NPR.org » World's Farmers Feel The Effects Of A Hotter Planet

http://m.npr.org/story/135952277?url=/2011/05/07/135952277/worlds-farmers-feel-the-effects-of-a-hotter-planet

"We're not saying yields have gone down, just to make this clear," Schlenker says. "What we're saying is yields are lower than they would have been without the climate trend. So yields have still been going up over the last 30 years."