The Statistics of Winter
Question: when is a trend not a trend?
Answer: when it’s been going barely a week.
Or, to put it more technically, for something to become a trend in any meaningful way, the event has to recur repeatedly. ‘Once or twice’ is not ‘repeatedly’. I’m writing this as we’re currently having an actual winter here in the UK, and we also had a winter last year. That’s two in a row, and it’s bound to summon climate-denialist nutcases soon. Two in a row must be a trend, right?
Really, really no.
In fact, we don’t even know yet if this winter even will be cooler than normal. The current cold snap started less than a fortnight ago, and winter doesn’t end until March. It’s barely even December now. For all we know, temperatures may well bounce back to their historical averages in January and February.
And even if we have two cold winters in a row, well, is that actually that odd? A quick glance at Wikipedia suggests that there have been at least five significantly-cold winters here in the last century (1946-47, 1962-63, 1990-91, 2009-10 and maybe 2010-11). And that’s just the ones that have made it to Wikipedia – anecdotal discussion with other people suggests that the winter of (IIRC) 1981 should count as well.
Straight away, we’re heading for half a dozen cold winters. It seems they’re not actually that rare.
Anyway, going with the above list, we have 5 definitely-cold winters in 100 years, which naively implies a 5% chance of any given year being cold. Granted, 5% seems like a small number, but it isn’t that small. So this means that although two randomly-cold ones in a row aren’t very likely, it’s not vanishingly unlikely either. Or, to phrase my point a bit more clearly, cold winter DOES NOT EQUAL no climate change.
Now, if the winter of 2010-11 stays cold and next year’s winter is also cold as well, then maybe there’ll be a trend starting to emerge. (I’ll consider the odds of three cold winters in a row as getting a bit on the low side.) But, at the time being, I don’t feel that any conclusion can be drawn from the current weather, except that you shouldn’t go outside without a hat!