If current demographic trends continue, moreover, the world could level off at about 9 billion persons in 2050, and nearly 1/3 of those could well be Muslim. The really big Muslim populations are not in the Middle East, which is largely arid and wouldn't support such populations. It is in relatively well-watered places such as Pakistan, India, Bangladesh, and Indonesia in Asia where the bulk of Muslims live. Pakistan now has about 170 million people but is likely to rival the current US population of 300 million in the next few decades. (When I first went to Pakistan in 1981 I think its population was, like, 70 million).