Abstract
Permafrost thaw and the subsequent mobilization of carbon (C) stored in previously frozen soil organic matter (SOM) have the potential to be a strong positive feedback to climate. As the northern permafrost region experiences as much as a doubling of the rate of warming as the rest of the Earth, the vast amount of C in permafrost soils is vulnerable to thaw, decomposition and release as atmospheric greenhouse gases. Diagnostic and predictive estimates of high-latitude terrestrial C fluxes vary widely among different models depending on how dynamics in permafrost, and the seasonally thawed ‘active layer’ above it, are represented. Here, we employ a process-based model simulation experiment to assess the net effect of active layer dynamics on this ‘permafrost carbon feedback’ in recent decades, from 1970 to 2006, over the circumpolar domain of continuous and discontinuous permafrost. Over this time period, the model estimates a mean increase of 6.8 cm in active layer thickness across the domain, which exposes a total of 11.6 Pg C of thawed SOM to decomposition. According to our simulation experiment, mobilization of this previously frozen C results in an estimated cumulative net source of 3.7 Pg C to the atmosphere since 1970 directly tied to active layer dynamics. Enhanced decomposition from the newly exposed SOM accounts for the release of both CO2 (4.0 Pg C) and CH4 (0.03 Pg C), but is partially compensated by CO2 uptake (0.3 Pg C) associated with enhanced net primary production of vegetation. This estimated net C transfer to the atmosphere from permafrost thaw represents a significant factor in the overall ecosystem carbon budget of the Pan-Arctic, and a non-trivial additional contribution on top of the combined fossil fuel emissions from the eight Arctic nations over this time period.
Highlights
25% of the northern hemisphere land surface is underlain by perennially frozen ground (Zhang et al 1999).1748-9326/14/045005+12$33.00 c 2014 IOP Publishing LtdThe northern high-latitude permafrost region has the potential for substantial influence on the global climate system through ecosystem feedbacks in energy, water and carbon (C) cycling (McGuire et al 2006)
The imbalance is a result of the sink effect of active layer dynamics on CO2 uptake ( f net primary production (NPP); 0.20 and 0.076 Pg C in the continuous and discontinuous zones, respectively) being outweighed by the source effect on CO2 ( f heterotrophic respiration (HR); 2.9 and 1.2 Pg C) and CH4 ( f CH4; 0.017 and 0.013 Pg C) emissions tied to thawing of the frozen soil C pool from 1970 to 2006
The state and flux of each component of the permafrost carbon feedback vary over time and space across the high-latitude region, reflecting the complex interaction of the key system drivers including climate, geomorphology, hydrology, vegetation dynamics and the physical properties of the permafrost regime
Summary
25% of the northern hemisphere land surface is underlain by perennially frozen ground (Zhang et al 1999).1748-9326/14/045005+12$33.00 c 2014 IOP Publishing LtdThe northern high-latitude permafrost region has the potential for substantial influence on the global climate system through ecosystem feedbacks in energy, water and carbon (C) cycling (McGuire et al 2006). Many coupled climate–carbon models project that the northern high latitudes will serve as a substantial land C sink during the 21st century because both climate warming and elevated global atmospheric CO2 concentration favor increased vegetation productivity and C uptake in the region (Qian et al 2010). These models do not wellrepresent, or have not accounted for, important ecosystem dynamics associated with warming-driven permafrost thaw (Koven et al 2012). These models simulate permafrost thaw as a thickening of the active layer, but estimates of the associated C emissions do not distinguish between C in the active layer and C from the permafrost below
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