The vulnerability of Amazonian rainforest, and the ecological services it provides, depends on an adequate supply of dry-season water, either as precipitation or stored soil moisture. How the rain-bearing South American monsoon will evolve across the twenty-first century is thus a question of major interest. Extensive savanization, with its loss of forest carbon stock and uptake capacity, is an extreme although very uncertain scenario1,2,3,4,5,6. We show that the contrasting rainfall projections simulated for Amazonia by 36 global climate models (GCMs) can be reproduced with empirical precipitation models, calibrated with historical GCM data as functions of the large-scale circulation. A set of these simple models was therefore calibrated with observations and used to constrain the GCM simulations. In agreement with the current hydrologic trends7,8, the resulting projection towards the end of the twenty-first century is for a strengthening of the monsoon seasonal cycle, and a dry-season lengthening in southern Amazonia. With this approach, the increase in the area subjected to lengthy—savannah-prone—dry seasons is substantially larger than the GCM-simulated one. Our results confirm the dominant picture shown by the state-of-the-art GCMs, but suggest that the ‘model democracy’ view of these impacts can be significantly underestimated.
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This research received financial support from the European Union Seventh Framework Programme, under the project AMAZALERT (Grant Agreement No. 282664). J.P.B. is grateful for support from the CONICYT (Chile) grants FONDAP (Center for Climate and Resilience Research, No. 15110009) and FONDECYT (No. 3150492). 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 (listed in Supplementary Table 1 of this paper) for producing and making available their model output. For CMIP the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. The precipitation data used in this paper were extracted from publicly available sites and are described, with references, in the supporting material. The MODIS MCD12C1 product was obtained through the online Data Pool at the NASA Land Processes Distributed Active Archive Center (LP DAAC), USGS/Earth Resources Observation and Science (EROS) Center, Sioux Falls, South Dakota (https://lpdaac.usgs.gov/data_access).
The authors declare no competing financial interests.
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Boisier, J., Ciais, P., Ducharne, A. et al. Projected strengthening of Amazonian dry season by constrained climate model simulations. Nature Clim Change 5, 656–660 (2015) doi:10.1038/nclimate2658
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