Tropical rainfall patterns control the subsistence lifestyle of more than one billion people. Seasonal changes in these rainfall patterns are associated with changes in the position of the intertropical convergence zone, which is characterized by deep convection causing heavy rainfall near 10∘ N in boreal summer and 3∘ N in boreal winter. Dynamic controls on the position of the intertropical convergence zone are debated, but palaeoclimatic evidence from continental Asia, Africa and the Americas suggests that it has shifted substantially during the past millennium, reaching its southernmost position some time during the Little Ice Age (AD 1400–1850). However, without records from the meteorological core of the intertropical convergence zone in the Pacific Ocean, quantitative constraints on its position are lacking. Here we report microbiological, molecular and hydrogen isotopic evidence from lake sediments in the Northern Line Islands, Galápagos and Palau indicating that the Pacific intertropical convergence zone was south of its modern position for most of the past millennium, by as much as 500 km during the Little Ice Age. A colder Northern Hemisphere at that time, possibly resulting from lower solar irradiance, may have driven the intertropical convergence zone south. We conclude that small changes in Earth’s radiation budget may profoundly affect tropical rainfall.
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Financial support was provided by the US National Science Foundation (J.P.S.), the US National Oceanic and Atmospheric Administration (J.P.S.), the Gary Comer Science and Education Foundation (J.P.S.) and the Alexander-von-Humboldt foundation through a Feodor-Lynen Research Fellowship (D.S.). Discussions with J. Chiang, K. Takahashi, A. Timmerman, M. Wallace, G. Philander, C. Wunsch, P. Colinvaux and C. Saenger improved this manuscript. M. Miller, C. Saenger, M. Dawson, L. Martin, P. Colin, L. Bell, The Coral Reef Research Foundation of Palau, J. Overpeck, J. Conroy, P. Colinvaux, M. Steinitz-Kannan, S. Fukada, K. Anderson, J. Briden and C. Corbett assisted with field work. O. Kawka, B. Demianew and R. Rottenfusser assisted in the laboratory. The Galápagos National Park, the Charles Darwin Foundation, the Republic of Kiribati and the Republic of Palau issued permits and provided assistance with field work.
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