Humanity still has a chance, close to the last, to prevent the worst of climate change’s future harms, a top United Nations panel of scientists said Monday. But doing so requires quickly slashing nearly two-thirds of carbon pollution by 2035, the Intergovernmental Panel on Climate Change said. The United Nations chief said it more bluntly, calling for an end to new fossil fuel exploration and for rich countries to quit coal, oil and gas by 2040.