Calling global warming "the defining issue of our time," UN Secretary General Ban Ki-moon urged immediate international action to counteract the "imminent threat" of climate change.
In a wide-ranging speech this afternoon at Harvard's Kennedy School of Government, Ban said it was crucial to ratify an international treaty on greenhouse-gas emissions before the Kyoto Accord expires in 2012.
Climate change "is no longer theory, it has become a fact," he told an audience of several hundred. "We cannot afford to delay any action."
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