Huge stretches of coral reef around the world are turning a ghostly white this year amid record warm ocean temperatures. On April 15, the US National Oceanic and Atmospheric Administration confirmed the world's fourth mass global bleaching event is underway—with serious consequences for marine life and for the people and economies that rely on reefs. Coral reefs cover less than one percent of the ocean floor, but have out-sized benefits for marine ecosystems and economies. A quarter of marine life will depend on reefs for shelter, finding food or spawning at some point in their lives and coastal fisheries would struggle without corals. Every year, reefs provide about US$2.7 trillion in goods and services, from tourism to coastal protection, a 2020 estimate by the Global Coral Reef Monitoring Network showed. About US$36 billion is generated by snorkeling and scuba diving tourists alone. Coral reefs also help coastal communities by forming a protective barrier against storm surges and large waves. This helps to avoid property damage for more than 5 million people worldwide, a 2022 study in the journal Marine Policy found. When water temperatures rise, jewel-toned corals get stressed. They cope by expelling their algae—causing them to turn bone white. Most corals live in shallow waters, where climate-driven warming is most pronounced. Whether a coral becomes heat-stressed depends on how long the high temperatures last, and how much warmer they are than usual. Scientists have found that corals generally begin to bleach when surrounding waters are at least 1 degree Celsius warmer than the maximum average temperature—or the peak of what corals are used to—and persist for four or more weeks. Corals can survive a bleaching event if the surrounding waters cool and algae return. Dead reefs can still offer shelter to fish or provide a storm barrier over several years for coastal communities. But eventually, these underwater graveyards of calcium carbonate skeletons will erode and break apart. The best chance for coral survival is for the world to cut greenhouse gas emissions to limit climate change. Many scientists think that at just 1.2 Celsius of warming above preindustrial level, the world has already passed a key threshold for coral reef survival. They expect between 70% and 90% of the world's coral reefs will be lost. Scientists and conservationists are scrambling to intervene. Local communities have cleanup program to remove litter from the reefs to reduce further stresses. Scientists are breeding corals in labs with the hopes of restoring degraded reefs. However, none of this is likely to work to protect today's corals from warming waters. Scientists are therefore trying to plan for the future by bringing coral larvae into cryopreservation banks, and breeding corals with more resilient traits.