Maybe most agriculture will have to happen indoors? Chile, Spain and many other year-round producers do it in greenhouses / hoop tunnels. Vertical farming/airoponics and so on to help scale food production.
I think the bigger problem is if we kill the ocean through sea level rise, warming, deoxygenation and acidification, then we kill 3/4 of oxygen production (but we would still have 100k-400k years supply) and 2%+ of the food supply.
We're the only species on the planet that lives in both 100+ degree deserts and frozen wastelands.
As long as it's still possible to grow crops, humanity will survive.