Long, Hot Summers: 2022, 1666 and all that

I compare summers in the Central England Temperature (CET) record in three ways: “High Summer” (July and August), Meteorological Summer (June, July and August) and “Full Summer” (June to September). Summer 2022 ranks in the top 6 of all three measures, but given global warming its mean temperatures are high, but not extreme for the era.

I am now using the latest version (2.0.0.0) of the CET data, so there may be minor differences (of the order of 0.1C) with data presented in previous posts.

I mentioned last month that focussing on the meteorological summer (June, July and August) is illogical. For sheer heat, “High Summer” (the two hottest months, July and August) makes more sense. For sustained heat, I suggested, June to September, which I’ll term the “Full Summer” may be most relevant. September is hotter than June more than 20% of the time (79 times in the 363 years of the CET record – I’ve counted), and occasionally hotter even than July (6 times) or August (9 times). Fun fact: 1890 is the one and only year when September was the warmest month.

The data are now available, and I can report that, because September was relatively mild this year, Full Summer 2022 “only” ranks 4th in the list of long, hot summers in the CET record, a full 0.6C cooler on average than 2006 (see Fig 2, below), when I remember purchasing my trusty evaporative cooler.

Fig 1 shows the changing Full Summer mean temperatures in the CET record over the last few centuries, highlighting some exceptional years:

As can be seen in Fig 1, summer 2022 was in fact not that exceptional a long, hot summer (although the unprecedented peak temperatures will be remembered, of course, at least until we see worse!), compared to, for example, 1826, 1846, 1911 and indeed 2006, given the increase in temperature of the average summer. September 2006 is the hottest in the CET record at 16.8C and September 2022 would have to have been even hotter for this year’s summer to claim the title of “longest and hottest”. It’s also interesting to see the historically significant heat of 1666 on the graph.

Long, hot summers are becoming much more frequent, of course (see also Fig 2, below). A mean Full Summer temperature of 16C or above was recorded once in the 41 years of the 17th century included in the CET; 4 times in the full 18th century, including 1779-81, when some claim there was an exceptional El Nino event; 5 times in the 19th century; 8 times in the 20th century; and already no fewer than 6 times (in 23 years, so more than 1 in every 4) in the 21st century. The frequency this century is more than 3 times that even in the 20th century, when global warming was already evident. Of course, the planet is still heating up, so we can expect long, hot summers even more often in the decades to come.

I thought it would be interesting to rank the hottest summers in the CET by temperature for High Summer, Meteorological Summer and Full Summer:

As shown in Fig 2, summer 2022 wasn’t the hottest on any measure, although it’s one of only 5 to feature in the top 10 on all three: 1976, 2003, 2006, 2018 and 2022.

That list indicates the effect of global warming and the impression of the most extreme summers tending to be more recent is also clear from other aspects of Fig 2. For example, the 11 hottest High Summers have occurred since the start of the 20th century, 1911 being the only one of the 10 hottest before the middle of that century. Only 1826 and 1846 from before the 20th century appear in the hottest 10 summers by any of the three measures in Fig 2.

This impression is reinforced by the graphs in Figs 1 (above) and 3 and 4 (below), which also show how extreme were some historical summers, most notably 1826, 1911 and 1976, compared to the average summers at the time.

As I’ve suggested previously, there’s plenty of scope for a summer hotter than that in 2022, even without further global warming.