Surface waters above methane seeps in the Arctic Ocean act as CO2 sinks, new PNAS study shows. This may, surprisingly, mean that methane seeps can have a cooling effect on climate.
Text: Maja Sojtaric
Methane release from the ocean floor is feared to contribute to greenhouse gas budget, not mitigate it. But a new study shows that several sites of methane release in the Arctic Ocean are actually CO2 sinks. The results are published in Proceedings of National Academy of Sciences of the United States of America (PNAS).
Surface waters above Arctic methane seeps absorbed 2,000 times more carbon dioxide from the atmosphere than the amount of methane that escaped into the atmosphere from the same waters.
“On Svalbard shelf, we find profuse methane seepage, as well as oceanographic mechanisms that facilitate algae bloom above these seeps. Algae consume CO2 as carbon source, which enhances CO2 drawdown from the surface ocean and consequently from the atmosphere above.” said CAGE researcher Anna Silyakova, co-author of the study.
Could have net cooling effect
CO2 uptake in the area of elevated methane release was enhanced, compared to surrounding waters. The cooling resulting from CO2 uptake overwhelmed the warming caused by methane output.
The cooling effect resulting from carbon dioxide uptake is actually up to 230 times greater than the warming effect expected from the emitted methane.
“If what we observed near Svalbard occurs more broadly at similar locations around the world, it could mean that methane seeps have a net cooling effect on climate, not a warming effect as we previously thought,” said USGS biogeochemist John Pohlman, who is the paper’s lead author. “We are looking forward to testing the hypothesis that shallow-water methane seeps are net greenhouse gas sinks in other locations.”
First study of its kind
Scientists continuously measured the concentrations of methane and carbon dioxide in near-surface waters off Svalbard and in the air just above the ocean surface. The measurements were taken over methane seeps at water depths ranging from 80 to 2600 meters.
Analysis of the data confirmed that methane was entering the atmosphere above the shallowest waters. However, the data also showed that significant amounts of carbon dioxide were being absorbed by the waters near the ocean surface.
Most previous studies have focused only on the sea-air flux of methane overlying seafloor seep sites. This is the first study that accounts for the drawdown of carbon dioxide, which could offset some of the atmospheric warming potential of the methane.
Not much methane escapes the ocean
A previous study by CAGE and NILU has shown that methane released from these areas of the Arctic seabed adds to the methane concentration in the ocean. But, also surprisingly, very little of the climate gas rising up through the sea reaches the atmosphere.
“These studies seem to be adding up to a different story than previously expected. Natural release of methane from the ocean floor may not be as influential on climate change as before.”
“But we do need to keep in mind that these studies are very local to this area of Svalbard. More global research and international collaboration is needed to see if these are global trends.” said CAGE director Jürgen Mienert, the director of the Centre for Arctic Gas Hydrate, Environment and Climate (CAGE) at the University of Tromsø.
“At CAGE, we are fortunate to have access to expertise, equipment, and a ship platform that allow us to launch sustained research focused on the Arctic Ocean. Collaborating with colleagues in US and Germany on the important issue of sea-air flux of greenhouse gases above seafloor methane seeps has been rewarding for all of the researchers involved.”
The study is a collaboration between USGS, and GEOMAR.
Reference: Pohlman, J et. al., Enhanced CO2 uptake at a shallow Arctic Ocean seep field overwhelms the positive warming potential of emitted methane. PNAS May 8, 2017. doi: Press release, USGS: Ocean Absorption of Carbon Dioxide More than Makes Up for Methane Emissions from Seafloor Methane Seeps