All of this is sure an interesting topic. On fuel cells, isn't the challenge obtaining the hydrogen? It takes a lot of energy to break water into its components. Not sure what the best sources are.
I've wondered what might happen if the entire world went to solar, for example. The CO2 greenhouse effect would be reduced, but we'd still be capturing a lot of solar energy instead of bouncing it back to space. It may be a fraction, but it's still capturing more energy than in past history. The only sustainable option is to use less energy. In our case, it's driving to a spot, but sitting there for many days, enjoying it, absorbing the sunshine and gaining vitamin D .
Anyway, the same argument could be made for wind (taking energy out of the atmosphere on a huge scale has to have impact), waves (same thing), nuclear (storing the waste, running out of fuel at some point). All seem to be a lot less impact than the greenhouse effect, but there's still impact. I hear most countries are having fewer babies, so maybe there's hope.