Evaluating methane dynamics under thawing permafrost using CLM4.5BGC

Permafrost soils characterize the Arctic landscape and are known to contain massive soil carbon (C) pools that are vulnerable to changing climate. The response of permafrost soils to Arctic warming represents a major source of uncertainty in projecting C-cycle climate feedbacks. Permafrost soils are highly heterogeneous, especially related to oxygen availability and redox conditions that govern the C-cycle response to changing soil conditions and the production and consumption of methane (CH4) and carbon dioxide (CO2). Uncertainty in the mechanisms controlling C mineralization is compounded by concurrent changes in soil hydrology associated with permafrost thaw. Until recently, the ESMs that incorporate global C cycle-climate feedbacks lacked sufficient structural completeness to realistically represent permafrost-C feedbacks. Developments in the most recent version of the Community Land Model (CLM4.5, released in June 2013, http://www.cesm.ucar.edu/models/cesm1.2/clm/) target a more complete representation of permafrost soil biogeophysical and biogeochemical processes and provide an exciting tool with which we can scale our understanding of hydrological influences on the C dynamics of thawing Arctic soils. Recent developments in CLM4.5BGC (the biogeochemistry subcomponent of the CLM4.5 that includes simulations of CH4 production and oxidation) aimed to improve representation of the complex biogeophysical and biogeochemical interactions characteristic of permafrost soils. These model development efforts, however, have outpaced collection of data in permafrost systems. To date, model validation efforts have largely relied on surface flux measurements of CO2 and CH4. These surface measurements alone cannot properly evaluate the processes represented in the vertically resolved structure of the model because they represent the net sum of CH4 production and oxidation throughout the soil column and are not directly linked to subsurface drivers that vary across steep vertical gradients. The objective of this study was to present the model structure related to aerobic and anaerobic CO2 and CH4 production and oxidation within the new version of CLM4.5BGC and suggest the types of observations that are necessary to evaluate and enhance the model. In addition, the newly developed excess ice features in the CLM4.5 was tested to show how permafrost-thaw associated land surface subsidence influences future simulations of CH4 from thawing permafrost.