Yesterday a new study came out: http://www.economist.com/node/18648350?story_id=18648350
The study seems to explain, to some extent, the fall globally, in crop yields. However,we seemed to have missed, so far, negative impacts. The article concludes:
That might be keenly felt if patterns of warming shift. One of the reasons that the climate effects Dr Lobell and his colleagues have dug out of the data are not worse is that, although the planet as a whole has warmed up during the past 30 years, growing seasons in the parts of America which produce 40% of the world’s maize and soyabeans have failed to follow suit. No one is quite sure why this might be and no one knows if it will last. That climate change has not yet done very much harm may be cheering, but the past offers no firm guarantees for the future.