Local and remote mean and extreme temperature response to regional aerosol emissions reductions
Authors: Westervelt Daniel M. , Mascioli Nora R. , Fiore Arlene M. , Conley Andrew J. , Lamarque Jean-François , Shindell Drew T. , Faluvegi Greg , Previdi Michael , Correa Gustavo , Horowitz Larry W. 
Published: March 2020

Abstract. The climatic implications of regional aerosol and precursor emissions reductions implemented to protect human health are poorly understood. We
investigate the mean and extreme temperature response to regional changes in aerosol emissions using three coupled chemistry–climate models: NOAA
GFDL CM3, NCAR CESM1, and NASA GISS-E2. Our approach contrasts a long present-day control simulation from each model (up to 400 years with perpetual
year 2000 or 2005 emissions) with 14 individual aerosol emissions perturbation simulations (160–240 years each). We perturb emissions of
sulfur dioxide (SO2) and/or carbonaceous aerosol within six world regions and assess the statistical significance of mean and extreme
temperature responses relative to internal variability determined by the control simulation and across the models. In all models, the global mean
surface temperature response (perturbation minus control) to SO2 and/or carbonaceous aerosol is mostly positive (warming) and statistically
significant and ranges from +0.17 K (Europe SO2) to −0.06 K (US BC). The warming response to SO2 reductions
is strongest in the US and Europe perturbation simulations, both globally and regionally, with Arctic warming up to 1 K due to a removal of
European anthropogenic SO2 emissions alone; however, even emissions from regions remote to the Arctic, such as SO2 from India,
significantly warm the Arctic by up to 0.5 K. Arctic warming is the most robust response across each model and several aerosol emissions
perturbations. The temperature response in the Northern Hemisphere midlatitudes is most sensitive to emissions perturbations within that region. In
the tropics, however, the temperature response to emissions perturbations is roughly the same in magnitude as emissions perturbations either
within or outside of the tropics. We find that climate sensitivity to regional aerosol perturbations ranges from 0.5 to 1.0 K (W m−2)−1 depending on the region and aerosol composition and is larger than the climate sensitivity to a doubling of CO2 in two
of three models. We update previous estimates of regional temperature potential (RTP), a metric for estimating the regional temperature responses to
a regional emissions perturbation that can facilitate assessment of climate impacts with integrated assessment models without requiring
computationally demanding coupled climate model simulations. These calculations indicate a robust regional response to aerosol forcing within the
Northern Hemisphere midlatitudes, regardless of where the aerosol forcing is located longitudinally. We show that regional aerosol perturbations
can significantly increase extreme temperatures on the regional scale. Except in the Arctic in the summer, extreme temperature responses largely
mirror mean temperature responses to regional aerosol perturbations through a shift of the temperature distributions and are mostly dominated by
local rather than remote aerosol forcing.