A team led by Pascal Yiou used the dates to reconstruct temperatures in Burgundy from 1370 to 2003, using as the benchmark the Pinot Noir grape, which has been grown in the central French region since the Middle Ages.

The later the harvest began, the cooler the summer, while the earlier the harvest, the warmer the summer -- a difference that Yiou says can be calculated to a hundredth of a degree.

By this estimate, the scorching temperatures of the 1990s have several local parallels, according to the study, published on Thursday in the British weekly journal Nature.

In the 1520s and in between the 1630s and 1680s, Burgundy experienced bouts of weather that were as warm as in the late 20th century.

After the 1680s event, there was a prolonged cooling which lasted until the 1970s.

The summer of 2003 -- when France was gripped by a heatwave that killed thousands of people -- was "an unprecedented event," the researchers say.

"(It) appears to have been extraordinary, with temperatures that were probably higher than in any other year since 1370," they note.

Other warm years were in the 1380s and the 1420s. Again, after the 1420s warmup, there was a very cold period that ran to the end of the 1450s.

The team say their estimates have been corroborated by local evidence from preserved tree rings.

Trees add a ring to their trunk for every year of their life. The bigger the gap between rings, the better the growth, and the likelier that the weather that year was favourable. A narrower gap points to worse climate conditions.

Previous research by scientists says that man-made global warming, inflicted by the unrestricted burning of carbon-based fossil fuels, began to occur in the 1970s.

The 1990s were, when measured as an average for the world, the hottest decade on record.

The French scientists make no comment on this. They merely say that grape harvest dates can be viewed as another useful tool for assessing regional climate patterns.

These dates have been recorded not just in Europe but also in parts of the Middle East where winegrowing is a tradition, they note.

Other yardsticks of climate patterns are in records of atmospheric temperatures and sea levels, although these are generally not considered accurate beyond the past century and a half, as well as ice cores, which hold dissolved carbon dioxide (CO2).

When thawed, the cores release the CO2 as a gas, which thus provides an assessment of carbon levels in the atmosphere over thousands of years.

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