Vast quantities of carbon remain locked in modern permafrost regions.
Long before industry altered the sky, the frozen earth itself was already a protagonist in the climate story. New research from the University of Gothenburg reveals that thawing permafrost — not the oceans alone — may have supplied nearly half of the atmospheric carbon dioxide rise that followed the last Ice Age, as ancient organic matter locked in frozen northern soils decomposed and exhaled millennia of stored carbon. Peatlands later absorbed much of that release, holding the atmosphere in a fragile equilibrium for thousands of years. Now, as human-driven warming thaws permafrost once more, scientists warn that the natural counterbalance which saved us then may not exist in the world we are making.
- A University of Gothenburg study overturns the long-held assumption that oceans were the primary engine of post-Ice Age CO2 rise, placing thawing permafrost at the center of the story instead.
- More than 300 billion metric tons of ancient carbon escaped frozen northern soils between 17,000 and 11,000 years ago, accounting for roughly 52 of the 90 parts-per-million rise in atmospheric CO2 during that transition.
- Peatlands quietly rescued the climate afterward, accumulating hundreds of billions of tons of carbon across the Holocene and stabilizing atmospheric concentrations for millennia — a natural correction that bought civilization its stable climate window.
- Today's permafrost thaw is accelerating under human-caused warming, but the compensating landscapes that absorbed carbon last time — new land freed by retreating ice sheets — will not reappear; rising seas will shrink available land instead.
- Climate models are now being revised to incorporate these findings, as scientists recognize that frozen soils are not passive bystanders but active amplifiers capable of delivering sudden pulses of greenhouse gas into the atmosphere.
Twenty-one thousand years ago, the Northern Hemisphere lay locked beneath vast ice sheets and permafrost so deep that organic matter accumulated in frozen soil for centuries without decomposing. Microbes could not work in the cold. Carbon that should have cycled back into the air stayed buried underground, building into an enormous subterranean reservoir across the frozen north.
For decades, scientists credited the oceans with driving the atmospheric CO2 rise that followed the last Ice Age — warmer water simply holds less dissolved gas. But a team at the University of Gothenburg, publishing in Science Advances, now argues that thawing permafrost was nearly an equal contributor. As temperatures climbed between roughly 17,000 and 11,000 years ago, frozen ground across the Northern Hemisphere began releasing its ancient stores. The researchers estimate that northern land areas shed more than 300 billion metric tons of carbon during this period, contributing approximately 52 of the 90 parts-per-million rise in atmospheric CO2 recorded in ice cores — a far larger share than land had previously been given credit for.
The team reconstructed this history by combining pollen records from ancient sediments with climate model data. Pollen grains preserve a fingerprint of past vegetation, and shifting plant communities signal shifting carbon storage. Watching tundra give way to grassland and then to forest across 21,000 years of snapshots, the researchers traced how each ecological transition altered how much carbon the soil could hold. The numbers confirmed that frozen ground was not a passive observer in Earth's climate — it actively amplified warming as it thawed.
Yet the story did not end in runaway release. During the Holocene, peatlands expanded across northern regions, their waterlogged conditions slowing decomposition and allowing dead plant matter to accumulate over millennia. These ecosystems absorbed hundreds of billions of tons of carbon, offsetting much of what permafrost had earlier exhaled and holding atmospheric CO2 relatively stable for thousands of years. Nature found its equilibrium.
The sobering implication is that this equilibrium may not be available to us now. Human activity has pushed atmospheric CO2 from roughly 280 parts per million at the start of the Industrial Revolution to about 420 parts per million today, while simultaneously warming the permafrost regions once more. Last time, retreating ice sheets exposed new land where peatlands could grow and carbon could be stored. This time, sea level rise will reduce available land rather than expand it. Lead researcher Amelie Lindgren noted plainly that it is difficult to see where the carbon released from thawing permafrost will go. The findings push scientists to build more realistic carbon estimates into climate models — and remind society that vast quantities of ancient carbon remain frozen in the ground, waiting on the temperature.
