WASHINGTON June 20, 2008 (Xinhua)-- World population will reach 7 billion in 2012, according to a new estimate released by the US Census Bureau Thursday.

Chart shows world population, 1950-2050.

The world population hit 6 billion in 1999 and another billion will be added to the total in only 13 years, the bureau said.

A reminder of the quick population growth is that the world's population even did not reach one billion in 1800.

The US Census Bureau attributed the growth to medical and nutritional advances in developing countries after World War II.

Nevertheless the latest projection shows that world population will grow at a slower pace during the first half of the 21st century than the latter half of the 20th century.

The world population doubled from 3 billion in 1959 to 6 billion in 1999, but is projected to increase by only 50 percent between 1999 and 2040.

Global population growth, about 1.2 percent per year, is projected to decline to 0.5 percent by 2050.

At the same time, the world population is growing older quickly.

The bureau estimated that by 2050, about 5 percent of the world 's population is projected to be 80 or older, in comparison with about 1.5 percent of the current global population is 80 or older.

It also projected that India will overtake China as the world's most populous nation by 2050.