One thing that I have always worried about Antartic ice melting and unleashing some ancient black death like plague for which we dont have any immunity.
I still believe the planetary ecosystem has a way of ridding itself of annoying biomass if a tipping point is reached. Who knows what will hit us and how fast it will hit us.
Permafrost viruses, dramatic temperature changes and generally more energy in the weather system due to global warming, catastrophic ocean acidification leading to the death of trillions and trillions of plankton, robbing us of oxygen to breathe.... there's likely a trove of "problems" we will face sooner rather than later.