http://en.wikipedia.org/wiki/Peak_oil#Agriculture_and_population_limits

Geologist Dale Allen Pfeiffer contends that current population levels are
unsustainable. To achieve a sustainable economy and avert disaster, he
maintains that the United States must reduce its population by at least
one-third, and world population will have to be reduced by two-thirds.

http://www.fromthewilderness.com/free/ww3/100303_eating_oil.html