Recent European heatwaves have raised interest in the impact of land cover conditions on temperature extremes. At present, it is believed that such extremes are enhanced by stronger surface heating of the atmosphere, when soil moisture content is below average. However, the impact of land cover on the exchange of water and energy and the interaction of this exchange with the soil water balance during heatwaves is largely unknown. Here we analyse observations from an extensive network of flux towers in Europe that reveal a difference between the temporal responses of forest and grassland ecosystems during heatwaves. We find that initially, surface heating is twice as high over forest than over grassland. Over grass, heating is suppressed by increased evaporation in response to increased solar radiation and temperature. Ultimately, however, this process accelerates soil moisture depletion and induces a critical shift in the regional climate system that leads to increased heating. We propose that this mechanism may explain the extreme temperatures in August 2003. We conclude that the conservative water use of forest contributes to increased temperatures in the short term, but mitigates the impact of the most extreme heat and/or long-lasting events.
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We are grateful to the members of the FLUXNET community (http://www.fluxdata.org/DataInfo/default.aspx) and in particular the CarboEuropeIP network for their efforts in acquiring the eddy covariance data. We acknowledge the financial support to the eddy covariance data harmonization provided by CarboEuropeIP, FAO-GTOS-TCO, iLEAPS, Max Planck Institute for Biogeochemistry, National Science Foundation, University of Tuscia, Université Laval and Environment Canada and US Department of Energy and the database development and technical support from Berkeley Water Center, Lawrence Berkeley National Laboratory, Microsoft Research eScience, Oak Ridge National Laboratory, University of California - Berkeley and University of Virginia. A.J.T. acknowledges financial support from Netherlands Organisation for Scientific Research (NWO) through Rubicon grant 825.07.009, ETH Zurich and the Swiss National Science Foundation through the NFP61 DROUGHT-CH project. We further acknowledge support from the European Commission Project CARBO-Extreme (FP7-ENV-2008-1-226701), the CCES MAIOLICA project and NCCR-Climate programme.
The authors declare no competing financial interests.
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Teuling, A., Seneviratne, S., Stöckli, R. et al. Contrasting response of European forest and grassland energy exchange to heatwaves. Nature Geosci 3, 722–727 (2010). https://doi.org/10.1038/ngeo950
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