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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 interacting with water. (This interferes with corals’ ability to build the protective skeletons that form the complex structure of a reef.) Since the late 1980s, ocean surface warming has quadrupled. Accordingly, in the last half century, half of the world’s live coral cover has disappeared.
“We’re no longer talking about future tipping points — there’s one happening right now,” Steve Smith, a research impact fellow at the University of Exeter’s Global Systems Institute and a coauthor of the report, told Grist. “Although our governments are used to planning for incremental, slow change, things do seem to be speeding up.”
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The more individual corals perish, the harder it gets for a reef to bounce back, destabilizing it and pushing it into a spiral of die-off. A quarter of all marine species rely on these bustling warm-water ecosystems — which cover some 350,000 square miles — but corals are bleaching as they release the symbiotic algae they need to harvest energy. Since 2023, more than 80 percent of the world’s reefs have suffered through the most widespread and intense bleaching event on record. Ever-higher acidification makes it even harder for corals to reproduce and then grow back from this kind of disturbance.
Warm-water corals are particularly vulnerable to climate change because they’ve made an evolutionary compromise. Being close to the ocean surface, their symbiotic algae soak up bountiful sunlight to provide energy, meaning they don’t need to rely as much on outside nutrients. But that positioning also means that during marine heat waves, hot water envelops the corals, stressing them to the point where they release their algae, causing bleaching.
“This is a tradeoff. They have a balance they have to strike,” said Gordon Zhang, a senior scientist at the Woods Hole Oceanographic Institution’s Reef Solutions group who wasn’t involved in the new report. “If the water doesn’t move much, and it’s a very shallow place, the water just keeps heating up.”
Beyond their critical role in hosting marine life,