Global environmental change is rapidly altering the dynamics of terrestrial vegetation, with consequences for the functioning of the Earth system and provision of ecosystem services1,2. Yet how global vegetation is responding to the changing environment is not well established. Here we use three long-term satellite leaf area index (LAI) records and ten global ecosystem models to investigate four key drivers of LAI trends during 1982–2009. We show a persistent and widespread increase of growing season integrated LAI (greening) over 25% to 50% of the global vegetated area, whereas less than 4% of the globe shows decreasing LAI (browning). Factorial simulations with multiple global ecosystem models suggest that CO2 fertilization effects explain 70% of the observed greening trend, followed by nitrogen deposition (9%), climate change (8%) and land cover change (LCC) (4%). CO2 fertilization effects explain most of the greening trends in the tropics, whereas climate change resulted in greening of the high latitudes and the Tibetan Plateau. LCC contributed most to the regional greening observed in southeast China and the eastern United States. The regional effects of unexplained factors suggest that the next generation of ecosystem models will need to explore the impacts of forest demography, differences in regional management intensities for cropland and pastures, and other emerging productivity constraints such as phosphorus availability.
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This study was supported by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant XDB03030404), National Basic Research Program of China (Grant 2013CB956303), National Natural Science Foundation of China (Grant 41530528), the 111 Project (Grant B14001), and the European Research Council Synergy grant ERC-SyG-610028 IMBALANCE-P. We thank all people and institutions who provided data used in this study, in particular, the TRENDY modelling group. R.B.M. is funded by NASA Earth Science. J.G.C. is grateful for support from the Australian Climate Change Science Program. A.A. and T.A.M.P. acknowledge support through EC FP7 grants LUC4C (Grant 603542) and EMBRACE (Grant 282672) and the Helmholtz Association ATMO programme, Y.W. acknowledges CSIRO strategic funding for CABLE science, E.K. was funded by ERTDF (S10) from the Ministry of Environment, Japan. J.M. is supported by the US Department of Energy (DOE), Office of Science, Biological and Environmental Research. Oak Ridge National Laboratory is managed by UT-BATTELLE for DOE under contract DE-AC05-00OR22725. B.D.S. is supported by the Swiss National Science Foundation and FP7 funding through project EMBRACE (282672).