  Von Hoerner (1975) examined the effects of human population growth and
predicted agricultural, environmental, and other problems from observed growth
rate trends. Using straightforward calculations, VH75 predicted the "doomsday"
years for these scenarios (2020-2050), when we as a species should run out of
space or food, or induce catastrophic anthropogenic climate change through
thermodynamically unavoidable direct heating of the planet. Now that over four
decades have passed, in this paper we update VH75. We perform similar
calculations as that work, with improved data and trends in population growth,
food production, energy use, and climate change. For many of the impacts noted
in VH75 our work amounts to pushing the "doomsday" horizon back to the
2300s-2400s (or much further for population-driven interstellar colonization).
This is largely attributable to using worldwide data that exhibit smaller
growth rates of population and energy use in the last few decades. While
population-related catastrophes appear less likely than in VH75, our continued
growth in energy use provides insight into possible future issues. We find
that, if historic trends continue, direct heating of the Earth will be a
substantial contributor to climate change by about 2260, regardless of the
energy source used, coincident with our transition to a Kardashev type-I
civilization. We also determine that either an increase of Earth's global mean
temperature of 12K will occur or an unreasonably high fraction of the planet
will need to be covered by solar collectors by about 2400 to keep pace with our
growth in energy use. We further discuss the implications in terms of
interstellar expansion, the transition to type II and III civilizations, SETI,
and the Fermi Paradox. We conclude that the "sustainability solution" to the
Fermi Paradox is a compelling possibility.
