This year is the hottest on record, and evidence is growing that climate systems are hitting dangerous tipping points. That backdrop is intensifying a fight over the future of fossil fuels that is set to dominate the annual United Nations climate conference over the next two weeks. New studies have found that several tipping points—from a collapse of Atlantic Ocean currents to drying of the Amazon rainforest—could be passed sooner than anticipated, some around the middle of this century. Loss of much of the West Antarctic ice sheet may already be unavoidable. Global temperatures, meanwhile, set record highs this year,…