There have been a number of arguments here before about how excess can be justified: specifically, how emitting more greenhouse gasses than is sustainable per-capita based on the present human population can be morally justified. A new logical possibility occurred to me today: it is possible that we are already doomed. By that, I mean that pretty much all aspects of life that we consider to be deeply meaningful or important are already destined to be obliterated as a result of past action or inevitable future actions. For instance, the amount of climate change already locked into the climate system as the result of lags and positive feedbacks may be sufficient to make human civilization untenable.
If this is true, it changes the dynamic somewhat. The standard view of climate change is that we are all on a big ship in the middle of the sea, completely isolated from any help, and a serious hull breach has occurred. If most of us work together selflessly, we can plug it and save the ship. There is, however, the logical possibility that the leak is so bad that even the complete commitment of everyone aboard will not stop the rising water, and will not save a single one among us.
If we have crossed that threshold of inevitability, we are released of our obligations to prevent the sinking of the ship. Of course, the extent to which the sinking was preventable or not can only be known after the fact. Either we find ourselves in the position of being saved, on the basis of whatever efforts were made, or we find ourselves in oblivion, in spite of whatever was done to encourage an alternative outcome.