Permafrost affects roughly a quarter of the landmass in the Northern Hemisphere and stores large quantities of organic carbon in the form of dead plant matter. As long as it remains frozen, this carbon is fixed in the ground – but when permafrost thaws, microorganisms begin breaking it down, releasing carbon into the atmosphere as carbon dioxide and methane. Accordingly, rising temperatures worldwide could activate this massive reservoir and substantially worsen climate change through additional greenhouse gas emissions. Consequently, in the public debate you will frequently encounter the idea of a “ticking carbon timebomb”. This is based on the assumption that the permafrost, like the Greenland Ice Sheet, is one of several tipping elements in the Earth system. In this view, permafrost will initially experience only gradual thawing in response to global warming; then, once a critical threshold value is surpassed, the thawing processes will suddenly begin amplifying one another, leading to the rapid and irreversible collapse of permafrost across the Arctic. Though many have speculated that this type of thawing scenario is possible, to date it has remained unclear whether there really is any such threshold value, and if so, what the corresponding temperature limit is.
An international research team involving Sebastian Westermann from the University of Oslo and CBA has now gotten to the bottom of this question. Based on available academic literature and their own data analysis the team has assessed all current findings on thawing processes in terms of whether and, if so, on which spatial scale – local, regional, global – they could lead to self-perpetuating thawing and therefore to a ‘tipping’ in connection with a given level of warming.
The results of the study are clear: although there are geological, hydrological and physical processes that are self-reinforcing and in some cases irreversible, these only have local or regional effects. One example is the formation of so-called thermokarst lakes. The ice in the permafrost soils melts and depressions form. The meltwater collects in the depressions, which in turn increases the warming of the permafrost under the water and causes a self-reinforcing thawing process in and around the lake. They also found similar reinforcing feedbacks in other permafrost-relevant processes, such as the loss of boreal coniferous forests due to fire - but here too only at a local to regional level. “There is no evidence of self-reinforcing internal processes that would affect the entire permafrost at a certain degree of global warming and accelerate its thawing globally,” the researchers explain. It is therefore misleading to portray permafrost as a global tipping element.
But this does not mean that there is nothing to worry about in terms of Arctic permafrost - on the contrary, the study clearly shows that the permafrost zone is very heterogeneous. Consequently, numerous small, local tipping points will be crossed at different times and warming levels that will accumulate over time. As a result, the global thawing of permafrost will not be a gradual increase followed by a sudden surge, but will intensify in step with global warming and eventually end with the total loss of permafrost when global warming reaches 5 to 6 degrees Celsius. “This means that more and more regions will inevitably be affected by thawing already now or in the near future,” says the research team. “In other words, there is no safety margin of warming - as the image of a tipping point might suggest - that we can still exploit as long as we do not exceed the threshold. We therefore need to keep a close eye on the permafrost regions through better monitoring, gain a deeper understanding of the processes and incorporate them into climate models in order to further reduce the uncertainties in forecasts. And another thing is clear with regard to permafrost loss and greenhouse gas-induced permafrost loss: the sooner humanity can achieve net zero emissions, the more regions can be preserved as unique habitats and carbon reservoirs.”
Original publication:
Nitzbon, J., Schneider von Deimling, T., Aliyeva, M., Chadburn, S. E., Grosse, G., Laboor, S., Lee, H., Lohmann, G., Steinert, N., Stuenzi, S., Werner, M., Westermann, S., & Langer, M.: No respite from permafrost-thaw impacts in the absence of a global tipping point. Nature Climate Change (2024). https://doi.org/10.1038/s41558-024-02011-4