Something strange is happening to our weather

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Did you know, the Met Office now posts UK weather station data online? It’s pretty easy to rummage around in the numbers, so we've been having a go. Take a look at this, our Carbon Census Chart of the Month for March 2011.

This chart (which is from the Heathrow weather station, but typical of weather stations around the UK) shows three rather strange things, as follows:

First strange thing: our winters have been getting colder. The coldest temp has got colder for 4 years in a row. That's only happened once before at Heathrow since records began. 

Second strange thing : our summers have been getting hotter. The hottest temp has got hotter for 3 years in a row, and that's also only happened once before in this dataset

But the third really, really strange thing is that the first two strange things are happening at the same time, which appears to be a genuinely unprecedented event for these isles.

In the past, when there has been any kind of run of warming or cooling in the weather, it has either got warmer for a few years, or got colder for a few years. There is no previous example of the hottest month getting hotter and the coldest month getting colder, for three and four years in a row, in either the Heathrow data since 1948 (plotted above) or in the data from older weather stations such as Oxford which go all the way back to the 1850s. (Take a minute to look at the analysis we've done of the Oxford dataset, if you like this kind of thing you'll get the idea straight away.)

So the next question is, could this be a coincidence? Well perhaps it could, but to answer that question we would need to have a Theory of the Weather. We would need to say something like this: here is how we expect the weather to behave, here is what it's been up to recently, and here is the chance of that happening for no particular reason, i.e. just by fluke. In statistical terms, in other words, we need a null hypothesis.

Of course, we're not meteorologists over here, so we haven't got a Theory of the Weather, so we haven't got a proper null hypothesis. But heck, let's just make one up and see what happens. Let's assume that there are no long term changes in the weather going on, and so the annual high and lo temperatures just bounce around a bit from year to year, with a normal distribution and standard deviation around some long term average value. We know that's not true, exactly, but it's not far off... and here's the interesting observation: if this cartoon model of the weather were true, then the chance of drawing the chart above by fluke would be less than 1 in 100,000.

So on that basis, I'd like to make a wager. I'll wager that this pattern is not a fluke at all, that it's an extremely unusual anomaly in historical terms, and that within 12 months of today someone will publish a peer reviewed article showing that this is the case. And just a hunch, of course, but I also think we'll hear a lot of news this year that climate change is accelerating and the impacts are worse than we thought. Increasing volatility (widening extremes, rather than rising averages) will be the climate change story of 2011.



For those who are interested, the Met Office weather station data can be downloaded here, and the Oxford analysis going back to 1852 is here. Many thanks to Lora Brill who helped me with a previous draft of this article.