Grape harvest records have enabled French scientists to track variations in summer temperatures in France since the 14th century.

Records of the harvest dates, which are closely related to temperature and have been registered in local archives for hundreds of years, show that last year's sweltering summer days were probably the hottest in more than 600 years.

"Grape harvest dates offer the potential to trace geographical variations in temperature over large parts of Europe and the Middle East over past centuries," said Pascal Yiou, of LSCE-CEA-CNRS, the Laboratory of Sciences for Climate and the Environment in Gif-sur-Yvette, France.

In research reported in the science journal Nature on Wednesday, he and his colleagues said there were warm spells in the 1380s and 1420s followed by a cold period from the mid-1430s to the end of the 1450s.

Temperatures also soared in the 1520s, 1630s and 1680s to levels similar to the end of the 20th century. But the summer of 2003 in France was very unusual, according to the scientists.

They said their results correlated with summer temperatures deduced from tree rings in central France and other climate records.

"The summer of 2003 appears to have been extraordinary, with temperatures that were probably higher than in any other year since 1370," Yiou added.