Carbon offsetting is the practice of doing something that increases carbon emissions, and then “offsetting” those carbon emissions by doing something that decreases carbon emissions by an amount that equals or exceeds the increase. For example, you might take a flight, and then donate to a charity that works to reduce carbon emissions.
Moral offsetting is harder to explain, but it is in some ways analogous to carbon offsetting. For example, a meat eater might try to morally offset her consumption of meat by donating to an animal welfare charity. The donation is supposed to produce a benefit that equals or exceeds the harm caused by her meat eating.
Some people think that carbon offsetting is not just analogous to moral offsetting, but that it is an instance of moral offsetting. Some of these people also think that moral offsetting is wrong. If you combine these views, then you get a straightforward argument for the conclusion that carbon offsetting is wrong. Though I've never seen this argument presented so explicitly, I think that it underlies much of the opposition to carbon offsetting.1
The problem with the argument is that carbon offsetting is not an instance of moral offsetting. To see this, think about how the donation to the animal welfare charity is supposed to offset your meat eating. It doesn't prevent the animals you eat from being harmed. It just benefits some different animals. The same goes for all instances of moral offsetting: the offset action causes some harm, and the offsetting action produces some benefit that equals or exceeds that harm.
But this is not how carbon offsetting works. It's not that the emissions from your flight cause some harm, and the carbon offset you buy produces some benefit. Instead, the carbon offset prevents the emissions from your flight from being harmful in the first place. After all, carbon emissions aren't intrinsically harmful. They are harmful only to the extent to which they contribute to climate change. And if, as a result of buying carbon offsets, you are carbon neutral, then you have not contributed to climate change.
Here is what I now think is a better analogy for carbon offsetting. Suppose that there is a car whose breaks work poorly, and so is likely to harm people if it's driven. Driving the car without fixing its breaks is wrong. But if you fix the car's breaks, and thereby prevent this likely harm, then driving it is permissible. This is not because fixing the breaks produces some benefit that outweighs the harm of driving the car. It's because fixing the breaks prevents driving the car from being harmful in the first place.
This is not a new point. MacAskill makes it clearly in Doing Good Better.2 But a lot of people who read my ‘Moral Offsetting’ paper seem to think that I was saying that carbon offsetting is an instance of moral offsetting, or that I was working out the moral principles behind carbon offsetting.3 So I wanted to go on record as saying that's not at all what I was doing, and that I wish I had been clearer about this in the paper.