How does global climate change affect agriculture?
A very broad question, can be advantageous but mostly not.
Firstly, with the average temperature of the Earth slowly increasing, you could suggest that this would increase the rate of photosynthesis of some important crops, thus increasing yields. Furthermore, latitudes which were once too cold to grow crops may now be able to successfully produces higher yields. Also, foods which require warmer temperatures could be grown in larger areas as a result of the temperature increase.
However, the change in temperature may also drive the movement of pests where they are not usually found, possible affecting yield. The increasing temperature and changing weather patterns may also contribute to increasing soil erosion. Finally, and most obviously, the increasing severity of tropical storms may cause substantial damage large quantities of areas which are used for farming.
There will be many more effects, but they are possibly the main ones.
Possibly the biggest effect would be more areas of drought. The amount of water vapor that air can hold as vapor is based on temperature. The warmer the air the more water it holds as vapor. More water vapor means less liquid water which means less precipitation.