Not only must human population growth be stopped, but reversed and as fast as possible. Having no kids for around 20 years, then one per family until a stable population of one billion is reached. Along with a 90% reduction if fossil fuel burning within a decade or less, with going to a steady state environmental economy.
A "limited" nuclear war might work, or putting one deep into Yellowstone and forcing a super-volcano eruption. Also, drilling and putting some in other seismic fracture zones, so it just seems like natural events which are already due.
Humans (in general) have shown an insane desire to keep growing while destroying the biosphere.
Otherwise, do nothing, or not enough, and the population will crash followed by thermageddon from CO2 to self-sustained methane releases, and an ELE worse than the dinosaur extinction event.