From the opposite angle, Romanian American economist Nicholas Georgescu-Roegen, a progenitor in economics and a paradigm founder of ecological economics, has argued that Malthus was too optimistic, as he failed to recognize any upper limit to the growth of population — only, the geometric increase in human numbers is occasionally slowed down (checked) by the arithmetic increase in agricultural produce, according to Malthus' simple growth model; but some upper limit to population is bound to exist, as the total amount of agricultural land — actual as well as potential — on Earth is finite, Georgescu-Roegen points out. Georgescu-Roegen further argues that the industrialised world's increase in agricultural productivity since Malthus' day has been brought about by a mechanisation that has substituted a scarcer source of input for the more abundant input of solar radiation: Machinery, chemical fertilisers and pesticides all rely on mineral resources for their operation, rendering modern agriculture — and the industrialised food processing and distribution systems associated with it — almost as dependent on Earth's mineral stock as the industrial sector has always been. Georgescu-Roegen cautions that this situation is a major reason why the carrying capacity of Earth — that is, Earth's capacity to sustain human populations and consumption levels — is bound to decrease sometime in the future as Earth's finite stock of mineral resources is presently being extracted and put to use. Political advisor Jeremy Rifkin and ecological economist Herman Daly, two students of Georgescu-Roegen, have raised similar neo-Malthusian concerns about the long run drawbacks of modern mechanised agriculture.