CLIMATE IMPLICATIONS OF BIOMASS BURNING SINCE THE 19TH-CENTURY IN EASTERN NORTH-AMERICA

Recent predictions that tropospheric aerosols have counterbalanced gre enhouse warming assume aerosol emissions were low before AD1850 and th en increased dramatically with industrialization of the Northern Hemis phere and biomass burning in the Tropics. We assembled the lake sedime nt record of emissions across northeastern North America, where temper atures are predicted to have been substantially affected by industrial aerosols. Sediment evidence suggests a systematic shift in source and an overall decline in emissions since the 19th century. The geographi cal shift results from high presettlement emissions from wildfires in the Midwest that collapsed with tillage and fire suppression. Meanwhil e, emissions were increasing in the North-east with European settlemen t. These regional changes produced a shift from the continental interi or to the North-east. An overall decline results because decreases in the Midwest more than compensate for increases in the North-east. Resu lts suggest the Central Plains as an important source of emissions in the recent past, consistent with pioneer accounts of dense smoke cloud s emanating from prairie in the 19th century. Contrary to recent model s that suggest increased 20th century combustion emissions could have offset warming effects of rising greenhouse gases, our data suggest th at aerosols could have actually decreased over this interval. Although we cannot directly quantify aerosols from our methods, the emissions of large particles suggest assumptions of 20th century aerosol decline s should be reconsidered.