It doesn't have to turn the Earth into Venus for unusually-rapid climate shifts to destabilize geopolitics badly, exceed system tolerances in infrastructure, consume an ever-growing portion of the economy either by increased loss and waste or in efforts at mitigation, and thereby effectively amp up the power of many other forms of global X-risk (or just contribute directly to X-risk by numerous small factors, none of which would by itself be capable of overwhelming the system, but which collectively undermine it).