“We now use about eighty percent of the net primary product of land-based photosynthesis,” he said. “One hundred percent is probably impossible to reach, and our long range carrying capacity has been estimated to be thirty percent, so we are massively overshot, as they say.

We have been liquidating our natural capital as if it were disposable income, and are nearing depletion of certain capital stocks, like oil, wood, soil, metals, fresh water, fish, and animals. This makes continued economic expansion difficult.”

“Difficult!” Art wrote. “Continued?”

“We have to continue,” Fort said, with a piercing glance at Art, who unobtrusively sheltered his lectern with his arm.

“Continuous expansion is a fundamental tenet of economics. Therefore one of the fundamentals of the universe itself. Because everything is economics. Physics is cosmic economics, biology is cellular economics, the humanities are social economics, psychology is mental economics, and so on.”

His listeners nodded unhappily.

“So everything is expanding. But it can’t happen in contradiction to the law of conservation of matter-energy. No matter how efficient your throughput is, you can’t get an output larger than the input.”

…

“…manmade capital and natural capital are not substitutable. This is obvious, but since most economists still say they are substitutable, it has to be insisted on. Put simply, you can’t substitute more sawmills for fewer forests. If you’re building a house you can juggle the number of power saws and carpenters, which means they’re substitutable, but you can’t build it with half the amount of lumber, no matter how many saws or carpenters you have. Try it and you have a house of air. And that’s where we live now.”

…

No matter how efficient capital is, it can’t make something out of nothing.” “New energy sources…” Max suggested. “But we can’t make soil out of electricity. Fusion power and self-replicating machinery have given us enormous amounts of power, but we have to have basic stocks to apply that power to. And that’s where we run into a limit for which there are no substitutions possible.”

Green Mars, Kim Stanley Robinson