Over a third of the Arctic boreal region now releases more carbon than it stores

A new study shows the Arctic boreal region is changing in ways that affect the entire planet. It still takes up carbon dioxide overall, but more than 30 percent of this vast area now releases more than it absorbs.

This matters because much of the northern ground is permafrost, soil that stays frozen for at least two years straight. When it warms, long-stored carbon can escape as carbon dioxide.

Study tracks Arctic carbon shifts

Study lead author Anna Maria Virkkala of Woodwell Climate Research Center (WCRC) and her colleagues analyzed records from 200 field sites across Alaska, Canada, Siberia, and the Nordic countries. They tracked how forests, wetlands, and open tundra exchange CO2 with the air.

The results are clear. The region was a net sink from 2001 to 2020, but 34 percent of the land area acted as an annual source, and fires reduced the overall sink size.

“More than 30 percent of the region was a net CO2 source,” noted Anna Maria Virkkala, Woodwell Climate Research Center.

Fires erase carbon gains

Fire years can dominate the carbon math. In 2023, an analysis estimated Canada’s wildfires produced about 480 million metric tons of carbon and roughly 23 percent of global wildfire carbon emissions.

Long-term evidence backs this up. Modeling of Alaska and Canada from 1986 to 2016 found North American boreal forests were a net carbon source when fires were counted, confirming fire’s outsized role in flipping the sign of the budget.

“The permafrost region becomes CO2 neutral,” wrote Virkkala, explaining that once fire emissions are added, the permafrost region is roughly carbon neutral. 

Winter releases hidden carbon

Summer growth pulls CO2 from the air, but the colder months often give it back. A 2019 report estimated the northern permafrost region loses about 1,662 billion kilograms of carbon each winter, which can outweigh growing season uptake in many places.

This winter loss is not a minor side note. It helps explain why some landscapes that look green and productive in July still end up as carbon sources over a full year.

Warmer shoulder seasons expand the period when microbes stay active in thawed soils. That lengthens the window for respiration, pushing annual budgets toward the positive side.

Thawing wetlands fuel methane

Carbon dioxide is not the only concern in the Arctic. Thawing permafrost and wetlands also release methane, a greenhouse gas more than 25 times as effective as CO2 at trapping heat over a century.

In some boreal wetlands, methane output has already increased with warming soils and longer thaw periods.

Studies using satellite data and ground stations suggest methane emissions from northern wetlands have risen over recent decades.

A 2024 analysis showed that warming and wetter summers in Arctic landscapes amplify methane release, adding another feedback that reinforces the climate signal.

Field data meets satellites

The researchers used a standardized monthly database called ABCflux that gathers on-the-ground measurements of carbon flows. Those include net ecosystem exchange (NEE), which is the balance between plant uptake and all ecosystem release.

They also separated the two big halves of that balance. Gross primary productivity is plant uptake through photosynthesis, and ecosystem respiration is the release from plants and microbes.

By comparing these with satellite indicators like the normalized difference vegetation index (NDVI), they could scale site data across the whole region.

Machine learning helped fill gaps where field towers are sparse. That approach improves regional estimates, though the team notes larger uncertainty in parts of Siberia that lack monitoring.

Permafrost holds vast carbon

Regions without trees, especially open tundra, appear to be shifting fastest. The study’s maps show many northern sites with persistent non-summer emissions that defeat short summers.

Boreal forests still act as sinks on average. Even so, extreme fire years can erase that benefit and leave a regional source for that year.

The scale of carbon stored in frozen soils raises the stakes. Northern permafrost holds hundreds of billions of tons of organic carbon, a large share of the world’s below-ground stock.

Monitoring Arctic carbon cycles

Monitoring needs to expand in places that are hard to reach. The study emphasizes the value of long running sites that capture full annual cycles, not just the summer peak.

Fires are changing with climate. Years with widespread burning in the Arctic can push the carbon budget over the line, so tracking fire seasons and recovery is essential for an accurate picture.

The mix of warmer soils, longer thaw seasons, and more disturbances will decide whether the northern lands help slow warming or speed it up. Better coverage in data-poor regions will make those calls more certain.

The study is published in the journal Nature Climate Change.

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