One of the two elements of human environmental impact. Without restraint, all animal populations grow geometrically (exponentially). The human population has been increasing since neolithic times. The UN estimates the world population in mid-2013 to have been 7.2bn. Its medium projection, which largely assumes a decline in fertility in countries where large families are still prevalent, is 9.6bn in 2050 and 10.9bn in 2100.