Twenty-one thousand years ago, the Northern Hemisphere was a different planet. Ice sheets the size of continents buried Scandinavia and Canada. Siberia, China, and central Europe lay locked beneath permafrost so deep and permanent that the ground itself became a vault. Plants died and fell. Grasses withered. Organic matter accumulated in the soil year after year, century after century, but barely decomposed. The cold was too complete. Microbes could not work. Carbon that should have returned to the air stayed trapped underground, layer upon layer, building into a reservoir of ancient carbon spread across the frozen north.
For decades, climate scientists believed they understood where that carbon went when the world warmed again. As temperatures climbed and glaciers retreated, the oceans released their stored carbon back into the atmosphere. Warmer water holds less dissolved gas than cold water. The math was straightforward. The oceans, scientists thought, were the primary engine of change.
But a team at the University of Gothenburg has now proposed a different answer to one of Earth's oldest climate mysteries. Their research, published in Science Advances, suggests that thawing permafrost—not the oceans alone—may have driven nearly half of the atmospheric carbon dioxide increase that followed the last Ice Age. Between roughly 17,000 and 11,000 years ago, as the climate warmed substantially, frozen ground across the Northern Hemisphere began to thaw. The organic matter buried for millennia started to decompose. Carbon that had been locked away since before human civilization entered the atmosphere as carbon dioxide. The researchers estimate that northern land areas released more than 300 petagrams of carbon during this period—that is, more than 300 billion metric tons. By 11,000 years ago, northern terrestrial carbon storage had fallen by more than 300 billion metric tons compared with Ice Age levels.
To reach these conclusions, the research team combined pollen records preserved in ancient sediments with climate model data. Pollen grains are like fingerprints of the past. Different plants produce unique signatures. By examining pollen layers, scientists can reconstruct what vegetation dominated a landscape thousands of years ago, and from that, estimate how much carbon the soil held. The team took snapshots every thousand years across 21,000 years of history, watching as tundra gave way to grassland, grassland to shrub, shrub to forest. Each shift in vegetation meant a shift in how much carbon the ground could store. The numbers they calculated were striking. The study estimates that cumulative land-based carbon losses may have contributed roughly 52 parts per million of atmospheric carbon dioxide increase. Atmospheric concentrations rose by about 90 parts per million during the broader transition from glacial to interglacial conditions. That means northern soils supplied more than half the total increase—a far larger share than scientists had previously attributed to land.
Ice core records confirm the basic timeline. At the height of the last Ice Age around 21,000 years ago, atmospheric carbon dioxide measured approximately 180 parts per million. By about 11,000 years ago, concentrations had climbed to roughly 270 parts per million. The study helps explain several rapid spikes in greenhouse gases recorded in those ancient ice cores. During periods of abrupt warming, thawing permafrost may have delivered large pulses of ancient carbon into the atmosphere, contributing to swift climate shifts. The frozen ground was not a passive observer in Earth's climate story. It was an active participant, amplifying warming as temperatures rose.
Yet the story did not end with carbon release. After the major thawing phase subsided, another natural process emerged that gradually helped restore balance. Peatlands expanded across many northern regions during the Holocene, the current warm period that began about 12,000 years ago. Peatlands form when waterlogged conditions slow decomposition, allowing dead plant material to accumulate over thousands of years. These ecosystems are remarkably effective at storing carbon. According to the study, peatlands accumulated hundreds of billions of tons of carbon during the Holocene. Their growth offset much of the carbon released earlier from thawing frozen ground. This natural balancing mechanism helps explain why atmospheric carbon dioxide remained relatively stable for thousands of years after the initial post-Ice Age rise. Nature found equilibrium.
