The arctic permafrost stores large amounts of carbon – more than the entire atmosphere. Using sediment studies, researchers have now reconstructed three major global warmings over the past 27,000 years. As they were able to prove, in the past warming of just a few degrees was enough to melt large parts of the permafrost and release large amounts of greenhouse gases. The greenhouse gases from the permafrost, in turn, accelerated further global warming – a scenario that threatens again in the near future in view of man-made climate change.
The permafrost in the Arctic remains frozen all year round. Some deep layers of the soil have not been thawed for millennia. As a result, the organic material stored in it – remains of animals and plants – has been saved from decomposition. However, when the permafrost thaws as a result of global warming, microorganisms begin to decompose the organic material. The carbon it contains is released into the atmosphere within a short time in the form of the greenhouse gases carbon dioxide (CO2) and methane (CH4). These in turn help the atmosphere to continue to heat up and melt more permafrost, which releases more greenhouse gases – there is a dangerous positive feedback.
Look into the past for future forecasts
A team headed by Jannik Martens from Stockholm University has now used sediment studies to show that such events have occurred several times in the past. To do this, the researchers examined an eight-meter-long sediment core from Siberia, which was recovered from the seabed more than 1000 meters below the surface of the Arctic Ocean during an expedition in 2014. This enabled them to take a look back at the causes and effects of global warming towards the end of the last ice age.
“For the first time, our study shows the full story of how warming at the end of the last ice age triggered the thawing of permafrost in Siberia. This also indicates the release of large amounts of greenhouse gases, ”describes Martens. “It is probable that the thawing of the permafrost during global warming around 14,700 and 11,700 years ago is partly related to the increase in CO2 concentrations that can be observed in the Antarctic ice cores for these times. It seems that warming the Arctic by just a few degrees Celsius is enough to destroy large areas covered by permafrost and potentially affect the climate. “
Acceleration of coastal erosion
With the help of the radiocarbon method and the analysis of molecular biomarkers, the researchers were able to date organic remains from the various layers of the sediment core and determine whether the material originally came from the sea or from the land. “From this core we also learned that the erosion of the permafrost coasts was an important driving force for the destruction of the permafrost at the end of the last ice age,” says Martens colleague Örjan Gustafsson. “Coastal erosion continues to this day, albeit ten times more slowly than in this earlier warming phase. With the recent warming trends, however, we are again seeing an acceleration in coastal erosion in some parts of the Arctic, which is expected to release greenhouse gases as the organic matter contained therein breaks down. “
On the basis of their investigations, the researchers warn of the massive effects of the resulting positive feedback. “Every release from the thawing permafrost means that there is even less space in the household of the earth’s climate system for the release of anthropogenic greenhouse gases before dangerous threshold values are reached,” says Gustafsson. “The only way to limit the release of greenhouse gases from permafrost is to mitigate global warming by reducing anthropogenic greenhouse gas emissions.”
Source: Jannik Martens (Stockholm University) et al., Science Advances, doi: 10.1126 / sciadv.abb6546