By Science/AAAS
Global warming has lowered crop yields for corn and wheat between 3 and 5% since 1980, but not for soybeans and rice, a study published in the journal Science finds. "Climate changes are already exerting a considerable drag on yield growth," the paper says. From 1980-2008 global corn production increased, but it would have gone up a further 3.8% without climate change. The percentage for wheat was 5.5%, compared to a model with no climate change. For example, recent studies in France suggest that climate change is an important factor in why wheat yields there have not risen since 1990. Wheat has been especially hard hit in Russia, where wheat yields are down almost 15%, as well as in Turkey and Mexico. Corn yields in China are also down, the paper says.

http://content.usatoday.com/communities/sciencefair/post/2011/05/climate-change-lowering-wheat-corn-yields/1