Stockholm University researchers on the potential climate impacts of thawing subsea permafrost

To most people, subsea permafrost is a counterintuitive idea—how can frozen ground exist beneath an unfrozen ocean for thousands of years? But it does, under vast swaths of the seas surrounding the Arctic Ocean. It is also known that this region is leaking climate-forcing gases, including methane, to the atmosphere. Previous estimates of how much gases are emitted have varied so wildly that it has been challenging to put firm numbers into climate discussions and models. Now, the first broad expert assessment study on subsea permafrost has quantified the certainties and uncertainties about the present and future of this enigmatic part of Earth’s marine environment, which will help shape research going forward.

The new assessment, published in Environmental Research Letters, included 25 international scientists, of which five are from Stockholm University. The study suggests that subsea permafrost is already releasing substantial amounts of greenhouse gas. However, this release appears due to the overall climate change since the last glaciation rather than current human activity. Currently, each year subsea permafrost is estimated to release approximately 140 million tons of carbon dioxide and 5.3 million tons of methane to the atmosphere. That is roughly the same overall greenhouse gas footprint as Spain.

Virtually absent from policy discussions

Knowledge of terrestrial permafrost climate feedbacks have grown substantially in the past decade, thanks to new observations and multiple syntheses. In contrast, subsea permafrost system has remained a “known unknown” because of difficulty acquiring samples and measurements. Consequently, subsea permafrost is virtually absent from policy discussions and reports such as the Intergovernmental Panel on Climate Change (IPCC). Many permafrost studies and reports have pointed to the subsea permafrost feedback as one of the wildcards in the Earth’s climate system, and this high level of uncertainty about the subsea permafrost feedback has precluded its inclusion in policy discussions and in climate change reports.

Expert assessment

The expert assessment was led by PhD student Sara Sayedi, and senior researcher Ben Abbott of Brigham Young University (BYU), Utah, USA, and coordinated through the Permafrost Carbon Network, an international group of scientists studying Earth’s permafrost regions. The assessment combined findings from published and unpublished studies to estimate the size of the past and present subsea carbon stock and how much greenhouse gas it might produce over the next three centuries. Using a methodology called expert assessment, which combines multiple, independent plausible values, the researchers estimated that the subsea permafrost region currently traps 60 billion tons of methane and contains 560 billion tons of organic carbon in seafloor sediments and soil—it is a potential giant pool of carbon. For comparison, humans have released a total of about 500 billion tons of carbon into the atmosphere since the Industrial Revolution.

Different angles

“This large participation from Stockholm University is a hugely positive reflection of the leading role Sweden and Bolin Centre researchers have played in permafrost and Arctic Ocean research over the past decades,” said Brett Thornton. “Understanding how much methane and carbon dioxide is being emitted, and could be emitted in the future from the Arctic Ocean, especially the East Siberian Arctic Shelf north of Siberia, has been the focus of several marine expeditions led by Stockholm University researchers in the past decade. Stockholm University researchers have approached this problem from different angles—my work directly measured the sea-to-atmosphere fluxes of methane and carbon dioxide. Others have studied the stability of methane hydrates—a solid mixture of methane and water—in the subsea permafrost region of the Arctic Ocean.”

Five-year effort

“Completing this assessment represents a five-year effort to truly understand the state of knowledge about the subsea permafrost domain,” said Brett Thornton. “This effort actually began at the American Geophysical Union meeting in San Francisco in December 2015; assembling an international group of scientists who have dissenting opinions and bringing them together for one study is not easy!”

“Knowledge about subsea permafrost is limited for many reasons, but mostly difficulty in actually accessing the harsh Arctic regions where it is found. The assessment incorporates a wide range of viewpoints about the future of subsea permafrost and its potential effects on climate, and quantifies where the greatest gaps in our knowledge are.”

Even if this climate feedback is relatively gradual, the researchers pointed out that subsea permafrost is not included in any current climate agreements or greenhouse gas targets.

Reference:

Sayedi et al., 2020, Subsea permafrost carbon stocks and climate change sensitivity estimated by expert assessment, Environmental Research Letters, https://doi.org/10.1088/1748-9326/abcc29

Contact Information for Stockholm University participants

Brett Thornton brett.thornton@geo.su.se (marine and atmospheric methane measurements)

Gustaf Hugelius gustaf.hugelius@natgeo.su.se (terrestrial permafrost and carbon inventories)

Martin Jakobsson martin.jakobsson@geo.su.se (marine geology; glacial history, sea ice & historic sea level)

Matt O’Regan matt.oregan@geo.su.se (Arctic marine geology and paleoceanography)

Christian Stranne christian.stranne@geo.su.se (methane hydrates and modelling)