It is said that the loss of the Siberian permafrost, uncovering land, would release large amounts of greenhouse gases to the atmosphere. How does this happen, what’s our best guess on its impact, and which uncertainties are involved?
Permafrost means perennially frozen ground. Approximately 20% of the Northern Hemisphere land surface is underlain by permafrost, including large tundra areas in Siberia. These areas are highly relevant to climate change, since over thousands of years they have stored vast amounts of organic carbon, presently containing roughly twice the amount in the atmosphere. As a result of warming-induced thawing of permafrost soils, a fraction of this storage may be released to the atmosphere as methane (CH4) and carbon dioxide (CO2), which are the key greenhouse gases (GHGs).
The atmosphere has warmed faster within the Arctic than elsewhere, and permafrost temperatures have also increased over the last decades. Additional GHG emissions would warm the atmosphere further, thus creating a self-enforcing cycle, i.e. positive feedback to climate change. The processes involved are complex, and our knowledge depends on the observations that do not cover uniformly the vast Arctic and are rather limited in time.
Thawing permafrost gradually exposes organic carbon compounds to decomposition by microbes, resulting in production of GHGs. Methane is produced in environments where oxygen is unavailable, and it may be partly oxidized to CO2 before entering the atmosphere. CH4 emissions increase as a direct response to increasing temperature, as the active soil layer that thaws every summer deepens, but so does the compensating CO2 uptake by plants. A key question is how much of the ‘old’ carbon from the newly thawed layer will mobilize.
Thawing may also occur abruptly when ground ice melts collapsing the land surface locally. This process leads to formation of the so-called thermokarst landforms. These affect surface water flows, which may amplify thawing. Very high CH4 emissions have been observed from thermokarst lakes and wetlands formed with abrupt thaw. In addition, disintegration of permafrost has been observed to facilitate escape of the geologic CH4 trapped in natural gas and hydrate deposits. Recent observations indicate that also the large nitrogen storage in the permafrost may be vulnerable to both gradual and abrupt thaw, resulting in the release of nitrous oxide (N2O), another potent GHG.
It is estimated that roughly 10% of the organic carbon stored within the northern permafrost zone may be released to the atmosphere by 2100 due to Arctic warming. There are understandably large uncertainties in the magnitude and timing of these emissions, and enhanced plant uptake of CO2 offsets part of them. A recent modelling study concluded that the northern permafrost area could act as a net carbon sink if climate change is mitigated substantially. Under less aggressive mitigation efforts, the region will likely act as a carbon source to the atmosphere. However, large net losses of ecosystem carbon are not expected to occur until after 2100.
Schuur et al. (2015) Climate change and the permafrost feedback. Nature 520, 171–179. https://www.nature.com/articles/nature14338