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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 22 | Copyright
Atmos. Chem. Phys., 15, 12681-12703, 2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 16 Nov 2015

Research article | 16 Nov 2015

Radiative forcing and climate response to projected 21st century aerosol decreases

D. M. Westervelt1, L. W. Horowitz2, V. Naik3, J.-C. Golaz2, and D. L. Mauzerall1,4 D. M. Westervelt et al.
  • 1Program in Science, Technology, and Environmental Policy, Woodrow Wilson School of Public and International Affairs, Princeton University, Princeton, NJ, USA
  • 2Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, Princeton, NJ, USA
  • 3UCAR/NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA
  • 4Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ, USA

Abstract. It is widely expected that global emissions of atmospheric aerosols and their precursors will decrease strongly throughout the remainder of the 21st century, due to emission reduction policies enacted to protect human health. For instance, global emissions of aerosols and their precursors are projected to decrease by as much as 80 % by the year 2100, according to the four Representative Concentration Pathway (RCP) scenarios. The removal of aerosols will cause unintended climate consequences, including an unmasking of global warming from long-lived greenhouse gases. We use the Geophysical Fluid Dynamics Laboratory Coupled Climate Model version 3 (GFDL CM3) to simulate future climate over the 21st century with and without the aerosol emission changes projected by each of the RCPs in order to isolate the radiative forcing and climate response resulting from the aerosol reductions. We find that the projected global radiative forcing and climate response due to aerosol decreases do not vary significantly across the four RCPs by 2100, although there is some mid-century variation, especially in cloud droplet effective radius, that closely follows the RCP emissions and energy consumption projections. Up to 1 W m−2 of radiative forcing may be unmasked globally from 2005 to 2100 due to reductions in aerosol and precursor emissions, leading to average global temperature increases up to 1 K and global precipitation rate increases up to 0.09 mm day−1. However, when using a version of CM3 with reduced present-day aerosol radiative forcing (−1.0 W m−2), the global temperature increase for RCP8.5 is about 0.5 K, with similar magnitude decreases in other climate response parameters as well. Regionally and locally, climate impacts can be much larger than the global mean, with a 2.1 K warming projected over China, Japan, and Korea due to the reduced aerosol emissions in RCP8.5, as well as nearly a 0.2 mm day−1 precipitation increase, a 7 g m−2 LWP decrease, and a 2 μm increase in cloud droplet effective radius. Future aerosol decreases could be responsible for 30–40 % of total climate warming (or 10–20 % with weaker aerosol forcing) by 2100 in East Asia, even under the high greenhouse gas emissions scenario (RCP8.5). The expected unmasking of global warming caused by aerosol reductions will require more aggressive greenhouse gas mitigation policies than anticipated in order to meet desired climate targets.

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Decreases in aerosols over the 21st century as projected by the Representative Concentration Pathways (RCPs) lead to increases up to 0.5 - 1 ºC in global temperature and up to 0.05 - 0.1 mm/day in global precipitation, depending strongly on present-day aerosol radiative forcing. In East Asia, future aerosol decreases could be responsible for 10-20% of the total temperature increase (30-40% with strong present-day aerosol forcing), even under the high greenhouse gas emissions scenario (RCP8.5).
Decreases in aerosols over the 21st century as projected by the Representative Concentration...