||Humans are altering the global cycle of carbon (C) on Earth by burning fossil fuels and altering the land surface. The addition of billions of tons of C greenhouse gases to the atmosphere is changing its heat-trapping capacity, which, in turn, is changing the Earth’s climate. While a large proportion of the modern increase in the size of the atmospheric C pool is due to human activities, the future trajectory of the atmosphere also depends, in part, on the response of terrestrial and ocean systems to climate change. Recently, attention has been drawn to permafrost (permanently frozen ground) thaw as a mechanism that could move significant quantities of land C into the atmosphere in response to a changing climate. There are 1672 billion tons of C stored in soils in the northern permafrost zone. This represents more than a third of the soil C stored in terrestrial ecosystems globally, and is several orders of magnitude greater than current annual anthropogenic CO2 emissions. Release of permafrost C to the atmosphere in a warming world may have significant implications for the trajectory of future climate change, but this depends on the vulnerability of the permafrost C pool.
Our overarching question is: Will the response of permafrost ecosystems to warming and thaw cause significant changes in C cycling that can feed back to affect atmospheric CO2 concentrations and future climate? We hypothesize that the transfer of old soil C to the atmosphere will occur as a result of permafrost thaw and the microbial decomposition of soil organic matter. Most importantly, this highly significant change in ecosystem C cycling will be detectable in the Δ14C and δ13C isotopic signature of respired C. We also predict that old C loss will offset increases in plant C uptake with warming (so-called ‘greening’ of the Arctic) and cause this ecosystem to be a net C source to the atmosphere during this century. We propose to test these hypotheses using a combination of field and laboratory experiments to measure isotope ratios and C fluxes from a tundra ecosystem in Alaska where permafrost is degrading. Field measurements will center on two experimental systems at this site. First, an established, two-factor warming manipulation using snow fences and open top chambers to increase winter and summer temperatures alone, and in combination. Our second inter-related experimental system is a permafrost thaw gradient where permafrost thaw and ground subsidence has been documented over the decadal time scale that is relevant to change in these northern ecosystems.
Three important measurement tools will be used to address our hypotheses. The first is a dual isotope approach with Δ14C and δ13C to quantitatively partition respiration and detect old C loss. The second is an eddy covariance tower to quantify whole ecosystem C balance across the permafrost thaw gradient over longer time and larger spatial scales. The third is data assimilation techniques and an ecosystem model to make projections of the response of tundra ecosystem C balance to future climate scenarios. Key parameters derived from this will be targeted for future incorporation into regional and Earth System models. The response of permafrost C is one of the potentially large feedbacks from terrestrial ecosystems to climate, and as such, these proposed research activities to quantify this feedback will directly support the Long Term Performance Measure of DOE climate change research, and also represent an immediately deployable test case that can rapidly provide tools and information that will assist the DOE Next-Generation Climate Change Ecosystem Experiment planned for Arctic tundra.