But the implications of this ancient history extend directly into the present, and they are sobering. Human activity has dramatically altered the carbon cycle over the last 250 years. Since the Industrial Revolution, atmospheric carbon dioxide has increased from about 280 parts per million to roughly 420 parts per million today. At the same time, permafrost regions are warming again. The difference between then and now is crucial. Ancient warming created new landscapes where carbon could accumulate—peatlands expanded, ice sheets retreated, new land became available. Future warming may not provide the same opportunities. Sea level rise will reduce available land for carbon sequestration. The carbon that will be released from thawing permafrost has nowhere new to go. Amelie Lindgren, the lead researcher, put it plainly: there are extremely high levels of carbon dioxide in the atmosphere right now, and the permafrost is thawing as temperatures rise. What helped humanity the last time the permafrost decreased was increased carbon storage in peatlands and new land areas becoming available when the continental ice sheets retreated. In the future, we will have less land due to sea level rise, and it is difficult to see where we will store the carbon that will be released. The study improves scientists' understanding of how land ecosystems influence atmospheric carbon dioxide over long periods. It suggests that thawing permafrost is not merely a consequence of warming but can also become a powerful source of greenhouse gas emissions. The findings will help researchers improve climate models by incorporating more realistic estimates of carbon stored in frozen soils. Better models can lead to more accurate projections of future climate change. For society, the research serves as a reminder that vast quantities of carbon remain locked in modern permafrost regions. As temperatures continue to rise, some of that carbon could enter the atmosphere, creating additional challenges for climate mitigation efforts.
Notable Quotes
Land north of the Tropic of Cancer emitted a lot of carbon when the average temperature rose in the northern hemisphere after our last ice age. This carbon exchange may have accounted for almost half of the rising carbon dioxide levels in the atmosphere.— Amelie Lindgren, researcher in ecosystem science at the University of Gothenburg
There are extremely high levels of carbon dioxide in the atmosphere right now, and the permafrost is thawing as temperatures rise. In the future, we will have less land due to sea level rise, and it is difficult to see where we will store the carbon that will be released.— Amelie Lindgren
The Hearth Conversation Another angle on the story
Why did scientists miss this for so long? The permafrost explanation seems obvious in hindsight.
It wasn't obvious because the oceans are genuinely important—they do release carbon as they warm. The permafrost story was harder to see because it requires reconstructing ancient vegetation from pollen grains and then modeling how much carbon different plant communities stored. You need both the paleobotany and the climate modeling to work together. For decades, the ocean explanation was sufficient enough that nobody pushed harder on the land side.
So the permafrost released 300 billion metric tons of carbon. That's a real number from 17,000 to 11,000 years ago. What does that mean in terms we can grasp?
It means that as the climate warmed, the ground itself became a source of greenhouse gas emissions—not because anyone was burning anything, but because decomposition finally became possible again. The microbes woke up. The carbon that had been frozen and inert for millennia started to oxidize and escape into the air. It was a feedback loop: warming caused thawing, thawing released carbon, released carbon caused more warming.
And then peatlands saved the day?
Not saved exactly, but balanced it. Peatlands accumulated hundreds of billions of tons of carbon over the next several thousand years. They're incredibly efficient at storing carbon because waterlogged conditions prevent decomposition. So the atmosphere stabilized. But that only worked because there was new land available—the ice sheets had retreated, creating space for these ecosystems to expand. That's the crucial difference with today.
Because we don't have new land to work with.
Right. Sea level rise will actually reduce available land. And the permafrost is thawing again, but this time there's nowhere for the carbon to be reabsorbed at the same scale. We're releasing ancient carbon without the natural mechanism that balanced it before.
Is the research saying we're doomed?
No. It's saying we need to understand what we're dealing with. The study improves climate models. Better models mean better predictions. And it highlights the importance of protecting peatlands and other natural carbon sinks—they may be more valuable than we realized. But it's honest about the asymmetry: ancient warming created new opportunities for carbon storage. Modern warming is destroying them.