Thawing permafrost raised carbon dioxide levels after the last ice age, study shows

Carbon dioxide levels in the atmosphere vary naturally between ice ages and interglacial periods. A new study by researchers at the University of Gothenburg shows that an unexpectedly large proportion of carbon dioxide emissions after the ice age may have come from thawing permafrost.

For a long time, it was the shifts between ice ages and interglacial periods that determined how much carbon dioxide was in the atmosphere. During ice ages, CO₂ levels fell, only to rise by around 100 ppm (parts per million) during interglacial periods.

Previously, the main reason for this was thought to be that warmer and more mixed oceans cannot store as much carbon and therefore release it into the atmosphere between ice ages.

However, new research from the University of Gothenburg shows that thawing permafrost may have accounted for a significant proportion of carbon dioxide emissions. The work is published in the journal Science Advances.

“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. We estimate that this carbon exchange may have accounted for almost half of the rising carbon dioxide levels in the atmosphere,” says Amelie Lindgren, researcher in ecosystem science at the University of Gothenburg.

Carbon froze into the ground

Researchers believe that large amounts of carbon were stored during the Ice Age when grass and other plants simply froze into the ground, with wind-borne rock dust settling on top. Such deposits, known as “loess,” are created during ice ages and can reach tens of meters in thickness. They are found across large areas of Europe and Asia, but also in North America.

Permafrost is required for the trapping of organic material in these deposits, and even normal soil with permafrost contains more organic carbon than unfrozen soil because the cold slows decomposition rates.

Pollen analyses

By combining analyses of pollen from the last 21,000 years with climate data from models, the researchers have been able to estimate the types of vegetation that existed in different places throughout history.

“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,” says Lindgren.

Around 21,000 years ago, the continental ice sheets reached their maximum extent in the Northern Hemisphere. The whole of Scandinavia and what is now Canada were covered by ice at that time, and permafrost prevailed in large parts of Siberia, China and parts of central Europe.

During the period 17,000–11,000 years ago, it became warmer. This led to the thawing of the permafrost, which resulted in increased emissions of CO₂ from the ground to the atmosphere.

Natural variation

Previous analyses of ice cores show that the carbon dioxide content in the atmosphere rose as follows:

• 180 ppm (parts per million) CO₂ 21,000 years ago, when the ice age reached its peak.
• 270 ppm CO₂ 11,000 years ago, during a normal interglacial period.

According to researchers, this is a natural variation between ice ages and interglacial periods. However, despite the shrinking ice sheet and continued thawing of new areas of permafrost, the carbon dioxide content did not rise much more after that.

“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,” says Lindgren.

Humans disrupt the carbon cycle

However, over the past 250 years, humans have disrupted the natural carbon cycle by burning large amounts of fossil carbon, mainly coal and oil. Since the Industrial Revolution in the 19th century, the carbon dioxide content in the atmosphere has increased from 280 ppm 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. 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,” says Lindgren.