The Great Barrier Reef is the largest structure on Earth built by living organisms. It extends for more than 2,300 kilometres along the northeastern coast of Australia, covers an area roughly the size of Italy, and contains more biodiversity than any comparably sized habitat on the planet. It is visible from space — a fact usually cited to convey wonder, though there is a second way to read it: we have damaged something large enough to see from orbit.
The reef has lost more than half its coral since 1995. Mass bleaching events in 2016, 2017, 2020, and 2022 caused unprecedented damage, with the most recent survey from the Australian Institute of Marine Science suggesting that roughly two thirds of the reef showed significant bleaching. This is not a slow decline happening over geological time. It is happening within a human lifetime, and it is happening fast enough for individual researchers to have witnessed healthy reefs become skeletal within their careers.
What a Reef Actually Is
The Great Barrier Reef is not a single thing. It is a collection of roughly 3,000 individual reef systems and 900 islands, built from the accumulated calcium carbonate skeletons of billions of tiny animals — coral polyps — over thousands of years. Each polyp is a small, soft creature that builds a hard external skeleton and lives in colonies. The reef's appearance — the branching, mounding, plate-like structures in shades of purple, orange, and white — is the accumulated architecture of these colonial animals.
Coral polyps do not survive alone. They live in symbiosis with microscopic algae called zooxanthellae, which take up residence inside the coral's tissue. The algae photosynthesise, converting sunlight to energy, and share up to 90 percent of what they produce with the coral. In return, the coral provides the algae with shelter and the carbon dioxide they need. The vivid colours of healthy coral largely come from these algae.
Bleaching and What Causes It
When water temperatures rise above what coral can tolerate — typically by just one or two degrees Celsius sustained over several weeks — the symbiosis breaks down. The coral expels its zooxanthellae. Without the algae, the coral loses both its colour and most of its energy supply. What remains is the white calcium carbonate skeleton showing through the transparent tissue: bleached coral.
Bleached coral is not dead coral. If temperatures return to normal quickly enough, the algae can recolonise and the coral can recover. But recovery takes time — typically ten to fifteen years to rebuild mature coral structure — and repeated bleaching events before recovery is complete cause lasting damage. The back-to-back events of 2016 and 2017 hit parts of the reef that had no time to recover between them. The 2022 event was the first to cause significant bleaching in La Niña conditions, when ocean temperatures are typically cooler, which suggests the baseline temperature has risen enough that even relatively cool years now cause damage.
What Lives There
The biodiversity of the reef is difficult to convey in numbers, but numbers help: more than 1,500 species of fish, 4,000 species of mollusc, 240 species of birds, six of the world's seven species of marine turtle, more than 30 species of whale and dolphin, and over 600 species of coral. The reef supports commercial and recreational fisheries that contribute billions of dollars to the Australian economy each year. It draws more than two million tourists annually. For many of the communities along the Queensland coast, it is the entire basis of the local economy.
Beyond economics, the reef is a nursery for much of the marine life in the surrounding ocean. Species that mature on the reef migrate outward and support fish populations that feed millions of people across the Pacific and beyond. The ecological connectivity between the reef and the wider ocean means that what happens within those 344,000 square kilometres ripples far beyond its boundaries.
What Can Still Be Done
The reef's future depends primarily on one variable: whether global average temperatures can be held below two degrees Celsius above pre-industrial levels. At two degrees, the models suggest the reef will lose most of its living coral but will likely retain its structural integrity and a reduced level of biodiversity. Above two degrees, the projections become much more severe. This makes the reef one of the most direct illustrations of what the difference between one and a half and two degrees of warming actually means at the species level.
Local actions — reducing agricultural runoff, limiting fishing pressure, controlling coastal development, conducting coral gardening programs — help the reef resist additional stresses and recover faster from bleaching. They buy time. But they cannot compensate for rising sea temperatures. The reef will survive or it will not largely depending on decisions made about emissions in the next decade — decisions made not in Queensland, but in capitals and boardrooms around the world that have never seen a coral reef.