The biggest group of climate change denialists are those scientists that insist we can cap things at 2°, which at this point is about the same as thinking that Santa is real.
Such a terrible prospect to consider but that hasn't stopped scientists and engineers planning for the worst. An ill-conceived quick fix scares me, yet seems to be more likely than global agreement to reduce emissions in the fractured world of international politics.
It's only when things get so bad there's no option but to do it, by which it's probably too late, when everyone might decide to fuck it and go solo instead of waiting for consensus.
Climate change, overpopulation, mass extinction, resource exhaustion, the threats of nuclear war and winter, self-imposed mass epidemic (particularly biowarfare), long-term chronic pollution (especially lead, mercury, and dioxins), ozone layer, mutagens, genetic drift.
Others aren't directly imposed but are self-inflicted: systemic global supply-chain collapse risk, global financial system collapse risk, creating potential breeding grounds and vectors for global pandemics, etc.
The risk of exogenous events -- supervolcanos, asteroid impacts, solar storms, nearby supernovae -- aren't affected by human activities. But in the case of endogenous events, we're directly affecting the probabilities.