How Permafrost Thaw Diminishes the Remaining Carbon Budget

The Paris Climate Agreement stipulates that global warming should be limited to well below 2°C above pre-industrial levels. According to recent calculations by the Global Carbon Project, for a 50 percent chance of not exceeding 2°C, a maximum of around 305 gigatons (billion tons) of anthropogenic carbon (equivalent to 1,110 gigatons of carbon dioxide) may be released into the atmosphere in the future, including this year. This figure is, of course, considerably lower for more ambitious temperature goals or higher probabilities to achieve them. However, there are large uncertainties. One poorly constrained process is the release of carbon from thawing permafrost—perennially frozen ground that stores large stocks of carbon. As the climate warms and permafrost thaws, this carbon can be released into the atmosphere as greenhouse gases. Researchers have shown that if this release of permafrost carbon is taken into account, the carbon budget remaining for this century for 2°C warming is reduced by a fifth.

The team, led by Goran Georgievski from the Max Planck Institute for Meteorology (MPI-M), prepared climate projections up to the year 2300 using the Max Planck Institute Earth System Model (MPI-ESM). The researchers compared simulations that take permafrost carbon into account with those that do not, and used the Adaptive Emission Reduction Approach (AERA) algorithm to calculate the anthropogenic carbon emissions compatible with a specific temperature target. These calculations revealed not only the remaining budgets in each case, but also emission pathways which show how quickly emissions need to drop.

Additional carbon from thawing permafrost

The warming of permafrost regions is causing frozen soil to thaw down to deeper layers. The study shows that this will affect around 122 gigatons of carbon by the end of the investigated period if warming reaches 2°C. If it got 3°C warmer, the figure would increase to 229 gigatons. Roughly three-quarters of this thawed permafrost carbon are projected to be emitted to the atmosphere. The rest is decomposed slowly and remains in the soil until the end of the simulation period.