The risk of crossing climate tipping points — a term that spans melting ice sheets, thawing permafrost, disruption of monsoons, coral bleaching, boreal forest loss and the degeneration of the Amazon rainforest — increases as the world warms. In October, researchers at the University of Exeter said the world is facing a “new reality” after having reached the first tipping point, namely the “widespread dieback” of warm-water coral reefs.