If these things do happen (are they really any reliable predictions?), perhaps they're compensated for by the fact that we can grow enough food to feed the planet thanks to fertilizers and fossil fuel powered transportation. The situation would probably be worse without fossil fuels (limited to mostly medieval technology) than even a doomsday scenario with them.

Don't forget ocean ecosystems have already more or less collapsed due to fishing and mass human migration has already happened due to urbanization and population growth. Most city buildings that exist today will have been constructed in the last few decades or certainly the last century. We're quite fast at moving and developing civilizations.