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Coral reefs occupy less than one percent of the ocean floor, yet they support roughly a quarter of all marine species, from microscopic plankton to reef sharks. This extraordinary concentration of life has earned reefs the nickname rainforests of the sea. They also sustain human societies: more than five hundred million people depend on reefs for food, income from tourism or protection from storm waves. Economists have estimated the annual value of reef services at around 375 billion dollars, a figure that dwarfs the cost of conserving them.
The foundation of every reef is the coral animal itself, a tiny relative of jellyfish called a polyp. Most reef-building corals host microscopic algae, known as zooxanthellae, inside their tissues. The algae photosynthesise, passing as much as ninety percent of the energy they produce to their host, while the coral provides shelter, carbon dioxide and nutrients. This partnership explains how reefs flourish in tropical waters that are otherwise remarkably poor in nutrients, and it also gives corals their vivid colours.
The symbiosis, however, is delicately balanced. When sea temperatures rise even one or two degrees Celsius above the normal summer maximum for several weeks, corals become stressed and expel their algal partners. Because the algae supply most of the coral's colour, the white skeleton shows through the transparent tissue, and the colony is said to be bleached. A bleached coral is not immediately dead, but without the energy the algae provide it grows slowly, stops reproducing and becomes vulnerable to disease. If cooler water returns within weeks, the coral can take up algae again and recover.
Before the 1980s, mass bleaching was virtually unknown to science. Since then it has become a recurring global phenomenon, with severe events in 1998, 2010 and 2016 affecting reefs across the Pacific, Indian and Atlantic oceans. The 2016 event was the most damaging on record for Australia's Great Barrier Reef, where surveys found that around ninety percent of individual reefs had bleached and nearly a third of the coral in the northern section died. Scientists attribute the increasing frequency of such events primarily to the warming of the oceans caused by rising greenhouse gas emissions.
Heat is not the only threat. As the ocean absorbs carbon dioxide from the atmosphere, its chemistry shifts towards greater acidity, which makes it harder for corals to build their calcium carbonate skeletons. On many coasts, fertiliser and sewage flowing from farmland and cities feed algal blooms that smother reefs, while overfishing removes the grazing fish that would otherwise keep those algae in check. In the Indo-Pacific region, outbreaks of the crown-of-thorns starfish, a predator that consumes coral tissue, have caused further damage, and some researchers link these outbreaks to nutrient pollution.
Not all corals respond identically to stress, and this variation has become the focus of intense research. Some colonies tolerate heat better than others, and a few species can shuffle the types of algae they host towards more heat-resistant strains. Marine biologists are testing whether such naturally resilient corals can be bred or transplanted to restore damaged reefs, an approach sometimes called assisted evolution. Coral gardening, in which fragments are grown in underwater nurseries and later attached to degraded reefs, has already restored small areas in countries such as Indonesia and the Philippines.
Protecting reefs delivers benefits well beyond conservation. Reefs act as natural breakwaters, and studies suggest they reduce wave energy by up to ninety-seven percent, shielding coastlines from erosion and flooding. Healthy reefs also support fisheries that provide the primary source of protein for many coastal communities. Recognising this, governments have expanded marine protected areas, and international programmes now fund reef monitoring and restoration in more than one hundred countries.
Scientists caution, however, that local measures cannot compensate for a warming climate. Projections indicate that if global temperatures rise by two degrees Celsius above pre-industrial levels, the vast majority of the world's coral reefs could be lost. Limiting warming to one and a half degrees would still cause severe damage but would leave enough reef habitat for ecosystems to regenerate. The fate of the reefs, researchers conclude, will be decided less by what happens in the ocean than by how quickly humanity reduces its emissions on land.