The implications of world population growth for future fertilizer use and impact on the environment have scarcely been predicted. We investigated world protein production using a global nitrogen balance model. The model assumes that world population continues to grow and crop yield per unit area is closely related to nitrogen fertilizer use. About five times more cereal protein is needed to produce meat protein through farming. Advanced countries consume more meat than people in developing countries. Assuming the world's arable land remains constant in area in the next century, and if the meat consumption in advanced countries is matched by the rest of the world, the demand for fertilizer will increase and reach 220 Tg y-1 by the middle of the next century. This is approximately three times more fertilizer than is currently used, and would accelerate environmental deterioration. The world would be suffering from conflict between supplying sufficient protein and greater nitrogen pollution of the environment.