Fundamental Dynamics of the Permafrost Carbon Feedback

Permafrost in the Arctic contains 950-1670 Gt of organic matter, frozen since the last ice age. As permafrost thaws in the 21st century, some of this organic matter will thaw and decay, increasing atmospheric CO2 concentrations and amplifying the warming rate, creating a positive Permafrost Carbon Feedback (PCF) on climate. Using model projections of the terrestrial carbon cycle, we estimate the PCF timing and magnitude. We add permafrost carbon dynamics to the Simple Biosphere/Carnegie-Ames-Stanford Approach (SiBCASA) model and use the ERA40 reanalysis as input weather. To represent future climate change, we scale the ERA40 air temperature assuming a linear, 4 °C century-1 temperature increase in the Arctic between 2000 and 2200. The PCF tipping point occurs when respiration due to the decay of thawed organic matter overpowers enhanced plant uptake due to longer growing seasons and warmer temperatures, changing the Arctic from a carbon sink to a source. The tipping point marks an abrupt change in high latitude carbon balance and signals the start of the PCF. We estimate the PCF magnitude in terms of the total flux of permafrost carbon into the atmosphere and potential changes in atmospheric CO2 concentration. We compare the PCF magnitude to projected fossil fuel emissions, the estimated global terrestrial carbon sink, and other carbon-climate feedbacks. We identify and discuss various sources of uncertainty in the estimated PCF tipping point and magnitude. Preliminary results indicate a PCF tipping point by 2047 and a total strength of 114 Gt C by 2200 (equivalent to a 52 ppm change in atmospheric CO2 concentration).