Asymmetric forcing from stratospheric aerosols impacts Sahelian rainfall

Abstract

The Sahelian drought of the 1970s–1990s was one of the largest humanitarian disasters of the past 50 years, causing up to 250,000 deaths and creating 10 million refugees1. It has been attributed to natural variability2,3,4,5, over-grazing6 and the impact of industrial emissions of sulphur dioxide7,8. Each mechanism can influence the Atlantic sea surface temperature gradient, which is strongly coupled to Sahelian precipitation2,3. We suggest that sporadic volcanic eruptions in the Northern Hemisphere also strongly influence this gradient and cause Sahelian drought. Using de-trended observations from 1900 to 2010, we show that three of the four driest Sahelian summers were preceded by substantial Northern Hemisphere volcanic eruptions. We use a state-of-the-art coupled global atmosphere–ocean model to simulate both episodic volcanic eruptions and geoengineering by continuous deliberate injection into the stratosphere. In either case, large asymmetric stratospheric aerosol loadings concentrated in the Northern Hemisphere are a harbinger of Sahelian drought whereas those concentrated in the Southern Hemisphere induce a greening of the Sahel. Further studies of the detailed regional impacts on the Sahel and other vulnerable areas are required to inform policymakers in developing careful consensual global governance before any practical solar radiation management geoengineering scheme is implemented.

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Figure 1: Observational analysis of SPAs.
Figure 2: Model precipitation response to volcanic simulations.
Figure 3: The June–October SPA in HadGEM2 as a function of year over the geographic area 10°–20° N, 20° W–10° E.
Figure 4: Model precipitation and NPP response to geoengineering simulations.

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Acknowledgements

N. Rayner is thanked for providing the HadISST SSTA data. T. Mitchell is thanked for maintaining the JISAO SPA records. P. Cox and T. Lenton are thanked for their encouragement. O. Morton and J. Knight are thanked for comments regarding the content, style and presentation of the work. This work was supported by the SPICE programme (http://www2.eng.cam.ac.uk/~hemh/climate/Geoengineering_RoySoc.htm), the IAGP programme (http://www.iagp.ac.uk/) and the Joint DECC/Defra Met Office Hadley Centre Climate Programme.

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J.M.H. came up with the idea for the experiments, analysed the experimental and model data, and wrote the paper. A.J. set up and ran the experiments, and analysed the model and observational data. N.B. performed analysis of the model and observational data. D.S. performed statistical analysis of the observational data.

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Correspondence to Jim M. Haywood.

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The authors declare no competing financial interests.

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Haywood, J., Jones, A., Bellouin, N. et al. Asymmetric forcing from stratospheric aerosols impacts Sahelian rainfall. Nature Clim Change 3, 660–665 (2013). https://doi.org/10.1038/nclimate1857

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