Climate change made 2022’s UK heatwave ‘at least 10 times more likely’

§ ‘Exceptional heat’

On 23 June, the European Centre for Medium-Range Weather Forecasts released a single model run predicting unusually high temperatures for 18-25 July over the UK and US. This was the first indication of what would eventually become an unprecedented (pdf) nation-wide heatwave, attracting global media attention.

Initially, scientists stressed that temperatures of 40C were unlikely to materialise in the UK. However, as the days passed, more and more models began to converge on higher temperatures – and the likelihood of seeing extreme heat rose steadily.

The UK Met Office implemented an “amber” weather warning a full six days before the heatwave struck – the earliest heat warning that it has ever released. And, by 15 July, the Met Office released its first ever “red” heat warning.

For the first time temperatures of 40°C have been forecast in the UK and the first ever Red warning for exceptional heat has been issued.

Find out more in our press release 👇

— Met Office (@metoffice) July 15, 2022

Later that day, the UK Health Security Agency issued its first-ever level 4 heat-health alert, signifying a “national emergency”. This level is reached “when a heatwave is so severe and prolonged that its effects extend outside the health and social care system”.

On Monday 18 July, temperatures soared across much of the UK, reaching a maximum of 38.1C in a village in Suffolk. There was little respite overnight, as the UK saw its hottest night on record. With temperatures remaining 25C in many places, the country experienced a “tropical night”.

On 19 July, the little-known village of Coningsby in Lincolnshire was catapulted into the headlines when it recorded a UK record high temperature of 40.3C – breaking the previous maximum temperature of 38.7C, recorded at the Cambridge Botanic Garden on 25 July 2019.

The maps below show air temperature at two metres above the surface for 19 July. They show daily maximum temperature (left) and the difference between this and the average daily maximum for 2000-2020 (right).

Red indicates hotter temperatures. Note that the colour bars on the two maps indicate different values. The box indicates the region analysed in the study – including London and Lincolnshire.

In total, the heatwave saw 46 weather stations meeting or exceeding the previous record high, in a band stretching from Kent to north Yorkshire, according to the study. And by the end of the day, four additional weather stations in the UK had crossed the 40C threshold, cementing 19 July 2022 as the UK’s hottest day on record.

Here’s a look at today’s provisional maximum temperatures at 16:00 👇#heatwave2022 #heatwave pic.twitter.com/A957lUFdOL

— Met Office (@metoffice) July 19, 2022

§ ‘Unprecedented’

Newspapers called the heatwave “unprecedented” and “record shattering”, noting that temperatures of 40C had never been recorded in the UK before.

To put the heatwave into its historical context and determine how unlikely it was, the authors analysed a timeseries of observational temperature data over 1950-2022.

The plot below shows a timeseries of the hottest daily maximum temperature in each year (left) and hottest two-day average temperature each year (right). The green line shows the 10 year running average.

Observations indicate that the 19 July maximum temperatures is a one-in-1,000 year event in the present-day climate, meaning that “such an event would have been almost impossible in a world without climate change”, the study finds. Meanwhile, the two-day average temperature over 18-19 July is a one-in-100 year event.

The authors also looked at the daily maximum temperature for three weather stations in England. These are St James’ Park in London, Cranwell in Lincolnshire – close to where the new UK record was set – and Durham. These datasets span 1944-2022, 1880-2022 and 1978-2022, respectively.

The plot below shows the historical temperature record of the three stations, which are used as another line of evidence. The green line shows the running 10-year average, while the red line shows the hottest daily maximum temperature in each year.

The authors find that the 19 July maximum temperatures at St James Park, Durham and Cranwell are likely to recur once every 500, 1,000 and 1,500 years, respectively, in the present day climate.

§ Attributing heat

Attribution is a fast-growing field of climate science that aims to identify the “fingerprint” of climate change on extreme-weather events, such as heatwaves and droughts. In this study, the authors investigated the impact of climate change on the extreme heat that hit the UK over 18-19 July.

To conduct attribution studies, scientists use models to compare the world as it is today to a “counterfactual” world without human-caused climate change. This study aimed to distinguish the “signal” of climate change in the UK heatwave over 18-19 July.

Overall, the authors find that current warming made the July 2022 UK heatwave at least 10 times more likely. However, the authors call this a “conservative” estimate, stressing that climate change probably increased the likelihood of the event by more than a factor of 10.

Prof Fredi Otto – a senior lecturer at the Grantham Institute for Climate Change and the Environment at Imperial College London and author on the study – told a press briefing that this underestimate is due to a limitation in the climate models used:

“We have a problem with the climate models that we are using to simulate the anthropogenic signal…They underestimate the trend in extreme temperatures in summers in western Europe.”

This issue has led to differences between conclusions based on models, as opposed to those based on observations.

While models suggest that current greenhouse gas concentrations made the heatwave 2C hotter than it would have been in a pre-industrial climate, historical weather records suggest a much larger effect of 4C.

Meanwhile, the model results suggest that similar heatwaves have a 1% chance of happening each year in the present day climate – in agreement with an earlier study. However, weather records again suggest that this is an underestimate, and that similar heatwaves are more likely to happen than climate models suggest.

(These findings are yet to be published in a peer-reviewed journal. However, the methods used in the analysis have been published in previous attribution studies.)

§ ‘Invisible risk

The UK “has very little experience with extreme high temperatures”, making the country “particularly exposed and vulnerable to devastating heat impacts with unadapted infrastructure, limited systems and knowledge about behaviours to adopt during extreme heat”, according to the study. This was part of the rationale for running an attribution study on the UK, Otto says.

The heatwave caused severe and widespread impacts. Trains operated at reduced speeds, a melting runway at Luton airport caused severe delays and the London Fire Brigade declared a “major incident”, with forces across the country facing “unprecedented” blazes.

While the precise number of casualties will not be available for weeks, estimates point to hundreds of heat-related deaths, the study says.

Days before the heatwave, Met Office chief executive Prof Penny Endersby released a video, warning that “the extreme heat we are forecasting right now is absolutely unprecedented”. She added:

“When climate change has driven such unprecedented severe weather events around the world, it can be difficult for people to make the best decisions in these situations, because nothing in their life experience has led them to know what to expect…Our lifestyles and our infrastructure are not adapted to what is coming.”

Study author Dr Emmanuel Raju – director of the Copenhagen Centre for Disaster Research – adds that heat is an “invisible risk”, noting that the most “vulnerable and marginalised” in our society are “disproportionately impacted”. For example, he notes that people in prison and those who are homeless suffer during heatwaves, but are rarely talked about.