Global warming could slash productivity in prime US wine-making areas like California's Napa and Sonoma valleys by century's end, leaving some of the best temperatures for grape-growing in New England, researchers reported. "We're projecting dramatic decreases in total production in the continental United States and this is really due to the increase in the frequency of extremely hot days in response to elevated greenhouse gas concentrations," climate scientist Noah Diffenbaugh of Purdue University said by telephone. A study published in the Proceedings of the National Academy of Sciences forecast an 81 percent drop in the US areas suitable to grow premium wine grapes by the year 2099.