Figure 1 Population projection variants, 1900–2100.

and the U.S. Bureau of the Census all generate, however, remarkably similar demographic futures, as shown in Figure 1. By the end of the 21^st century, we can expect a world population anywhere from about 8 billion to nearly 20 billion, depending on high, medium, or low assumptions of slow, medium, or very rapid reductions of human fertility. In these projections, mortality is assumed to continue its long-term secular decline—not an altogether safe assumption given the many human crises we are experiencing around the world. The annual number of new additions to planet earth is now peaking at about 90 million, and although this growth will continue for another decade, it will decline steadily throughout the next century. The peak rate of growth of the world's population was about 2.1 percent per year around 1970, having declined to today's rate of 1.6 percent. The net increment of population increase today is equivalent to adding one new Mexico or three new Canadas each year to the world's total population.

World population growth reflects a remarkable decline in fertility in the developing world, from an average of six children to four children between 1965 and 1985 (Figure 2). An unprecedented pace of decline has been experienced in much of Asia and Latin America. Slower declines were witnessed in South Asia and the Middle East, while little or no decline has been experienced in sub-Saharan Africa, although there is evidence that fertility has finally begun to decline in about four African countries. Throughout North America and Western, Central, and Eastern Europe, fertility has been near or even below replacement. The fertility declines have been associated with a marked increase in the prevalence of contraceptive use, from 9 to 45 percent of all eligible couples in the developing world.

Our demographic future over the next 25 years appears to be already with us. We will definitely have more people in the world, in large measure because of "population momentum," which is built-in growth due to reproduction among children already born. In other words, even if all couples were to achieve replacement fertility immediately, the youthful age structure of the world's population would still generate future growth because the