How habitable is the Earth? It’s a trick question, Charlie Stross warns:
So here’s the upshot: of the 4.6 Gy [gigayears] of Earth’s known history, there’s only been enough oxygen in the atmosphere for us to survive for about 0.5 Gy. For roughly 90% of the Earth’s history we couldn’t even breathe the air. And about 10–25% of the time, there have been ice ages so savagely fierce that the glaciers reached the tropics: odds are good that any meat probe [unequipped human explorer] landing on solid ground during these periods would rapidly die of exposure.So historically, Earth has only been inhabitable about 8% of the time — assuming you are lucky enough to find some solid ground. Once you factor in the random surface distribution, we’re down to about 2% survivability.