Extreme weather has already had effects on crops, e.g., wheat in Russia in 2010 and in Australia in 2012, both from lack of rainfall. And this year olive crops in France, Spain, and Italy. These events mattered a lot to the farmers affected but had little impact on the world supply of wheat and olives. But this is only a hint of what will come if nothing is done about global warming. Extreme weather will have a widespread negative effects on crops. The Guardian reports that the climate change panel, IPCC, says that "global warming is fueling not only natural disasters, but potentially famine - and war". If nothing is done, the 8 billion people on this planet fifteen years from now will have less food than we have now for 6 billion people. There is no other implication we can take from this than that there will be mass starvation.
But there's a worse food problem. It turns out that "the global livestock industry actually produces the same amount of greenhouse gas emissions as the transportation sector (both at 15 percent of total emissions)". This is saying that we can turn all our cars into renewable energy cars but we are still on our way to the end of civilization if we continue to eat meat.
This is a frightening conclusion, but I believe, a problem that will take care of itself. As global warming advances, and even if we succeed in bringing it to a threshold we can live with, meat and dairy will be very expensive and only available to the very rich. They'll be like Beluga Caviar. The rest of us will be vegetarians.