This is both annoyingly inaccurate and surprisingly insightful. The inaccuracy comes in the historical part--what opponents of Catholicism call "buying" indulgences were really indulgences granted for almsgiving, for a variety of purposes (including, but not limited to, building new churches--a practice that gave rise to abuses, which played a part in triggering the Reformation; thus, indulgences are no longer granted for almsgiving of any kind).
The insightful part comes in what's different between the two.
The idea behind indulgences is that Christ, in His life and death, amassed a superabundance of merit--and this merit (along with that of the saints, though next to Christ's, it's insignificant) can then be applied by the Church for the partial or complete remission of temporal punishment for a person's sins (though only after repentance--an indulgence is most emphatically NOT a get-out-of-Hell-free card).
What's different from this about carbon credits (other than not needing to repent first--for that matter, environmentalists can continue to sin as long as they keep buying credits) is that, while the environmentalists have brokers to take the role of the Church in the above equation, they DON'T have any equivalent for Christ, Whose superabundance of merit is the whole reason indulgences could exist in the first place. So if there's no enviro-Christ to dole out remission of environmental emissions, then where are the offsetting environmental benefits coming from?
Charles Krauthammer has the answer:
[I]t is a way for the rich to export the real costs and sacrifices of pollution control to the poorer segments of humanity in the Third World. (Apparently, Hollywood's plan is to make up for that by adopting every last one of their children.) For example, GreenSeat, a Dutch carbon-trading outfit, buys offsets from a foundation that plants trees in Uganda's Mount Elgon National Park to soak up the carbon emissions of its rich Western patrons. Small problem: expanding the park encroaches on land traditionally used by local farmers. As a result, reports the New York Times, "villagers living along the boundary of the park have been beaten and shot at, and their livestock has been confiscated by armed park rangers." All this so that swimming pools can be heated and Maseratis driven with a clear conscience in the fattest parts of the world.
It's a hideous parody of the original--where Catholics rely on the free gift of their Lord to save them, carbon credit consumers forcibly shunt the costs of their sins onto the poorest of their brethren.
And it's a pretty good example of everything that's wrong with environmentalism.