WASHINGTON (AP) -- The length of European heat waves has doubled and the number of extremely hot days has nearly tripled over the last 100 years, adding further evidence that hotter summers are a sign of global climate change, according to a study released Friday.

Heat waves, such as the one in the summer of 2003 blamed for the deaths of tens of thousands of people in Europe and millions of euros in agricultural losses, are expected to become more common still as the climate warms, said Paul Della-Marta, the head of a research team at the University of Bern in Switzerland.

"These results add more evidence to the belief among climate scientists that western Europe will experience some of the highest environmental and social impacts of climate change and continue to experience devastating hot summers like the summer of 2003 more frequently in the future," Della-Marta said in a statement.

The results appear in the Aug. 3 edition of the Journal of Geophysical Research-Atmospheres, a publication of the American Geophysical Union.

Researchers compiled evidence across Europe and found that heat waves now last an average of three days, with some lasting up to 13 days. That compares to an average of about 1.5 days in 1880, they said.

Past temperature records were probably overestimated, because thermometers were not accurate, researchers said.

The study found that even small temperature changes will drastically increase the length of heat waves.

The European Union warned in June that the continent has already warmed by 1 degree Celsius over the past century, faster than the global average, and that Europe must get ready for even small temperature changes that will have "very big effects" in the future.