Walter Greiling projected in the 1950s that world population would reach a peak of about nine billion, in the 21st century, and then stop growing, after a readjustment of the Third World and a sanitation of the tropics.
上世纪五十年代，沃尔特·格雷林预测世界人口会到达90亿峰值，到二十一世纪随着第三世界调整及热带地区为生改善会停止增长。