The earth is a closed system. Only sunlight provides an inflow of external energy. Humans have to create their environment from the resources of the earth. Until late in the 20th century it seemed to be believed that the resources on earth were unlimited, and that human influence had no major impact on the state of the earth. With the increase in living standards and population as well as new insights, it became evident however, that human influence had a great impact after all and that there was a limit to the resources on earth as far as humans were concerned.