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The fashionable answer to the problem is a transition to a 'bioeconomy'. This 'clean', 'green', 'renewable', 'sustainable' economy of the future will be based on biomass from agricultural crops, forests and algae.
Even to those unversed in science, this alternative to fossil fuels is quite clearly no more sustainable than our current energy source. It's simply dependent on other finite natural resources which are already limiting: suitable land area, soil nutrients, and fresh water.