Permafrost carbon feedback represents a very slow, but irreversible climatic tipping point. Permafrost will thaw slowly over many years, but once it thaws, you cannot refreeze it, writes KEVIN SCHAEFER.

KEVIN SCHAEFER is a research scientist at the National Snow and Ice Data Center (NSIDC), University of Colorado. This article appeared in The Circle 04.15.
PERMAFROST is perennially frozen ground remaining at or below 0°C for at least two consecutive years. Regions with extensive permafrost occupy about 24% of the land area in the Northern Hemisphere. The active layer is the surface layer of soil above the permafrost that thaws each summer and refreezes each winter. The thickness of the permafrost layer depends upon a delicate balance between freezing from surface due to cold winter temperatures and warming from the Earth’s molten interior. Permafrost is thickest along the Arctic coastline where temperatures are coldest, extending down to depths as great as 1500 meters. Air temperatures increase southward from the Arctic Ocean and the thickness of the permafrost layer becomes progressively thinner, eventually disappearing altogether at latitudes between 50 and 60 degrees north.
The effects of warming temperatures due to global climate change have begun to thaw the permafrost. The effects of climate change are especially strong north of the Arctic Circle, where the warming rate is roughly double the global average. The rising temperatures have caused permafrost to disappear entirely in some regions, moving the southern boundary of the permafrost domain northward. The active layer thaws deeper each year as summer temperatures rise. The temperatures within the permafrost layer itself remain below freezing, but are rising at rates as high as 1°C per decade. These current temperature increases are truly alarming considering that permafrost can take hundreds of years to respond to variations in climate such as the little ice age 400 years ago.
Buildings, roads, and other infrastructure will be damaged or destroyed as permafrost continues to thaw. Ice within permafrost binds soil particles together like cement. Permafrost is hard, dense, and erosion resistant, but if the permafrost thaws, the ice turns to water and the permafrost turns to mud, destabilizing and collapsing buildings with remarkable rapidity. Retreating sea ice has increased wave intensity, resulting in rapid coastal erosion. Indeed, several villages have already been moved because the coast has simply eroded away. Climate change is affecting permafrost, but thawing permafrost will also affect the global climate. Organic matter frozen in permafrost contains enough carbon to easily double the carbon dioxide concentration in the atmosphere. Since the end of the last ice age about 15,000 years ago, this frozen carbon was buried by sedimentation and other processes. The soil depth increased as sediment built up, but the surface thaw depth stayed constant such that organic matter at the bottom of the active layer became frozen into the permafrost. The organic matter will remain stable as long as the permafrost remains frozen, but, like broccoli removed from a freezer, once the organic matter thaws it will decay and release carbon dioxide and methane into the atmosphere. Once released into the atmosphere, this carbon dioxide and methane will amplify warming due to the burning of fossil fuels in a process called the permafrost carbon feedback.
For the ‘business as usual’ scenario where we continue to burn fossil fuels at current rates or higher, thawing permafrost will release ~120 gigatons of carbon by 2100 (a gigaton is a unit of explosive force equal to one billion tons of trinitrotoluene or TNT), equivalent to ~5.7% of anthropogenic emissions. This will increase global temperatures by 0.29 degrees centigrade above the 7-8 degrees expected due to the burning of fossil fuels. Half of the total emissions will occur after 2100 with a total of ~240 gigatons by 2300 resulting in a global temperature increase of ~0.6 degrees centigrade.
The permafrost carbon feedback will complicate the negotiation of the climate change treaty. The international community is currently negotiating a treaty to stop global climate change based on a target of 2 degrees centigrade global warming above preindustrial levels. If we reduce fossil fuel emissions to hit the 2 degree centigrade warming target, the rate of permafrost thaw and associated emissions will go down to ~60 gigatons by 2100 with an additional global warming of ~0.1 degrees. Again, half of the emissions will occur after 2100 with a total of ~120 gigatons by 2300 resulting in a global temperature increase of ~0.2 degrees centigrade. While this is small compared to fossil fuel emissions, if the international climate change treaty does not account for emissions from thawing permafrost, we will overshoot our 2 degree warming target.
The permafrost carbon feedback represents a very slow, but irreversible climatic tipping point. Permafrost will thaw slowly over many years, but once it thaws, you cannot refreeze it. The decay of the thawed organic matter occurs slowly over hundreds of years because the Arctic soils will still be fairly cold and wet. However, once the organic matter decays away, there is no way on human time scales to put it back in the permafrost. In essence, once the permafrost carbon feedback starts, it will persist for centuries.