This week, two conflicting projections of the world's future population were released. As National Geographic's Rob Kunzig writes here, a new United Nations and University of Washington study in the journal Science says it's highly likely we'll see 9.6 billion Earthlings by 2050, and up to 11 billion or more by 2100. These researchers used a new "probabalistic" statistical method that establishes a specific range of uncertainty around their results. Another study in the journal Global Environmental Change projects that the global population will peak at 9.4 billion later this century and fall below 9 billion by 2100, based on a survey of population experts. Who is right? We'll know in a hundred years.

Population debates like this are why,