Coral Reefs: Value and Threats

Coral reefs provide immense benefits to humanity worldwide. They support fisheries, boost tourism, protect coastlines from storms, and even help scientists develop new medicines. The economic value of these advantages is estimated at around $9.8 trillion annually.

However, scientists now report that global ocean warming has triggered mass coral bleaching, affecting about half of the planet’s reefs. These findings, made by researchers at the Smithsonian Institution, for the first time allow for an accurate assessment of the scale of bleaching during such events. A new wave of marine heat, which began in 2023, is still ongoing. The results of the study were published in the journal Nature Communications.

Causes of Coral Bleaching

Corals exist thanks to a close symbiosis between two organisms: tiny animals related to jellyfish that form the reef’s hard structure, and microscopic algae living within their tissues. The algae use sunlight to produce energy, which feeds the coral. When ocean temperatures become too high, this partnership breaks down: corals expel the algae, losing their main energy source, and turn white—this is bleaching. Without algae, corals grow more slowly, reproduce less, and may die if the heat stress is too intense or lasts too long.

The Scale of the Third Global Bleaching Event (2014–2017)

To assess the damage during the third global coral bleaching event (2014–2017), scientists from dozens of countries joined forces. The project was led by researchers from the Smithsonian Tropical Research Institute, James Cook University (Australia), and the former director of the NOAA Coral Reef Watch program. The team combined satellite data on ocean surface temperatures with reef surveys and aerial photography collected worldwide. This approach allowed them to link satellite data on heat exposure with the actual condition of reefs.

According to Sean Connolly, senior scientist at the Smithsonian Institution, this is the most comprehensive geographic analysis of coral bleaching ever conducted: nearly 200 co-authors from 143 institutions in 41 countries and territories contributed their data.

Consequences: Coral Damage and Mortality

Analysis of over 15,000 reef surveys showed that 80% of reefs experienced moderate or severe bleaching, and 35% suffered moderate or high levels of coral death. After establishing the link between heat stress and reef damage at surveyed sites, the team used satellite data to estimate the impact on reefs that had not been directly studied. As a result, it was found that more than half of the world’s coral reefs underwent significant bleaching, and 15% suffered substantial coral mortality. As reefs degrade, the services they provide—such as tourism revenue and seafood supplies that millions depend on—also decline.

According to Mark Eakin, former director of Coral Reef Watch, the level of heat stress during this event was so extreme that new, higher alert levels for bleaching had to be created, which were not previously required.

Professor Scott Heron of James Cook University added that about half of the reefs exposed to heat stress experienced bleaching two or more times within three years, often with devastating consequences. For example, the Great Barrier Reef in Australia suffered two consecutive events, followed by three more. The reefs do not have time to recover before the next bleaching event occurs.

Ocean Warming and the Fourth Global Bleaching Event

Over the past 30 years, the planet has lost about 50% of its corals. The oceans absorb most of the excess heat generated by burning fossil fuels. Without this absorption, air temperatures on Earth would have reached about 50 degrees Celsius. Data collected worldwide now show that Earth is experiencing a fourth global coral bleaching event, which began in early 2023.

Sean Connolly notes that the third global bleaching event was the most severe and widespread in recorded history, but now reefs are facing an even more serious fourth event.

The Importance of Global Coral Reef Monitoring

Joshua Tewksbury, director of STRI, emphasizes that local, regional, and global economies heavily depend on the health of natural systems like coral reefs, yet we often take them for granted. It is crucial for scientific communities to unite, as this global team has done, to track changes in these critically important systems. This requires combining geographies and technologies—from satellite remote sensing to underwater surveys—which calibrate space-based observations and reveal the true scale of the damage.