A visibly angry Greta Thunberg berated world leaders as she addressed a UN climate summit on Monday, accusing them of betraying her generation by failing to tackle greenhouse gas emissions and asking "How dare you?"  The Swedish teen, who has become the global face of the growing youth movement against climate inaction, began by telling her audience: "My message is that we'll be watching you," eliciting laughter. Watch the heart-wrenching speech.