A world with 11 bln people?

And for the first time, through the use of a "probabilistic" statistical method, the Science paper establishes a range of uncertainty around its central estimate-9.6 billion Earthlings in 2050, 10.9 billion by 2100. There's an 80 percent chance, the authors conclude, that the actual number of people in 2100 will be somewhere between 9.6 and 12.3 billion, National Geographic reports. That range "is the truly innovative part," says John Wilmoth, head of the UN Population Division and one of the authors of the Science paper. "It's a much more plausible analysis of uncertainty-but we may still be off by two billion." According to other demographers, the UN has missed the mark by just about that amount. In a paper in press at Global Environmental Changeand in a forthcoming book, Wolfgang Lutz and his colleagues at the International Institute for Applied Systems Analysis (IIASA) in Vienna, Austria, use a very different method-one that involves canvassing a large group of experts-to argue that population is likely to peak at 9.4 billion in 2075 and fall to just under 9 billion by 2100. The UN team estimates there's no more than a 5 percent chance of that rosier scenario coming to pass. Both groups foresee India becoming the world's most populous country, with its numbers peaking around 2070 and declining to around 1.5 or 1.6 billion by 2100. Where they differ most is in their estimates of the coming population decline in China and of the coming population explosion in Africa south of the Sahara-where most of the world's growth is going to occur. According to the UN, the population in that region could quadruple, from less than one billion to nearly four billion. Africa in 2100 would be as densely populated as China is today. "These are not predictions," says Wilmoth. "These are projections of what will happen if current trends continue. There is still an opportunity to intervene." For full version go to