"It tries to understand how the fruit of a tree gets mature," Yiou said. "It is easy to understand that when it is very warm in the spring and summer, the fruits will become ripe earlier."
For the Nature study, Yiou and his colleagues used a phenology model developed for pinot noir grapes, the main variety grown in Burgundy since the 14th century.
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Scientists also make temperature reconstructions by interpreting other biological signals, such the width of tree rings or the chemicals and gases trapped in the Arctic and Antarctic ice sheets. But according to Yiou, the grape-harvest records are superior, because there are no ambiguities about the date.
Menzel agrees that the method is robust but said "there are some discrepancies, because the grape harvest is determined by man and his special purposes."
For example, vintners may decide to harvest their grapes earlier than usual to avoid the risk of their grapes becoming infected with a pathogen. Or they may keep them on the vine longer, so as to allow the fruit to more fully ripen, which enhances the flavor of wine.
To correct the record for these human factors, Yiou's team includes historians. "It really is collaborative work between historians and climatologists," he said.
According to Yiou, his team's Burgundy temperature reconstruction from the pinot noir grape harvests is similar to temperature reconstructions from tree rings and other sources, confirming the warming trend of the last 50 years. Most scientists attribute the trend in part to an increase in greenhouse gases from cars, commercial aircraft, and power plants.
Yiou and his colleagues are less certain about the cause of the unprecedented heat wave experienced in 2003.
"My feeling is that 2003 is so unusual that we cannot comment on if it's just by chance or due to human activity," Yiou said. "If we have more of such events in the future, then it would be a sign we are changing the climate. So far the only thing we can say is it has never been seen."
According to Menzel, what is clear is that the climate is warming and the impact can be seen clearly via changes in phenology, such as plants sprouting and flowering earlier, experiencing a longer growing season. "In the Northern Hemisphere at mid-latitudes and higher latitudes nearly all seasons are affected, especially the spring phases," she said.
As for the impact the warming is having on the quality of wine, Gregory Jones said that for the time being, wine quality is improving. Jones is a professor of geography at Southern Oregon University in Ashland.
At the November 3, 2003, annual meeting of the Geological Society of America in Seattle, Washington, he and his colleagues presented results of a study that correlated temperature and wine quality over the past 50 years in the world's top 27 wine regions.
They found an average temperature rise of 3.6 degrees Fahrenheit (2 degrees Celsius) and higher vintage ratings according to the hundred-point vintage-rating scale used by Sotheby's auction house. But rising temperatures aren't necessarily good news for wine lovers. If temperatures continue to rise, they could cause fruit to over-ripen, add water stress, and prompt an increase in disease, the researchers noted.
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