A precursor eruption of Pinatubo, June 1991 (USGS)
Large volcanic eruptions can affect global climate — ask the folk who starved to death in Europe in 1816, after the massive eruption of Tambora in faraway Indonesia the year before. Explosive eruptions often inject vast quantities of ultrafine material high into the stratosphere, where it spreads around the planet blocking incoming sunlight. Because the stuff is so fine it takes a long time to settle out, and while it’s there it cools the planet, improving our snow seasons¹.
Anecdotally, we often refer to the June 1991 eruption of Mt Pinatubo in the Philippines affecting our snow. Pinatubo was the biggest stratospheric eruption of the twentieth century², probably resulting in our last great three metre snow season (perhaps last ever), way back in 1992. But what about some numbers? As usual I’m just going to look for simple correlations.
We need a data series. Not all volcanoes affect weather and climate. We’re interested in very large explosive eruptions³ that inject millions of tonnes of sulfur compounds — particularly sulfur dioxide — high into the stratosphere, where they generate vast quantities of sub-micron sized droplets and particles, mostly of sulfuric acid⁴. The simplest measure is to look directly at those “stratospheric aerosols”. Aerosols are of course routinely measured today, but historically some extrapolation and guesswork is required. For convenience, I’m going to use the combined measured and guesstimated aerosol time series adopted to drive the NASA GISS global climate model⁵. That is available in both global and hemispheric versions, so I’ll of course focus on the Southern Hemisphere series.
Southern Hemisphere stratospheric aerosols
The plot is of “optical thickness” (opacity) at 550 nm wavelength, near the middle of the visible spectrum. Note the magnitude. The measure used is actually logarithmic (the negative natural logarithm of transmissivity), but the highest peaks are not far from simple fractional reductions. At its peak, Krakatoa in 1883 is thought to have produced nearly a 20% reduction in visible light transmission. This is no small effect.
As best we can tell big volcanic eruptions are just random arrivals, with no pattern. They produce rapid upward spikes in aerosol optical thickness which decay roughly exponentially, with a half life of less than a year. Most are completely gone within 3 – 4 years. Here’s a close up of the record since our best snow depth series began in the 1950s, with global average temperature overlaid for comparison:
Southern Hemisphere aerosols and global temperature
You can see the short-lived dips in global temperature after the eruptions of Agung on Bali and Pinatubo in the Philippines. It’s interesting that Agung in 1963-64 correlates with another of our three metre snow seasons, the second biggest ever in 1964.
Snow depth correlation
To the correlation. As before, I’m just going to look at the correlation with season peak snow depth at Spencers Creek, midway between Perisher Valley and Thredbo, NSW (data from Snowy Hydro). After some trial and error, I eventually opted to use the winter average Southern Hemisphere optical thickness, consistent with the averaging intervals used with most of the other parameters in my snow depth prediction model. The result looks like this:
Spencers Creek season peak snow depth vs aerosol optical depth
There’s substantial scatter, but the correlation coefficient is relatively strong at 8% … compared to the others. It’s also likely to be independent of them. I’ll be including aerosols in a new version of my snow depth prediction model for this year.
Notes and references
- Well of course that’s been thought of. Inject some aerosols and reverse our snow decline … and fix global warming while you’re at it. That’s certainly feasible, but not trivial. The quantities are not small. The Pinatubo volcanic eruption injected about 17 million tonnes into the stratosphere for a temperature drop of just 0.2°C, lasting maybe two years. We’d presumably be more efficient, but no, just adding sulfur to jet fuel won’t do it (most passenger aircraft fly too low). Then there’s the nasty side effects. Like blue skies being the stuff of memory; clear skies everywhere would be washed out white. And worried about changing weather? This doesn’t necessarily fix it, because the distribution of the imposed change is different to that from carbon dioxide (CO₂ warms the poles; this cools the tropics). And suppose you’re now sick of this, costing too much, want to stop … better not, because the world could suddenly warm by a lot (locally by many degrees), as your stuff settles out. More at Slate.
- The eruption of Novarupta in Alaska in 1912 was bigger in terms of total material expelled, but it had relatively little stratospheric impact, especially in the Southern Hemisphere.
- The stratosphere ranges from around 10 km to about 50 km above sea level. Generally we’re talking “ultra-Plinian” eruptions of “stratovolcanoes“. “Plinian” for Vesuvius and Pliny the Younger, who observed its 79 CE eruption; “strato” for a layered cone built up from expelled ash and viscous sliceous lavas (not for “stratosphere”, despite that they often affect it…).
- Also called “sulfate aerosols”, even though H₂SO₄ is not usually thought of as a sulfate. The name derives from other natural aerosols, which are predominantly sulfates.
- Sato, M., Hansen, J. E., McCormick, M. P., & Pollack, J. B. (1993). Stratospheric aerosol optical depths, 1850–1990. Journal of Geophysical Research: Atmospheres (1984–2012), 98(D12), 22987-22994.
