Just How Exceptionally Mild Was December 2015 in the UK?

“Very” is the answer, based on the 350+ year long Central England Temperature (CET) record.  Here’s a graph of all the CET December temperatures since 1659:

As is readily apparent from the graph, the mean temperature of 9.7C in December 2015 was much higher than in any previous year.  In fact, only twice before had the average exceeded 8C.  Decembers 1934 and 1974 were previously tied as the mildest on 8.1C.

But how much was the recent mild weather due to global warming and how much to normal variability? Apart from anything else the mild spell has to coincide with a calendar month to show up in this particular dataset.  And it so happened that the weather turned cooler even as the champagne corks were in the air to celebrate the end of 2015.

To help untangle trends from freak events, I’ve included some running means on the graph above.  The green line shows the mean December temperature over 5 year periods.  For example, thanks in large part to December 2015, the 5 Decembers from 2011 to 2015 are the mildest in succession, though other periods have come close.

The red and black lines show 11 and 21 year running means, respectively.  The black line therefore represents the long-term trend of December temperatures.  These are close to the highest they’ve ever been, though in some periods, such as around the start of the 19th century, the average December has been as much as 2C colder than it is now.  Perhaps some exceptionally mild Decembers back then – such as 1806 – were as unusual for the period as December 2015 was compared to today’s Decembers.

I therefore had the idea to plot the deviation of each December from the 21 year mean centred on that year, represented by the black line on the graph above.  If you like, I’ve simply subtracted the black line from the blue line.

A health warning is necessary.  I’ve had to extrapolate the 21 year mean, since we don’t yet know what weather the next 10 Decembers (2016 to 2025) will bring.  We’ll have to wait until 2025 to see precisely how unusual December 2015 will prove to have been.  In the meantime, I’ve set the mean temperature for 2016 through 2025 to the last 21 year mean (i.e. the one for the years 1995 through 2015).

With that proviso, here’s what we get:

The green line now shows the difference between the mean December temperature for a given year and the mean December temperature for the 21 years including the 10 before and the 10 after the given year.

We can see that December 2015 was, at 4.91C much more mild than contemporary Decembers than was any other December, with the proviso that I’ve not been able to take Decembers after 2015 into account.

The next most freakish December was the aforementioned 1806 which was 3.86C warmer than the mean of Decembers 1796 through 1816.

What’s going on? Is it just weather – something to do with the ongoing El Nino, perhaps – or is something else afoot?

One hypothesis might be that, with the climate out of equilibrium due to global warming, greater variability is possible than before. Our weather in 2015 may have been driven by a heat buildup somewhere (presumably in the ocean) due to global warming. On average this perhaps doesn’t happen – we may suppose our weather to be often determined by regions of the planet where the temperature hasn’t changed much, at least at the relevant time of year. Specifically, the Greenland ice-sheet hasn’t had time to melt yet.

It won’t have escaped the notice of my eagle-eyed readers that the graph above also shows 2010 to be the most freakishly cold December in the entire CET record.

Perhaps, until the ice-sheets melt, the deep oceans warm and the planet reaches thermal equilibrium, we’ll find that when it’s cold it’s just as cold as it used to be, but when it’s warm it’s a lot warmer than it used to be.   Just a thought.

It might be worth mentioning a couple of other, not necessarily exclusive, possibilities:

• Maybe the situation will continue even when the planet is in thermal equilibrium.  Maybe, for example, assuming there is some limit to global warming and the Arctic seas still freeze in winter, we’ll still get cold weather in winter just or nearly as cold as it ever was, but we’ll get much warmer weather when there’s a tropical influence.
• It could be that weather patterns are affected by global warming, especially through the later freezing of Arctic ice.

Or December 2015 could just have been a freak weather event.