Grapes on the vine crave sunshine, but wine growers in France and elsewhere are starting to worry that global warming is giving them too much of a good thing.

And worry they should, if scientists are right.

Predicted increases in average temperatures of 2, 3 or even 4 C this century could spell the ruin of many of Europe's most fabled vineyards.

Already, warning signs are abundant.

Serious wine makers embrace the fickle weather that yields more-or-less memorable vintages. Dealing with yearly fluctuations is part of the alchemy of turning grape juice into elixir.

But veterans who have nurtured and nursed vineyards over a period of decades are today reporting changes that chime perfectly with these threatening climate scenarios.

"Overall, we are seeing a rise in alcohol content and a drop in acidity. That's a problem," said Laurent Audeguin, head of research and development at the French Institute for Vineyards and Wine, a national repository for grape varietals.

"Acidity is crucial for the balance and the taste of wine, as well as for proper ageing," he explained.

Too much heat and sun, in other words, means that grapes mature too quickly, throwing the delicate ratio between sugar and acid out of sync.

Ironically, previous generations of grape growers ripped up vines that matured slowly and replaced them with slightly different variants of the same grapes - mainly merlot, cabernet sauvignon, chardonnay and half-a-dozen others - that yielded ripe fruit more quickly.

"It's true that we've focused a lot on precocity in the last forty years," said Pascal Bloy, who oversees the more than 400 grape types grown at the Institute, near Montpellier.

"The idea was to get a high maturity quickly. We realize now that this was probably a mistake."

That could leave French wine growers, especially in the south, scrambling for long-term solutions.

Bloy is eyeing varieties found in even warmer climes such as Greece and Portugal.