All the other energy sources need some kind of advanced technology to make them efficient enough to use… so assuming we have some moderately advanced technology we could maybe use wind farms (they’re not too different from windmills!!)
If we had used renewables, like wind turbines or waves power, we might not have produced so much energy so quickly and maybe the population wouldn’t have grown so quickly. The rate of population growth is related to energy produced. So maybe we would be in a situation that is more manageable.
It’s not nice to think about lots of the world’s population disappearing though.. we’re all here now and we have to look after each other 🙂
Knowing what we do now, I’d design our buildings differently to be more passive (means they don’t need a heat source – or not as much). Then whatever energy we still needed I’d generate from water, wind, tides etc.