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1 UMass Amherst - University of Massachusetts Amherst 2 Alaska Cooperative Fish and Wildlife Research Unit 3 University of Montana 4 NCAR - National Center for Atmospheric Research Boulder 5 University of Washington 6 CNRM-GAME - Groupe d-étude de l-atmosphère météorologique 7 IAE - Institute of Applied Energy 8 Center for Global Environmental Research, National Institute for Environmental Studies, Tsukuba, Japan 9 University of Victoria 10 LSCE - Laboratoire des Sciences du Climat et de l-Environnement Gif-sur-Yvette 11 LGGE - Laboratoire de glaciologie et géophysique de l-environnement 12 College of Global Change and Earth System Science 13 HelmholtzZentrum München 14 JAMSTEC - Japan Agency for Marine-Earth Science and Technology 15 Lund University Lund 16 SESE - ASU School of Earth and Space Exploration 17 Météo-France Paris 18 Department of Civil and Environmental Engineering, University of Washington 19 Beijing Normal University

Abstract : A warming climate is altering land-atmosphere exchanges of carbon, with a potential for increased vegetation productivity as well as the mobilization of permafrost soil carbon stores. Here we investigate land-atmosphere carbon dioxide CO 2 cycling through analysis of net ecosystem productivity NEP and its component fluxes of gross primary productivity GPP and ecosystem respiration ER and soil carbon residence time, simulated by a set of land surface models LSMs over a region spanning the drainage basin of Northern Eurasia. The retrospective simulations cover the period 1960–2009 at 0.5 • resolution, which is a scale common among many global carbon and climate model simulations. Model performance benchmarks were drawn from comparisons against both observed CO 2 fluxes derived from site-based eddy covariance measurements as well as regional-scale GPP estimates based on satellite remote-sensing data. Published by Copernicus Publications on behalf of the European Geosciences Union. 4386 M. A. Rawlins et al.: CO 2 Exchange Across Northern Eurasia The site-based comparisons depict a tendency for overestimates in GPP and ER for several of the models, particularly at the two sites to the south. For several models the spatial pattern in GPP explains less than half the variance in the MODIS MOD17 GPP product. Across the models NEP increases by as little as 0.01 to as much as 0.79 g C m −2 yr −2 , equivalent to 3 to 340 % of the respective model means, over the analysis period. For the multimodel average the increase is 135 % of the mean from the first to last 10 years of record 1960–1969 vs. 2000–2009, with a weakening CO 2 sink over the latter decades. Vegetation net primary productivity increased by 8 to 30 % from the first to last 10 years, contributing to soil carbon storage gains. The range in regional mean NEP among the group is twice the multimodel mean, indicative of the uncertainty in CO 2 sink strength. The models simulate that inputs to the soil carbon pool exceeded losses, resulting in a net soil carbon gain amid a decrease in residence time. Our analysis points to improvements in model elements controlling vegetation productivity and soil respiration as being needed for reducing uncertainty in land-atmosphere CO 2 exchange. These advances will require collection of new field data on vegetation and soil dynamics, the development of benchmarking data sets from measurements and remote-sensing observations, and investments in future model development and intercomparison studies.

Autor: M. A Rawlins - A. David Mcguire - J. Kimball - P Dass - D. Lawrence - X. Chen - C. Delire - C. Koven - A Macdougall - S. Peng - A



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