Global warming contributes to permafrost thawing in the Arctic. Photo: Örjan Gustafsson

A new assessment published in Nature Reviews Earth & Environment provides a comprehensive synthesis of the Arctic carbon cycle, highlighting how rising temperatures may be weakening the region's ability to sequester carbon.

The study, led by an international team of scientists, including Örjan Gustafsson, Professor at the Department of Environmental Science, and Jorien Vonk, former PhD student at the department and now Professor at Vrije Universiteit Amsterdam, evaluates the Arctic’s current role as a net carbon sink. At present, oceanic CO₂ uptake (127 ± 36 Tg C year⁻¹) and organic carbon burial in shelf sea sediments (112 ± 41 Tg C year⁻¹) contribute to this sink function. However, some estimates suggest this balance is fragile due to fluctuations in terrestrial emissions from inland waters, uncertainties around methane release—particularly from marine and subsea sources—and disturbance events that add complexity to the region’s overall greenhouse gas budget.

A changing Arctic landscape

The Arctic is warming nearly four times faster than the global average, driving permafrost thaw, increased coastal erosion, and enhanced riverine discharge of organic carbon. While Arctic greening—stimulated by longer growing seasons—can contribute to carbon uptake, these gains are offset by rising soil respiration, increasing methane emissions from inland waters, and disturbances such as wildfires.

Professor Jorien Vonk. Photo: Vrije Universiteit Amsterdam

“Both the land and water masses in the Arctic store considerable amounts of carbon," says Jorien Vonk. "If you add strong warming to that, you can imagine that this will increasingly affect stocks, fluxes, and cycles of carbon with potential consequences for ecosystems and our global climate.”

Erosion, emissions, and uncertain projections

The study quantifies the vast carbon reservoirs in Arctic soils and marine sediments. Terrestrial soils alone contain 877 ± 16 Pg of carbon—over a quarter of the world’s total soil carbon—while the surface sediments of the Arctic Ocean hold an additional 82 ± 35 Pg. Moreover, vast amounts of methane are trapped within the largely uncharted subsea permafrost system.

With accelerating warming and permafrost thaw—particularly in ice-rich and subsea permafrost regions—these carbon stores are increasingly vulnerable to release. Extreme events, such as storm-driven coastal erosion and collapsing methane hydrates, further amplify uncertainty in Arctic carbon cycle projections. These non-linear processes can rapidly mobilize previously stable carbon stores into the atmosphere or ocean, with potentially profound climate impacts.

“The subsea permafrost is thawing ten times faster than nearby land permafrost,” explains Örjan Gustafsson. “Projecting the increasing trajectory of already extensive methane release from the thawing subsea permafrost and shallow hydrates is a major challenge.”

The need for global attention

The review underscores the need for interdisciplinary collaboration to address critical knowledge gaps in Arctic carbon flux. Subsea shelf regions, small catchments, and terrestrial-to-marine transfer zones remain underrepresented in carbon assessments, limiting the accuracy of climate projections.

Professor Örjan Gustafsson. Photo: Eva Dahlin

As global climate policy increasingly emphasizes carbon budgets and emissions reductions, understanding the Arctic’s evolving carbon dynamics is crucial. If the Arctic’s carbon sink function continues to weaken, it risks transitioning into a net source of emissions—further accelerating global warming and complicating international climate goals.

“The Arctic stores a disproportionately large amount of carbon compared to other regions on the globe, making it highly relevant to the future of our planet,” says Vonk. “Beyond that, I personally find the Arctic a fantastic place to be. Its wildlife, wilderness, remoteness, and wonderful people make it a region we should all take great care to preserve.”
 

Read the original study in Nature Reviews Earth & Environment