It's also argued that, fossil fuels being literally the most energy-dense per unit-of-infrastructure-applied energy source in the solar system, our societal complexity is likely to decrease in the future as the hard-to-get ones are themselves drawn down and there becomes no way to keep drawing upon the sheer levels of energy per capita we have become accustomed to over the last 200 years in the wealthier nations.

I recommend Tom Murphy's "do the math" blog for a frank discussion of energy densities and quantities and the inability of growth or likely even stasis in energy use to continue.