The human population is expected to increase from the current 6.9 billion to more than 9 billion by 2050. But some argue that the planet cannot cope with so many people.
Roger Martin is the chair of a British environmental charity, the Optimum Population Trust. He told Carrie Gracie that the best way to limit the pressure on natural resources is to limit the number of children we have.
You can watch the full interview on Monday 13 December 2010 on BBC World News at 0430, 0930, 1530 & 2130 GMT and also on the BBC News Channel at 0430 and 2330 GMT.
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