But we also depend on vastly higher yields. [1] IMHO it is not clear, that additional agriculture in currently cold areas could offset all losses in warmer areas. Additionally we are already not very good at the distribution of food on a global scale.
Indeed, and we are dependent on natural gas (for fertilizer) and petroleum supply (all manner of chemicals, fuels, etc) to grow things now.
Additionally I would point out that famines today are almost never problems of global distribution but rather local breakdowns in distribution. I.e. a small nation falls victim of civil war and consequently famine happens as well.
There are very real and open questions what our current approaches to agriculture can do as oil continues to become more expensive.
Basically, I think the thing is that we humans tend towards as high a population as we can manage and expect famines to control our population. We've been pushing the envelope by utilizing vast amounts of fossil fuels for a while, and GMO's are often sold on the promise of being even more energy intensive (spray more pesticides and your crops will survive).
I wonder how sensitive our modern society is to similar agriculturally relevant factors.