Global temperature rise may feel like it’s gradual, but the changes it brings can turn out to be sudden, massive, and self-reinforcing. These changes are what scientists call tipping points . When a tipping point is reached, an Earth system abruptly and dramatically changes, often irreversibly, like the Amazon rainforest turning into a savanna — a point of no return that is already perilously close .

But today, a group of 160 scientists from 23 countries is announcing that the planet has already reached its first major tipping point: the widespread death of warm-water coral reefs. That’s due primarily to rapidly rising marine temperatures — the seas have absorbed 90 percent of the excess heat we’ve created — but also the acidification that comes from more atmospheric CO2 interacti

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