Keep in mind that we plan to colonize Mars. If we can pull that off then certainly we can survive global warming. You can build self contained ecosystems where you grow food. But that isn’t even needed. There is nothing wrong with our atmosphere. Many eras may become to dry for farming. But it is not like the whole world will run out of water. And desalination is getting ever cheaper.

If all the he fish died in the water that will certainly suck, but you don’t have to eat fish to survive. And besides fish farming in tanks on land is possible.

This is mainly an economic question. Environmental destruction will make food growing more expensive, not impossible. Thus it will hurt poor people. This may sound cynical but there is a big difference between the whole human race getting exterminated and the poorest dying.

We should push for a green shift because we care about the poor and our own economy, not because we are in danger of being wiped out. A species which can survive in hostile environment of outer space can certainly survive global warming.