A new United Nations report projects that the world population could reach nearly 11 billion by 2100, about 8 percent more than predicted just two years ago. The projected increase largely stems from the fact that the fertility rate in Africa has declined more slowly than expected, with demographers now forecasting that the number of people on the continent could nearly quadruple this century, from from about 1.1 billion today to about 4.2 billion. “The fertility decline in Africa has slowed down or stalled to a larger extent than we previously predicted, and as a result the African population will go up,” said Adrian Raftery, a professor of statistics and sociology at the University of Washington, who helped develop the statistical method used in the report. The total world population passed 7 billion in 2011. According to the new report, 8 of the top 10 increases in national populations by 2100 will occur in Africa, led by Nigeria, where the number of people is expected to jump from 184 million to 914 million.