In the game each tile gives some type of resource. If I remember right, you had food, shields (productivity) and trade (money). A farm land would produce a lot of food but little trade but a mine would produce lots of gold but no food. You gather this amount of resource each turn but it doesn't tax the land at all. Nukes just cost a certain amount of productivity so as long as you hold land you can keep building them. The game isn't really meant to last much past modern times so the model kind of breaks down in a game like OPs. Its worth noting that in later games special resources were added to help make it a little more realistic. Like you had to have a uranium mind somewhere to produce nukes. It lead to really cool strategies, like wars for oil or iron.

The reason why he had world resources dwindling at all is because its possible for global warming to cause sea levels to rise and take away bits of land and in the parts of land that don't sink their type can change (plains can become deserts, jungles to forests).