Seminar Abstract

Julian Sachs

Tuesday, February 9, 2010

Large Natural Changes in Tropical Pacific Rainfall Prior to the 20th Century

Tropical rainfall patterns directly influence the subsistence lifestyle of more than a billion people and indirectly influence climate globally. Their seasonal changes are associated with the position of the Intertropical Convergence Zone (ITCZ) where deep convection causes heavy rainfall near 10°N in boreal summer and 3°N in winter. Dynamic controls on the ITCZ position are debated but paleoclimate evidence on and near continental Asia, Africa and the Americas suggests it has shifted substantially during the last millennium, reaching its southern-most position some time during the Little Ice Age (LIA, 1400-1850 AD). However, without records from the meteorological core of the ITCZ in the Pacific Ocean quantitative constraints on its position are lacking. We demonstrate with microbiological, molecular and hydrogen isotope evidence from lake, lagoon and bog sediments from islands across the tropical Pacific Ocean that the Pacific ITCZ was south of its modern position for most of the last millennium, by as much as 500 km during the LIA. A colder Northern Hemisphere at that time, possibly resulting from lower solar irradiance, may have driven the ITCZ south, implying small changes in Earth’s radiation budget can profoundly impact tropical rainfall.