Ensemble projections of wildfire activity and carbonaceous 1 aerosol concentrations over the western United States in the 2 mid - 21 st century

15 We estimate future wildfire activity over the western United States during the mid-21 st 16 century (2046-2065), based on results from 15 climate models following the A1B 17 scenario. We develop fire prediction models by regressing meteorological variables from 18 the current and previous years together with fire indexes onto observed regional area 19 burned. The regressions explain 0.25-0.60 of the variance in observed annual area burned 20 during 1980-2004, depending on the ecoregion. We also parameterize daily area burned 21 with temperature, precipitation, and relative humidity. This approach explains ~0.5 of the 22 variance in observed area burned over forest ecoregions but shows no predictive 23 capability in the semi-arid regions of Nevada and California. By applying the 24 meteorological fields from 15 climate models to our fire prediction models, we quantify 25 the robustness of our wildfire projections at mid-century. We calculate increases of 26 24-124% in area burned using regressions and 63-169% with the parameterization. Our 27 projections are most robust in the southwestern desert, where all GCMs predict 28 significant (p<0.05) meteorological changes. For forested ecoregions, more GCMs 29 predict significant increases in future area burned with the parameterization than with the 30 regressions, because the latter approach is sensitive to hydrological variables that show 31 large inter-model variability in the climate projections. The parameterization predicts that 32 the fire season lengthens by 23 days in the warmer and drier climate at mid-century. 33 Using a chemical transport model, we find that wildfire emissions will increase 34 summertime surface organic carbon aerosol over the western United States by 46-70% 35 and black carbon by 20-27% at midcentury, relative to the present day. The pollution is 36 most enhanced during extreme episodes: above the 84 th percentile of concentrations, OC 37 increases by ~90% and BC by ~50%, while visibility decreases from 130 km to 100 km 38 in 32 Federal Class 1 areas in Rocky Mountains Forest.