Two "Liberal" Stances: Abortion and Obligations to Future Generations

I've recently been musing about the relationship between the ethics of abortion and our obligations to future generations.

It's often supposed that our responsibility to look after the environment comes from our duties towards future generations. However, according to liberal stances on abortion, we don't have obligations to a fetus that (who) is not yet a person; the person simply doesn't exist at that stage, so there is no question of them having a right to life or any such thing.

So, can we have duties towards people who don't exist (yet)? If not, is there some other way to understand environmental ethics? If so, can we really justify abortion on the grounds that the relevant person simply doesn't (yet) exist?

I can see various ways in which the tension here can be reduced, but I'm not convinced it can be eliminated without serious revision of one of the two stances.