“So the hard news is we’ve got maybe a billion years left. If we’re lucky.”
Scientists are fairly certain that 500 million years from now, the sun will start expanding and the Earth will no longer be comfortably within the habitable zone due to heat.
So I’ve heard before. The sun is heating up as it ages, and anywhere between the time you mentioned and twice that long the rate of evaporation will exceed the rate of water condensation, and the oceans will disappear. Goodbye life on Earth. Humans will have been gone long before that anyway. The sun itself will be a degenerate core of slowly cooling carbon and oxygen a few billion years later. The andromeda galaxy will collide with the Milky Way at about the same time, turning both into starbursts, depleting all the star-forming hydrogen gas. In a few billion more years there will only be old red dwarf stars populating the night sky, none of them even visible to the human eye, not that we’ll be here anyway. In another one or two trillion years those will also be gone, only their dark degenerate cores remaining. I’m trying to think of a bright side here, but all I can come up with is that God is eternal, and death has no sting.