I agree with this, we can already see it. The global warming hysteria is pushing us to burn our crops, and as a result food prices have gone up. We really need to get algae or cellulosic ethanol up and running and stop burning our food.
The problem isn't so much not enough food as too many people. Something will happen to drastically reduce population before we see any major food shortages. For example, massive war breaking out in the Middle East.
This is true, they are already rationing bags of rice at the Sam's Club. The free market will take care of it and it will determine who eats and who starves. That is the only logical solution.
Your question is asking about time, so it depends on when you consider global warming a problem. It is not like the Earth will disappear because of global warming. Honestly, I think the lack of food will harm us before global warming will. In fact, starvation already is a huge problem. Approximately 24,000 people die every day because of hunger or hunger-related causes and about 1 billion people suffer from malnutrition. That is just about 1 out of every 6 people. Yup, a lack of food has "bit[ten]" much before global warming.