UK Heatwaves: What are the Global Warming Risks? (1) A Really Freak Summer

I suggested last time that 2018 could beat 1976 to the title of UK’s hottest summer ever in the whole Central England Temperature (CET) series which goes back to 1659. I now doubt that will happen since a change in the weather is on its way. Here are Weathercast’s projections, albeit a couple of days out of date as their site isn’t updating at the moment (I guess someone will be in the office on Monday morning!):

And here’s what the Met Office have to say:

The CET mean for August so far is impressive:
but a period of average daily mean temperatures (around 16C) will soon drag the monthly mean down below the 18C necessary for the June through August average to be higher than in 1976. Note that, after a cool start, it is now very possible that 2018 as a whole will be hotter than the previous hottest year in the entire CET record, 2014.

2018 has been exceptional, though. The problem is the arbitrary period (June, July and August) defined as the meteorological summer. But for 2018 to break all-time records it’s not even necessary to split months (e.g. by taking a period ending on 7th August). As I pointed out a fortnight ago, 2018 has been easily the hottest “early summer” – May, June and July – as well as easily the hottest for the period April through July. This seems very significant to me, because it illustrates the effect of global warming so starkly, but has been ignored or unnoticed by other commentators, so here again is a graph of mean annual May through July CET (as previously published, but with the final July figure of 19.1C incorporated):

The question is, how much worse could UK heatwaves get, with global warming?

Now, I find statements such as the following from a recent Guardian front-page lead to be extremely unsatisfactory:

“Events worse than the current heatwave are likely to strike every other year by the 2040s, scientists predict.”

Curiously, this sentence does not appear in the online version of the same article.

How can it be possible for summers as hot as 2018’s to occur every other year in less than 30 years?

The planet is warming at “only” somewhere around 0.2C per decade, so by “the 2040s” is unlikely to warm by more than 0.6C. And my patent graph above shows that May through July 2018 has been around 1.5C warmer than the same period has been on average in recent years (and more than 2C warmer than it used to be in the average year).

So it would seem that, even by the 2040s, another summer as hot as 2018 would be quite unusual if average summer temperatures are only 0.6C warmer than at present: the graph above shows few summers – less than 10% – 1C or more warmer than the 21-year running mean (the black line).

It seems the “heatwave every other year” statement refers to temperatures across Europe as a whole. This is problematic for two reasons.

First, as the European Environment Agency reports, land temperatures are rising nearly twice as fast as ocean temperatures:

“According to different observational records of global average annual near-surface (land and ocean) temperature, the last decade (2008–2017) was 0.89 °C to 0.93 °C warmer than the pre-industrial average, which makes it the warmest decade on record.

The average annual temperature for the European land area for the last decade (2008–2017) was between 1.6 °C and 1.7 °C above the pre-industrial level, which makes it the warmest decade on record.”

Second, the “every other year” claim may be based on the average for a large area. As pointed out by King and Karolyi in Nature (“Climate extremes in Europe at 1.5 and 2 degrees of warming” (pdf)):

“The highest changes in frequency are projected for the largest regions as the year-to-year variability is lower on these spatial scales…”

The UK is some islands next to Europe sticking out into the Atlantic at a latitude where weather systems usually move from west to east. Its climate is therefore influenced more by the ocean than the nearby continental landmass. Thus average UK summer temperatures have not been and will not by the 2040s rise as fast as for the land area of Europe as a whole.

The statement in the Guardian should not be taken as applicable to the UK.

Nevertheless, as summer 2018 shows, there are times when the UK is in step with Europe, climate-wise. And that’s when we get those exceptionally hot days and even entire summers.

The simple way to look at the risk of something worse than 2018 is to consider by how much freak UK summers in the past have been warmer than the average contemporaneous summer.

Let’s consider July and August first, since that’s when the heat is most intense. The following chart shows mean summer temperatures since 1659 at the top, with 11- (red) and 21-year (black) running means. At the bottom, in green, it shows the deviation of each year from the 21-year running mean centred on that date (extrapolated at the ends of the graph, by assuming no change from the first or last available year):

The most exceptional year in the entire 350 year series is 1911, when, as I mentioned in a previous post, the nobility reportedly played tennis in the altogether at their country estates, while riots broke out in the cities. The mean temperature in the CET series for July and August 1911 was 2.73C above the mean for 1901-1921.

