Southern Ocean confirmed as strong carbon dioxide sink

New research utilizes airborne measurements of carbon dioxide to estimate ocean uptake

Research published in recent years has suggested the Southern Ocean might be absorbing less carbon dioxide (CO₂) from the atmosphere than previously thought.  But a new study published this week in the journal Science confirms the role of the Southern Ocean as a significant carbon sink.

Using observations from research aircraft flown during three field projects over nearly a decade, as well as a collection of atmospheric models, researchers found that the Southern Ocean absorbs 550 million tonnes of carbon from the atmosphere every year, or about 18 percent of the amount absorbed by all the world’s oceans. The study highlighted the value of airborne observations when investigating 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.”

Samples captured over the Southern Ocean during three airborne research missions, including ATom, helped to confirm that the Southern Ocean is a major carbon sink. Credit: Sam Hall/NCAR.

NOAA’s Global Monitoring Laboratory measured CO₂ from the samples collected during the airborne projects and provided atmospheric CO₂ records from the surface monitoring stations of its Global Greenhouse Gas Reference Network.

“Airborne measurements have the advantage of a very large footprint, over the scale of thousands of kilometers,” said Colm Sweeney, a scientist with NOAA’s Global Monitoring Laboratory and a co-author. “These measurements allow us to see the strong summer uptake and near-neutral winter outgassing over the Southern Ocean.”

Uncertainty about the role of the Southern Ocean

Once CO₂ emissions enter the atmosphere, some is taken up by plants, and some is absorbed into the ocean. These land and ocean “sinks” slow the increases in atmospheric CO₂ levels. 

As society continues to emit more CO₂, understanding the location, scale and variability of carbon sinks is crucial to understanding the future trajectory of climate change, and evaluating the impact of future emission reductions measures and CO₂ removal technologies.

Piecing airborne measurements from three different field projects (HIPPO, ORCAS, ATom) with deployments stretching over nearly a decade, the scientists showed that the Southern Ocean takes in significantly more CO₂ in the summer than it loses during the winter. In the summer, blooms of photosynthetic algae, or phytoplankton, play a key role in driving CO₂ uptake into the ocean.

“It’s critical that we have a finger on the pulse of the carbon cycle as we enter a period when global society is taking action to reduce CO₂ in the atmosphere,” Long said. “These observations can help us do just that.”

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

This story was adapted from an NCAR press release.

For more information contact Theo Stein, NOAA Communications, at theo.stein@noaa.gov.