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Biomass burning drives changes in greenhouse gases, climate-forcing aerosols, and global atmospheric chemistry. There is controversy about the magnitude and timing of changes in biomass burning emissions on millennial time scales from preindustrial to present and about the relative importance of climate change and human activities as the underlying cause. Biomass burning is one of two notable sources of ethane in the preindustrial atmosphere. Here, we present ice core ethane measurements from Antarctica and Greenland that contain information about changes in biomass burning emissions since 1000 CE (Common Era). The biomass burning emissions of ethane during the Medieval Period (1000-1500 CE) were higher than present day and declined sharply to a minimum during the cooler Little Ice Age (1600-1800 CE). Assuming that preindustrial atmospheric reactivity and transport were the same as in the modern atmosphere, we estimate that biomass burning emissions decreased by 30 to 45% from the Medieval Period to the Little Ice Age. The timing and magnitude of this decline in biomass burning emissions is consistent with that inferred from ice core methane stable carbon isotope ratios, but inconsistent with histories based on sedimentary charcoal and ice core carbon monoxide measurements. This study demonstrates that biomass burning emissions have exceeded modern levels in the past and may be highly sensitive to changes in climate. 

This PDF file includes: Supplementary text Figs. S1 to S7 (Figs. S1 through S5 are referenced in the main manuscript) Tables S1 to S6 (Tables S1 through S3 are referenced in the main manuscript) References for SI reference citations