We now use about 35% of the net product of photosynthesis. 100% is probably impossible to reach. 30% has been estimated as our long range-carrying capacity so we have already overshot. 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 population and economic expansion difficult.
Continuous expansion is a fundamental tenet of economics. However, the laws of thermodynamics inform us that continuous expansion is impossible throughout an entire system. No matter how efficient our throughput is, you can't get an output equal to the input, let along greater than the input.
In the capitalist world, the word capital has taken on more and more uses. People talk about human capital, which is what labor accumulates through education and work experience. Human capital differs from the classic kind in that you can't inherit it, and it can only be rented, not bought or sold. The concept of natural capital actually resembles the traditional definition more than human capital. It can be owned and bequeathed, and divided into renewable and nonrenewable, marketed and unmarketed.
Manmade capital and natural capital are not substitutable. Put simply, you can't substitute more sawmills for fewer forests. If you're building a house, you can juggle the number of carpenters and power saws, which means they are substitutable, but you can't build a house with half the amount of lumber, regardless of how many carpenters and power saws you use.
There is a fallacy that if you improve manmade capital to use less natural capital, that is a substitution. Rather, that's efficiency. Capital is a quantity of input, and efficiency is the ratio of input to output. No matter how efficient capital is, it can't make something out of nothing.
Sources:
Sheffield, Charles; Limits of Capitalism: Princeton University Press, Princeton, 1994.
Ceresole, Peter and Lisa Nowell; Natural Resources: Landoll Inc, New York, 1996.