Permafrost in a warmer world: net ecosystem carbon imbalance

Abstract

Arctic tundra and boreal forest have accumulated a vast pool of organic carbon, twice as large as the atmospheric carbon pool and three times as large as the carbon contained by all living things. As the permafrost region warms, more of this carbon will be exposed to decomposition, combustion, and hydrologic export. This permafrost carbon feedback has been described as the largest terrestrial feedback to climate change as well as one of the most likely to occur; however, estimates of its strength vary by a factor of thirty. Models predict that some portion of this release will be offset by increased arctic and boreal biomass, but the lack of robust estimates of net carbon balance increases the risk of further overshooting international emissions targets with serious societal and environmental consequences. In this dissertation I investigate the potential and actual response of Arctic and boreal carbon balance to climate change. First, I present estimates from 98 permafrost-region experts of the response of circumarctic biomass, wildfire, and hydrologic carbon flux to warming over the next several centuries. Because precise estimates of the factors driving arctic and boreal carbon balance are unlikely in the near future, these qualitative estimates provide a holistic summary of current scientific understanding and provide a framework for assessing uncertainty and risk. Assessments indicate that little agreement exists on the magnitude and even sign of change in high-latitude biomass, and that end-of-the-century organic carbon release from arctic rivers and collapsing coastlines could increase three-fold while carbon loss via burning could increase seven-fold. Second, I test the impact of permafrost collapse (thermokarst) on carbon and nutrient release from upland tundra on the North Slope of Alaska. The biogeochemical consequences of thermokarst are not adequately conceptualized or characterized to incorporate into numerical models, though thermokarst may impact a third of the permafrost region by the end of the century. I employ a coupled aquatic and terrestrial experimental design to address this knowledge gap, measuring the displacement of soil organic carbon, surface flux of CO₂, CH₄, and N₂O, and hydrologic export of dissolved carbon and nutrients. Results show that thermokarst can stimulate or suppress ecosystem respiration depending on feature morphology; remove a large portion of ecosystem carbon; mobilize highly biodegradable dissolved organic carbon; disrupt the nitrogen cycle resulting in N₂O production and hydrologic nitrogen losses; and influence offsite organic matter decomposition by the release of labile dissolved organic carbon nitrogen, and other nutrients. Spatial patterns of carbon and nutrient export from thermokarst suggest that upland thermokarst may be a dominant linkage between terrestrial and aquatic ecosystems as the permafrost region warms. I conclude that the strength of the permafrost climate feedback depends largely on coupled carbon and nutrient dynamics, which will interact with disturbance such as wildfire and thermokarst. My results indicate that three-quarters of permafrost carbon release could be avoided if human emissions are actively reduced, though the window of opportunity to keep that carbon in the ground is rapidly closing.