By MEAD GRUVER
The U.S. Census Bureau estimates the world’s population has topped 8 billion for the first time. The agency said Thursday that it estimates the global population exceeded the threshold Sept. 26. The bureau says the overall trend is longer lifespans offsetting fewer births. Even so, the global population growth rate continues a long-term trend of decline. The world’s population of 6 billion in 2000 was double the number in 1960. The Census Bureau says out world population estimates vary widely because of discrepancies in how countries count their own populations. The United Nations estimated that the world’s population topped 8 billion almost a year ago.