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Increasing risk of Amazonian drought due to decreasing aerosol pollution

Nature volume 453, pages 212–215 (2008)Cite this article

Abstract

The Amazon rainforest plays a crucial role in the climate system, helping to drive atmospheric circulations in the tropics by absorbing energy and recycling about half of the rainfall that falls on it. This region (Amazonia) is also estimated to contain about one-tenth of the total carbon stored in land ecosystems, and to account for one-tenth of global, net primary productivity1. The resilience of the forest to the combined pressures of deforestation and global warming is therefore of great concern2, especially as some general circulation models (GCMs) predict a severe drying of Amazonia in the twenty-first century3,4,5. Here we analyse these climate projections with reference to the 2005 drought in western Amazonia, which was associated6 with unusually warm North Atlantic sea surface temperatures (SSTs). We show that reduction of dry-season (July–October) rainfall in western Amazonia correlates well with an index of the north–south SST gradient across the equatorial Atlantic (the ‘Atlantic N–S gradient’). Our climate model is unusual among current GCMs in that it is able to reproduce this relationship and also the observed twentieth-century multidecadal variability in the Atlantic N–S gradient7, provided that the effects of aerosols are included in the model8. Simulations for the twenty-first century using the same model3,8 show a strong tendency for the SST conditions associated with the 2005 drought to become much more common, owing to continuing reductions in reflective aerosol pollution in the Northern Hemisphere9.

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Figure 1: Anomalies in SSTs for July–October 2005, relative to the July–October mean values over the standard climatological period 1961–1990.
Figure 2: Comparison of observed and modelled climate variables relevant to the Amazonian drought of 2005.
Figure 3: Relationship between the July–October mean values of western Amazonian rainfall and the ANSG index.
Figure 4: Predicted change in the probability of a 2005-like drought in Amazonia, based on results from the HadCM3LC GCM run with aerosols8.

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Acknowledgements

The authors acknowledge funding from the NERC CLASSIC programme and Great Western Research (P.M.C. and T.E.J.); the CEH Science Budget (C.H. and P.P.H.); the UK Department for Environment, Food and Rural Affairs and the UK Ministry of Defence (R.A.B, C.D.J. and M.C.); and the Brazilian Research Council and the Global Opportunities Fund from the UK Foreign and Commonwealth Office (C.A.N. and J.A.M.). We also acknowledge the modelling groups the Program for Climate Model Diagnosis and Intercomparison and the WCRP Working Group on Coupled Modelling for their roles in making available the WCRP CMIP3 multimodel data set. Support for this data set is provided by the Office of Science, US Department of Energy.

Author Contributions P.M.C. coordinated the work, identified the role of aerosols in delaying Amazonian drying in HadCM3LC, and drafted the paper; P.P.H. and C.H. defined the SST indices and analysed the relationships between these indices and western Amazonian rainfall, in both the observations and the model runs; C.D.J. and R.A.B. carried out the HadCM3LC runs and provided output data from them; J.A.M. and C.A.N. provided observational data and insights on the nature of the 2005 drought in western Amazonia; M.C. extracted and then intercompared data on predicted changes in Amazonia from the CMIP3 models; C.H. and T.E.J. provided guidance on statistical significance.

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Authors and Affiliations

  1. School of Engineering, Computing and Mathematics, University of Exeter, Exeter EX4 4QF, UK

    Peter M. Cox & Tim E. Jupp

  2. Met Office Hadley Centre, Exeter EX1 3PB, UK

    Peter M. Cox, Richard A. Betts, Matthew Collins & Chris D. Jones

  3. Centre for Ecology and Hydrology, Wallingford OX10 8BB, UK

    Phil P. Harris & Chris Huntingford

  4. Brazilian Centre for Weather Forecasting and Climate Studies, CPTEC/INPE, Sao Paulo, Brazil ,

    José A. Marengo & Carlos A. Nobre

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Cox, P., Harris, P., Huntingford, C. et al. Increasing risk of Amazonian drought due to decreasing aerosol pollution. Nature 453, 212–215 (2008). https://doi.org/10.1038/nature06960

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