Global warming is upending the class distinctions in Germany's wine world.

The country has long had a dual wine personality.  About 2/3 of all German wine production is trocken (dry) or halb-trocken (off-dry, literally "half dry"), while more than 80% of German wines exported to the US are sweet.

Now, though, global warming is breaking up that simplistic duality and forcing a re-evaluation of where the best German wines come from. 

There's no better example than two regions:  The Mosel and the Pfalz.
The Mosel has long been the aristocracy of German wine, and until World War II was among the best-regarded wine regions in the world.  A century ago, some sweet table wines from the Mosel sold for more in London than first-growth Bordeaux.

The Pfalz was for centuries literally a land apart:  It was part of a minor kingdom until after the Napoleonic Wars, when it became part of Bavaria even though it's geographically separate.  While locals enjoyed wine from the Pfalz, it wasn't in the same class as the Mosel.
But warmer summers have changed everything.

The reason the Mosel's wines have been sweet is because they had to be:  The acidity in grapes is so high that sugar is needed to counter it (imagine lemonade without sugar).  Acidity -- which fluctuates with temperature -- has always been lower in the warmer Pfalz, making dry Rieslings more viable, but the sweet wines have never been as interesting.

Nowadays, dry Rieslings in the Pfalz are better than ever, and sweet wines are becoming an afterthought.

"Climate change helped us," says Martin Franzen, winemaker for Müller-Catoir.  "It wasn't possible to vinify dry Rieslings of this quality in the 1980s.  The climate was too cold and it wasn't possible to get the structure we can today."

In the Mosel, global warming is either crisis or opportunity.  With sweet wines out of favor, the Mosel is only recognized as a great region by wine geeks, mostly non-Germans.

"Most German wine drinkers don't know what the Mosel is," says Johannes Haart, assistant winemaker at Reinhold Haart.  "They have a bad impression of sweet wines."

Today many Mosel vintners are able to tempt the domestic market by making drier wines.

However, after tasting dozens of dry Mosel Rieslings, I'm not convinced this is in their best interest.  Many of these wines are mouth-puckering, eye-watering wakeup calls that might taste great in a decade, but are a shot to the palate right now.  It's ironic that the Mosel is now a Pfalz wannabe; it's like the aftermath of a revolution, with the former aristocrats looking longingly at the elevated working class.

The Mosel still has the trappings of its former station.  I visited two different Mosel organizations that are ruthlessly exclusionary.  The Wagnerian-sounding Bernkasteler Ring, a group of wine estates by the Mosel river, was founded in 1899, is limited to 35 members, and does regular quality checks on its members.  If somebody wants in, an existing member must be kicked out.

A younger version, literally, is Mosel Junger, limited to people under 40 who must have a position of responsibility in a winery.  "If you apply, we go to visit the winery and see if we will accept you," said association co-head Verena Clüsserath, 30.  If they let you in, eventually you will be old enough to get kicked out.

I tasted a lot of good wines through both groups, and their requirement that members be up to snuff helps the consumer.

But there's just a different, less regimented feel in the Pfalz, which has been making mostly mass-produced wine for centuries until the new hand dealt by a warmer climate.

There are some aristocrats there, like the Catoir family that owns Müller-Catoir.  But there are also dynamic newcomers like Achim Niederberger, a mogul who made his money in advertising and publishing.  He bought Weingut von Winning in 2007 and is investing some of his fortune in upgrading all of its facilities.

Revitalizing is all the rage in the Pfalz.  The story of Dr. Bürklin-Wolf, a winery founded in 1597, is commonplace.

"In the late '70s and early '80s the German winemaking philosophy was, 'How much can I get out of the land?  How many different flavors can I get?' " said Dr. Bürklin-Wolf wine consultant Tom Benns.
When Bettina Bürklin-von Guradze took over the estate in 1990, "we had 130 different wines," Benns said, and not all of quality.  The winery eliminated brands, reduced yields, and today makes about 20 different Rieslings, depending on vintage.  Almost all of them are dry.

"That's what the Pfalz is doing now, more dry wines.  We're not the south of Spain, but it's a warm climate," Benns said.  "Come here in the middle of July.  If you sit on someone's sandstone, it will burn you.  If we try to make a medium-sweet wine here, it gets flabby.  They can make wonderful sweet wines in the Mosel.  On the knife edge, with delightful fruit.  You can argue whether the Mosel should be making dry wines at all."

The challenge for the Pfalz is not to overreact to global warming.  It's already the source of some red wines, but though 1/3 of the total vineyard area of Germany is planted to red-wine grapes, the thirsty domestic market can absorb more.  I did try some nice red wines at Weingut Knipser, particularly a world-class 2004 Spätburgunder (Pinot Noir).

But German red wines are likely to remain a curiosity for the foreseeable future, whereas if Pfalz growers could get some unity they could market themselves internationally as (with apologies to Alsace) the leading European source for dry Rieslings -- just as Riesling is taking off in popularity.

"We're talking about getting winemakers into a room and making a decision," Benns said.  "It's very difficult to tell anyone what to do.  But it is happening.  You can go into Pfalz and make sweet wine, and the cooperatives feel they have to make everything for everybody.  But we live from dry Riesling.  That's what we do, that's what we built our reputation from."

Meanwhile, in the Mosel, the challenge is also to not overreact to global warming.  It might yet be a source of great dry Rieslings, but it isn't now.  The acidity is still mostly too high, but wines with some sweetness are lovely, even if they're unfashionable.  Climate is changeable, but fashion changes even faster.