UK Heatwaves: Could They Get Even Hotter?

The UK record temperature of 40.3C set on 19th July 2022 could potentially be significantly exceeded, even in the current climate. Historically, daily maximum temperatures tend to be 0.5C-1C higher 10-20 days later in the year. Furthermore, the most exceptional temperatures have occurred during long, hot UK summers with drought conditions. That was not the case in 2022. In addition, not only are GHG levels increasing, the levels of cooling aerosols due to hydrocarbon burning will continue to decrease over the next few decades. An extended period of high-30C/low-40C days and 25C nights would be far more dangerous than the episodic heatwaves of the last few years, of which the 2022 event is the most notable and most recent. The question we should be asking is: what would happen now were a UK summer analagous to 1976 or 1911 occur? How hot would it be? And for how long?

Here’s an interesting graphic, tweeted by the Met Office a few days ago:

Fig 1 is completely out of date, of course. The hottest temperature, 40.3C, was recorded yesterday, 19th July 2022, at Coningsby. For good measure, the 4th hottest was the previous day, 18th July 2022, 38.1C at Santon Downham.

Of course, extreme days occurred before temperatures were properly measured, such as on “Hot Tuesday”, 19th July (by the modern calendar), 1707, when horses as well as people reportedly succumbed to heat stroke. Putting that event to one side, it’s not necessary to imagine points for 18th and 19th July 2022 in the top right corner of Fig 1 to be struck by the sudden acceleration in peak UK summer temperatures. In the period of instrumental temperature measurements, 37C was never exceeded before 1990 – and even in that year only by 0.1C. The record was smashed in 2003 and now again in 2022 (perhaps we should call 19th July 2022 “Hot Tuesday 2.0”!).

Global mean temperatures have increased by about 1C since pre-industrial times, but UK peak temperatures are over 3C higher! That’s a bit concerning given we’re likely to hit 2C of global warming. (See my recent post, Heatwave Horror, for one reason for the magnification of the mean global temperature rise during UK – and in general all extra-tropical – heatwaves).

How much worse could it get now, though?

There are a couple of reasons to suppose that higher temperatures are possible even with the current climate, let alone further global warming.

When are the highest temperatures?

Until 19th July 2022, the all-time highest UK temperature record had never been so early in the year since at least 1911 (excepting that 22nd July 1911 seems to have held the record for 18 days!). In fact, until 25th July 2019 the record had been in August for at least 108 years. The fact that the hottest temperatures for many years were 9th August 1911, 3rd August 1990 and 10th August 2003 suggests to me that the very hottest temperatures occur later than 19th July.

To test this hypothesis I calculated the 11-, 21- and 31-day running means of the daily maximum temperatures from June through August, and, sure enough, the peaks are in late July and early August. This simple analysis suggests that, had the recent heatwave occurred around a fortnight later, the temperature could have been around 0.5-1C higher!

1911 and 1976

The 1911 heatwave lasted from early July to mid-September. Eight daily temperature records scattered over this period were still unbroken by the time of the publication of The Wrong Kind of Snow (“WKS”) in 2007. The highest of these was the 36.7C on 9th August shown in Fig 1, the highest temperature recorded in the UK until 37.1C on 3rd August 1990, suggesting that early August is the time when the very highest temperatures occur.

If 1976 was such an extreme heatwave why weren’t higher temperatures recorded? Well, just as they say every pandemic is different, so too is every heatwave. Of the 12 daily temperature records for 1976 recorded in WKS (some since broken), no fewer than 11 are in the period 23rd June to 7th July. The other is on 25th August. Like in 1911, the 1976 heatwave and drought persisted all summer, but in 1976 temperatures moderated somewhat after early July.

The peak 1976 temperature of 35.9C on 3rd July was truly exceptional, however. I’ve previously stressed the importance of taking into account the date of a temperature record, and introduced the concept of a “date record” – the highest temperature so early in the year.

Now, when I discussed 21st June 2017, my table noted that (at the time) 1st July 2015, with a maximum temperature of 36.7C, was the hottest day earlier than 3rd August, a “33(!!) days” (as I put it) date record. But 1st July 2015 is a result of 39 years global warming over 1976. Prior to this harbinger of this year’s 40.3C, 3rd July 1976 was a 17 day date record, including at the time of publication of WKS.

Furthermore, the next date record was 36.5C on 19th July 2006 (a daily record itself exceeded this year and possibly also in 1707, of course), a result of 31 years of additional global warming over 1976. Discounting that would leave 36.0C on 22nd July 1911. But arguably (actually there’s little argument) 35.9C on 3rd July is more exceptional than 36C on 22nd July, so certainly at the time and at least until 37.1C on 3rd August 1990, 3rd July 1976 was one of, if not the, most exceptionally hot days ever in the UK.

To digress slightly, the 1976 heatwave could be an early indicator of global warming, given that CO2 levels were already somewhat above the pre-industrial level of around 280ppm:

Consideration of 1976 and 1911 suggests that the most exceptional temperatures occur during long, hot, dry UK summers. One reason is that drought conditions reduce evaporative cooling, so higher temperatures are reached at ground level (official measurements are taken at a height of 1.25-2m). 2022 has been dry, but not extremely so, so there may be scope for higher temperatures in another year (or even later this year!).

Waning Global Cooling

1976 may have been purely a freak natural event and not exacerbated by the early stages of global warming. It’s worth noting that September 1906 was almost as exceptionally hot, with 2nd a 14 day date record.

One reason for saying this is that 1976 may be an exception that proves the rule because it occurred during a period of global cooling (see Fig 3), attributable to industrial pollution and volcanic eruptions.

Looking at all the summer daily temperature records given in WKS (updated by myself in the margins, notes I’ve recently reconciled with a handy list on the TORRO site) it strikes me that so few are from the 1950s to 1980s. In fact, currently only 4 of the 122 June through September daily records are from the 1950s, none at all from the 1960s and just 2 from the 1980s. The 1970s are curious: there are still 11, all but one from 1976. For comparison there are 16 from the 1940s, 15 from the 2010s and already 10 from the 2020s. Fig. 4 provides a complete up to date list as well as the records published in WKS (which, seemingly in error, omitted some daily records for the 2000s):

The cooling in the post-war period is generally attributed to the masking of greenhouse gas (GHG) warming by aerosol cooling, as the IPCC put it (Fig 5). Only after about 1980 did the effect of GHG emissions outweigh the aerosol cooling (Fig 3). (Natural forcings, principally due to volcanic emissions are also a factor, of course).

As we transition away from hydrocarbon fuels, aerosol emissions will decline further, notably in the region where UK heatwaves are generated, i.e. not just the UK, but Western Europe. This, as well as increased levels of GHGs, will allow air masses causing UK heatwaves to warm even more.

So not only was “Hot Tuesday 2.0”, 19th July 2022, not the worst case given the current climate, the potential temperature of UK heatwaves is likely to increase even further over the next few decades.