Permafrost thaw and ecosystem carbon cycle under multifactor global change at a tundra site: A modeling approach

Background/Question/Methods

The permafrost region harbors a significant amount of organic carbon that is vulnerable to the predicted most pronounced climate warming this century.  Large uncertainties still are present on how permafrost carbon responds to multiple climate changes (temperature, CO2 concentration and precipitation) due to technical difficulty in field experiments.  This study applied a modeling method to examine the multiple climate changes alone or in combination on permafrost thaw and carbon cycle.  In a tundra site named Eight Mile Lake (EML) at Alaska, the Community Atmosphere Biosphere Land Exchange (CABLE) model was calibrated and validated to simulate thaw depth, gross primary productivity (GPP), ecosystem respiration (Reco) and net ecosystem exchange (NEE) under climate warming, rising atmospheric CO2, and altered precipitation during 2007-2011.

Results/Conclusions

Our results show that (1) on average, warming (2~10°C) increases thaw depth at 3.8- to 2.4-cm per degree C increase in 2009 and 2010, respectively.  Simultaneously, warming increases GPP at a greater rate than Reco, thus leading to a greater NEE (i.e. carbon accretion).  The percentage increase in thaw depth is generally larger than that of GPP and NEE under the same warming treatment; (2) winter warming has a smaller effect on thaw depth and soil temperature but a greater effect on GPP and NEE than summer warming; (3) warming, rising CO2 and decreased precipitation (i.e. snowfall) alone or in combination have greater positive effects on GPP than Reco thus positive effects on NEE; whereas, increased precipitation (i.e. snowfall) reduces Reco; (4) winter warming can largely increase winter time Reco that can potentially offset the GPP increase under warming.  Results suggest that larger responses of permafrost thaw under warming did not necessarily lead to greater ecosystem respiratory losses than gross primary productivity.  The extended growing period is a more important mechanism to contribute to ecosystem carbon gain than increased thawed carbon availability under climate changes.  These results demonstrate the vulnerability of organic C stored in near surface permafrost to increasing temperatures and the strong potential for warming tundra to serve as a positive feedback to global climate change.