Khatiwala, explained to the Earth Institute at Columbia University. Khatiwala is an oceanographer at Columbia University’s Lamont-Doherty Earth Observatory. “Because of this chemical effect, over time, the ocean is expected to become a less efficient sink of manmade carbon. The surprise is that we may already be seeing evidence for this, perhaps compounded by the ocean’s slow circulation in the face of accelerating emissions.”
Prior to this study, scientists have been forced to depend on complex modeling to estimate the oceans’ capacity to sequester carbon. Khatiwala and his colleagues employed complex equations with data from water temperatures, salinity, and manmade chlorofluorocarbons to reconstruct the oceans’ annual carbon intake.
Their reconstruction found that the Southern Ocean, which surrounds Antarctica, plays an especially important role in sequestering carbon. According to the study, this cold sea sequesters 40 percent of the oceans’ carbon.
“We’ve suspected for some time that the Southern Ocean plays a critical role in soaking up fossil fuel CO2,” said Khatiwala. “But our study is the first to quantify the importance of this region with actual data.”
As opposed to warmer seas, the Southern Ocean’s cold and dense water better facilitates carbon intake.
The role of the oceans in the carbon cycle has been little discussed in matters of policy, but environmentalists are working to change this. Just this week the IUCN released a new report on coastal carbon sinks at a climate change summit in Granada, Spain.
 Smokestack in China. Photo by: Rhett A. Butler. |
“While there have been a lot of discussions about major carbon sinks on land such as forests, we have not heard much about the missing sinks of carbon in the oceans. The marine world not only regulates our climate, supplies essential goods and services, but also helps us tackle climate change,” Carl Gustaf Lundin said, Head of IUCN Global Marine Programme. “Decision-makers at national and international level will have to look at policies and financing mechanisms for protection and management of our oceans, and this report is the best starting point.”
The IUCN report points out the importance of mangrove forests, seagrasses, and salt marshes in sequestering carbon in their soils. Scientists suspect that these ecosystems, which are highly threatened, also play a vital role in funneling carbon from the atmosphere into the oceans. However, Khatiwala warns that in a world where every year brings a new emissions record, humanity cannot simply depend on natural carbon sinks to fix their dirty work.
“What our ocean study […] suggests is that we cannot count on these sinks operating in the future as they have in the past, and keep on subsidizing our ever-growing appetite for fossil fuels,” Khatiwala said.
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