By Paul Rincon
Science reporter, BBC News
The duration of heatwaves in Western Europe has doubled since 1880, a study has shown.
Wildfires during heat waves can ravage countryside
The authors of the research also discovered that the frequency of extremely hot days has nearly tripled in the past century.
The study suggests many previous assessments of daily summer temperature underestimated the change in heatwaves in Western Europe by about 30%.
The research appears in the Journal of Geophysical Research - Atmospheres.
The team found that heatwaves lasted an average of three days now, with some lasting up to 13 days. This compares with an average of about 1.5 days in 1880.
Paul Della-Marta, from MeteoSwiss in Zurich, Switzerland, and colleagues analysed daily maximum temperature data from 54 recording stations across Europe.
Forty-six sets of records date back to the 19th Century; others go back to the early 1900s. The data sets come from as far north as Finland, as far south as Spain and as far east as Croatia.
In the past, however, thermometers were not kept in modern Stevenson screens.
These wooden shelters protect thermometers from direct sunlight and indirect radiation coming from the ground, both of which distort temperature readings.
Once the researchers had corrected for these effects, they found a "warm bias" in observations made prior to the introduction of these screens. In other words, temperatures were recorded as being hotter than they really were.
This in turn meant the increase in temperature over time appeared to be smaller than it actually was.
The authors of the latest study also corrected for other biases in the variability of summer temperatures.
"After looking at the records more closely, we believe... the change in heat waves have been underestimated by about 30% over the entire Western European region," Dr Della-Marta told the BBC News website.
We can expect more extremes of temperature in future
"We see a doubling of the length of heatwaves and we also see a tripling in the frequency of one-off events."
The results support the idea that the western European climate in summer is becoming more variable - that the range of temperatures had increased. "We're getting stronger heat waves or perhaps summers that are not so strong in terms of heat - relative to an increasing trend. This is a major cause for concern," Dr Della-Marta explained.
Co-author Malcolm Haylock, from re-insurer PartnerRe in Zurich, Switzerland, and formerly of the Climate Research Unit at the University of East Anglia, UK commented: "We expect to see the largest impact of global warming on extreme temperatures in the future, and this study shows that these large changes are already occurring now."
The heatwave experienced by Europe in 2003 had major adverse socio-economic and environmental effects.
Thousands of elderly people died. Forests were devastated by fire, water ecosystems were strained, and the total mass of Alpine glaciers shrank by 10%.
The authors say we can expect extreme weather events like this to occur more frequently in future.
The paper's other authors were Jurg Luterbacher and Heinz Wanner, both of the University of Bern, Switzerland.