The position of the northern boundary of the tropical belt affects the hydroclimate of many arid and semi-arid regions in the Northern Hemisphere. Widening of the tropical belt since the 1970s has largely been attributed to anthropogenic forcing. However, the relative influence of natural drivers of tropical belt expansion and contraction before this time is poorly understood. Here we use data on tree-ring widths from five mid-latitude regions in the Northern Hemisphere to reconstruct the movement of the northern boundary of the early spring tropical belt over the past 800 years (ad 1203–2003). Our reconstruction explains 45% of the interannual variance in the latitudinal extent of the Hadley circulation, a metric of the position of the tropical belt boundary. We find that the tropical belt contracted (expanded) during positive (negative) phases of the El Niño Southern Oscillation and Pacific North American teleconnection patterns. The tropical belt also contracted significantly following major volcanic events that injected sulfur into the stratosphere. The longest period of persistent tropical belt expansion occurred in the late sixteenth century, during one of the coldest periods of the Little Ice Age. Our results warn of potential socio-economic consequences of future variations in tropical belt width driven by natural climate variability or stratospheric aerosol injections, whether volcanic or artificial.
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The RHCEFMA will be housed at NOAA-Paleoclimatology/World Data Service for Paleoclimatology: https://www.ncdc.noaa.gov/paleo/study/25050. The data that support the findings of this study are available from the corresponding author upon request.
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This study was supported by the US National Science Foundation (NSF) CAREER grant AGS-1349942 and the NSF grant 1065790 from the Emerging Frontiers Section in the Division of Environmental Biology. R.A.S. is supported by the postdoctoral grant Juan de la Cierva-Formación-FJCI-2015-26848, from the Spanish Ministry of Economy, Industry and Competitiveness. S.K. acknowledges the support of USDA-AFRI grant 2016-67003-24944. R.K.M. acknowledges support from NSF Ecosystems Program grant 1754430. R.V. is partially supported by the BNP-PARIBAS Foundation.
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Alfaro-Sánchez, R., Nguyen, H., Klesse, S. et al. Climatic and volcanic forcing of tropical belt northern boundary over the past 800 years. Nature Geosci 11, 933–938 (2018). https://doi.org/10.1038/s41561-018-0242-1
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