Reduction in carbon uptake during turn of the century drought in western North America


Fossil fuel emissions aside, temperate North America is a net sink of carbon dioxide at present1,2,3. Year-to-year variations in this carbon sink are linked to variations in hydroclimate that affect net ecosystem productivity3,4. The severity and incidence of climatic extremes, including drought, have increased as a result of climate warming5,6,7,8. Here, we examine the effect of the turn of the century drought in western North America on carbon uptake in the region, using reanalysis data, remote sensing observations and data from global monitoring networks. We show that the area-integrated strength of the western North American carbon sink declined by 30–298 Tg C yr−1 during the 2000–2004 drought. We further document a pronounced drying of the terrestrial biosphere during this period, together with a reduction in river discharge and a loss of cropland productivity. We compare our findings with previous palaeoclimate reconstructions7 and show that the last drought of this magnitude occurred more than 800 years ago. Based on projected changes in precipitation and drought severity, we estimate that the present mid-latitude carbon sink of 177–623 Tg C yr−1 in western North America could disappear by the end of the century.

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Figure 1: FLUXNET sites and enviroclimatic indicators.
Figure 2: Basin runoff and crop productivity in the western conterminous United States from 1997 to 2007.
Figure 3: Carbon and energy fluxes observed at FLUXNET from 1997 to 2007.
Figure 4: Drought over western North America from 800 to 2100.


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C.R.S., C.A.W. and K.S. were supported by the US National Science Foundation grant ATM-0910766. C.A.W. was additionally supported through NASA Terrestrial Ecology award NNX10AR68G (2N041). B.E.L. was supported by AmeriFlux (the Office of Science (BER), US Department of Energy (DE-FG02-04ER63917 and DE-FG02-04ER63911)). K.T.P.U. was supported by the US National Science Foundation grant F1137306/MIT subaward 5710003122 to the University of California, Davis. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups (Supplementary Table S4) for producing and making available their model output. For CMIP the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provided coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. CarbonTracker 2011 results provided by NOAA ESRL, Boulder, Colorado, USA from the website at Jena CO2 inversion results provided courtesy of C. Rödenbeck, Max Planck Institute for Biogeochemistry, Jena, Germany.

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C.R.S., C.A.W. and K.S. designed the study and are responsible for the integrity of the manuscript; C.R.S. carried out the analysis and all calculations. C.R.S., with C.A.W. and K.S., wrote the manuscript. D.B., T.A.B., A.H.G., B.E.L., W.C.O., K.T.P.U. and R.L.S. contributed FLUXNET data. All authors discussed and commented on the manuscript.

Correspondence to Christopher R. Schwalm.

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Schwalm, C., Williams, C., Schaefer, K. et al. Reduction in carbon uptake during turn of the century drought in western North America. Nature Geosci 5, 551–556 (2012) doi:10.1038/ngeo1529

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