"It is a big change when you consider it is an average temperature," Della-Marta said.

The reason why previous estimates were too low was because before 1930 most temperatures were measured without the type of screens now in use that block radiation from the sun and ground that can skew a reading, he said.

The study's findings may provide further evidence that heat waves like the one that killed dozens this summer in Europe were a sign of global warming.

"It could be used as more evidence that temperatures are rising and we may be underestimating impacts of human activity," he said.

Weather forecasters may also use the new data to help build climate models to help better predict the impact rising temperatures will have, Della-Marta added.

"We would expect the frequency and duration of heat waves to increase with global warming at an accelerated rate," he said in a telephone interview. "It means it is getting hotter and that extreme events are getting more frequent."

The team, which also included researchers from the University of Bern, compiled temperature readings from more than 200 locations across Europe and found that heat waves now lasted an average of three days compared to around 1.5 days in 1880.

The researchers defined a heat wave as periods when temperatures rose above a certain level, which varied depending on the location. For example, the threshold was 30.7 degrees Celsius in Paris and 35 degrees in Madrid.

Because many of the stations -- which stretched from Sweden to Croatia -- recorded temperatures in the same regions, the team was able to strip out off-the-mark temperatures and come up with a more accurate reading, Della-Marta said.