Which informal fallacy is committed in the argument: 'No one has proved that global warming is actually occurring, so there is no reason to believe that it is actually occurring'?

Concluding that global warming is not occurring merely because no one has proved it is an appeal to ignorance.

The fallacy treats the absence of proof for a claim as proof of its falsity.

Lack of established evidence leaves the matter open, not settled against the claim.

Nothing here shows warming is absent, only that it has not yet been demonstrated.

Inferring falsity from want of proof is the appeal to ignorance (argumentum ad ignorantiam).