The pickers working their way along the hillside, clipping bunches of small, dark purple grapes from the rows of vines and dropping them into plastic buckets are harbingers of a warmer planet.

In recent years, aided by milder springs and autumns, a few British wineries have revived a red winemaking tradition which died around 600 years ago.

Wine aficionados are mixed about the results so far, but say the finest red wines may in future come from north of the English channel if a 190-nation conference in Copenhagen next month fails to agree a strong new U.N. climate change pact.

"We've benefited from global warming," said Chris White, General Manager of Denbies Vineyard, 24 miles south of London, watching plastic trays of Pinot Noir grapes being emptied into a stainless steel wine press in his winery.

"Climate determines the grape varieties you can grow."

Climate scientists have warned that global warming will shift growing patterns for crops, to the point that some developing countries may become too hot or dry to grow enough wheat and maize to feed themselves.

Most experts are too cautious to claim evidence of this theory in actual crop failures caused by droughts or flooding.

However, winemakers are clear that rising temperatures have already redrawn the international wine map, with wine regions developing characteristics of areas further to the south.

"Burgundy has got bigger and riper," wine writer Robert Joseph told Reuters. "Alsace, in North East France, which used to make very light red wine, now makes much fuller red wine. Germany which used to very light red wine, is now making fuller red wine."

Bellwether Crop

Wine is especially sensitive to weather and temperature because its only value lies in its taste.

If it's too cold, grapes will not develop fruity flavours or produce enough sugar, giving a wine that tastes acidic.

However, if it is too warm, the grapes produce too much sugar, giving a wine that tastes jammy and heavy.

"For a wine to work a grape should have a harmonious balance of sugar and acidity," said Simon Field, English wine buyer at wine merchant Berry Brothers and Rudd.

Sparkling wines are the most forgiving of under-ripe grapes: red wines need warmth and sunshine the most.

"In the sparkling, we don't need the complex flavours you'd expect in a red," White said.

This is why the Champagne region in France, which has a similarly cool climate to southern England, can make what are seen as the finest sparkling wines. The regions also have the same chalky soil, which explains the small chips of white stone in the soil at Denbies estate.

These similarities have helped English sparkling wines to beat their Champagne rivals in occasional blind tastings.

However, the UK is far north of the narrow bands in temperate latitudes where red grapes have traditionally grown.

Citing research from the University of Burgundy, Greenpeace said in a report in August the best latitudes for winemaking in the northern hemisphere may move 1,000 km (620 miles) north by the end of this century if nothing is done to stop global warming.

The report prompted 50 famous French chefs and sommeliers to write an open letter to President Nicolas Sarkozy urging action as fine wines, "jewels of French culture", were in danger.

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