“Five billion years. For most of us, this is a comfortable limit we can live with. Individual human life lasts approximately 80 years. We know about this, and we experience humans aging all around us. We are conditioned for this, too. But when our lifespan is lowered — that is individual life or life on Earth — our lifetime priorities must be changed.
The earth will last only 800 million years? This is something to worry about? Ha! From an evolutionary perspective, in relation to the time frame from the start of multicellular life to now, 800 million years is LOTS of time. Approximately 560 million years ago, the first remains of what we would call multicellular life appear as smudges and "pizza" fossils in the Ediacaran Period, barely recognizable as plants or animals. Within 30 million years or so, the Cambrian explosion appears; then we have animal guts and brains, even eyes. The point I wish to make is that large, non bacterial and algal forms of life are a recent invention; from the fossil record, animals and plants, in all their incredible variety, and in all the revolutions that have occurred on earth (from bugs and worms in the ocean, to fish, to amphibians, to reptiles and mammal-like reptiles, to dinosaurs, to large mammals, to apes and humans) has ALL occurred within the last 600 million years, give or take a little. Given all that change in life forms in the past, and considering that evolutionary change is driven by mass extinctions involving impacts, vulcanism, continental drift, changing ocean currents, glaciation, all which ultimately create MAJOR climate change, it seems to me life will be so different in 800 million years (including whatever humans have evolved into, likely multiple, multiple times!) that to be concerned over such a large time period is akin to worry about eternity! Concern over the next 100 years, with possible population increases, proliferation of nuclear weapons, pollution, destruction of plant and animal habitat; now there is something to think about AND something we can impact and will impact US! I regret not being able to meet our distant relatives; the Dickinsonia, the Anomalcarids, the Trilobita, etc., but that is SO far in the past and we have SO many things to think about now that will impact the immediate future, that one time scale seems (ALMOST!) to dwell on the hypothetical. Still, even that knowledge is important and useful. Just, other things may need to come first!
Want to worry about a big problem that will certainly strike a lot sooner, and for which currently no solution exists? Worry about what we will substitute for the enormous energy content of the fossil fuels, oil, natural gas, and coal after they are exhausted. At the rate they are being consumed, all except coal will be scarce before the current century ends. Uranium isn't all that abundant in concentrations that can be economically mined either. Think about what keeps the electricity on, and how you would survive if it goes off. Good luck with that. Good at farming with hand tools?
And if we survive anywhere near another million years, and somehow manage to conquer the coming fuel shortage, our technology may be advanced enough to relocate to planets around other stars.
Of course, super volcanoes are far more likely to cause our extinction than solar expansion. It will get really cold when the next one blows. I doubt that we will have to wait 800 million years for the next one to blow.