Aircraft reveal a surprisingly strong Southern Ocean carbon sink

The Southern Ocean is indeed a significant carbon sink — absorbing a large amount of the excess carbon dioxide emitted into the atmosphere by human activities — according to a new study led by the National Center for Atmospheric Research (NCAR).

The findings provide clarity about the role the icy waters surrounding Antarctica play in buffering the impact of increasing greenhouse gas emissions, after research published in recent years suggested the Southern Ocean might be less of a sink than previously thought.

The new study, published this week in the journal Science, makes use of observations from research aircraft flown during three field projects over nearly a decade, as well as a collection of atmospheric models, to determine that the Southern Ocean takes up significantly more carbon than it releases. The research also highlights the power that airborne observations have to reveal critical patterns in the global carbon cycle.

“You can’t fool the atmosphere,” said NCAR scientist Matthew Long, the paper’s lead author. “While measurements taken from the ocean surface and from land are important, they are too sparse to provide a reliable picture of air-sea carbon flux. The atmosphere, however, can integrate fluxes over large expanses. Airborne measurements show a drawdown of CO2 in the lower atmosphere over the Southern Ocean surface in summer, indicating carbon uptake by the ocean.”

The research is funded by the National Science Foundation (NSF), which is NCAR’s sponsor, as well as by NASA and NOAA.

Uncertainty about the role of the Southern Ocean

Once human-produced emissions of CO2 — from burning fossil fuels and other activities — enter the atmosphere, some of the gas is taken up by plants and some is absorbed into the ocean. While the overall concentration of CO2 in the atmosphere continues to increase, causing the global temperature to rise, these land and ocean “sinks” slow the effect.

A more precise understanding of where carbon sinks exist, how big they are, and how they may be changing as society continues to emit more CO2 is crucial to projecting the future trajectory of climate change. It is also necessary for evaluating the impact of potential emission reduction measures and CO2 removal technologies.

Scientists have long thought that the Southern Ocean is an important carbon sink. In the region around Antarctica, cold water from the deep ocean is transported to the surface. This upwelling water may not have seen the surface of the ocean for hundreds of years — but once in contact with the atmosphere, it’s able to absorb CO2 before sinking again.

Measurements of CO2 and related properties in the ocean suggest that 40 percent of all human-produced CO2 now stored in the ocean was originally taken up by the Southern Ocean. But measuring the actual flux at the surface — the back and forth exchange of CO2 between the water and the overlying air throughout a year — has been challenging.

The value of atmospheric measurements

In the new study, the research team sought to address the uncertainty by looking at carbon in the air instead of in the water. The atmosphere and the ocean exist in balance, and they are constantly exchanging CO2, oxygen, and other gases with each other.

The research team pieced together airborne measurements from three different field projects with deployments stretching over nearly a decade: the HIAPER Pole-to-Pole Observations (HIPPO) project, the O2/N2 Ratio and CO2 Airborne Southern Ocean (ORCAS) study, and the Atmospheric Tomography (ATom) mission.

While there are also surface monitoring stations that measure CO2 in the atmosphere over the Southern Ocean, these stations are relatively few and far between, making it difficult to characterize what is happening across the entire region.

After piecing together how CO2 typically varies in the atmosphere at a particular time of the year, the research team turned to a suite of atmospheric models to help them translate their atmospheric profiles into an estimate of how much CO2 the ocean was soaking up or releasing. Their conclusion was that the Southern Ocean takes in significantly more carbon in the summer than it loses during the winter, absorbing a whopping 2 billion tons of CO2 over the course of a year. In the summer, blooms of photosynthetic algae, or phytoplankton, play a key role in driving CO2 uptake into the ocean.

Journal Reference:

Strong Southern Ocean carbon uptake evident in airborne observationsScience, 2021; 374 (6572): 1275 DOI: 10.1126/science.abi4355

National Center for Atmospheric Research/University Corporation for Atmospheric Research (via ScienceDaily), 2 December 2021. Press release.

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