There are other possibilities in the longer term to all those...

There are other possibilities in the longer term to all those suggested.

Basically we are a plague of fleas living in a closed sytem on a tiny outer shell of a small planet. In the end Malthus has to be right, entropy will get us, and we will suffer the same fateas all those past civilisations that ran themselves out of resources - wars, barbarism and collapse of our civilisation into savagery

UNLESS

we escape the closed system and get off this little planet. Even pushing a few hydrocarbon comets into orbit outside the atmosphere might do the job.

But of course the costs of such enterprises are fantastic - far beyond floating the (so far) uneconomic alternative technologies mentioned here , and so it seems, unlikely to be even considered until resources are far too few to make extraterrestrial solutions any kind of possibility.