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Using volcanoes to unlock the secrets of aerosol-cloud interactions

Holuhraun fissure eruption (Credit: Anja Schmidt, University of Leeds).

July 2017 - Research led by Exeter University and in collaboration with the Met Office shows that a spectacular six-month Icelandic lava field eruption could provide the key for scientists to unlock the role aerosols play in climate change.

Aerosols are tiny particles suspended in the atmosphere. They have a potentially large cooling effect on climate via:

  1.  direct effects, whereby they reflect sunlight back to space;
  2.  indirect effects, which modify the properties of water droplets in clouds causing the clouds to become brighter and reflect more sunlight back to space.

Emissions of aerosols from human activities thus exert a cooling that, to some extent, counterbalances global warming from increased concentrations of greenhouse gases.  However, the strength of this cooling effect is not well known, particularly the cooling effect associated with cloud-aerosol interactions. This means they are responsible for a large amount of uncertainty in global warming projections. 

Large volcanic eruptions produce sulphur dioxide, which in turn produces aerosols and thus represent a natural experiment through which to quantify aerosol–cloud interactions. The massive fissure eruption of Holuhraun, Iceland, in 2014-2015 emitted sulphur dioxide.  At its peak, this was almost 10 times higher than that from all 28 EU countries added together. This provided a unique opportunity to investigate aerosol-cloud interactions. In a paper that appeared in Nature (Malavelle et al., 2017), with several Met Office Hadley Centre co-authors, we show that the fissure eruption reduced the size of liquid cloud droplets—consistent with expectations—but had no discernible effect on other cloud properties (Figure 1).  
 

Figure 1. Showing the change in the climatological a) effective size of water droplets and c) cloud liquid water amount detected by the MODIS satellite sensor in October 2014. The panels on the right hand side of these panels represent the zonal mean, with the grey envelope representing a single standard deviation derived from the period 2002-2013. Probability distribution functions of the two variables are shown in b) and d) respectively. From Malavelle et al (2017).
 

The reduction in cloud droplet size led to brighter clouds and a global cooling effect of around −0.2 watts per square metre for September to October 2014. Changes in cloud amount or cloud liquid water amount, however, were undetectable, indicating that these indirect effects, and cloud systems in general, are well buffered against aerosol changes.

Malavelle et al (2017) then tested the results from global climate models which aimed to simulate the impact of the Holuhraun volcano on cloud properties and found that, while the most up to date model (HadGEM3-UKCA) was able to represent both the impact on cloud droplet effective size and the minimal impact on the cloud liquid water, earlier variants of the model (HadGEM3-CLASSIC) and other climate models were not (Figure 2). This result will help to reduce uncertainties in future climate projections and provide more accurate information to decision-makers, because we are now able to reject results from climate models with an excessive liquid-water response.  

Figure 2. Showing the change in cloud effective radius (left column) and the change in the cloud droplet effective size (right column) caused by the emissions of sulphur dioxide from Holuhraun. The observations are shown in the ‘MODIS’ retrievals in the final row. Ticks and crosses are used to show whether the model simulations of the cloud effective radius and liquid water path are in agreement (ticks) or disagreement (crosses) with the observations from the MODIS satellite.

Full reference

Malavelle F, et al. 2017. Strong constraints on aerosol-cloud interactions from volcanic eruptions, Nature, 546, 485–491,doi:10.1038/nature22974.

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