The world population increased from 3 billion in 1959 to 6 billion by 1999, a doubling that occurred over 40 years. The Census Bureau's latest projections imply that population growth will continue into the 21st century, although more slowly. The world population is projected to grow from 6 billion in 1999 to 9 billion by 2045, an increase of 50 percent that is expected to require 46 years.
As of June 4, 2013 the world's human population is estimated to be 7.089 billion by the United States Census Bureau, and over 7 billion by the United Nations.
Most contemporary estimates for the carrying capacity of the Earth under existing conditions are between 4 billion and 16 billion.
Depending on which estimate is used, human overpopulation may or may not have already occurred.
Nevertheless, the rapid recent increase in human population is causing some concern. The population is expected to reach between 8 and 10.5 billion between the year 2040 and 2050.
In May 2011, the United Nations increased the medium variant projections to 9.3 billion for 2050 and 10.1 billion for 2100.