Two More Extreme UK Months: March 2013 and April 2011

My previous post showed how December 2015 was not only the mildest on record in the Central England Temperature (CET) record, but also the mildest compared to recent and succeeding years, that is, compared to the 21 year running mean December temperature (though I had to extrapolate the 21-year running mean forward).

December 2010, though not quite the coldest UK December in the CET data, was the coldest compared to the running 21 year mean.

I speculated that global warming might lead to a greater range of temperatures, at least until the planet reaches thermal equilibrium, which could be some time – thousands of years, maybe.  The atmosphere over land responds rapidly to greenhouse gases. But there is a lag before the oceans warm because of the thermal inertia of all that water. One might even speculate that the seas will never warm as much as the land, but we’ll discuss that another time. So in UK summers we might expect the hottest months – when a continental influence dominates – to be much hotter than before, whereas the more usual changeable months – when maritime influences come into play – to be not much hotter than before.

The story in winter is somewhat different.  Even in a warmer world, frozen water (and land) will radiate away heat in winter until it reaches nearly as cold a temperature as before, because what eventually stops it radiating heat away is the insulation provided by ice, not the atmosphere.  So the coldest winter months – when UK weather is influenced by the Arctic and the Continent – will be nearly as cold as before global warming.   This will also slow the increase in monthly mean temperatures.  Months dominated by tropical influences on the UK will therefore be warmer, compared to the mean, than before global warming.

If this hypothesis is correct, then it would obviously affect other months as well as December.  So I looked for other recent extreme months in the CET record.  It turns out that the other recent extreme months have been in late winter or early spring.

Regular readers will recall that I wrote about March 2013, the coldest in more than a century, at the time, and noted that the month was colder than any previous March compared to the running mean.  I don’t know why I didn’t produce a graph back then, but here it is:

Just as December 2010 was not quite the coldest December on record, March 2013 was not the coldest March, just the coldest since 1892, as I reported at the time.  It was, though, the coldest in the CET record compared to the 21-year running mean, 3.89C below, compared to 3.85C in 1785.  And because I’ve had to extrapolate, the difference will increase if the average for Marches 2016-2023 (the ones I’ve had to assume) is greater than the current 21-year mean (for 1995-2015), which is more than half likely, since the planet is warming, on average.

We’re talking about freak years, so it’s surprising to find yet another one in the 2010s.  April 2011 was, by some margin, the warmest April on record, and the warmest compared to the 21-year running mean:

The mean temperature in April 2011 was 11.8C.  The next highest was only 4 years earlier, 11.2 in 2007.  The record for the previous 348 years of CET data was 142 years earlier, in 1865, at 10.6C.

On our measure of freakishness – deviation from the 21-year running mean – April 2011, at 2.82C, was comfortably more freakish than 1893 (2.58C), which was in a period of cooler Aprils than the warmest April before the global warming era, 1865.  The difference between 2.82C and 2.58C is unlikely to be eroded entirely when the data for 2016-2021 is included in place of my extrapolation.  It’s possible, but for that to happen April temperatures for the next 6 years would need to average around 10C to sufficiently affect the running mean – the warmth in the Aprils in the period including 2007 and 2011 would need to be repeated.

So, of the 12 months of the year, the most freakishly cold for two of them, December and March, have occurred in the last 6 years, and so have the most freakishly warm for two of them, December and April. The CET record is over 350 years long, so we’d expect a most freakishly warm or cold month to have occurred approximately once every 15 years (360 divided by 24 records).  In 6 years we’d have expected a less than 50% chance of a single freakishly extreme monthly temperature.

According to the CET record, we’ve had more than 8 times the number of freakishly extreme cold or warm months in the last 6 years than would have been expected had they occurred randomly since 1659.

And I bet we get more freakishly extreme cold or warm months over the next 6 years, too.