for comparison the land requirements of today s global fossil fuel infrastructure are less than km which is about the size of belgium smil b solar power reaches spatial power densities that are two orders of magnitude higher than for wind and three orders of magnitude higher than for photosynthesis solar power can in principle reach power densities commensurate with demand densities in houses and some smaller cities however industry highrise buildings and megacities in which the majority of the worlds population will live require even higher power densities than solar could offer