Long, long decades of heat, water getting less accessible and overpopulated areas getting more overpopulated. I used to think the world would collapse all at once soon, but I changed my mind. Humanity will survive for years to come, each year harder than the before, until civilization slowly decays to a point of either extinction or rebuilding, when things get balanced out again (but nature more blighted than ever) and humans manage to slowly and painfully raise again. No sudden apocalypse, just long years of constant loss in quality of life.