Would thawing permafrost have a greater climate impact than expected?

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Field observations have revealed the presence of a hitherto unknown mechanism by which the thawing of polar soils leads to the release of carbon into the atmosphere. In other words, the alteration of permafrost could have a greater climate impact than expected. The results were recently published in the journal Nature communications.

Permafrost (permafrost in English) refers to the part of a soil that remains frozen for at least two consecutive years. It is found on nearly 17% of land surfaces, most of it located in the far north. This substrate constitutes theone of the major carbon reservoirs on Earth. And for good reason, it contains large amounts of organic matter for the moment preserved from oxidation by freezing. A fragile balance that is increasingly threatened by global warming.

In addition, scientists expect that some of the stored carbon will be released into the atmosphere as carbon dioxide (CO2) or methane (CH4). However, these two powerful greenhouse gases would amplify even more the rise in global temperatures. However, in practice, estimating the amount of carbon that is likely to be effectively removed from storage is a complex task. The processes involved being multiple and often antagonistic.

Thawing permafrost on the Alaskan coast. Credits: NASA.

Between iron and carbon: discovery of a mechanism unexpected

In this context, a study recently uncovered an unexpected mechanism by which melting permafrost causes the release of CO2. As often, it involves the action of activated microorganisms when the soil enters the thaw phase. The novelty is that the decomposition work carried out by microorganisms impairs carbon sequestration in mineral iron formations. Until now, scientists thought that it was sufficiently stabilized by the bonds between iron molecules so as not to be mobilized during the thaw.

“What we are seeing is that the bacteria simply use iron minerals as a food source. As they feed, the bonds that had trapped the carbon are destroyed and it is released into the atmosphere in the form of greenhouse gases ”, details Carsten W. Müller, co-author of the study. “Frozen soil has a high oxygen content, which stabilizes iron minerals and allows carbon to bind to them. But as soon as the ice melts and turns into water, oxygen levels drop and iron becomes unstable. Additionally, melted ice provides access to bacteria. Overall, this is what releases the stored carbon “.

Location and geography of the study area. Credits: Monique S. Patzner & al. 2020.

Thaw permafrost and release of carbon: a phenomenon still difficult to quantify

The work – based on field observations – has so far focused on a specific point in northern Sweden (Abisko). Nevertheless, the authors expect the results to be valid on a much larger scale. Indeed, indirect measurements from other parts of the globe suggest the presence of a similar process. ” It means that we have a significant new source of CO2 emissions which needs to be included in climate models and examined more closely ”, note Carsten W. Müller.

The quantity of carbon that can be mobilized by this mechanism would be equivalent to 5% of that already present in the atmosphere. Does this mean that with global warming, permafrost will release a much larger quantity of greenhouse gases than expected? On this point, the researchers do not go too far. Certainly, other as yet unknown processes must exist and the net effect on carbon flux remains difficult to assess. “The majority of Arctic climate research focuses on the amount of carbon stored and its sensitivity to climate change. We focus much less on the deeper mechanisms that trap carbon in the soil ”, relates the co-author. To be continued !

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