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 wine-making tradition that died around 600 years ago.

Wine aficionados give mixed reviews of the results so far, but say the finest red wines may in the future come from north of the English Channel if a 190-nation conference in Copenhagen, Denmark, next month fails to agree a strong new United Nations climate change pact.

"We've benefited from global warming," says Chris White, general manager of Denbies Vineyard, 39 kilometers 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, wine makers 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," says wine writer Robert Joseph. "Alsace, in northeast 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."

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," says 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 flavors you'd expect in a red," White says.

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 wine-making in the northern hemisphere may move 1,000 kilometers 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.

English wine makers see the trend partly as the pendulum swinging back in their favor.

Britain experienced a "Medieval warm period" in the centuries around AD 1000, says Philip Brohan, climate scientist at the UK's national weather service, the Metropolitan Office.

This allowed wine-making, introduced to Britain by the Romans, to thrive under the Normans before declining, says professor Richard Selley, author of "The Winelands of Britain".

Historians say an increase in trade with France and King Henry VIII's 16th-century dissolution of the monasteries - which operated many vineyards - were also likely factors.

Brohan says the current warming cycle is more severe than the medieval one, and scientists stress carbon dioxide emissions are behind the man-made climate change of the present day. Nonetheless, UK vineyards still struggle to ripen grape varieties that produce the most popular red wines.

Pinot Noir produce the fine Burgundy reds but it is only every few years those grown in English vineyards ripen sufficiently to allow the production of a single variety wine.

Most years in the past decade or so that UK growers have been making reds, they have blended Pinot Noir with other grapes such as Dornfelder, a Germanic variety which ripens more easily but which is not considered as flavorsome.

So far, producers have not managed to master the varieties behind the famous full-bodied wines of Bordeaux, such as Merlot, Cabernet Sauvignon and Cabernet Franc, although some have tried to grow these under plastic sheeting.

This has yielded reds that are generally lighter in color and taste, and less alcoholic than wines from warmer climates.

Visitors sampling the red wines in Denbies cellar say they were pleasantly surprised.

"It's tasty," says Mario Garcalo from Portugal, as he sipped a Pinot Noir-Dornfelder blend in the large, musty, barrel-lined underground tasting room.

Another factor working against English reds is cost. With annual red wine output of around 400,000 bottles and total output of just over 2 million bottles, against 7-8 billion bottles in France, the industry lacks economies of scale.

English red wines retail at around US$13 a bottle, against an average price of US$7 per bottle for wine in the UK, according to consumer data provider Nielsen.

For the time being, the economics favor production of sparkling wine, where English producers find it easier to compete both in terms of price and quality, says Chris Foss, head of the Wine Department at Plumpton College in East Sussex.

That may change if Copenhagen fails to curb carbon dioxide emissions. "It's a disaster in lots of other ways but I'm looking forward the UK making some interesting Pinot Noirs and Cabernet Francs," Foss says.