There are two kinds of difficult ethical problems: situations where it is genuinely hard to work out what the right course of action is, and situations where the right course of action is fairly clear but people are unwilling to do it.
Air travel is an example of the second type. I think it’s pretty unarguable that our excessive emissions of greenhouse gas pollution are unethical. Long flights produce excessive amounts of CO2, and many (perhaps most) of those long flights serve morally unimportant purposes. As such, people should fly less, because their decisions to fly harm innocent strangers. And yet, few people are willing to do that. They put their own preferences and convenience ahead of the interests of others. Eating most types of meat and dairy products probably falls into this category too – at least if you think the suffering of non-human animals has any moral importance.
The international distribution of the costs of dealing with climate change may be an ethical problem of the first type. It’s not entirely clear what the ethical status of historical emissions is, what the relevance of population is, the importance of per capita emissions, etc. While it may not be entirely clear who should pay exactly what, I do think it is clear that every country needs to take action – far more action than they are taking now.
There are flights that serve morally important purposes.
One exercise that may have value is writing a letter to members of future generations, justifying any particular trip. If you can come up with justifications that would be convincing to them, there is a good chance the trip serves a sufficiently important purpose to justify the pollution.
That does seem like a good method for working out whether travel (or some other emissions intensive activity) is justified.
Yes, this is a useful distinction. Of course, some issues fit both categories (the right thing to do may only be known with a low degree of confidence and yet still be difficult/unpalatable).
I also find anon’s suggestion about trying to justify one’s actions to future generations a very interesting proposal. It asks us to put ourselves in the shoes of a stranger, whose situation is significantly unknown (and unknowable) to us and to simulate their ethical judgements. This is not an easy task (in the first sense above), and the outcomes may not be palatable, meaning that flights that serve a prima facie morally important (or somewhat important) purpose may represent a good example of an issue that fits both categories.