| The permafrost soils of the northern high latitudes are estimated to contain 1700 Pg of
carbon, most of it sequestered below the active layer in perennially frozen soils. This pool of
carbon has been incorporated into the UVic Earth System Climate Model (ESCM) by
prescribing historically permanently frozen soil layers with a uniform permafrost carbon
density in the top 3.35 m of soil. When these layers thaw the permafrost carbon within
them is transferred to the active soil carbon pool and the carbon is subjected to
heterotrophic soil respiration. The UVic ESCM is forced under four emissions pathways
diagnosed from representative concentration pathways 2.6, 4.5, 6.0, and 8.5. An
uncertainty envelope for the likely strength of the permafrost carbon-climate feedback is
established by varying permafrost carbon density between 16 and 26 kg m-3 and
varying the equilibrium climate sensitivity (to a doubling of CO2) of the model
between 2 and 4.5 °C. The strength of the permafrost carbon-climate feedback is
estimated to be 0.25 (0.1 to 0.7) °C (relative to control runs with no permafrost
carbon) by the end of the 21st century regardless of emission scenario followed.
By the end of the 23rd century the strength of the feedback diverges by emission
pathway. Notably the upper bound of the feedback strength (in terms of additional
warming) is highest for the two emission pathways with the lowest cumulative
anthropogenic CO2 emissions. This counterintuitive result is linked to the lower
radiative efficiency of a unit of CO2 at higher atmospheric CO2 concentrations.
That is, the CO2 released by the permafrost has a greater ability to warm the Earth
under scenarios where there is less CO2 already in the atmosphere. If these model
simulations are accurate humanity may have already set into motion a positive climate
system feedback beyond our ability to mitigate with reductions in carbon emissions. |