If 2018 were to average 18C, as I previously assumed rather bullishly, it would only be 1.97C warmer than the mean nowadays (and maybe even less, since I’ve used the mean for 1998-2018 and not for 2008-2028, which won’t be available for another decade, but may well be higher). For 2018 to be as freakishly hot as 1911, the mean CET for August would have to be 19.6C, that is rather hotter than in the hottest August recorded (1995, at 19.2C), but following the third hottest July on record, at 19.1C. When there’s already a media frenzy about the heat in July, we’d have to experience an even hotter month – moreorless the current uncomfortable weather continuing for nearly another 4 weeks, rather than breaking up in a few days.  Not a pleasant prospect, but apparently possible, on the evidence of 1911.

And I mentioned earlier that May to July 2018 has been the hottest on record. But was it the hottest it could have been? It seems not. Other years were much more exceptional for their period, the record being 1976, which, relative to summers of the time, was more than 0.6C warmer from May to July than 2018:

So one real risk for the UK is that we could experience a truly freak summer, that is, a summer much hotter than the warmer summers we are experiencing because of global warming. 2018 has been exceptionally hot on some measures, but much of that has been due to global warming. It really hasn’t been a freak.

But has global warming changed what’s possible? Could there be even more serious heatwave risks for the UK than summer temperatures as much above the current norm as they were in 1911 and 1976? These questions will be addressed in the next exciting instalment!

Summer 2018: UK’s Hottest Ever?

2018 is already notable (pending final data for July) for the hottest May to July and the hottest April to July in the entire 360 years of the Central England Temperature (CET) record, as the graphs in my previous post show so eloquently. Nevertheless, besides foaming at the mouth that Englishmen are being advised not to go out in the mid-day sun (optional musical interlude), certain sections of the media are speculating rabidly as to whether 2018 could be the hottest summer ever. The Express, for example, announces that “Summer 2018 [is] on track to beat ALL RECORDS as HOTTEST day looms”, though neither the article, nor the accompanying Met Office video, quite say that.

When the media, and especially the Met Office, refer to “summer” they mean June, July and August. So pedantic. It can be nearly as hot in the UK in May and September, so comfortably the hottest May through July recorded in 360 years seems more significant to me than barely the hottest June through August.

Especially as the main feature of this summer is the lack of any significant breaks in the weather as opposed to the sheer heat – unless I’ve not been paying attention we don’t seem to have broken a single daily record for the UK as a whole so far. I went so far as to question in my post reporting on a fairly warm June whether we were experiencing a heatwave or just warm weather. I’ll grant, though, that this week, with peaks consistently over 30C here in London, does feel unpleasantly like a “heatwave”. And at least one daily record high may be anticipated.

So what are the chances of June to August, “summer”, being the hottest ever in the CET? Simple: the CET mean for August needs to be 18C or higher. I’ve put 18C in the data for August and produced this graph:

This shows that if the CET mean for August is 18C, summer 2018 will average 17.8C in that series, just pipping 1976’s 17.77C and the 17.6C recorded in 1826.

There are a couple of assumptions. First, I still have 19.3C as the figure for July. The Met Office page is currently showing 19.4C up to 23rd, so 19.3C should be safe enough, since the next few days are likely to drag up the average for the month, although the figure can change by a few tenths right at the end of the month. I understand this to be because data from remote weather stations comes in late and I’ve noticed that the monthly figure is usually adjusted downwards, at least in winter.  Second, the figure for June was adjusted down significantly (which caught me out somewhat), but that this has been queried. Obviously if that adjustment was erroneous and the June figure is revised upwards (which I don’t expect to happen), then August doesn’t need to be so hot for 2018 to break the summer record.

