How unusual was the cool UK summer of 2011?

Why do so many in the media feel they have to get their story in before the final whistle? It’s always a risk. Towards the end of August, a number of articles, typified by this one in the Guardian, trumpeted 2011 as “the coldest summer since 1993”. Political correctness is the order of the day – judging from the figures quoted, the record refers to the whole of the UK. I prefer to use the Central England Temperature (CET) record, which goes back further, to 1659. And I waited until the final data was in (there’s always a delay at the end of the month before the Met Office provide the final figure) and updated my spreadsheet. Here’s my latest summer temperature graph:

CET for Summers 1660-2011 (smoothing shown at central point of date range)

Note that my running means (smoothing) are shown centred, i.e. for the central of the 5, 11 or 21 years averaged. I tried the possible alternatives (i.e. trailing and forward – the latter to try to see the effect of events, such as eruptions), but this representation seems clearest to me. This way, you can most easily see the effect of, for example, the mystery eruption of 1809 and the Tambora eruption of 1815, with all curves dipping at about the same time.

I was expecting to be writing that a comparison with 1993 is not a level playing field, since the eruption of Pinatubo in 1991 cooled the whole planet for a few years (see the graphs from James Hansen that I posted in 2010), making 2011 more freakish, since there hasn’t been a recent eruption. But, in the CET record at least, summer 2011 was in fact colder than 1993.

As the graph shows, there were some colder summers in the mid 1980s, but, again, the whole planet was cooled a tad at that time by the eruption of El Chichon in 1982.

So you have to go back to the 1970s to find a summer cooler than 2011 that wasn’t induced by a volcanic eruption.

Still, you can expect the coldest summer in 40 years every 40 years, so on this reckoning 2011 was not that exceptional – compared to, say, December 2010.

But let’s go a little bit further and take global warming into account. Because of global warming we’d expect warmer summers. Indeed, as the graph shows, prior to 1933, the CET summer mean had only exceeded 17C twice (in 1826 and 1846). The mean CET touched 17C in 1933 and edged past it in 1947. But in the last 40 years it has passed that mark on 5 occasions: 1976, 1983, 1995, 2003 and 2006. The 5, 11 and 21 year running means have all broken new ground.

We should really judge the freakishness of 2011 against the prevailing summer temperature. The trouble is, we don’t know whether temperatures will continue to increase, level off for a decade or two, or even dip – that’s why the 11 and 21 year running mean curves stop before they get to the present day. If summers over the next few years are as warm as from 2003-6, then 2011 will look very unusual – perhaps the most atypical summer since 1860, which was more than 1.5C cooler than might have been expected.

On the other hand, if it turns out that the atypical summers were 2003-6, and temperatures level off for a while, then summer 2011 will just represent the sort of anomaly that might be expected every few decades, rather than a once a century or two event.

Regardless, 2011 is a long way from matching 1725 as the most disappointing summer in the CET record. 1725 was even cooler than 1816, the “year without a summer” following the Tambora eruption!

So, 2011 was surprisingly cool, but not unprecedented.

Incidentally, anyone who followed the link to my previous post which looked at global temperature data might have noticed that the graph of the mean summer UK CET record is uncannily similar in shape to that of (annual, not just summer) Northern Hemisphere (NH) temperatures as a whole.

For more convenient comparison here’s a more recent graph (i.e. including 2010) from the GISS graph site (we’re primarily interested in the solid red line representing the 5 year running mean NH temperature):

The hemispheric temperature record from GISS

Note how, in both graphs, the temperature peaks around 1900, then dips (usually attributed to the 1902 Santa Maria eruption), rises from around 1920 to a peak around 1940, dips again to 1970 or so, then rises into the new millennium. Overall, the magnitude of UK summer temperature changes is about the same as that for the NH as a whole, though the 1930s to 1940s peak is a little more pronounced. So it’s not just UK summer temperatures that vary – as I said, in comparing summer temperatures for freakishness (rather than trends), we need to take account of global warming.

Note also the effects of the eruptions of Pinatubo (1991) and El Chichon (1982) on the NH temperature record (or at least the dips in NH temperature following the dates of the eruptions!). This justifies my decision to exclude 1993 and the mid 1980s summers from the comparison with 2011.

I recently visited my storage unit and discovered that some boxes had fallen and damaged a fan I bought in response to the heat in, I think, 2005. The fan had been gathering dust for a few years – I haven’t needed it. The fact that I’ve paid to store the thing surely shows, though, that I certainly didn’t expect such a change from 2003-6, when all summers exceeded a mean CET of 16C, to 2007-11 when none have (the sudden dip in summer temperatures is clearly shown by the green 5 year running mean in the first figure, above). This weather/climate business is sure full of surprises!