A recent report from the International Food Policy Research Institute highlighted the degree to which climate change threatens global agricultural output:
In parts of the developing world some crop yields in 2050 could be only half of their 2000 levels. Irrigation may not help: climate change will hit irrigated systems harder than rain-fed ones. And the hope that gainers from climate change will outweigh losers looks vain: the damage from higher temperatures and erratic rainfall will be too big.
Couple that with ever-increasing population, and you have a recipe for a lot of suffering and strife.