Beneath vast stretches of the Arctic and sub-Arctic sits ground that has stayed frozen since before the last ice age ended, holding onto plant and animal material that never fully decomposed because the cold stopped the microbes that would normally break it down. As that ground warms and thaws, those microbes wake up and start consuming carbon that has been locked away for thousands of years, releasing it back into the atmosphere as carbon dioxide and methane. Scientists have spent decades trying to pin down how much carbon is at stake and how fast it could come out, because the answer affects how much additional warming the planet can expect on top of whatever comes from human emissions alone.
What Permafrost Actually Stores Underground
Permafrost is defined simply as ground that stays at or below freezing for at least two consecutive years, and it underlies roughly a quarter of the land area in the Northern Hemisphere, according to the overview of permafrost. What makes it climatically significant is not the ice itself but what is frozen inside it: dead plants, animal remains, and other organic matter accumulated over thousands of years that never fully decomposed because the cold halted the microbial activity that breaks organic material down elsewhere. The National Snow and Ice Data Center estimates that permafrost holds nearly a third of all the carbon stored in soil worldwide, an amount roughly twice what currently sits in Earth’s atmosphere. That carbon has been building for hundreds of thousands of years in the oldest of these deposits, accumulating layer by layer as vegetation died, was buried by wind-blown silt or peat, and froze in place before microbes could finish breaking it down. Because permafrost formed over such a long span, its carbon is not evenly distributed: parts of Siberia, Alaska, and northern Canada hold especially carbon-dense, ice-rich ground known to permafrost scientists as yedoma, formed during past glacial periods, which is considered particularly vulnerable once thaw begins because of how much organic material is packed into a comparatively thin layer near the surface.
How a Warming Arctic Thaws Ground That Held for Millennia
The Arctic has been warming several times faster than the global average, and that outsized warming is what has started pushing permafrost temperatures up toward, and in some places past, the freezing point in ground that stayed reliably frozen for thousands of years. Thaw does not happen uniformly; it tends to start at the “active layer” near the surface that already freezes and thaws seasonally, then gradually deepens into ground that has never thawed within human memory. Once that deeper, ancient layer begins to thaw, the organic material trapped inside becomes accessible to microorganisms for what is, in geological terms, the first time since it was buried.
Microbes Turn Ancient Carbon Into Methane and Carbon Dioxide
When permafrost thaws, soil microbes resume the decomposition process that the cold had paused, consuming the organic material and releasing greenhouse gases as a byproduct in much the same way composting releases gas from food scraps, just on a geological timescale. Which gas dominates depends heavily on conditions underground: decomposition in oxygen-rich, well-drained soil tends to produce mostly carbon dioxide, while decomposition in waterlogged, oxygen-poor conditions, common in thawing wetlands and collapsing ground, favors methane, a gas that traps significantly more heat per molecule than carbon dioxide over the short term. Because thawing permafrost frequently creates exactly the kind of soggy, low-oxygen ground that favors methane production, some of the most carbon-dense permafrost regions are also the ones best positioned to generate the more potent of the two gases.
Abrupt Thaw and the Risk of a Faster Release
Much of the concern among permafrost researchers centers on abrupt thaw, a faster and more localized process than the gradual deepening of the active layer, in which ice-rich ground collapses suddenly to form slumping terrain, thermokarst lakes, or landslides that expose thick layers of ancient soil all at once rather than a few inches at a time. That kind of collapse can expose in a single season organic material that gradual thaw would otherwise take decades to reach, and it often creates the wet, low-oxygen conditions that favor methane production specifically. Because abrupt thaw features cover a comparatively small share of the permafrost zone but can release carbon disproportionately fast, they have become a priority for researchers trying to build more accurate models of how much warming the thaw will ultimately add.
Why Scientists Treat This as a Feedback Loop Worth Watching
What worries climate researchers most is not permafrost thaw as an isolated event but as a feedback loop: warming caused mostly by human greenhouse gas emissions thaws permafrost, thawed permafrost releases its own carbon dioxide and methane, and that additional greenhouse gas adds further warming that can thaw still more ground. Research funded through NOAA’s Climate Program Office, conducted with the National Snow and Ice Data Center, estimated that emissions from thawing permafrost could add tens of trillions of dollars in cumulative global economic impacts by the next century if the feedback continues unchecked. That combination of a slow-moving process with a potentially large and self-reinforcing effect is why permafrost has become one of the standard benchmarks climate scientists track alongside more visible signs of Arctic change like sea ice loss and glacier retreat.
This article was produced with the assistance of AI and reviewed by Morning Overview editors.
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