The Science

University of Alberta and Berkeley Lab researchers used a mechanistic three-dimensional (3D) ecosystem model (ecosys) to project how vegetation cover changes in polygonal tundra will interact with soil temperatures and active-layer dynamics (Grant et al. 2019). The model was shown to very accurately match a wide range of Next-Generation Ecosystem Experiments (NGEE)–Arctic observations at the Utqiagvik, Alaska, site. Vegetation and landscape-scale hydrology strongly affect surface energy budgets and thereby active-layer deepening, implying that land models must accurately represent these processes in 21^st century simulations.

The Impact

Current land models applied for large-scale assessments of permafrost dynamics have poorly represented many of the processes known to affect these dynamics. Here the research team used a mechanistic 3D model to explore the roles that vegetation changes and landscape-scale hydrology over the coming decades will have on soil thermal dynamics. Research results point toward the importance of representing vegetation dynamics (e.g., density and composition) and hydrology at relevant spatial scales, and that doing so will result in smaller changes to soil temperatures and active layer deepening.

Summary

Model projections of permafrost thaw during the next century diverge widely. This study used ecosys to examine how climate change will affect permafrost thaw in a polygonal tundra at Utqiagvik (formerly Barrow), Alaska. The model was tested against observed diurnal and seasonal variation in energy exchange, soil heat flux, soil temperature (Ts) and active layer depth (ALD), and interannual variation in observed ALD from 1991 to 2015. During Representative Concentration Pathway (RCP) 8.5 climate change from 2015 to 2085, increases in air temperature and precipitation altered energy exchange by increasing the leaf area index (LAI) of dominant sedge relative to that of moss. Increased CO₂ concentrations and sedge LAI imposed greater stomatal control of transpiration and reduced soil heat fluxes, slowing soil warming, limiting increases in evapotranspiration, and thereby causing gradual soil wetting. Larger landscape drainage slowed ALD increases. The predicted rates are closer to those derived from current studies of warming impacts in the region but were smaller than those of earlier modelling studies, primarily because they did not account for vegetation changes. Therefore, accounting for climate change effects on vegetation density and composition, and consequent effects on surface energy budgets, will cause slower increases in active layer deepening over the 21st century.

Principal Investigator

William Riley
Lawrence Berkeley National Laboratory
wjriley@lbl.gov

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This research was supported by the Office of Biological and Environmental Research, within the U.S. Department of Energy Office of Science, under Contract No. DE-AC02-05CH11231 as part of the Next-Generation Ecosystem Experiments (NGEE)–Arctic project.

References