Arctic tundra that locked away carbon dioxide in frozen soil for thousands of years is now releasing it back into the atmosphere, according to NOAA’s 2024 Arctic Report Card. The shift from carbon sink to carbon source is driven by a chain of reinforcing processes: warming accelerates plant growth, which in turn alters fire regimes, darkens snow-covered ground, and destabilizes permafrost. Together, these feedbacks threaten to amplify the very warming that set them in motion.
Decades of Satellite Data Confirm a Greener Arctic
The greening signal is not subtle, and it is not new. NOAA’s tundra indicator tracks vegetation change across the circumpolar region using Normalized Difference Vegetation Index (NDVI) trends from four independent satellite records: MODIS (2000 to 2024), GIMMS (1982 to 2023), Landsat Collection 2, and VIIRS. All four show a persistent upward trend in plant cover, meaning the pattern holds regardless of which instrument or time window scientists examine. This convergence across datasets gives researchers high confidence that the Arctic landscape is, on average, becoming denser and more productive during the growing season.
A peer-reviewed study in Nature Communications put finer numbers on the trend by analyzing pan-Arctic tundra at 30-meter resolution using Landsat time series from 1985 to 2016. That analysis found greening at a clear majority of monitored sites, with the increase correlating strongly to summer air temperature, soil temperature, and soil moisture. Browning occurred at a smaller share of sites, often tied to localized disturbances such as insect outbreaks, flooding, or severe fire. The takeaway is straightforward: warmer, somewhat wetter summers are pushing shrubs and grasses into terrain that was, until recently, too cold or too dry to support them, reshaping habitats for wildlife and altering how snow accumulates and melts.
Wildfire and Shrub Growth Lock Into a Self-Reinforcing Loop
Greener tundra is not just a passive response to warmth; it is also reshaping how the Arctic burns. In Alaska, high-severity wildfire is actively changing what grows back and how flammable the landscape becomes. Research led by Dong Chen of the University of Maryland, College Park and published in Nature Plants documented a fire, greening positive feedback loop: severe burns clear existing vegetation and expose mineral soils, after which dense shrubs colonize the scorched ground. Those shrubs carry more biomass, intercept more snow, and tend to dry out faster in summer, raising the odds of another intense fire. Landsat-based NDVI trajectories in the study showed that post-fire greening was strongest where burn severity was highest, meaning the worst fires produce the most flammable regrowth and set the stage for future large blazes.
Separate modeling work published in Nature Communications projects that permafrost thaw could trigger a nonlinear jump in Arctic and subarctic wildfire activity. As frozen ground degrades, soils can drain and dry, and previously frozen organic layers become available as fuel. The fires that follow release stored carbon, which drives additional warming and further thaw. This is not a simple, gradual escalation: the models suggest threshold behavior, where modest additional warming can produce abrupt increases in burned area once key soil and vegetation conditions are met. The 2023 Canadian wildfire season, which produced exceptional smoke and carbon emissions, offered an unsettling preview that boreal and tundra fire regimes may already be edging toward these modeled tipping points.
Why More Green Does Not Mean More Carbon Storage
A common assumption holds that more vegetation should mean more carbon pulled from the air. In the Arctic, that logic breaks down in at least two ways. First, a synthesis of multi-decadal CO₂ flux measurements across northern permafrost ecosystems, published in Nature Climate Change, found that increased summer carbon uptake by growing plants is offset by rising respiration during the non-growing season. Microbes in warming soils stay active longer into autumn and winter, decomposing organic matter and releasing CO₂ even as snow covers the ground. The analysis concluded that, when autumn and winter losses are fully accounted for, many tundra landscapes are already close to carbon neutral or slightly net-emitting over the course of a year despite visible greening.
Second, denser shrub and tree cover in snowy high latitudes darkens the surface. Snow reflects a large share of incoming solar radiation, but vegetation poking through it absorbs heat instead. A foundational study in Nature established that this albedo reduction can offset some of the carbon-sequestration benefit of northern forestation in radiative forcing terms, effectively turning some “green gains” into additional warming. Observational work led by SDSU biologist Donatella Zona, summarized in the Arctic Report Card, added another wrinkle: earlier spring green-up in the Arctic is associated with greater CO₂ release rather than stronger uptake, partly because earlier snowmelt exposes soils sooner and causes plants to age and senesce earlier. The net carbon math, in other words, is far less favorable than satellite greenness alone would suggest, especially once changes in surface brightness and season length are factored in.
Permafrost Thaw Turns the Tundra Into an Emitter
Ground-level measurements confirm what the models predict about thawing ground. A 15-year eddy covariance record from a tundra site underlain by degrading permafrost showed persistent net carbon emissions to the atmosphere across the entire monitoring period. That is not a seasonal blip or a response to a single warm year; it is a sustained shift in the carbon balance of a landscape that once stored more carbon than it released. As ice-rich permafrost thaws, the ground subsides into hummocks and ponds, exposing previously frozen organic matter to oxygen and microbial decomposition. The resulting CO₂ and methane emissions add to atmospheric greenhouse gas concentrations and are effectively irreversible on human timescales.
The 2024 Arctic Report Card noted that permafrost temperatures at multiple Alaska monitoring sites reached their highest levels on record, while gridded flux data from NASA’s ABoVE program indicated the pan-Arctic was roughly CO₂-neutral from 2001 to 2020. Neutrality, though, masks a tug-of-war: boreal forests still absorb substantial carbon in summer, but tundra regions are tipping toward net release as thaw progresses and shoulder seasons lengthen. NOAA framed the stakes bluntly in its December 2024 news release, stating that Arctic tundra is becoming a source of carbon dioxide emissions rather than a sink, and warning that continued warming could unlock a portion of the vast permafrost carbon pool that rivals centuries of current human emissions.
Feedbacks Reach Beyond the Tundra Edge
The consequences of these changes are not confined to the high Arctic. A study of coupled atmosphere–ice–ocean dynamics found that greenhouse gases from human activities are driving more rapid sea-ice loss than would be expected from direct warming alone, in part because of reinforcing feedbacks involving clouds, ocean heat uptake, and atmospheric circulation. As tundra and boreal regions emit more carbon and darken through vegetation change and soot deposition from fires, they feed into this broader Arctic amplification loop that accelerates ice retreat. Loss of reflective sea ice, in turn, allows the ocean to absorb more solar energy, further warming the region and promoting additional permafrost thaw on adjacent coasts.
These intertwined feedbacks complicate efforts to project future climate using simple linear relationships between emissions and temperature. They also raise the risk that, beyond certain thresholds, parts of the Arctic system could begin adding greenhouse gases to the atmosphere even if human emissions start to decline. For policymakers and communities far from the tundra, the implication is that choices made on energy, land use, and adaptation in the next few decades will determine how much of the permafrost carbon pool remains safely frozen. For Arctic residents, who are already grappling with collapsing ground, shifting wildlife, and smoke-filled summers, the emerging science underscores that local environmental changes are now inseparable from the global climate trajectory.
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*This article was researched with the help of AI, with human editors creating the final content.
