Summer heatwaves in the UK are up to three times more frequent today than they were in 1878, a study has shown.
The duration of heatwaves has also increased two to three-fold since the end of the 19th century, according to the research.
Scientists based the findings on data from the Central England Temperature (CET) record, the most extensive library of daily temperature measurements in the world.
By applying statistical techniques, the researchers explored the changing relationship between frequency, duration and intensity of heatwaves.
The team focused on days when temperatures rose above 28C, the definition of overheating under current UK building regulations.
Six day-long heatwaves occurring every five years typically see temperatures higher than this threshold, the study found.
It highlighted a two to three-fold reduction in the “average return period, ” the average time separating two successive heatwaves, since 1878.
The duration of heatwaves with an average five year return period had as much as tripled over the same time span.
Lead scientist Professor Sandra Chapman, from the University of Warwick, said: “Heatwaves are by definition rare events, so that putting numbers on their frequency, duration and severity is a challenge.
“However, as hotter days become more frequent, heatwaves will on average become more likely and longer lasting and if we have the data, this is something we can quantify.
“How these temperature extremes are changing may not simply follow changes in the average temperature.
“We have seen intense heatwaves in the UK several times before, but at the same time we see heatwaves becoming more intense and severe on average.”
The findings are published in the journal Geophysical Research Letters.