"The world’s population will shortly reach 7 billion. The United Nations estimates the world will reach this milestone on October 31, 2011. The U.S. Census Bureau estimates the world population will pass 7 billion on March 12, 2012 (link to Census Bureau’s estimates of today’s population, as well as annual midyear estimates from 1950 to 2050). While the true world total population is not known, it is independently estimated by both the United Nations and the Census Bureau. Both organizations begin with separate population estimates for individual countries and areas throughout the world and then compile them into a world count. Country estimates typically are based on national census counts, which usually are taken only once a decade. Thus, to estimate each country population beyond the latest census year requires separate projections based on current demographic trends, which are typically determined using whatever demographic data (sometimes limited) are available. Clearly, there is some uncertainty in world population estimates. Despite some differences, the world population estimates of the United Nations and the Census Bureau are remarkably close, different by less than one half of one percent in 2011. The rapid increase in the Earth’s population in recent decades is because of an excess of births over deaths. Although birth rates and death rates have both declined sharply since the 1960s, death rates have declined more rapidly than birth rates. The cumulative effect of the excess of births over deaths in recent decades has led to a successive attainment of billion-person milestones every 12 or 13 years."