For those who haven't been paying attention (or have only recently begun seeing my blog at its spiffy new alternative location), I explained the relevance of the Non-Identity Problem like this:
If we were to act to prevent or mitigate climate change, we would bring it about that people would spend their money on different things, travel to different places, meet different people, get different jobs, and most importantly, have different children (just think how tiny are the chances of a particular spermatozoon fertilizing a particular egg!). In 100 years, it's likely (if not certain) that the world would be populated by an entirely different set of people.
As a consequence of this "fact" (I will accept it as one), we are pretty much forced to say that the people who inherit a world affected by climate change are no worse off than they could have been, because if we had caused less climate change, they wouldn't have existed. Accordingly, it seems difficult to see how we could say that climate change "harms" anyone; if we did anything differently "to" them, they'd simply not exist.
So if the people who would face climate change will be different people than the ones who would have existed if we didn't cause climate change, how can we reasonably talk about costs being incurred as a result of climate change? It seems like when we talk about costs, we do rely on some sort of counterfactual, based on what would have happened if the event in question hadn't happened. For example, let's say I'm talking about a cost imposed on me by a car accident. What I have in mind is that there is a difference between what actually happened to me and what would have happened to me if the accident hadn't happened.
And when we talk about costs imposed by climate change, it seems like we're using the same sort of thinking: the costs imposed by climate change represent the difference between what happens to people in a climate change scenario, and what would have happened to them in the absence of climate change. But as I've said, what would happen to them in the absence of climate change is that they wouldn't exist. So how can we say that a cost has been imposed?
It's my view that this is actually not a problem for cost-benefit analyses at all. When we talk about what would have happened if a particular event had not occurred, I don't think it's necessary that it would actually have been possible for the event not to have occurred. I might say, "What costs and benefits did I incur as a result of being born male instead of female?" I couldn't have been born female; if my parents had a female child, it wouldn't have been me. But I still think we can ask such a question without speaking utter gibberish.
Some might be quick to point out that doing so would involve a lot of serious difficulties, because we'd have to hypothesize exactly what kind of life "I" would have lived, and we'd need to somehow compare that life to the one I already have. In the same way, it's extremely difficult to establish what someone's life would have been like if climate change hadn't affected them, and probably harder still to compare that hypothetical life to the one that actually happens. But it's important to see that this problem isn't confined to situations characterized by the Non-Identity Problem. The same kind of difficulties seem to be present when we ask, "What costs and benefits did I incur as a result of majoring in philosophy?" And it seems to me that any cost-benefit analysis is going to have to face these problems.
So back to the real question: does the Non-Identity Problem create any new problems for cost-benefit analysis? It does if we think of costs as representing harmful deviations from alternative possibilities. As I pointed out earlier, the concept of harm seems to include the idea of being moved away from a baseline, and the sort of baseline we'd need to refer to here is one where the individual couldn't possibly be on the baseline. If you couldn't exist if certain things didn't happen, then it's hard to see why we would say that you're harmed by their happening. But costs don't need to be thought of as harmful to people. As I alluded to earlier, I wouldn't want to say that I was harmed by being born a male instead of a female. My being male seems to be a necessary condition for my existence. But I can still try to determine what costs being a male has imposed on me.
So the fact that we can't consider the costs involved in future cost-benefit calculations to be harmful doesn't prevent us from being able to conduct the cost-benefit analysis. But one thing we have to keep in mind is whether the costs that we'd be measuring have any ethical significance. I want to think more about that, so I'll stop here.