Coral reefs cover less than one percent of the ocean floor and support roughly a quarter of marine species. They shelter fish that feed hundreds of millions of people, protect coastlines from storm waves, and bring in tourism income for dozens of countries. They're also dying faster than almost any other ecosystem, and the main cause is heat.
What bleaching is
Reef-building corals are animals that live in partnership with tiny algae called zooxanthellae. The algae live inside the coral's tissue, make food from sunlight, and give the coral its colour. When seawater stays a degree or two above the usual summer maximum for several weeks, the algae become harmful to the coral, which expels them. The coral turns white. That's bleaching.
A bleached coral isn't dead. It can recover if temperatures fall and the algae return. But it's starving and weak while it waits, and a long or severe heatwave kills it.
A pattern that keeps repeating
Before the 1980s large-scale bleaching was rare. Since then it has become routine. The Great Barrier Reef suffered mass bleaching in 1998, 2002, 2016, 2017, 2020, 2022 and 2024. A study of the 2016 and 2017 events found that roughly half of the shallow-water corals on the reef had died. Gaps between events are now too short for slow-growing corals to rebuild.
The problem isn't limited to Australia. In 2024 US and international monitoring bodies confirmed a fourth global bleaching event, and heat stress reached reefs in every major ocean basin. Preliminary estimates suggested that a large majority of the world's reef area had experienced bleaching-level heat by 2025.
The warming math
The Intergovernmental Panel on Climate Change estimated in 2018 that if global warming reaches 1.5 degrees Celsius above preindustrial levels, 70 to 90 percent of warm-water reefs would be lost. At 2 degrees, more than 99 percent would be. The world has already warmed by about 1.2 to 1.3 degrees, and the margin is narrow.
Other pressures
Heat is the biggest driver, though not the only one. Runoff of fertiliser and sediment from farms clouds the water and feeds seaweed that crowds out coral. Overfishing removes the grazers that keep algae in check. Destructive methods such as blast fishing do direct damage. Ocean acidification, caused by the sea absorbing carbon dioxide, makes it harder for corals to build their skeletons.
Reefs in good local health do tend to bounce back better, which is why managing these local pressures still matters even when the heat can't be controlled locally.
What can be done
Restoration work is growing. Coral gardening involves growing fragments in nurseries and transplanting them to damaged sites. Researchers are also breeding or selecting corals that tolerate more heat. These approaches can help specific places, and they're expensive and small in scale compared with the size of the losses.
Scientists are blunt that nothing replaces cutting greenhouse gas emissions. Protecting reefs from local damage buys time, and restoration can save particular sites, but if ocean temperatures keep rising, the reefs we know today won't be there for later generations to see.
No comments:
Post a Comment