Climate is a difficult thing to predict because of the number of different factors that have to be taken into account.  The WhyFiles blog has a recent detailed article which describes some of the things that climate scientists have to think about when making projections of climate in the future.  This posting includes several paragraphs discussing the links between climate and agriculture.  (Full disclosure: Dr. Steve Vavrus, one of the scientists quoted in the article, was a classmate of mine in graduate school at the University of Wisconsin-Madison.)

Source: NOAA
Source: NOAA