Everybody compares carbon offsets with the indulgences of the medieval Catholic Church. Indeed, a good number of people seem to treat the comparison as the decisive point against them. Offsets allow one person to ‘sin’ by flying or driving a big car, then pay for it by having someone else reduce emissions by a similar amount. While there is certainly potential for abuse, the real issue here is about the intuitive sense of fairness people possess.
Obviously, if someone buys an offset that produces no real reduction in emissions, they have been bilked and the climate has suffered. There are plenty of cases of dubious offsets, including all those based around planting trees. Furthermore, it is necessary not only for the sale of the offset to lead to reduced emissions: it must lead to a reduction of emissions equivalent to the face value of the offset and, crucially, these must consist entirely of reductions that would not otherwise occur. The perfect offset is something like this: (a) a farm releases large amounts of methane, a powerful greenhouse gas (b) in the normal run of things, the farm would have no incentive to stop doing so (c) the sale of offsets changes the economics of the situation, making it most economically efficient to capture the methane, perhaps using it to generate electricity (d) this produces a quantity of real and verifiable reductions that can be sold at the marginal cost of capturing the methane.
In this situation, the argument of ineffectiveness does not apply. What we are left with is the offence against fairness – allowing one person to ‘take more than their share.’ While there is intuitive force behind this position, I don’t think it is very convincing. While it would be better to both moderate one’s consumption and help others to do so, it does seem less objectionable to emit and purchase credible offsets than to emit and simply ignore the consequences of your actions. The critical difference between offsets and indulgences is that offsets (when used properly) actually have a mitigating effect on total greenhouse gas emissions; indulgences never did anything at all, except raise money for those selling them and the ire of those opposed.