Clouds play a critical role in modulating the Earth’s radiation balance and climate. Anthropogenic aerosol particles that undergo aging processes, such as soot, aid cloud droplet and ice crystal formation and thus influence the microphysical structure of clouds. However, the associated changes in cloud radiative properties and climate effects remain uncertain and are largely omitted in climate models. Here we present global climate simulations of past and future effects of both ozone-aged soot particles acting as cloud condensation nuclei and sulfuric acid-aged soot particles acting as ice-nucleating particles on the structure and radiative effects of clouds. Under pre-industrial conditions, soot aging led to an increase in thick, low-level clouds that reduced negative shortwave effective radiative forcing by 0.2 to 0.3 W m−2. In the simulations of a future, warmer climate under double pre-industrial atmospheric carbon dioxide concentrations, soot aging and compensating cloud adjustments led to a reduction in low-level clouds and enhanced high-altitude cirrus cloud thickness, which influenced the longwave radiative balance and exacerbated the global mean surface warming by 0.4 to 0.5 K. Our findings suggest that reducing emissions of soot particles is beneficial for future climate, in addition to air quality and human health.
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The ECHAM-HAMMOZ model is made freely available to the scientific community under the HAMMOZ Software License Agreement, which defines the conditions under which the model can be used. More information can be found at the HAMMOZ website (https://redmine.hammoz.ethz.ch/projects/hammoz, last access: 16 June 2020). Scripts are available from ref. 79.
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We thank A. Ekman for her valuable input and feedback, S. Münch for providing his implementation of the cirrus scheme applied in this study and O. Sourdeval for providing multi-annual mean DARDAR NICE data. Funding was provided by the SNSF Ambizione Grant #PZ00P2_161343 (A.A.M. and F.F.), the ETH research grant ETH-25-15-1 (Z.A.K. and F.M.), the SNSF Early Postdoc Mobility Grant No. P2EZP2_191837 (F.M.) and the EU’s Horizon 2020 research and innovation programme FORCeS Grant No. 821205 (D.N.). The ECHAM-HAMMOZ model is developed by a consortium composed of ETH Zurich, the Max Planck Institute for Meteorology, Research Center Jülich, the University of Oxford, the Finnish Meteorological Institute and the Leibniz Institute for Tropospheric Research and managed by the Center for Climate Systems Modeling (C2SM) at ETH Zurich. The computing time for this work was supported by the Swiss National Supercomputing Centre (CSCS) under project ID s903 (U.L. and D.N.).
The authors declare no competing interests.
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Simulated ICNC in the 1 x CO2 climate and its modelled change in the future. Zonal, annual mean ICNC per liter of air in the 1xCO2 for the simulations REF (a), and Soot-CCN+INP (b), and their difference (c). Changes in zonal, annual mean ICNC between 2 x CO2 and 1 x CO2 for simulations REF (d), and Soot-CCN+INP (e), and their difference (f). Statistically insignificant changes are marked by dots. Source data
Shortwave (SW) and longwave (LW) effective radiative forcing (ERF) including both aerosol-radiation (ari) and aerosol-cloud interactions (aci) between present-day (2008) and pre-industrial (1850) times and their interannual standard deviations multiplied by 1.96 for the different simulations performed with ECHAM-HAM (see Fig. 4b). IRFari and cloud effects are computed following Ghan9, using an additional call to the radiation routine for aerosol-free conditions. “Cloud” is the sum of cloud adjustments (semi-direct aerosol effect) and ERFaci. The bottom three rows denote the equilibrium climate sensitivity (ECS), the change in precipitation rate and the hydrological sensitivity between the 2 x CO2 and 1 x CO2 simulations (see Fig. 4a) and their interannual standard deviations. Source data
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Lohmann, U., Friebel, F., Kanji, Z.A. et al. Future warming exacerbated by aged-soot effect on cloud formation. Nat. Geosci. 13, 674–680 (2020). https://doi.org/10.1038/s41561-020-0631-0