By 2100 Africa’s population could be three billion to 6.1 billion, up sharply from 1.2 billion today, if birth rates remain stubbornly high

This unexpected rise will stress already fragile resources in Africa and around the world. In Africa, women give birth on average to 4.7 children and the population is rising nearly three times faster than in the rest of the world. By the end of this century, demographers now project, Africa’s inhabitants will triple or quadruple. For years the prevailing projections put Africa’s population at around two billion in 2100. Those models assumed that fertility rates would fall fairly rapidly and consistently. Instead the rates have dropped slowly and only in fits and starts. The United Nations now forecasts three billion to 6.1 billion people — staggering numbers. Even conservative estimates, from places such as the International Institute for Applied Systems Analysis in Austria, now see Africa at 2.6 billion. The U.N. has in recent years continually raised its midline projection for 2100 world population, from 9.1 billion in a 2004 estimate to 11.2 billion today. Almost all of the unanticipated increase comes from Africa. Extreme growth threatens Africa’s development and stability. Many of its inhabitants live in countries that are not especially well endowed with fertile soils, abundant water or smoothly functioning governments. Mounting competition for nourishment and jobs in such places could cause strife across the region and, in turn, put significant pressure on food, water and natural resources around the world, especially if Africans leave their nations in droves, which is already happening. As many as 37% of young adults in sub-Saharan Africa say that they want to move to another country. Today only 29% of married African women of childbearing age use modern contraception. On all other continents the rate is solidly more than 50%. Surveys also show that more than a third of African pregnancies are unintended; in sub-Saharan Africa 58% of women aged 15 to 49 who are sexually active but do not want to become pregnant are not using modern contraception. In Niger in West Africa, one of the world’s poorest nations, the average fertility is 7.5 children per woman, and it has barely dipped since measurements began in 1950. Women and men surveyed in Niger say that the ideal family is even larger.