But how usual is an 18C mean in the CET for August? That presents the opportunity for another graph!:

As can be seen, 18C in August was pretty unusual for two or three centuries and not even achieved in 1976, which only managed 17.6C. But in the global warming era 18C is very possible, provided, of course, that current weather patterns continue.  If they don’t everyone will be wishing they’d trumpeted the record spring-into-summer heat!

July 2018 UK Weather: CET Records Set

Last month I jumped the gun to report the hottest UK June since 1976 in the Central England Temperature (CET) record.  I was slightly undone by a slight downward revision so that in the event June 2018 was only equal with that in 2003 as the warmest since 1976.  Despite that, the forecast for another week of temperatures reaching the 30Cs and the CET for July to date of significantly over 19C prompts me to call July 2018 even earlier as one of the three hottest on record in the CET.  Here’s a graph (the first of many, so be prepared!):

Only 2006 (19.7C), 1983 (19.5C) and now 2018 (the CET so far this month was 19.3C when I prepared this graph) have exceeded 19C in the CET (thanks, as ever, to the Met Office for the data). In fact, since the next hottest July was in 1783 at 18.8C – which should possibly even be discounted on the grounds that the heat was in part the effect of volcanic smog from the Icelandic volcano Laki – some wintry weather indeed would be necessary for July 2018 to now not be one of the three warmest, justifying my early call (though there’s a huge getting round to it factor in that!).

What is also striking about the July temperature graph is that the three hottest Julys – 2006, 1983 and 2018 – are all in the global warming era. Of course.

I’ve also labelled some notable years in this and subsequent graphs. In particular, I read articles drawing 1955 and 1911 to my attention. Ian Jack wrote nostalgically about 1955, though I do wonder if its impact was magnified by his age at the time. I’d personally rank 1983 – one of the few summers when I played tennis regularly – as up there with 1976. And I’m backed up by the CET data!

A brilliant Weatherwatch column in the Guardian (better even than the one of 2011 on the same topic) reports on the summer of 1911. It’s worth quoting:

“The long hot summer of 1911 is credited with changing fashions, with women shedding whalebone corsets and brassieres becoming the rage. Edwardian [sic, though Edward VII died in 1910] aristocrats are said to have taken up nude tennis at their country estates…

There was record heat in August and the sunshine continued until September, by which time the countryside was also in severe distress and riots had broken out in the cities.”

Time will tell if we’re in for a repeat!

So onto the graph-fest.

I was going to follow up last month’s post with one of the April to June CET, having noticed that the hot June had followed a distinctly mild mid to late spring (despite cold snaps continuing). Anyway, here’s that one, a little belatedly:

Yep, that’s right, April to June this year has been one of the three warmest such periods in the CET record, exceeded only by 1762 and 1798. Crikey!

Then, of course, a hot June followed by an exceptionally hot July must make the early to mid summer graph (June and July) quite interesting:

It is, but 2018 is still only the third hottest year, after 1976 and 2006 this time (though 2018 could still also fall behind 1826, I suppose).

Surely there must be some measure on which 2018 is (provisionally) the warmest ever?

Yes, you’ve guessed it. A mild late spring and hot early to mid summer makes 2018 a record-breaker for May to July mean CET:

And that’s not it. If we add in April as well, sort of mid-spring to mid-summer, it’s not even close:

Bingo!

Hottest UK June Since 1976 (and Weather Reporting Hype)

It always baffles me that the Met Office reports notable weather months 2 or 3 days before their end – you’d have thought they’d wait to finalise the “official” data – so this time I’m facetiously reporting before they do (assuming I type fast enough)!

I know it’s only the 27th and CET (that’s the Central England Temperature for any newbies) data has been published only up to the 25th (thanks again to the Met Office for this resource):

but it’s already a nailed-on slam-dunk that the CET mean for June 2018 will exceed the 16.1C recorded in the exceptionally hot summer of 2003, making this June the warmest since the legendary summer of 1976 (17C).

