One of the most worrisome unknowns about climate change is the behavior of positive feedbacks that amplify warming. The ultimate fear is that we could cross tipping points, where the warming gains a momentum of its own that would continue for a time even if human emissions were drastically reduced.
The shrinking ice and snow cover in the Arctic is one example—white, reflective areas are being replaced by dark land or water that absorbs much more radiation from the Sun. That's a positive feedback on its own but it could also trigger a separate one: the massive store of carbon in Arctic permafrost could become vulnerable as the region thaws.
The thawing of permafrost soils (which cover nearly a quarter of the land in the Northern Hemisphere) has both local and global effects. Locally, buildings and infrastructure can be damaged as the ground softens and shifts. Coastal erosion can accelerate rapidly once the soil is no longer frozen rock-hard, and lakes and wetlands can change dramatically.
The broader impact is that the permafrost contains roughly twice as much carbon as the atmosphere does at present. Much as the food in your freezer will start to rot if you unplug it, all that organic carbon could be pulled back into the carbon cycle as it thaws out. Microbes in the permafrost respire—breathing in oxygen, munching on organic matter, and expelling CO2. That moves some of the carbon back into the atmosphere, where it acts as a greenhouse gas. Of course, that raises temperatures and thaws more permafrost—that nasty positive feedback loop.
Climate modelers have tried to estimate the impact that added carbon would have on global warming. However, existing models weren’t able to link the processes in the permafrost and the climate system together and simulate them both. That made it difficult to truly work out the extent of the feedback. The latest generation of climate models has added that capability. In a study published in Nature Geoscience, researchers from the University of Victoria experimented with their model to estimate how much carbon will be released from permafrost, how fast, and much additional warming that will cause.
The climate model was run using a range of possible values for the amount of carbon in the permafrost and four scenarios for future human emissions of greenhouse gases. In addition, the model was configured to simulate a range of climate sensitivities—the amount of climate warming that results from a given increase in carbon dioxide—by manually altering the amount of heat energy allowed to pass through the top of the atmosphere and into space. This allowed the permafrost feedback to be evaluated at climate sensitivities other than the one the model naturally generates on its own.
The best estimate of warming caused purely by the permafrost carbon feedback by 2100 was about 0.25°C (0.45°F), with a possible range of 0.1 – 0.7°C. This occurs because the carbon released from the thawing permafrost adds between 40 and 100 parts per million to atmospheric CO2 (which is just under 400 parts per million at present).
By the end of the simulation, in 2300, the results varied depending on the emissions scenario used, with best estimates of the permafrost's contribution to warming coming in between 0.37 and 0.73°C (and a possible range of 0.13 – 1.69°C). Even though the amount of carbon released from permafrost is smaller for the low anthropogenic emissions scenarios, it could still pack a big punch when it comes to warming. That’s because the heat-trapping efficiency of each molecule of CO2 drops as the total concentration in the atmosphere increases (the more greenhouse gas molecules you have, the more likely some other one will absorb a photon first).
The model results are also a reminder that current emissions have long-term impacts that can’t easily be undone. Even in the best-case emissions scenario, where atmospheric CO2 peaks around 2050 and is actively removed after that, the model shows that carbon continues to be released from the permafrost over the next three centuries.
Although this model‘s estimate of the amount of carbon released from permafrost by 2100 is considerably higher than previous estimates, the researchers caution that their method “is in a number of ways conservative.” Some of the processes by which more carbon could be released from permafrost (including erosion, thermokarst pockets, and fires) are not simulated by the model. The decay of organic matter is also simulated entirely as respiration to CO2, leaving out the contribution of anaerobic bacteria that produce methane, which is a more potent greenhouse gas. On the other hand, the model also misses the effect of increased nutrient availability on plant growth over the thawing permafrost, which could take up carbon that would otherwise end up in the atmosphere.
Over time, projections of future warming with increasing emissions have become more and more sophisticated. As meaningful societal responses to this knowledge continue to lag behind, we can only hope that the climate system doesn’t cross any of the tipping points the researchers are working to understand.