My impression is that most CO2 offsets are bogus, basically the climate change version of “just 25 cents will help save a child’s life”. If you subject them to a GiveWell style analysis, I would guess most of these offset programs fall apart, or at least deliver way less than the promised counterfactual impact.
Also logically I think it would make sense to lump offsets in with other charitable giving and subject them to the same scrutiny, and when you do that it just doesn’t make sense to buy offsets. Even within the climate cause area, I really doubt that buying offsets would be cost effective, and I also doubt that climate is the most cost effective cause area right now.
See also Notes on “The Art of Gathering” from a few years back.