I say this simply because the forecast for the rest of the month is for fairly hot conditions to persist (thanks this time to Weathercast):

Simple arithmetic suggests that daily mean temperatures of around 20C (London’s are not atypical of England as a whole, slightly cooler if anything) will drag up the average for the month from 15.9C for the first 25 days to over the 16.1C recorded in 2003.  Here’s a graph showing June CET since 1659, assuming (conservatively) a mean of 16.2C this year:

Having said all that, this June and May (which I’ll come to) have not been notable for exceptional temperatures.  For example, the current “heatwave”, though fairly unpleasant, has come nowhere near breaking daily records for the CET area (though some local records may be broken, in Wales, for example).  Temperatures have so far only edged above 30C in one or two places, with 30.1C at Hampton Water Works on 25th (Monday) not a patch on the 33.5C at East Bergholt on the same date in 1976.  Even the 30.7C at Rostherne No 2 yesterday, 26th, is well below 1976’s 35.4C at North Heath.

I might even go so far as to say it’s a little bit of an exaggeration to call the current conditions a “heatwave” (at least in southern England).  The term is being devalued by tabloid reporting.  It’s an outrage!  (To use another overused word).  It’s just “hot weather”.

Given that we had several days in succession over 35C in June 1976, and we’ve had 42 years of global warming since then, and warming affects extreme events disproportionately, I wonder what temperatures we’d hit if we had similar conditions to 1976?  Presumably then (as in 2003), high pressure didn’t just sit over the UK, but drew in air from the warmest direction in summer, that is from the south-east (or even just from further east).

What has been notable this year has been the persistence of dry, sunny, windless, anticyclonic conditions, with only a small interlude of westerlies in June. That persistent high pressure conditions are fairly unusual in June is presumably the reason why, on average, June CET temperatures have risen less than other months in the global warming era (the black line in the graph above shows that, averaged over 21 years, the recent period has not been exceptional, though global warming will inevitably drag the mean temperature up over the coming decades).  Because the oceans warm only slowly, periods of weather dominated by westerlies are likely to be only a little warmer than before global warming set in.  The 5 year periods in the mid 2000s and most recently (the green line in the graph) show the potential for generally hotter Junes.

And the historical record (check out 1676 and 1846!), suggests that a truly freakish June these days (with global warming) would average well over 18C, possibly even touching 19C.  Much worse than the low 16Cs this year.

At least this June has been reasonably hot.  May was widely reported as the hottest and sunniest on record.  It was exceptionally sunny (as may also be the case for June), but nowhere near the hottest.  Here’s my latest graph of May CET:

In fact, at 13.2C in the CET, May 2018 was only equally as warm as May 2017 and less warm than in 2004 (13.4C), 1992 (13.6C) and quite a few others!

So how can May 2018 be reported as the “hottest on record”?

Well, obviously it might be because statistics are being used for a different region e.g. the UK as a whole, but I don’t think that’s the main reason.  The CET is fairly representative.

No, if you read the small print you’ll find that the “hottest May” claim is based on daily maximum temperatures only.  When you take night-time temperatures into account, as is almost universal practice, May 2018 was not exceptionally warm.  The reason for the difference lies in all that sunny weather, which tends to lead to warm days and cool nights, so that the day-time average temperature is higher than the overall average.

If that weren’t enough, weather record reporting is also afflicted by “Year Zero Syndrome”.  The CET record back to 1659 is not used, or even referred to.  Instead records are based on the period since 1910, when more comparable records begin.  It’s a bit like the way football records in England are now based on the period since the start of the Premier League in 1992, so that we no longer realise that goal-scoring feats comparable (rather than equal, because there are now 2 fewer top-flight teams) to Dixie Dean‘s 60 goals in 1927-28 are still possible.  Clearly, from the chart above, no recent May mean temperature approaches that of 1833, or even 1848.

That leads me to my usual warning.  May 1833 was about 3.5C hotter at 15.1C than the mean for the period (given by the black 21-year running mean on the graph).  Because, in line with global warming, an average May (unlike an average June) is now warmer than at any time since 1659 (the black line again), a similarly freakish May would be somewhere in the mid-15Cs.

Unless the last few years are exceptional, it’s curious that June shows the global warming signal so weakly.  I’ll have to look more closely at the data to see if that for any other months exhibits a similar feature.

