20 Aug 2021

20 Aug 2021

Review status: this preprint is currently under review for the journal ACP.

Opportunistic Experiments to Constrain Aerosol Effective Radiative Forcing

Matthew Christensen1,2, Andrew Gettelman3, Jan Cermak4,5, Guy Dagan1, Michael Diamond6,7,8, Alyson Douglas1, Graham Feingold7, Franziska Glassmeier9, Tom Goren10, Daniel Grosvenor11, Edward Gryspeerdt12, Ralph Kahn13, Zhanqing Li14, Po-Lun Ma2, Florent Malavelle15, Isabel McCoy16,17, Daniel McCoy18, Greg McFarquhar19,20, Johannes Mülmenstädt2, Sandip Pal21, Anna Possner22, Adam Povey1,23, Johannes Quaas10, Daniel Rosenfeld24, Anja Schmidt25,26, Roland Schrödner27, Armin Sorooshian28,29, Philip Stier1, Velle Toll30, Duncan Watson-Parris1, Robert Wood6, Mingxi Yang31, and Tianle Yuan32,33 Matthew Christensen et al.
  • 1Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom
  • 2Atmospheric Science & Global Change Division, Pacific Northwest National Laboratory, Richland, WA 99352, Washington, USA
  • 3National Center for Atmospheric Research, Boulder, CO, USA
  • 4Karlsruhe Institute of Technology (KIT), Institute of Meteorology and Climate Research, Karlsruhe, Germany
  • 5Karlsruhe Institute of Technology (KIT), Institute of Photogrammetry and Remote Sensing, Karlsruhe, Germany
  • 6Department of Atmospheric Sciences, University of Washington, Seattle, USA
  • 7NOAA Chemical Sciences Laboratory (CSL), Boulder, Colorado, USA
  • 8Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado, USA
  • 9Department Geoscience and Remote Sensing, Delft University of Technology, PO Box 5048, 2600GA Delft, Netherlands
  • 10Universität Leipzig, Institute for Meteorology, Leipzig, Germany
  • 11National Centre for Atmospheric Sciences, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
  • 12Space and Atmospheric Physics Group, Imperial College London, UK
  • 13Earth Science Division, NASA Goddard Space Flight Center, Greenbelt MD USA
  • 14Dept of Atmospheric and Oceanic Science, University of Maryland, USA
  • 15Met Office, Atmospheric Dispersion and Air Quality, Fitzroy Rd, Exeter EX1 3PB, UK
  • 16Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA
  • 17University Corporation for Atmospheric Research, Boulder, CO, USA
  • 18Department of Atmospheric Sciences, University of Wyoming, Laramie, USA
  • 19Cooperative Institute for Mesoscale Meteorological Studies (CIMMS) and School of Meteorology
  • 20University of Oklahoma, Norman, OK
  • 21Department of Geosciences, Texas Tech University
  • 22Institute for Atmospheric and Environmental Sciences, Goethe University Frankfurt, Frankfurt/Main, Germany
  • 23National Centre for Earth Observation, University of Oxford, Oxford, OX1 3PU, United Kingdom
  • 24The Hebrew University of Jerusalem, Israel
  • 25Department of Geography, University of Cambridge, Cambridge, UK
  • 26Department of Chemistry, University of Cambridge, Cambridge, UK
  • 27Leibniz Institute for Tropospheric Research, Leipzig, Germany
  • 28Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ, USA
  • 29Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ, USA
  • 30University of Tartu, Institute of Physics, Tartu, Estonia
  • 31Plymouth Marine Laboratory, Prospect Place, Plymouth, PL1 3DH, United Kingdom
  • 32Joint Center for Earth Systems Technologies, University of Maryland, Baltimore County
  • 33Earth Science Division, NASA Goddard Space Flight Center, Greenbelt, MD

Abstract. Aerosol-cloud interactions (ACI) are considered to be the most uncertain driver of present-day radiative forcing due to human activities. The non-linearity of cloud-state changes to aerosol perturbations make it challenging to attribute causality in observed relationships of aerosol radiative forcing. Using correlations to infer causality can also be challenging when meteorological variability also drives both aerosol and cloud changes independently. Natural and anthropogenic aerosol perturbations from well defined sources provide “opportunistic experiments” (also known as natural experiments) to investigate ACI in cases where causality may be more confidently inferred. These perturbations cover a wide range of locations and spatio-temporal scales, including point sources such as volcanic eruptions or industrial sources, plumes from biomass burning or forest fires, and tracks from individual ships or shipping corridors. We review the different experimental conditions and conduct a synthesis of the available satellite data sets and field campaigns to place these opportunistic experiments on a common footing, facilitating new insights and a clearer understanding of key uncertainties in aerosol radiative forcing. Strong liquid water path increases due to aerosol perturbations are largely ruled out by averaging across experiments. Cloud albedo perturbations are strongly sensitive to background meteorological conditions. Opportunistic experiments have significantly improved process level understanding of ACI, but it remains unclear how reliably the relationships found can be scaled to the global level, thus, demonstrating a need for deeper investigation in order to improve assessments of aerosol radiative forcing and climate change.

Matthew Christensen et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Referee comment on 'Opportunistic Experiments to Constrain Aerosol Effective Radiative Forcing' by Christensen et al.', Anonymous Referee #1, 09 Sep 2021
  • RC2: 'Review of Christensen et al.', Anonymous Referee #2, 13 Sep 2021

Matthew Christensen et al.

Matthew Christensen et al.


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Short summary
Trace gases and aerosols (tiny airborne particles) are released from a variety of point sources around the globe. Examples include volcanoes, industrial chimneys, forest fires and ship stacks. These sources provide opportunistic experiments with which to quantify the role of aerosols in modifying cloud properties. We review the current state of understanding on the influence of aerosol on climate built from the wide range of natural and anthropogenic laboratories investigated in recent decades.