26 Apr 2022
26 Apr 2022
Status: this preprint is currently under review for the journal ACP.

Robust evidence for reversal in the aerosol effective climate forcing trend

Johannes Quaas1, Hailing Jia1, Chris Smith2,3, Anna Lea Albright4, Wenche Aas5, Nicolas Bellouin4,6, Olivier Boucher4, Marie Doutriaux-Boucher7, Piers M. Forster2, Daniel Grosvenor2, Stuart Jenkins8, Zig Klimont3, Norman G. Loeb9, Xiaoyan Ma10, Vaishali Naik11, Fabien Paulot11, Philip Stier7, Martin Wild12, Gunnar Myhre13, and Michael Schulz14 Johannes Quaas et al.
  • 1Universität Leipzig, Institute for Meteorology, Leipzig, Germany
  • 2University of Leeds, School of Earth and Environment, Leeds, U.K.
  • 3International Institute for Applied Systems Analysis, Laxenburg, Austria
  • 4Laboratoire de Météorologie Dynamique, Institut Pierre Simon Laplace, Sorbonne Université, Paris, France
  • 5Norwegian Institute for Air Research, Kjeller, Norway
  • 6University of Reading, UK
  • 7EUMETSAT, Darmstadt, Germany
  • 8University of Oxford, Atmospheric, Oceanic and Planetary Physics, Oxford, U.K.
  • 9NASA Langley Research Center, Hampton, USA
  • 10Nanjing University of Information Science & Technology, School of Atmospheric Physics, Nanjing, China
  • 11Geophysical Fluid Dynamics Laboratory, Princeton, USA
  • 12ETH Zürich, Department of Environmental Systems Science, Zürich, Switzerland
  • 13CICERO, Oslo, Norway
  • 14Norwegian Meteorological Institute, Oslo, Norway

Abstract. Anthropogenic aerosols exert a cooling influence that offsets part of the greenhouse gas warming. Due to their short tropospheric lifetime of only up to several days, the aerosol forcing responds quickly to emissions. Here we present and discuss the evolution of the aerosol forcing since 2000. There are multiple lines of evidence that allow to robustly conclude that the anthropogenic aerosol effective radiative forcing – both aerosol-radiation and aerosol-cloud interactions – has become globally less negative, i.e. that the trend in aerosol effective radiative forcing changed sign from negative to positive. Bottom-up inventories show that anthropogenic primary aerosol and aerosol precursor emissions declined in most regions of the world; observations related to aerosol burden show declining trends, in particular of the fine-mode particles that make up most of the anthropogenic aerosols; satellite retrievals of cloud droplet numbers show trends consistent in sign, as do observations of top-of-atmosphere radiation. Climate model results, including a revised set that is constrained by observations of the ocean heat content evolution show a consistent sign and magnitude for a positive forcing relative to 2000 due to reduced aerosol effects. This reduction leads to an acceleration of the forcing of climate change, i.e. an increase in forcing by 0.1 to 0.3 W m-2, up to 12 % of the total climate forcing in 2019 compared to 1750 according to IPCC.

Johannes Quaas et al.

Status: open (until 27 Jun 2022)

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Johannes Quaas et al.

Johannes Quaas et al.


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Short summary
Pollution particles cool climate and offset part of the global warming. But they are washed out by rain and thus their effect responds quickly to changes in emissions. We show multiple datasets to demonstrate that in many regions that are influenced by human emissoins, aerosol emissions and their concentrations declined, as did the effects on clouds. In consequence, the cooling impact on the Earth energy budget became smaller. This change in trend implies a relative warming.