Climatic Effects of 1950-2050 Changes in US Anthropogenic Aerosols

28 We use the GEOS-Chem chemical transport model combined with the GISS general 29 circulation model to calculate the aerosol direct and indirect (warm cloud) radiative 30 forcings from US anthropogenic sources over the 1950-2050 period, based on historical 31 emission inventories and future projections from the IPCC A1B scenario. The aerosol 32 simulation is evaluated with observed spatial distributions and 1980-2010 trends of 33 aerosol concentrations and wet deposition in the contiguous US. The radiative forcing 34 from US anthropogenic aerosols is strongly localized over the eastern US. We find that it 35 peaked in 1970-1990, with values over the eastern US (east of 100°W) of -2.0 W m for 36 direct forcing including contributions from sulfate (-2.0 W m), nitrate (-0.2 W m), 37 organic carbon (-0.2 W m), and black carbon (+0.4 W m). The aerosol indirect effect 38 is of comparable magnitude to the direct forcing. We find that the forcing declined 39 sharply from 1990 to 2010 (by 0.8 W m direct and 1.0 W m indirect), mainly reflecting 40 decreases in SO2 emissions, and project that it will continue declining post-2010 but at a 41 much slower rate since US SO2 emissions have already declined by almost 60% from 42 their peak. This suggests that much of the warming effect of reducing US anthropogenic 43 aerosol sources may have already been realized by 2010, however some additional 44 warming is expected through 2020. The small positive radiative forcing from US BC 45 emissions (+0.3 W m over the eastern US in 2010) suggests that an emission control 46 strategy focused on BC would have only limited climate benefit. 47