Aerosols have a potentially large effect on climate, particularly through their interactions with clouds, but the magnitude of this effect is highly uncertain. Large volcanic eruptions produce sulfur dioxide, which in turn produces aerosols; these eruptions thus represent a natural experiment through which to quantify aerosol–cloud interactions. Here we show that the massive 2014–2015 fissure eruption in Holuhraun, Iceland, reduced the size of liquid cloud droplets—consistent with expectations—but had no discernible effect on other cloud properties. The reduction in droplet size led to cloud brightening and global-mean radiative forcing of around −0.2 watts per square metre for September to October 2014. Changes in cloud amount or cloud liquid water path, however, were undetectable, indicating that these indirect effects, and cloud systems in general, are well buffered against aerosol changes. This result will reduce uncertainties in future climate projections, because we are now able to reject results from climate models with an excessive liquid-water-path response.
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J.M.H., A.J., M.D., B.T.J., C.E.J., J.R.K. and F.M.O.C. were supported by the Joint UK BEIS/Defra Met Office Hadley Centre Climate Programme (GA01101). The National Center for Atmospheric Research is sponsored by the US National Science Foundation. S.B. and L.C. are respectively Research Fellow and Research Associate funded by FRS-FNRS. P.S. acknowledges support from the European Research Council (ERC) project ACCLAIM (grant agreement FP7-280025). J.M.H., F.F.M., D.G.P. and P.S. were part-funded by the UK Natural Environment Research Council project ACID-PRUF (NE/I020148/1). A.S. was funded by an Academic Research Fellowship from the University of Leeds and NERC urgency grant NE/M021130/1 (‘The source and longevity of sulphur in an Icelandic flood basalt eruption plume’). R.A. was supported by the NERC SMURPHS project NE/N006054/1. G.W.M. was funded by the National Centre for Atmospheric Science, one of the UK Natural Environment Research Council’s research centres. D.P.G. is funded by the School of Earth and Environment at the University of Leeds. G.W.M. and S.D. acknowledge additional EU funding from the ERC under the FP7 consortium project MACC-II (grant agreement 283576) and Horizon 2020 project MACC-III (grant agreement 633080). G.W.M., K.S.C. and D.G. were also supported via the Leeds-Met Office Academic Partnership (ASCI project). The work done with CAM5-Oslo is supported by the Research Council of Norway through the EVA project (grant 229771), NOTUR project nn2345k and NorStore project ns2345k. We thank the following researchers who have contributed to the development version of CAM5-Oslo used in this study: K. Alterskjær, A. Grini, M. Hummel, T. Iversen, A. Kirkevåg, D. Olivié, M. Schulz and Ø. Seland. The AQUA/MODIS MYD08 L3 Global 1 Deg. data set was acquired from the Level-1 and Atmosphere Archive and Distribution System (LAADS) Distributed Active Archive Center (DAAC), located in the Goddard Space Flight Center in Greenbelt, Maryland (https://ladsweb.nascom.nasa.gov/). This work is dedicated to the memory of co-author Jón Egill Kristjánsson who died in a climbing accident in Norway.
This animation shows 60 days (from the 1st of September 2014) of IASI data (top) and HadGEM3 simulation results (Bottom) of the SO2 atmospheric column content expressed in Dobson units (DU).
This animation shows 30 days (from the 1st of September 2014) of MODIS data from daily Aqua orbits above Iceland processed to look like true-color photographs of the planet's surface.
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Nature Communications (2018)