Global Warming will obviously increase the rate of evaporation of water and so increase rainfall. Unfortunately it is also likely to shift the pattern of rainfall so though some areas will get much more rain some will get much less. The way this will happen is way beyond any simple model. The same applies to the changes in temperature that occur during any growing season. If you have a particular set of changes in mind then you may be able to create a credible way of getting there once you define the current "position" of agriculture.
