Three metre season?

Update: Corrected for skew…

In February 1969 an excited schoolboy from the subtropics visited Australia’s highest mountain, Mt Kosciuszko¹. I remember grilling the friends we stayed with in Canberra, “Is there any snow left” (I’d never seen snow of course). In February? That would be unlikely, surely, but they told me there was plenty, as indeed there was. The drifts were still metres deep beside the road as we drove our EH up the narrow dirt track to the top. You see, 1968 was one of those “3 m seasons” (greater than 3 m peak snow depth at Spencers Creek — beside the summit road near Charlotte Pass, midway between Perisher Valley and Thredbo, NSW).

Snow near the Kosciuszko summit in January 1969, from Bruce Thomas, via Flickr


Three metre seasons are the really big ones in Australia. They used to be relatively common; there were three in the first twenty years of the 60-year long Spencers Creek snow depth record. But that has changed: not one of the last 20 years even approached 3 m peak depth, and the last time we made it to 3 m, back in 1992, it was very likely the result of the short-lived dip in global temperatures following the huge Mt Pinatubo volcanic eruption in the Philippines in June of the previous year. That eruption — the biggest of the century — injected about 17 million tonnes of microscopic sulfate aerosols into the stratosphere, where they remained for a couple of years reflecting incoming sunlight and cooling the planet.

What are the odds that we’ll see another 3 m season in our lifetimes, or, say, the lifetime of someone born on the infamous 17 July 2014? I’ve posted at length this year on the future of Australian snow cover. The analysis is pretty rigorous, and the view is far from pretty:

Future trend of Australian snow depth

Spencers Creek snow depth future


You can see that 3 m is a long way away from where we are in 2014, but how far? That’s fairly easy to calculate², and it looks like this:



That is roughly a 30% chance of ever seeing another 3 m season in Australia. That’s the total chance, not per year, and most of that accrues over the next couple of decades. After that, the chances are near enough to zero³.



1. Actually that would be Mawson Peak on Heard Island, which even the CIA agrees is part of Australia. But please please please, it’s “Cos-chews-co” … as my Polish friend and lover of fine vodkas would attest. Look at the damn spelling.

2. To get this right it turns out we need to allow for the small positive skew in the residuals — the observation that excursions on the high side of trend tend to stretch a little further than those on the low side. I’ve done that by fitting a Pearson type III distribution (also called a 3-parameter gamma or shifted gamma distribution), which is like a normal distribution (ordinary bell curve) with the top pushed to the left a bit, raising the right tail (“positive skew” … yeah I know, it’s a definitional thing).


3. If no significant emission reduction measures are adopted, which seems a pretty safe bet in the current climate. And if you’re pedantically thinking “ever” is a very long time, consider that on the best evidence about a third of this year’s increase in atmospheric CO2 concentration will still be in the atmosphere in 10,000 years. Yes, that is right, 10,000 years.