Average terrestrial carbon dioxide balance from 2001–2020 in the Arctic Greg Fiske / Woodwell Climate Research Center.
Climate Change

After millenia as a carbon sink, over a third of Arctic Boreal region turns into carbon emitter

Permafrost thaw and wildfires push Arctic ecosystems into net carbon emission territory

Rohini Krishnamurthy

The Arctic Boreal Zone, encompassing treeless tundra, boreal forests, and wetlands spread over 26 million square kilometres, has undergone a significant transformation. New research has revealed that 40 per cent of this region has become a carbon source, releasing more carbon dioxide into the atmosphere than it absorbs, marking a shift from its role as a carbon sink for millennia.

A carbon sink absorbs more carbon from the atmosphere than it releases. A carbon source, on the other hand, releases more carbon than it absorbs.

“The Arctic boreal region is known for being soil organic carbon reservoirs. There is this concern that part of the soil stock will be released as carbon dioxide,” Anna Virkkala, a research scientist at the Permafrost Pathways initiative at Woodwell Climate and lead author of the study, told Down To Earth.

The study, published in the journal Nature Climate Change, attributed this transition to longer growing seasons, increased microbial activity and an uptick in the frequency and intensity of wildfires. The researchers suggest the shift may have started before 1990.

We are seeing that longer growing seasons and more microbial activity in winter are gradually shifting carbon trajectories
Marguerite Mauritz, assistant professor at the University of Texas-El Paso and co-author of the study, said in a statement

The findings align with NOAA’s 2024 Arctic Report Card, which also highlighted that the Arctic tundra, a treeless region, is becoming a net carbon source, largely due to warming temperatures and escalating wildfire activity in the Northern polar region..

“Our observations now show that the Arctic tundra, which is experiencing warming and increased wildfire, is now emitting more carbon than it stores, which will worsen climate change impacts,”  NOAA Administrator Rick Spinrad, said in a statement. “This is yet one more sign, predicted by scientists, of the consequences of inadequately reducing fossil fuel pollution.” 

The study gathered data from 200 monitoring sites between 1990 and 2020, analysing year-round changes in carbon dynamics alongside shifts in climate and fire patterns. While the region acted as a terrestrial carbon dioxide sink during the 2001–2020 period, more than 30 per cent of it was a net carbon dioxide source. Factoring in fire emissions increased this figure to 40 per cent.

The carbon source areas were distributed across Alaska (44 per cent), northern Europe (25 per cent), Canada (19 per cent), and Siberia (13 per cent). A critical finding was that emissions from the prolonged non-summer season (September to May) in the tundra outweighed the carbon dioxide absorbed during the short summer months (June to August).  

“While we found many northern ecosystems are still acting as carbon dioxide sinks, source regions and fires are now cancelling out much of that net uptake and reversing long-standing trends,” Anna Virkkala, a research scientist at the Permafrost Pathways initiative at Woodwell Climate and lead author of the study, said in a statement.

The Arctic Boreal Zone has experienced significant “greening,” with 49 per cent of the region showing longer growing seasons and increased vegetation. However, only 12 per cent of the region acts as a net carbon sink annually. This limited carbon absorption is offset by the effects of warming temperatures, which are accelerating permafrost thawing.  

Permafrost, defined as soil or rock that remains at or below 0 degrees Celsius for at least two consecutive years, stores vast amounts of carbon locked in ice. As it thaws, these greenhouse gases are released, exacerbating global warming.  

Most of the treeless Tundra high up in the Arctic have permafrost, which is starting to thaw. That means there is more organic matter available for decomposition by microbes, which was earlier safe in this frozen ground and that can be released, Virkkala explained.

The authors have a theory on why 12 per cent remains a sink despite greening trends in nearly half of the region. One potential reason could be that the non- growing season (autumn, winter and spring) could be too cold for plants to ohostosymtheis, but the conditions might be warm enough for microbes to release CO2, Virkkala added.