Oenophiles love global warming?

According to a report in Scientific American:

Gregory Jones of Southern Oregon University and his colleagues analyzed data from 27 of the top wine-producing regions worldwide from the last 50 years... The scientists studied Sotheby's vintage rating...and compared the trends to climate records. Overall, they found an average temperature increase of two degrees Celsius for the wine regions and higher vintage ratings for their products. "There were no negative impacts," Jones notes.

Armchair economics makes me suspicious. The "top wine-producing regions" should be located in places where the weather is optimal for grape growing. If higher temperatures are better, why weren't the top wine regions located further to the South to begin with? Higher temperatures could make previously inhospitable areas good for wine-making and even raise wine quality on net but the best regions ought to be made worse by climate change. I suspect the authors did not control well for other factors that are improving wine.