Aerosol forcing of the position of the intertropical convergence zone since ad 1550

Abstract

The position of the intertropical convergence zone is an important control on the distribution of low-latitude precipitation. Its position is largely controlled by hemisphere temperature contrasts1,2. The release of aerosols by human activities may have resulted in a southward shift of the intertropical convergence zone since the early 1900s (refs 1, 3, 4, 5, 6) by muting the warming of the Northern Hemisphere relative to the Southern Hemisphere over this interval1,7,8, but this proposed shift remains equivocal. Here we reconstruct monthly rainfall over Belize for the past 456 years from variations in the carbon isotope composition of a well-dated, monthly resolved speleothem. We identify an unprecedented drying trend since ad 1850 that indicates a southward displacement of the intertropical convergence zone. This drying coincides with increasing aerosol emissions in the Northern Hemisphere and also marks a breakdown in the relationship between Northern Hemisphere temperatures and the position of the intertropical convergence zone observed earlier in the record. We also identify nine short-lived drying events since ad 1550 each following a large volcanic eruption in the Northern Hemisphere. We conclude that anthropogenic aerosol emissions have led to a reduction of rainfall in the northern tropics during the twentieth century, and suggest that geographic changes in aerosol emissions should be considered when assessing potential future rainfall shifts in the tropics.

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Figure 1: YOK-G δ13C record and chronology.
Figure 2: YOK-G δ13C record.
Figure 3: Scatterplot of YOK-G δ13Cwet versus NHT from the Esper reconstruction.
Figure 4: Annual mean YOK-G δ13C and links to sulphate aerosols.

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Acknowledgements

This research was supported by funding from: the European Research Council (grant 240167 to J.U.L.B.); National Science Foundation of the United States (grant BCS-0620445 to K.M.P., grant HSD-0827305 to K.M.P. and Y.A., grant HSD-0827275 to D.J.K., and grant BCS-0940744 to D.J.K. and M.Z.); the Alphawood Foundation (grant to K.M.P.); the Schweizer National Fund, Sinergia (grant CRSI22 132646/1 to S.F.M.B.). Research permits were issued by the Belize Institute of Archaeology. Hydromet Belize are thanked for meteorological data.

Author information

H.E.R. produced the stable isotope record with assistance from C.G.M. and J.L.P. Y.A., V.P. and V.V.A. were responsible for developing the uranium-series chronology. D.J.K. and B.J.C. performed radiocarbon analyses. H.E.R., K.M.P. and J.U.L.B. contributed significantly to fieldwork. M.Z. and T.X. performed GHCN analyses. F.A.L. and S.F.M.B. performed high-resolution stable isotope reanalyses. H.E.R. and J.U.L.B. wrote the manuscript. J.A. contributed to logistics associated with fieldwork. All named co-authors contributed to the project, discussed manuscript ideas and approved the final manuscript.

Correspondence to Harriet E. Ridley.

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The authors declare no competing financial interests.

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Ridley, H., Asmerom, Y., Baldini, J. et al. Aerosol forcing of the position of the intertropical convergence zone since ad 1550. Nature Geosci 8, 195–200 (2015). https://doi.org/10.1038/ngeo2353

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