US special climate envoy John Kerry told AFP that the world's population will not be sustainable in 2050, when it is expected to reach nearly 10 billion, but he stopped short of asking Americans to give up steak. As of November, the global population officially surpassed 8 billion, more than triple the number in 1950. That has already stretched food and energy needs and supplies. United Nations forecasts say that number will rise to 9.7 billion by mid-century. "I personally don't think it's sustainable," he said in an interview on Tuesday (June 6).

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