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Warning of the risks of “runaway” global warming, the United Nations secretary general, António Guterres, on Monday called on global leaders to rein in climate change faster.

“If we do not change course by 2020, we risk missing the point where we can avoid runaway climate change,” Mr. Guterres said at United Nations headquarters in New York.

“Climate change is the defining issue of our time, and we are at a defining moment,” he said. “Scientists have been telling us for decades. Over and over again. Far too many leaders have refused to listen.”

His remarks came with countries around the world far short of meeting the goals they set for themselves under the 2015 Paris accord to reduce the emissions that have warmed the planet over the last century. The next round of climate negotiations is scheduled for this year in Poland.