World population may hit 6 billion today, according to U.S. Census Bureau estimates, meaning human numbers have doubled in less than 40 years. The United Nations predicts the 6 billion mark will be hit on October 12. Either way, humanity is increasing by 78 million people a year, despite a decelerating growth rate, and scientists predict that the human population will double again to 12 billion by 2050. In 61 of the world’s 191 nations, fertility rates have dropped below the replacement level of 2.1 children per woman, but it takes a nation about 70 years to stabilize its population even after a balanced fertility rate is achieved.