ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-3349-2012 Climatic effects of 1950–2050 changes in US anthropogenic aerosols – Part 2: Climate response Leibensperger E. M. 1 9 Mickley L. J. 1 Jacob D. J. 1 Chen W.-T. 2 Seinfeld J. H. 3 Nenes A. 4 Adams P. J. 5 Streets D. G. 6 Kumar N. 7 Rind D. 8 School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA School of Earth & Atmospheric Sciences and School of Chemical & Biological Engineering, Georgia Institute of Technology, Atlanta, GA, USA Department of Civil & Environmental Engineering and Department of Engineering & Public Policy, Carnegie Mellon University, Pittsburgh, PA, USA Argonne National Laboratory, Argonne, IL, USA Electric Power Research Institute, Palo Alto, CA, USA NASA Goddard Institute for Space Studies, New York, NY, USA now at: Dept. of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA 10 04 2012 12 7 3349 3362 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/3349/2012/acp-12-3349-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/3349/2012/acp-12-3349-2012.pdf

We investigate the climate response to changing US anthropogenic aerosol sources over the 1950–2050 period by using the NASA GISS general circulation model (GCM) and comparing to observed US temperature trends. Time-dependent aerosol distributions are generated from the GEOS-Chem chemical transport model applied to historical emission inventories and future projections. Radiative forcing from US anthropogenic aerosols peaked in 1970–1990 and has strongly declined since due to air quality regulations. We find that the regional radiative forcing from US anthropogenic aerosols elicits a strong regional climate response, cooling the central and eastern US by 0.5–1.0 °C on average during 1970–1990, with the strongest effects on maximum daytime temperatures in summer and autumn. Aerosol cooling reflects comparable contributions from direct and indirect (cloud-mediated) radiative effects. Absorbing aerosol (mainly black carbon) has negligible warming effect. Aerosol cooling reduces surface evaporation and thus decreases precipitation along the US east coast, but also increases the southerly flow of moisture from the Gulf of Mexico resulting in increased cloud cover and precipitation in the central US. Observations over the eastern US show a lack of warming in 1960–1980 followed by very rapid warming since, which we reproduce in the GCM and attribute to trends in US anthropogenic aerosol sources. Present US aerosol concentrations are sufficiently low that future air quality improvements are projected to cause little further warming in the US (0.1 °C over 2010–2050). We find that most of the warming from aerosol source controls in the US has already been realized over the 1980–2010 period.

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