For a few days in the southern hemisphere every spring, the world’s biggest and most famous coral reef explodes. This is coral spawning season – a time when the Great Barrier Reef creates the next generation of corals. In fact, as some scientists explain it, it’s when the whole reef has sex. This year’s coral spawning began on November 2. During this period, different species of corals release sperm and eggs, which meet each other on the water’s surface and form coral polyps. But this isn’t just about witnessing an interesting natural experience. Coral spawning, which happens ahead of the southern hemisphere’s summer, gives clues about the health of the 133,000-square-mile (345,000-square-kilometer) reef. “The annual coral spawning is not only one of the most extraordinary natural phenomena on the planet,” says Anna Marsden, managing director of the Great Barrier Reef Foundation. “It provides us with an opportunity to fast-track world-leading research to safeguard its future from the impacts of climate change.” UNESCO, which maintains the list of World Heritage sites, has gone back and forth over whether to add the reef to its list of “sites in danger” over the past few years. While the reef escaped the designation in 2023, UNESCO stated that “sustained action to implement the priority recommendations of the mission is essential in order to improve (the reef’s) long-term resilience” and has requested an update on the reef’s health by February 2024. The warming waters caused by climate change have led to large-scale coral bleaching, which is when “stressed” corals turn white, revealing their carbonate skeletons. A scientific survey released by the Australian government in May 2022 reported a sixth “mass bleaching event,” with 91% of reefs affected.