Deserts are spreading and we are losing farmland to it in earth's midband.  Sudan, Somalia, Syria, Iraq etc.   Plants may respond to increases in CO2 but their grain yield does not improve.  Crops need 20" of rainfall/irrigation and without that the CO2 discussion is moot.  Sparse wild plants may well benefit in this regime but they aren't much in the way of food potential.

Global warming is broadening the arid/desert belts further north as the Hadley cells dump their moisture further north.  Hadley cells (rain patterns) will be the next hot topic in the public domain probably.

http://www.metoffice.gov.uk/learning/learn-about-the-weather/how-weather-works/global-circulation-patterns

Cheers,
John Meyer