The cataclysmic death of a billion people or more. This is equivalent to one thousand megadeaths. This does not need to occur in a single event, but from a connected cause. This word was derived from "megadeath", which was coined in 1953 to describe the results of nuclear war. In contrast, "gigadeath" was coined to describe the more massive catastrophe of industrial civilization combined with peak oil, global warming, ecological degradation, and explosive population growth overshooting the global carrying capacity.
"Abrupt climate change would inevitably lead to gigadeath."
"Humans can live sustainably on this planet, but only after a few gigadeaths."