About 80 countries want to increase their climate pledges ahead of schedule under the Paris climate accord, the United Nations said Tuesday, signaling that some of them would do so at a summit of world leaders in September. Luis Alfonso de Alba, a Mexican diplomat who is currently the United Nations Secretary General’s envoy on climate change, declined to say which countries were expected to announce higher climate ambitions, nor whether they would be enough to make a difference to the fate of the world. Even if every country were to meet the pledges already made under the 2015 Paris agreement — and they are nowhere on track to do so — it would be insufficient to avert the worst effects of climate change. An “exponential increase in ambition,” Mr. de Alba said, is needed. “We need to step up ambition quite radically. […]