We should remember that our planet is not even approximately a closed system
with respect to energy; we gain energy from the sun. The entropy law does remind
us, however, that the flow of solar energy establishes an upper limit on the flow of
available energy that can be sustained. Once the stocks of stored energy (such as
fossil fuels and nuclear energy) are gone, the amount of energy available for useful
work will be determined solely by the solar flow and by the amount that can be
stored (through dams, trees, and so on). Thus, in the very long run, the growth
process will be limited by the availability of solar energy and our ability to put it
to work.