I mean... we eat biomass? Mass starvation will occur when we can't grow food anywhere. We literally grow the food we eat in a year in a year. I don't think it's ridiculously optimistic to assume that if starvation was on the horizon, we'd be able to spin up food production in the places where the climate allowed it, and this stuff is going to happen on the scale of years at worst, but probably the scale of decades. I think it's a hell of a pessimistic assumption to make that half the world's population is going to starve to death.