Some climate scientists (for example Gaia theorist, James Lovelock and grandfather, James Hansen), have embraced nulclear power as the carbon-neutral answer to our high energy needs in this modern world. They feel we’ve gone too far to turn back now–a depressing thought if there ever was one. Of course, almost no one is willing to point out that maybe there’s just too many people here on the planet today…
The fact that nuclear power plants will inevitably all have meltdowns if no one’s around to keep them going and keep them cool with thousands of gallons of water seems clearly the biggest threat to the future of Earth’s other species if humans were to suddenly check out (from say, nuclear war or pandemic, or starvation due to climate change, etc.).
Also, if jet traffic with its global cooling, cloud-like contrails in the stratosphere were to come to an abrupt end (the oil is going to run out some day, after all), the planet would quickly warm up to temperature levels not seen in hundreds of thousands of years (known as the Venus effect).
Sadly, there’s no easy answer.
We’re addicted to speed, madly shovelling coal into a freight train headed for a brick wall. It’s too late for the wish that humans would just go away and let nature take back over, putting things right again. At this point, the train is going to crash and there’s no place left for humans to safely jump off.