An environmental warning signal has returned in the Atlantic, Pacific and Indian oceans: Coral is turning white and even dying as record sea heat takes a devastating toll. That sign, known as bleaching, happens when living coral gets stressed and loses its vibrant colors because the water it lives in is too warm – especially in shallow areas. Coral health is vital because the submerged formations sustain ocean life, fishing and generate tourism revenue. Nicknamed the sea's architect, coral builds vast structures in tropical areas that house one-quarter of marine species. They are natural barriers that absorb the force of waves and storm surges, keeping coastal communities safer.

Climate change drives up sea surface temperatures as oceans absorb warming gases emitted when we burn oil, coal and gas. El Niño – a natural climate event – has also contributed to warmer levels since last June. The global average ocean temperature broke its all-time record last August and has stayed above average almost every day. Water off Florida last summer reached the high 90s and even exceeded 100 dgrees in some shallow areas -- hot tub levels. Reefs are an early warning system for the impacts of a warming planet on nature. "As the world's oceans continue to warm, coral bleaching is becoming more frequent and severe," says Derek Manzello, coordinator of a Coral Reef Watch program for the U.S. National Oceanic and Atmospheric Administration (NOAA).

This is the second global mass coral whitening in 10 years and the fourth in recent decades, according to that federal agency, which adds that 54 percent of waters with coral reefs are affected. Coral can recover from heat stress, but ideally needs several years. When weakened, it is susceptible to disease and can starve to death. "Coral mortality hurts the people who depend on the coral reefs for their livelihoods," says Manzello.

The last mass global bleaching was in 2014-16. Since then, ocean temperatures have become so much warmer that NOAA introduced three new heat alert levels. Only a rapid and global reduction in greenhouse gas emissions that limits ocean warming will guarantee we have at least some coral left, scientists say. "We need to learn from this to not do this to other ecosystems," says ecologist David Obura of Kenya, a country in east Africa on the Indian Ocean coast.

Scientist says: "For the first time ever, we've documented very high levels of bleaching in all three areas of the Great Barrier Reef Marine Park [a UN heritage site off Australia]." – Neal Cantin, senior researcher at the Australian Institute of Marine Science

Federal expert says: "From February 2023 to April 2024, significant coral bleaching has been documented in both the Northern and Southern Hemispheres of each major ocean basin." – Derek Manzello, NOAA coordinator

State specialist says: "We can't ignore climate change." -- Stephanie Schopmeyer, associate research scientist with Florida's Fish and Wildlife Conservation Commission