How Not to Report a Weather Record: 21st June 2017

Well, well, well.  Less than a year on from an exceptionally hot mid-September day (at least exceptionally hot for the UK, if not, perhaps, for Kuwait), and it’s only gone and happened again.

Yeap, the presumably less poisonous than mercury red liquid in my re-purposed fridge thermometer has only gone and reached 34.5C this week, on what was widely reported as “the hottest June day for 41 years”, that is, since the summer of 1976.  And curiously I was close to the epicentre of the heatwave back in ’76, in FA Cup-winning Southampton, then the hottest place in the country, just as where I am now, a few miles from Heathrow, has been this time.

And once again the record has been somewhat understated.   I explained in my post on the topic last September that the true significance of the 13th September 2016 was that it was the hottest day that had been recorded in the UK so late in the year.

You’ve guessed it.  The 34.5C recorded at Heathrow this summer solstice was the hottest daily maximum so early in the year.  Back in 1976 the temperatures over 35C (peaking at 35.6C in Southampton on 28th) were later in the month.  In other words, 21st June 2017 saw a new “date record”.

Admittedly, it was not a particularly notable date record, since 34.4C was recorded at Waddington as early as 3rd June during the glorious post-war summer of the baby-boom year of 1947.  And 35.4C at North Heath on 26th June 1976 also seems somewhat more significant than nearly a whole degree less on 21st June.  Furthermore, unlike in 1947, 1976, and, for that matter, 1893, only one “daily record” (the hottest maximum for a particular date) was set in the 2017 June heatwave.

Nevertheless, 21st June 2017 set a new date record for 5 days (21st to 25th June, inclusive) and that is of statistical significance.  The point is that without global warming you would expect there to be approximately the same number of date records each year, or, more practically, decade.  The same is true of daily records, of course – providing a recognised statistical demonstration of global warming – but my innovation of date records provides for a more efficient analysis, since it takes account of the significance of daily records compared to those on neighbouring dates.  It makes use of more information in the data.

Supporting the “hypothesis” of warming temperatures, the 5 day date record set on 21st June 2017 exceeds what you would expect in an average year, given that daily temperature records go back over 150 years.  On average you’d expect less than 3 days of date records in any given year.  But we can’t read too much into one weather event, so how does it look for recent decades?

Last September, I provided a list of UK date records from the hottest day, 10th August, when 38.1C was recorded in Gravesend in 2003 through to October 18th, promising to do some more work next time there was a heatwave.  So, keeping my word, we have the following date records:

34.4C – 3rd June 1947 – 18(!) days

34.5C – 21st June 2017 – 5 days

35.4C – 26th June 1976 – 1 day

35.5C – 27th June 1976 – 1 day

35.6C – 28th June 1976 – 3 days

36.7C – 1st July 2015 – 33(!!) days

37.1C – 3rd August 1990 – 7 days (through 9th August)

Obviously, weighting for how exceptionally hot they were, the 2010s have had way, way over their share of exceptionally hot days for the time of year during the summer months.  I’m timed out for today, but I will definitely have to get round to an analysis of the whole year!  Watch this space.

 

How Not to Report a Weather Record: 13th September 2016

Last Sunday, the Guardian website suggested Tuesday 13th September would be jolly warm:

“If the mercury rises above 31.6C, the temperature was [sic] reached at Gatwick on 2 September 1961, it will be the hottest September day for 55 years.”

“No, no, no!!”, I was obliged to point out, adding, by way of explanation that:

“If the temperature rises above 31.6C it will be the hottest September day for more than 55 years, since 1961 was 55 years ago.

For it to be the hottest September day for 55 years it will only have to be hotter on Tuesday than the hottest September day since 1961.”

Good grief.

After that I was hardly surprised – since your average journo seems not even to be an average Joe, but, to be blunt, an innumerate plagiarist – to read in the Evening Standard on the 13th itself:

“If the heat rises above 31.6C, which was reached at Gatwick on September 2, 1961, then it will be the hottest [September] day for 55 years.”

See what they’ve done there?  With a bit of help from Mr Google, of course.

In the event, it reached 34.4C on 13th, making it the hottest September day for 105 years.

