[cryptonector]

> the problem is, thermodynamics is so generic that it's hard to imagine how to sidestep it - and people do try all the time.

Population is already set to decrease.

Speaking of thermodynamics...

The life-carbon cycle on the planet is not a reversible process. Photosynthetic life, and sea life (sea shells require a lot of carbon) pull down a lot of CO2 from the atmosphere. Sea life in particular leads to carbon being sequestered in limestone over millions of years. Land life leads to carbon being buried in soil. This process trades low-entropy, high-energy sunlight for high-entropy, low-energy light emitted back to space by the planet (keeping us in equilibrium), with the entropy going into things on Earth, like the conversion of CO2 into limestone.

The biggest and shortest-term natural threat to life on this planet is dwindling atmospheric CO2. At the end of each glacial period (thus the beginning of each interglacial) CO2 is lower than at the end of the previous glacial period, and it has been thus since the beginning of the current ice age, and it seems like a pattern that will keep repeating. Eventually the Earth will fall below the photosynthesis starvation level of atmospheric CO2. Before the Industrial Revolution, and the oil&gas revolution in particular, that was going to be just a few more glacial periods. By raising atmospheric CO2 to 400+ppm we've bought the planet a few million years, and even so, if humans were to disappear in the next glacial period, we'd be looking at a closer end to life on the planet through exhaustion of atmospheric CO2 than through astronomic catastrophes like a large asteroid hitting the planet.