Human dieoff

Read about human dieoff at [1]. There are many theories of how this will or could happen, too many to list here, but here are some major ones:

(totally credible) nuclear winter theories re the disruption of climate by nuclear weapons being used in large numbers; Risk of this is believed to be less now that USSR is gone, and the USA is no longer on hair-trigger alert.
(extremely credible) climate change theories re: extinction and mass social disruption as over 3 billion people are directly affected by disasters - see IPCC numbers and letters signed by ALL Nobel Prize winners on Earth.
(somewhat credible) civilization lifecycle theories re: energy subsidy rising to the point that a civilization can no longer afford to keep gathering energy, at which point it dies, see Joseph Tainter in particular on this.
(much less credible) peak oil theories regarding the loss of fossil fuel supply and perhaps rising conflict over who will control it
(credible only to trolls) theories regarding sysopism taking over and making dissent impossible to express, so that whatever disasters occur, no one will be able to point them out in advance or take any steps to prevent them; a sysop vandalism mentality could prevail where bureaucrats destroy thoughts in order to frustrate and drive away those whose thoughts disturb them; this may be a variant of the civilization lifecycle theories - there are also (trivial) wiki lifecycle theories, e.g. at Meatball Wiki, that more or less say something like this.