To paraphrase Shakespeare, in the vineyards of the world, something worrisome this way comes. Over the last decade, global warming has started affecting those narrow zones best suited for growing wine grapes.

Warmer temperatures are a mixed blessing for winemakers. In colder climates like Bordeaux and Burgundy, more heat can increase sugars in the grapes.

Richard Snyder, a biometeorology specialist at the University of California, Davis, speaking at last year's Unified Wine & Grape Symposium, said a three degrees centigrade increase in temperature would create more carbon dioxide in the air, aiding photosynthesis and prolonging frost-free growing seasons.

The bad news, said Snyder, is that higher temperatures can bring more droughts to the Mediterranean and California.

Unlike so-called broad acre crops such as soybeans and wheat, wine grapes are a "niche crop that can only been grown in certain areas," says Gregory V. Jones, professor and research climatologist in the Department of Environmental Studies at Southern Oregon University.

"The issue today is, when we talk of global warming, we talk about humans' contribution, which is occurring at a much faster rate than in recorded history. What we used to consider a one-in-50-year drought is now more commonplace. The extreme heat of 2012 in the U.S. was a one-in-1,600- year event," he says.

Axel Heinz, director of production for Super Tuscan wine Ornellaia, said the prolonged European heat wave in the summer of 2003 was what alerted first producers to what was going on.

"The weather is now getting more and more extreme and unpredictable with sudden heat spikes, long lasting drought periods and violent and unpredictable rainfalls," he said.

Such spikes are forcing winemakers to adapt. Heinz has observed an accelerating trend of increased sugar levels leading to higher alcohol levels in the past five years.

Whereas conventional vineyard techniques had evolved in northern growing areas where optimal ripening was a struggle, now, ripeness is achieved more easily and quickly.

"We are returning to a more conservative approach, diminishing canopy sizes, picking slightly earlier and reducing vine vigor to allow the vines to better manage the resources in the soil," he said.

In Burgundy, where sun and heat can be a boon, there is some cautionary optimism about global warming. "The vines flower very early now," said Marie-Andree Mugneret, co-owner of Domaine Georges Mugneret-Gibourg in Vosne-Romanee. "If I told my grandfather we were picking grapes in early September, he would say it's impossible.

"For now, it seems a good thing; the concentration of flavors is there. But we just don't know what the future will hold."

Burgundians worry that alcohol levels may rise high above 14 percent in pinot noir, the dominant grape there. Possible soil damage is also a nagging concern for enologist Gautier Romani of Chateau de Pommard.

"I fear that global warming will affect the soil underneath the top layer. It will become compacted and micro- organisms in the soil will be affected," he told me. "The vineyards will change but we don't know how."

In Portugal's Douro Valley, home of Port, winemakers are confident that hundreds of years of experience will enable them to cope, says Robert Bower, sales and export manager for the Fladgate Partnership Vinhos, maker of Taylor, Croft, and Fonseca Ports.

Though there's no definitive evidence yet, "if global warming comes to the Douro, the Port producers can adapt with the variety of elevations and various aspects to the sun available in a port vineyard," he says. "If the year is warmer than they would like, winemakers can use more grapes from higher elevations or use more grapes from a northerly aspect to the sun."

Jones of Southern Oregon University urges vineyard owners to pay close attention to every aspect of their micro climate. "We have to be good stewards of the land, looking at micro fungi in the vine roots, what kind of fertilizer should be used, how the elimination of pests affects everything," he said.

"With niche crops like chocolate, coffee and wine grapes, small changes can have a large impact. The most magical things happen when a grape ripens at the margin of the climate."

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John Mariani writes about wine for Muse, the arts and culture section of Bloomberg News.