[tlb]

A lot! There's no serious debate that it's happening and the main cause is human burning of fossil fuels, but numerical estimates of future temperature change have wide error bars and estimates of ecosystem damage are all fairly speculative since they depend on species not being able to adapt (which they may or may not.)

The debate has been poisoned by deniers pointing to any uncertainty as evidence of it all being fake. The intellectually honest approach is to consider the expected costs across all likely outcomes, and spend money on mitigations accordingly.