The question he asks is what evil in plain sight today will be abhorred by our descendants. Interestingly, he frames this in terms of political ideologies - liberalism, conservatism, and libertarianism. He claims that liberalism offers us the problem of cruelty to animals, conservatism offers us the problem of abortion, and libertarianism offers us the problem of our treatment of foreigners.
The really odd pair are the liberal/libertarian options. Before getting to that I want to say that he makes a good point on abortion. I'm pro-choice, but I've come around to the view that pro-lifers (I used to be one of them too!) have a potentially legitimate case. Of course it all depends on personhood which is not an easy thing to know what to think of. Having been through a pregnancy and having a child now I still don't know what to think of the question of personhood. Indeed that's a big part of why I'm pro-choice. In the face of such a contested question I find it hard to tell everyone to follow one viewpoint. But if the conservative view is correct this is a monstrosity.
OK, back to liberalism and libertarianism.
The liberalism one was weak and the libertarian one was confusing.
We're animals, and animals eat other animals. Most liberals are happy to eat other animals for exactly this reason. Tough luck - you're below us on the food chain. Of course outright cruelty to animals isn't a good thing, but that's not what Bryan is talking about and do you really have to be a liberal to think that cruelty to animals is bad? I hope not. Bryan takes some unusual life-style choices that are plausibly more common among liberals and invents a moral problem where there really isn't one.
The libertarian case - our treatment of foreigners - is confusing because I think of this as being a liberal thing as much as a libertarian thing. And indiscriminate pacifism on the part of some libertarians (and liberals) is not better treatment of foreigners, it's worse treatment. It's not that this isn't characteristic of a lot of libertarians (although many are anti-immigration), but it just doesn't seem as distinctive as Bryan would like it to be.
There is a very obvious liberal option that is different from conservatism or libertarianism: concern for individuals who happen to be disadvantaged due to the circumstances of their birth. I'm not just talking about feeling bad for the disadvantaged any more than Bryan is talking about just feeling bad for slaves. Everyone feels bad for the disadvantaged. I'm talking about using the coercive power of government to put a stop to a barbarity. Presumably Bryan is supportive of legally barring the ownership of other human beings. He is contemplating legally barring abortion.
The willingness to use the power of the state to help those who are disadvantaged by chance is something that I think our descendants are going to look back on and be shocked that we didn't do more to address.