stratospheric aerosol

Small volcanic eruptions and the global warming ‘pause’

Wellcome Library, London  Mount Vesuvius emitting a column of smoke after its eruption on 8 August 1779. Coloured etching by Pietro Fabris, 1779.

Wellcome Library, London
A small eruption of Mount Vesuvius on 8 August 1779, part of a sequence that culminated in a moderate eruption. Coloured etching by Pietro Fabris, 1779. Copyrighted work available under Creative Commons Attribution only licence CC BY 2.0

A new paper in Nature Geoscience by Santer and colleagues revisits the volcanic scenarios used in modern climate model simulations. The authors consider the effects of including a ‘more realistic’ model for the influence of small volcanic eruptions on the climate system over the past two decades. Of course, more realistic means more difficult.. and one of the long-standing and unresolved problems with small volcanic eruptions is that not only are they small, but their consequences are unpredictable. These complications arise, in part, from the fact that the part of the volcanic system that is responsible for the climate impact are the emitted gases (notably, sulphur dioxide or SO2), and not the volcanic ash. In real volcanoes, these two parameters don’t seem to be very well correlated – and it has been well known for some time that small but explosive eruptions of sulphur-rich magmas might well have a disproportionate effect on the climate system (see, for example, Rampino and Self, 1984; Miles et al., 2004). For this reason, models of volcano-climate impact that only use information on eruption size (as measured by the Volcanic Explosivity Index) will usually only be a poor approximation to reality. A better representation might instead be a volcanic sulphur dioxide climatology, building on the extensive work of the volcanic emissions satellite-remote sensing community since the first volcanic plume satellite measurements in 1979. The currently most up to date compilations of volcanic SO2 emissions since 1996 can be found in Carn et al., (2003) and McCormick et al., (2013).

Reading between the lines, it looks as though Santer and colleagues have come to a similar conclusion – finding that their model simulations get a little closer to observations of tropospheric temperature trends when they introduce a ‘realistic’ volcanic scenario to simulate the past 25 years of eruptions. What a pity that the volcanic dataset they relied on to line up particular eruptions with aerosol optical depth perturbations was patched together from secondary sources.  Clearly, as they suggest, more work is needed – but why not start by bringing the  climate modeling community and volcanologists together to find out what we each think that we know ?

Further reading.

Carn SA et al. 2003 Volcanic eruption detection by the Total Ozone Mapping Spectrometer (TOMS) instruments: a 22-year record of sulphur dioxide and ash emissions, In: Oppenheimer et al. (eds), Volcanic Degassing, Geological Society, London, Special Publications 213, 177-202.

McCormick BT et al. 2013 Volcano monitoring applications of the Ozone Monitoring Instrument, In: Pyle DM et al. (eds), Remote Sensing of Volcanoes and Volcanic ProcessesGeological Society, London, Special Publications 280, 1259-291.

Miles GM, Grainger RG and Highwood EJ 2004 The significance of volcanic eruption strength and frequency for climate Q. J. R. Met. Soc. 130 2361–76

Rampino MR and Self S 1984 Sulphur-rich volcanic eruptions and stratospheric aerosols, Nature 310, 677 – 679

Santer B et al, 2014, Volcanic contribution to decadal changes in tropospheric temperature Nature Geoscience (2014) doi:10.1038/ngeo2098

Related posts.

For more information on William Hamilton and Vesuvius, try this delightful blog post by Karen Meyer-Roux.

Time to move scientific debate into the open?

A few months ago, I got a routine request to review a paper about the fate of the plume formed during the 2011 eruption of Nabro volcano, Eritrea. The topic looked interesting, and so I agreed and duly reported. A few weeks later, the journal asked if I might write a commentary to introduce the paper, essentially as a bit of advertising. It wasn’t too hard to agree to that either; after all, you don’t often get the chance to write short opinion pieces for journals, and this was an area where I had a few things to say. So I duly wrote a short piece on ‘small volcanic eruptions and stratospheric aerosol‘ and it went live a few days later.

Within a matter of days, the journal editor emailed again. Someone had submitted a comment on my piece, and the editor wished it to be corrected. My first instinct was  surprise – I had written an opinion piece and I thought I had been quite careful in checking the facts.  But the comment picked up on a single throwaway statement in the discussion. In my original article I had widened the discussion about the Nabro plume to bring in the interpretation from a recent paper in Science which had suggested that atmospheric circulation, related to the Asian monsoon, may have helped to loft the initial volcanic plume from the upper levels of the troposphere into the stratosphere. I perhaps gave way when I should not have done, but duly wrote a correction (corrigendum) to clarify the point, and that was eventually published. Now even though my commentary and correction are all in the public domain, there is no record of the discussion that went on behind the scenes. The originator of the comment is not identified, and the comment that prompted the correction was not published. This week, however, it has all become a little clearer: a pair of formal comments to the original Science paper, and a reply, have now been published in Science. The comments were submitted in August, about the time that my original piece came out – which explains why the rebuke arrived so quickly. But of course, as is usual with these things the comments and reply illuminate the problem (did the Nabro plume reach the stratosphere of its own accord, or not?) but not the solution.

The whole process  of writing the response, checking the proofs and signing the copyright forms to write a one paragraph response to an anonymous comment on what was only ever intended as an opinion piece took a few weeks of to-and-fro. This seems both rather archaic, and unneccessarily formal, compared to the ease with which one can have civilised and informed debate in blogs and other social media. Is it time for journals to move this sort of  ‘comment’ and ‘reply’ out of the constraints of formal ‘publication’, and into the public domain?


Bourassa A E et al., 2012 Large volcanic aerosol load in the stratosphere linked to Asian monsoon transport Science 337 78–81

Sawamura P et al 2012 Stratospheric AOD after the 2011 eruption of Nabro volcano measured by lidars over the Northern Hemisphere Environ. Res. Lett. 7 034013