The official definition of a heatwave is to be changed in a band of English counties from Surrey to east Yorkshire, in response to the warming climate.
The Met Office said it was increasing heatwave temperature thresholds in eight counties before the summer. The change reflected “an undeniable warming trend” for the UK that had made the original thresholds obsolete.
For hot weather to be officially considered a heatwave, the temperature in a specific area has to reach a threshold set by the Met Office for three consecutive days. This varies by UK county.
Greenhouse gas emissions from burning fossil fuels, forest destruction and other human activities are trapping heat and putting more energy into the climate system.
Hotter air means heatwaves are much more likely. For example, scientists now say the unprecedented heat and wildfires across the northern hemisphere in 2018 “could not have occurred without human-induced climate change”. In Australia, the scorching summer of 2016-17 in New South Wales was made at least 50 times more likely by global heating, linking it directly to climate change.
Hotter air can also carry more water vapour, meaning more intense rain and more flooding.
Another important factor in the northern hemisphere is the impact of changes in the Arctic. The polar region is heating more rapidly, reducing the temperature difference with lower latitudes. There is strong evidence that this is weakening the planetary waves (including the jet stream) that normally meander over Europe, Asia and North America.
When these waves stall, weather gets fixed over regions and becomes extreme. This has been linked to past floods in Pakistan, heatwaves in Russia and drought in California.
Most of the planet’s trapped heat goes into the oceans and rising sea temperatures mean more energy for hurricanes and typhoons. The deluge delivered in the US by Hurricane Harvey in 2017 was made three times more likely by climate change. Rising sea level also means storms cause more coastal damage.
Natural variability would cause some extreme weather, even without global heating, but human impacts on the climate make such extremes more likely. Carbon Brief analysed more than 230 studies and found 95% of heatwaves were made more likely or worse by climate change. For droughts, 65% were definitely affected by our hotter world, while the figure for floods was 57%.
In Surrey, Berkshire, Buckinghamshire, Bedfordshire, Hertfordshire and Cambridgeshire, the temperature now has to linger at 28C for three days or more to be officially classed as a heatwave, up from 27C previously.
In Lincolnshire, temperatures must reach 27C, an increase from 26C, and in East Riding of Yorkshire it has changed to 26C, from 25C.
Dr Mark McCarthy, who manages UK climate records at the Met Office, said statistics showed the UK was getting warmer. The rise was greatest in parts of central and eastern England, where temperatures in some locations had increased by more than 1C. In Scotland and Northern Ireland, the rise was closer to 0.7C.
The Met Office said its definitions were always intended to be flexible and to adapt to a changing climate. The original heatwave thresholds were based on the climate between 1981 and 2010. The new thresholds are based on the climate from 1991-2020.
Climate change was also making heatwaves more likely, McCarthy said.
“A scientific study by the Met Office into the summer 2018 heatwave in the UK showed that it was 30 times more likely to occur now than in 1750 because of the higher concentration of carbon dioxide in the atmosphere,” he said.
“As greenhouse gas concentrations increase, heatwaves of similar intensity are projected to become even more frequent, perhaps occurring as regularly as every other year.”
The 2018 heatwave caused widespread drought, hosepipe bans, crop failures, and a number of wildfires in the UK. Nearly 700 more deaths than average were recorded during the heatwave’s 15-day peak.
