Thawing permafrost could tip Earth's climate balance

The air we breathe today carries a story millions of years in the making. Carbon dioxide, a gas central to life and climate, has always risen and fallen with Earth’s rhythm of ice ages and warm interglacial periods.

A new study from the University of Gothenburg reveals that this story may have an unexpected twist. A large share of carbon dioxide released after the last ice age likely came from thawing permafrost rather than oceans alone.

Ancient climate patterns

For centuries, scientists linked carbon dioxide changes mainly to the oceans. During ice ages, CO2 levels dropped, then rose by nearly 100 parts per million as interglacial warmth returned.

Warmer oceans could not hold as much carbon, so they released it. That was the accepted explanation. But new evidence suggests land north of the Tropic of Cancer also played a huge role when the northern hemisphere warmed.

“We have concluded that land north of the Tropic of Cancer, 23.5 degrees north, emitted a lot of carbon when the average temperature rose in the northern hemisphere after our last ice age,” said study lead author Amelie Lindgren.

“We estimate that this carbon exchange may have accounted for almost half of the rising carbon dioxide levels in the atmosphere.”

Permafrost soils and Earth’s climate

During the Ice Age, vast landscapes turned into natural vaults of carbon. Grasses and plants froze into the ground and were buried under thick layers of windblown dust called loess. These deposits stretched across Europe, Asia, and North America, sometimes tens of meters thick.

Permafrost preserved the organic matter within, slowing decomposition and holding more carbon than unfrozen soils could ever store.

To understand this hidden reservoir, the researchers combined pollen records spanning 21,000 years with climate model data. This allowed them to reconstruct vegetation patterns and estimate how much carbon soils stored over time.

“We have chosen to take a snapshot every thousand years. Once we know what type of vegetation prevailed, we can estimate how much carbon were stored in the soil. In this way, we can model how carbon exchange between the soil and the atmosphere has looked since the last ice age,” said Lindgren.

As the ice sheets retreated between 17,000 and 11,000 years ago, northern soils thawed. This sudden release of carbon dioxide added to the atmosphere, shifting the balance of Earth’s climate.

Thawed permafrost and lost carbon

The new study provides numbers that highlight the scale of change. Loess deposits stored about 363 petagrams of carbon at the peak of the last glaciation. After thaw, only about 57 petagrams remain today.

Most losses occurred before 10,000 years ago, representing one of the largest carbon shifts of the period.

At the same time, peatlands spread and became powerful carbon sinks. Over the Holocene, they absorbed about 450 petagrams of carbon – more than any other land system. Their expansion marked the only sustained drawdown of atmospheric carbon in this long timeline.

Ice retreat and seas

The retreat of massive ice sheets changed landscapes in several ways. Soils under glaciers lost carbon, but new ground emerged for vegetation to colonize, creating fresh carbon stores. Rising seas also submerged large continental shelves.

Whether submerged soils released carbon quickly or preserved it as subsea permafrost remains uncertain.

The study estimates that at least some carbon cycled back into the atmosphere during inundation, but much may have been buried in marine sediments.

Natural rise in carbon dioxide

Ice cores confirm the bigger picture. Around 21,000 years ago, carbon dioxide levels hovered at 180 ppm during the peak of the ice age. By 11,000 years ago, they had climbed to 270 ppm, marking the interglacial phase.

Afterward, increases slowed, thanks to peatlands that absorbed significant amounts of carbon and balanced out permafrost emissions.

‘We see that peatlands stored large amounts of carbon during the Holocene. Over time, the uptake in peatlands has actually compensated for the emissions that occurred from the permafrost,” said Lindgren.

Land as source and sink

The researchers show that northern lands acted both as sources and sinks of carbon. Deglacial losses were abrupt, linked to thawing loess and permafrost. Later, peatland growth reversed the flow.

The team’s reconstructions suggest that these land processes were strong enough to shape atmospheric changes, alongside ocean shifts.

The timing of losses and gains created a complex signal in the atmosphere, sometimes masking the full impact of each process.

Humans broke the natural cycle

This natural cycle held steady until humans began altering it. Over the past 250 years, the combustion of coal, oil, and gas has added immense amounts of fossil carbon into the air. Levels have surged from 280 ppm during the Industrial Revolution to 420 ppm today.

“There are extremely high levels of carbon dioxide in the atmosphere right now, and the permafrost is thawing as temperatures rise. What helped us the last time the permafrost decreased was increased carbon storage in peatlands and new land areas becoming available when the continental ice sheets retreated,” said Lindgren.

“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.”

Lessons for tomorrow

The study highlights how fragile Earth’s carbon balance can be. While natural systems once offset permafrost losses, today’s world faces shrinking land and rising seas.

Understanding the past may not provide comfort, but it gives clarity. The carbon locked in frozen soils could again shape our climate, only this time against a backdrop of human-driven warming.

The study is published in the journal Science Advances.

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