I don't think humans will *be* on Earth more than a billion years from now.

Yeah, well, it isn't as if there's precedent for any life form more complicated than a sponge (and don't get me wrong, any macroscopic critter that can be put through a blender and survive has my utmost respect) remaining recognizable over the course of a billion years anyway.

I'm figuring if they're still recognizably human, then they've worked enough things out that a piddly thing like too much solar radiation really won't be a problem. At that point, a really big parasol may be possible...