at the solar power level, although fuel cells get more efficient the larger they are and while their closed system was OK for one household in the burbs, it wouldn't work at an urban scale. The real point is that we are surrounded by energy that is harvestable and that could be used for electrolysis. We're simply in no position to do it yet and will likely never be there at an urban scale. Start looking at powering tower blocks with renewables and the problems pop up pretty quickly.
The problem is that the systems of the future will be messy and fragmented, more so than they are now, especially if we don't want to cook the planet enough that we have to figure out how to breathe methane.
I'm not quite as pessimistic as you are, the planet will survive us. Remember, Canada has been tropical rain forest in the past, as were the Great Plains, hippos in Nebraska and all that. I think we will likely survive, although certainly not 7 billiion of us at once, and we will be different, we will have to be.
Then again, Campi Flegrei might blow up and save our collective bacon as far as warming goes. We just don't know. What is clear is that we are a product of nature just as much as the blue green algae that froze the planet solid some 650 million years ago.
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