Much was also made of the fact that we had 3 days in a row last week when the temperature broke 30C for the first time in September in 87 years.

But the significance of the 34.4C last Tuesday was understated.

The important record was that the temperature last Tuesday was the highest ever recorded so late in the year, since the only higher temperatures – 34.6C on 8th September 1911 (the year of the “Perfect Summer”, with the word “Perfect” used as in “Perfect Storm”) and 35.0C on 1st rising to 35.6C on 2nd during the Great Heatwave of 1906 – all occurred earlier in the month.  By the way, in 1906 it also reached 34.2C on 3rd September.  That’s 3 days in a row over 34C.  Take that 2016.  They recorded 34.9C on 31st August 1906 to boot, as they might well have put it back then.

No, what’s really significant this year is that we now know it’s possible for the temperature to reach 34.4C as late as 13th September which we didn’t know before.

I’m going to call this a “date record”, for want of a better term.  Any date record suggests either a once in 140 years freak event (since daily temperature records go back that far, according to my trusty copy of The Wrong Kind of Snow) or that it’s getting warmer.

One way to demonstrate global warming statistically is to analyse the distribution of record daily temperatures, i.e. the hottest 1st Jan, 2nd Jan and so on.  Now, if the climate has remained stable, you’d expect these daily records to be evenly distributed over time, a similar number each decade, for example, since 1875 when the records were first properly kept.  But if the climate is warming you’d expect more such records in recent decades.  I haven’t carried out the exercise, but I’d be surprised if we haven’t had more daily records per decade since 1990, say, than in the previous 115 years.

It occurs to me that another, perhaps quicker, way to carry out a similar exercise would be to look at the date records.  You’d score these based on how many days they apply for.  For example, the 34.4C on 13th September 2016 is also higher than the record daily temperatures for 12th, 11th, 10th and 9th September, back to that 34.6C on 8th September 1911.  So 13th September 2016 “scores” 5 days.

Here’s a list of date records starting with the highest temperature ever recorded in the UK:

38.1C – 10th August 2003 – counts for 1 day, since, in the absence of any evidence to the contrary, we have to assume 10th August is the day when it “should” be hottest

36.1C – 19th August 1932 – 9 days

35.6C – 2nd September 1906 – 14 days

34.6C – 8th September 1911 – 6 days

34.4C – 13th September 2016 – 5 days

31.9C – 17th September 1898 – 4 days

31.7C – 19th September 1926 – 2 days

30.6C – 25th September 1895 – 6 days

30.6C – 27th September 1895 – 2 days

29.9C – 1st October 2011 – 4 days

29.3C – 2nd October 2011 – 1 day

28.9C – 5th October 1921 – 3 days

28.9C – 6th October 1921 – 1 day

27.8C – 9th October 1921 – 3 days

25.9C – 18th October 1997 – 9 days

And you could also compile a list of date records going back from 10th August, i.e. the earliest in the year given temperatures have been reached.

The list above covers a late summer/early autumn sample of just 70 days, but you can see already that the current decade accounts for 10 of those days, that is, around 14%, during 5% of the years.  The 2000s equal and the 1990s exceed expectations in this very unscientific exercise.

Obviously I need to analyse the whole year to draw firmer conclusions.  Maybe I’ll do that and report back, next time a heatwave grabs my attention.

It’s also interesting to note that the “freakiest” day in the series was 2nd September 1906, with a daily record temperature hotter than for any of the previous 13 days.  2nd freakiest was 19th August 1932 – suggesting (together with 2nd September 1906) that perhaps the real story is an absence of late August heatwaves in the global warming era – joint with 18th October 1997, a hot day perhaps made more extreme by climate change.

Am I just playing with numbers?  Or is there a serious reason for this exercise?

You bet there is.

I strongly suspect that there’s now the potential for a sustained UK summer heatwave with many days in the high 30Cs.  A “Perfect Summer” turbocharged by global warming could be seriously problematic.  I breathe a sigh of relief every year we dodge the bullet.

 

 

 

Greater Interannual Seasonal Temperature Variability in a Warming World?

