My understanding is that it's not so much about undoing past decades, but being able to even reach zero additional warming in the first place. Cows won't stop farting (that sounds like they're the butt of a joke but it's not actually, even if some feed additives reduce it), so we need to compensate for the methane. Or air travel for relevant distances (where a train won't do), we don't have the battery tech to make it zero emission but we can totally capture the carbon. (It's not super economical but it's cleaning up after oneself, it's either that or accepting further warming and much greater costs later.) Or compensating whatever emissions occur as part of making those batteries and stuff we need to capture and use that zero-emission solar/wind/water energy