population

As of 19 January 2011, the world population is estimated by the United States Census Bureau to be 6.894 billion.^[2]
According to papers published by the United States Census Bureau, the world population hit 6.5 billion (6,500,000,000) on 24 February 2006. The United Nations Population Fund designated 12 October 1999 as the approximate day on which world population reached 6 billion. This was about 12 years after world population reached 5 billion in 1987, and 6 years after world population reached 5.5 billion in 1993. The population of some countries, such as Nigeria and China is not even known to the nearest million,^[3] so there is a considerable margin of error in such estimates.^[4]
Population growth increased significantly as the Industrial Revolution gathered pace from 1700 onwards.^[5] The last 50 years have seen a yet more rapid increase in the rate of population growth^[5] due to medical advances and substantial increases in agricultural productivity, particularly beginning in the 1960s,^[6] made by the Green Revolution.^[7] In 2007 the United Nations Population Division projected that the world's population will likely surpass 10 billion in 2055.^[8] In the future, world population has been expected to reach a peak of growth, from there it will decline due to economic reasons, health concerns, land exhaustion and environmental hazards. There is around an 85% chance that the world's population will stop growing before the end of the century.^{[citation needed]} There is a 60% probability that the world's population will not exceed 10 billion people before 2100, and around a 15% probability that the world's population at the end of the century will be lower than it is today. For different regions, the date and size of the peak population will vary considerably.^[9]
The population pattern of less-developed regions of the world in recent years has been marked by gradually declining birth rates following an earlier sharp reduction in death rates.^[10] This transition from high birth and death rates to low birth and death rates is often referred to as the demographic transition.^[10]