There are potential tipping points that very much could result in much of the world becoming an unlivable hell-scape, at least for humans. We probably can't turn ourselves into Venus, thankfully, but we could end up raising global temperatures by 11-12 C, which would be enough to render most of the Earth unsuitable for human habitation.

https://en.wikipedia.org/wiki/Tipping_points_in_the_climate_...