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Journal of Climate , Volume 26 Issue 10, ( 2013), P3139-P3158.
Zachary M. Subin, Charles D. Koven, William J. Riley, and Margaret S. Torn, David M. Lawrence and Sean C. Swenson.
Princeton Environmental Institute, Princeton, New Jersey, and Lawrence Berkeley National Laboratory, Berkeley, California and
Lawrence Berkeley National Laboratory, Berkeley, California and
National Center for Atmospheric Research, Boulder, Colorado.
Abstract
At high latitudes, changes in soil moisture could alter soil temperatures independently of air temperature changes by interacting with the snow thermal rectifier. The authors investigated this mechanism with model experiments in the Community Land Model 4 (CLM4) with prescribed atmospheric forcing and vegetation state. Under equilibrium historical conditions, increasing CO2 concentrations experienced by plants from 285 to 857 ppm caused local increases in soil water-filled pore space of 0.1–0.2 in some regions throughout the globe. In permafrost regions that experienced this moistening, vertical- and annual- mean soil temperatures increased by up to 3°C (0.27°C averaged over all permafrost areas). A similar pattern of moistening and consequent warming occurred in simulations with prescribed June–September (JJAS) rainfall increases of 25% over historical values, a level of increase commensurate with projected future rainfall increases. There was a strong sensitivity of the moistening responses to the baseline hydrological state. Experiments with perturbed physics confirmed that the simulated warming in permafrost soils was caused by increases in the soil latent heat of fusion per unit volume and in the soil thermal conductivity due to the increased moisture. In transient Representative Concentration Pathway 8.5 (RCP8.5) scenario experiments, soil warming due to increased CO2 or JJAS rainfall was smaller in magnitude and spatial extent than in the equilibrium experiments. Active-layer deepening associated with soil moisture changes occurred over less than 8% of the current permafrost area because increased heat of fusion and soil thermal conductivity had compensating effects on active-layer depth. Ongoing modeling challenges make these results tentative.
Additional Information:
In this paper the authors investigated the mechanism of how changes in soil moisture could alter soil temperatures independently of air temperature changes using the CLM4 model.
The Community Land Model is the land model for the Community Earth System Model (CESM) and the Community Atmosphere Model (CAM). It is a collaborative project between scientists in the Terrestrial Sciences Section (TSS) and the Climate and Global Dynamics Division (CGD) at the National Center for Atmospheric Research (NCAR) and the CESM Land Model Working Group. Other principal working groups that also contribute to the CLM are Biogeochemistry, Paleoclimate, and Climate Change and Assessment. The model formalizes and quantifies concepts of ecological climatology. Ecological climatology is an interdisciplinary framework to understand how natural and human changes in vegetation affect climate. It examines the physical, chemical, and biological processes by which terrestrial ecosystems affect and are affected by climate across a variety of spatial and temporal scales. The central theme is that terrestrial ecosystems, through their cycling of energy, water, chemical elements, and trace gases, are important determinants of climate. Model components consist of: biogeophysics, hydrologic cycle, biogeochemistry and dynamic vegetation.
For more information about CLM4: http://www.cgd.ucar.edu/tss/clm/
