Submarine permafrost is still waking up after 12,000 years

In the far north, the swelling Arctic Ocean engulfed vast swathes of coastal tundra and steppe ecosystems. Although the ocean water was only a few degrees above freezing, it began to thaw the permafrost below, exposing billions of tons of organic matter to microbial degradation. The decomposing organic matter started to produce CO₂ and CH₄, two of the most important greenhouse gases.

Although researchers have been studying degrading undersea permafrost for decades, difficulties in collecting measurements and sharing data across international and disciplinary areas have prevented a general estimate of the amount of carbon and the rate of release. A new study led by Ph.D. candidate Sara Sayedi and senior researcher Dr. Ben Abbott at Brigham Young University (BYU) published in IOP Publishing journal Environmental research letters, sheds light on submarine permafrost climate feedback and generates initial estimates of circumarctic carbon stocks, greenhouse gas emissions and possible future responses of the submarine permafrost zone.

Sayedi and an international team of 25 permafrost researchers worked under the coordination of the Permafrost Carbon Network (PCN), which is supported by the US National Science Foundation. The researchers combined findings from published and unpublished studies to estimate the size of the undersea carbon stock past and present and how much greenhouse gas it could produce over the next three centuries.

Using a methodology called expert assessment, which combines multiple, independent plausible values, the researchers estimate that the submarine permafrost region currently retains 60 billion tons of methane and contains 560 billion tons of organic carbon in sediment and soil. For reference, humans have emitted a total of about 500 billion tons of carbon into the atmosphere since the industrial revolution. This makes the submarine permafrost carbon stock a potential gigantic ecosystem feedback for climate change.

“Subsea permafrost is truly unique in that it is still responding to a dramatic climate transition from more than ten thousand years ago,” said Sayedi. “In some ways, it could give us a glimpse into the potential reaction of permafrost thawing from human activity today.”

Estimates by Sayedi’s team suggest that submarine permafrost is already emitting significant amounts of greenhouse gas. However, this release is mainly due to ancient climate change rather than current human activity. They estimate that submarine permafrost emits about 140 million tons of CO₂ and 5.3 million tons of CH4 per year in the atmosphere. This is comparable in size to Spain’s total greenhouse gas footprint.

The researchers found that if human-caused climate change continues, the release of CH₄ and co₂ of undersea permafrost can increase significantly. However, this response is expected to occur over the next three centuries, rather than abruptly. Researchers estimate that the amount of future greenhouse gas emissions from submarine permafrost depends directly on future human emissions. They found that under a business-as-usual scenario, warming submarine permafrost emits four times more CO₂ and CH₄ compared to when human emissions are reduced to keep warming below 2 ° C.

“These results are important because they indicate substantial but slow climate feedback,” says Sayedi. “Some of the coverage of this region has suggested that human emissions could cause catastrophic release of methane hydrates, but our study suggests a gradual increase over many decades.”

Although this climate feedback is relatively gradual, the researchers point out that subsea permafrost is not included in the current climate commitments or greenhouse gas targets. Sayedi emphasized that there is still a lot of uncertainty about submarine permafrost and that additional research is needed.

“Compared to how important submarine permafrost could be to the future climate, we know shockingly little about this ecosystem,” said Sayedi. “We need more sediment and soil samples, as well as a better monitoring network to detect when greenhouse gas emissions are responding to current warming and how quickly this giant carbon pool is waking up from its frozen sleep.”