a) Like "overpopulation," the "sustaining capacity" of the earth is a totally relative notion. The limits of the earth's "sustaining capacity" are strictly undefinable, because strictly speaking, they are indefinite; it is impossible to determine them.
Why is it impossible to determine them? Very simply because it is fortunately impossible to assign any limit whatsoever to mans ability to intervene in the world.
Without forcing the paradox, one can say with the economist Sheldon Richman, in the final analysis there are no natural resources.
b) The Indians in Texas lived for centuries above oil deposits which they didn't know how to exploit. As long as it was simply there, oil was just a thing. It didn't become a natural resource until the moment men took interest in it, and made of it a source of energy and the basis for innumerable chemicals.
Titanium, discovered at the end of the eighteenth century, did not become a natural resource until 1947, once its light weight, its hardness and its resistance to corrosion began to be exploited in the aerospace industry and later in surgery. It is one of the most abundant of all the chemicals found in the earth (ninth most common). What made it a natural resource was the genius of man.
Silicone was discovered at the end of the eighteenth century. After oxygen, it is the most abundant chemical element on the earth, where it is present notably as sand. Traditionally, it was used for ceramics, now it is widely employed in metallurgy. However, for decades it has been at the basis of the electronic revolution. More recently still, under the form of fiber optics, it has revolutionized methods of medical diagnosis and telecommunications.
Engine manufacturers are trying to produce airplane engines that consume less gasoline. When they produce an engine that uses 30% less gasoline than even the engine of a preceding generation, they increase the oil reserves all the more.
The wind has been used in Holland for centuries, first of all to dry out the lowlands (the land flooded by the sea) and to grind wheat, then for producing electricity.
Research in agronomy and zootechnics is still progressing. In countries of the Third World, only those holding on to an archaic vision of agriculture and breeding continue to manage the land as though men were cattle and as though the yield of the soil was condemned to be what it has always been.
c) Japan understood very early on that the primary resource -as if it were the only resource- of which it could dispose was man. That is why it made -and continues to make- an exemplary effort in education and professional training of its youth.
d) In conclusion, one can say that the principal, even unique resource of man is his intelligence and his free will, by which his resemblance to God is most explicitly manifested. Thanks to these eminent gifts, man has the capacity of constantly ameliorating his relationship to nature, of bringing additional value to the elements, of transforming materials into goods, of better organizing society. It offends his dignity to present man as a consumer predisposed to destroy his surrounding environment, or as a predator programmed to defend his living space.