One of the responses to last week's post about climate change was asking whether, if you donate to the AMF or another charity that saves lives, should you buy carbon offsets for the estimated emissions that causes?
I see this as a question of norms. The strongest case I see for offsetting here, is if you think people have an obligation to make their choices carbon neutral. Perhaps anyone who makes a decision which entails emitting greenhouse gasses is then on the hook for averting or capturing some equivalent amount. For example, if you choose to fly round trip from Boston to LA, and this entails emitting 1.3T CO2e, then you should buy 1.3T CO2e of offsets. On the other hand, if this is business travel for a company that already is offsetting its employees' emissions, then you wouldn't need to buy offsets.
It's a bit of a stretch to extend this to averting deaths: shouldn't the people whose lives you save offset their own emissions? But if you think they aren't going to, a reasonable assumption for the extremely poor people that, ex, the AMF is helping, then perhaps you still are responsible? 
This also runs into complicated questions around estimating the impact of your decisions. If I buy a shirt at a thrift store, what is the chance that this causes someone else to not find a shirt they like and so buy a new one instead? If I buy leather, how much of an impact does that have on how many cows are raised? If I rent in a place where I can take the subway to work, but then the city refuses to allow more construction and so someone else ends up living farther out and driving more, how do I count this?
I think this way of thinking about emissions is coherent, but requires people to put a lot of effort into estimating the emissions impact of their decisions. Instead, I think carbon taxes are a much better approach. If we tax emissions at their full social cost, then instead of people tracking the counterfactual climate impact of their decisions they just need to look at prices.
For example, the airlines would be taxed for emitting carbon from flights, taking into account that high-altitude emissions are worse. Someone buying a plane ticket will see this as a higher price, but doesn't need to try to estimate the airline's load factor or the fuel efficiency of its jets.
This does prompt the question, though, of what we should do now, since most places don't have carbon taxes yet. Building a norm that you should offset down to carbon neutrality is hard work, following the norm would also be hard work, and the norm wouldn't be needed once we got the tax in place, so I don't think this is a good path. Instead I think we should work on building a much more general norm: spend 10% of your money (or time) on whatever you think will most make the world better.
If you think that climate change is the most urgent thing, then you might consider putting your 10% towards carbon offsets, but I suspect that some estimates of relative impact might lead you to put your resources towards advocacy instead. On the other hand, people who decide that malaria nets, animal welfare, nuclear non-proliferation, or something else is more urgent might choose to fund or work on that instead. This is a much more robust norm: there's so much to do that there will be valuable things for people to put their 10% towards for a very long time.
 In the particular case of averting deaths leading to people emitting more carbon, two things you'd want to consider are that (a) these are very poor people with very small carbon footprints, and (b) that averting deaths doesn't actually increase the population size very much because these are mostly deaths of young children and parents seem to have a target family size.
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