You attempt to use the correct scientific jargon and then realise that sometimes the English language is insufficiently precise.  What I mean by the title is to ask the important question as to whether, as global warming proceeds, we will see a greater variation between summers, winters, springs and autumns from year to year.  Or not.

My previous two posts used Central England Temperature (CET) record data to show how exceptional the temperatures were in December in 2010 (cold) and 2015 (warm) and highlighted two other recent exceptional months: March 2013 (cold) and April 2011 (warm).  I speculated that perhaps, relative to current mean temperatures for a given period, in these examples a calendar month, both hot and cold extreme weather conditions are becoming more extreme.

What prompted today’s follow-up post was an update from the venerable James Hansen, Global Temperature in 2015, to which a link appeared in my Inbox a few days ago.  This short paper documents how 2015 was by a fair margin globally the warmest year on record.  But it also includes a very interesting figure which seems to show increasingly greater variability in Northern Hemisphere summers and winters:

I’ve added a bit of annotation to emphasise that the bell curves for both summer and winter have widened and flattened. That is, not only have the mean summer and winter temperatures increased, so has the variance or standard deviation, to use the technical terms.

If true, this would be very concerning. If you’re trying to grow food and stuff, for example, it means you have to worry about a greater range of possible conditions from year to year than before, not just that it’s getting warmer.

I was about to suggest it might be time to panic. But then it occurred to me that there must surely have been some debate about this issue. And sure enough Google reveals that Hansen has written about variability before, and more explicitly, such as in a paper in 2012, titled Perception of climate change, which is free to download.  Hansen et al note “greater temperature variability in 1981-2010” compared to 1951-80.

Trouble is Hansen et al, 2012 was vigorously rebutted by a couple of Harvard boffs.  Andrew Rhines and Peter Huybers wrote to the Proceedings of the National Academy of Sciences, where Hansen et al had published their paper, claiming that Frequent summer temperature extremes reflect changes in the mean, not the variance [my stress].  They attributed Hansen’s flattening bell curves were due to various statistical effects and asserted that mean summer and winter temperatures had increased, but not the standard deviation, and therefore the probability of relative extremes.

That left me pretty flummoxed, especially when I found that in Nature that another bunch of eminent climate scientists also claimed, No increase in global temperature variability despite changing regional patterns (Huntingford et al, Nature 500, p327–330, 15 August 2013).

Just so we’re clear, what the guys are saying is that as global warming proceeds – not even when we reach some kind of steady state – temperatures will just on average be shifted up by a certain amount.

I have to say I find this very difficult to believe, and indeed incompatible with the fact that some parts of the world (continental interiors, say) warm faster than others (deep oceans) and the observation that the wind blows in different directions at different times!

Furthermore we’ve just seen, between Decembers 2010 and 2015 in the  CET record, a much greater spread of temperatures than in any comparable period (actually in any period, period, but we’re concerned here with variability over a few years – less than a decade or two, say – when the climate has had little time to change) in the previous 350 years.  I take the liberty of reproducing the graph from my previous post:

December 2015 was 10C warmer than December 2010, 2C more than the range between December temperatures in any other era.

And I also recollect figures like this one, showing the freakishness of summer 2003 in Switzerland, where, like the UK, there is a long history of weather records:

This appears on the Climate Communication site, which shies away from any mention of increased variability.  But the original Nature paper in which it appeared, Schär et al, 2004 is very clear, and is even titled The role of increasing temperature variability in European summer heatwaves. The synopsis (which is all I can access – pay-wall) notes that:

Instrumental observations and reconstructions of global and hemispheric temperature evolution reveal a pronounced warming during the past approx 150 years. One expression of this warming is the observed increase in the occurrence of heatwaves. Conceptually this increase is understood as a shift of the statistical distribution towards warmer temperatures, while changes in the width of the distribution are often considered small. Here we show that this framework fails to explain the record-breaking central European summer temperatures in 2003, although it is consistent with observations from previous years. We find that an event like that of summer 2003 is statistically extremely unlikely, even when the observed warming is taken into account. We propose that a regime with an increased variability of temperatures (in addition to increases in mean temperature) may be able to account for summer 2003. To test this proposal, we simulate possible future European climate with a regional climate model in a scenario with increased atmospheric greenhouse-gas concentrations, and find that temperature variability increases by up to 100%, with maximum changes in central and eastern Europe. [My stress].

