1G: Regional climate impacts from regional aerosol emissions

As reported in Myhre et al. (2013), the radiative forcing of well-mixed greenhouse gases in 2011 relative to 1750 is in the range [2.22 to 3.78] Wm-2.  Over the same period, the total radiative forcing of aerosols (including aerosol-cloud interactions) is in the range [-1.9 to -0.1] Wm-2.  The aerosol radiative forcing constitutes the largest forcing uncertainty and most of the uncertainty is in the representation of the interaction of clouds with aerosols.

In this project, we examine local and remote forcing, aerosol burdens, and surface temperature responses to anthropogenic SO2 from the United States specifically. While we will ultimately explore the climate response to regional emissions from different areas (North America, Europe, Asia, …) and different sources (SO2, black carbon, biomass burning), we focus the present research on the impacts of US SO2 emissions.

To evaluate those, we use results from two (perturbation and control) 200-year (or more) present-day simulations which are identical except for the removal of anthropogenic US emission of SO2 in the perturbation run. Because we want to identify the robustness (across models) of the results, we use simulations using the CESM1-CAM5, GFDL-CM3 (from LDEO), and GISS-E2 (from Duke) chemistry-climate models.  We focus our analysis on radiative forcing, responses in annual-mean SO4 burden, radiative forcing, aerosol optical depth, dust, and surface temperature response. All of these models couple aerosol and precursor emissions to climate forcing and climate response interactively, in contrast to other climate models that specify aerosol distributions as a climatology.  In particular, we use CESM1-CAM5 with interactive chemistry, as described in Tilmes et al. (2015).

While many aspects of the models are diverse, we find the models show similarities in their pattern of temperature responses over land.  The temperature responses are strongest near the North Pole, but are positive for most of the Northern Hemisphere land regions.   In particular, the magnitude of global average temperature response is within a factor of 2 between models (Conley et al., 2017).

Figure 1: SO4 burden change in mg/m^2
Figure 1: SO4 burden change in mg/m^2. The SO4 burden changes most strongly over the region of SO2 emission changes.  For CESM, the deficit is transported zonally to the east.  GFDL has a similar zonal transport, however, there is an increase in SO4 over the Sahel region and Southern Asia.  The GISS model has a less pronounced decrease over the source region and a more diffuse transport of the burden change towards the North pole.

References

1. Conley, A. J., D. Westervelt, J.-F. Lamarque, A. M. Fiore, D. Shindell, G. Correa, G. Faluvegi, L.W. Horowitz. Multi-model surface temperature responses to removal of U.S. sulfur dioxide emissions.  Submitted for publication in J. Geophys. Res., 2017.

2. Myhre, G., D. Shindell, F.-M. Bréon, W. Collins, J. Fuglestvedt, J. Huang, D. Koch, J.-F. Lamarque, D. Lee, B. Mendoza, T. Nakajima, A. Robock, G. Stephens, T. Takemura and H. Zhang, 2013: Anthropogenic and Natural Radiative Forcing. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

3. Tilmes, S., Lamarque, J.-F., Emmons, L. K., Kinnison, D. E., Ma, P.-L., Liu, X., Ghan, S., Bardeen, C., Arnold, S., Deeter, M., Vitt, F., Ryerson, T., Elkins, J. W., Moore, F., and Spackman, R.: Description and evaluation of tropospheric chemistry and aerosols in the Community Earth System Model (CESM1.2), Geosci. Model Dev., 8, 1395-1426, doi:10.5194/gmd-8-1395-2015, 2015.