Hmm. Contradictory findings, scientific debate.

My money’s on an increase in variability. I’ll keep an eye on that CET data.

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.

 

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.

Could 2013 Still be the Warmest on Record in the CET?

Blossom is appearing and buds are opening. The front garden magnolias of West London are coming into flower. The weather is turning milder in the UK. Spring is here at last.

So perhaps I’ll be coming to the end of posts on the subject of unusual weather for a while. Until there’s some more!

We’ve seen that March 2013 was, with 1892, the equal coldest in the CET since 1883, which is particularly significant given the generally warmer Marches in recent decades.

The first quarter of 2013 was the coldest since 1987, and the cold has now continued into April. This is where we now are, according to the Met Office:

So far this year it’s been 1.44C colder than the average over 1961-90, which is the basis for CET “anomalies” here.

The rest of the year would have to be 2.37C warmer than usual, on average, for 2013 to be the warmest in the record.

Is it possible for 2013 to still be the warmest year in the CET? I’m saying no – or, to be more measured, it’s extremely unlikely.

Last year, it was July 13th before I felt able to make a similar statement.

But now I’ve realised that I can simply plot a graph of the later parts of previous years and compare them to the required mean temperature in 2013.

Here’s the graph of mean CET for the last 9 months of the year:

Perhaps the most notable feature is that the last 9 months of 2006, at 13C were a whole 0.5C warmer than the last 9 months of the next warmest year, 1959, at 12.5C!

It’s easy enough to calculate that for 2013 to be the warmest year in the CET, the mean temperature for the last 9 months of the year would have to be 13.38C.

To be warmer than the warmest year in the CET, also 2006, the last 9 months of 2013 would need to be 0.38C warmer than the last 9 months of 2006. That’s a big ask.

But let’s look a little more carefully at 1959. The last 9 months of 2009 were about 1.4C warmer than the prevailing mean temperatures at the time, given by the 11 year (red line) and 21 year (black line) running means. The last 9 months of 2006 were “only” about 1.1 or 1.2C warmer than an average year at that time.

If 2013 were 1.4C warmer than the running means in previous years (obviously we can only determine the running means centred on 2013 with hindsight) then it would not be far off the warmest year in the CET.

No other year in the entire CET spikes above the average as much as 1959, so we have to suppose the last 9 months of that year were “freak” – say a once in 400 year event – and extremely unlikely to be repeated.

So on this basis it seems 2013 is extremely unlikely to be the warmest in the CET.

Now we have a bit of data for April we can also carry out a similar exercise for the last 8 months of the year.

The Met Office notes (see the screen-grab, above) that the first 8 days of April 2013 were on average 3C cooler than normal in the CET (“normal” with respect to the CET is always the 1961-90 average). If we call those 8 days a quarter of the month, the rest of the month needs to be 1C warmer than usual for April as a whole to be average. Let’s be conservative, though, and assumes that happens.

It’s easy enough now to calculate that for 2013 to be the warmest year in the CET, the mean temperature for the last 8 months of the year would have to be 14.07C, assuming the April temperature ends up as the 1961-90 average.

On this basis, we can then compare the last 8 months of previous years in the CET with what’s required for this year to be the warmest on record:

Here 2006 seems more exceptional, and 1959 not quite such an outlier. (April is not now included: in 1959 the month was warm at 9.4C whereas in 2006 it was warmer than average at 8.6C, but not unusual).

Clearly, the spike above the running means would have to be a lot higher than ever before for 2013 to be the warmest year in the CET. Those 8 cold days seem to have made all the difference to the likelihood of 2013 breaking the record.

That’s it for now – though if April is particularly cold this year, a comparison of March and April with those months in previous years will be in order. The plot-spoiler is that 1917 was the standout year in the 20th century for the two months combined.