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Major outbreaks of ‘bleaching’ of coral reefs in the past decade were almost always caused by unusually high sea temperatures, probably linked to global warming, according to the first global study of the phenomenon.

Coral is ‘bleached’ pink or white whenever algae are expelled from coral polyps. The algae provide coral with both its carbon and its colour. Usually, bleached coral recovers in the next cool season, but if all the algae are lost, the coral will die.

Thomas Goreau, a leading researcher into coral reefs, warns that global warming will increase the bleaching of coral and could seriously damage the diversity of marine organisms. He wants a worldwide programme to monitor bleaching and ocean ‘hot spots’, as part of the UN Climate Change Convention’s commitment to protect ecosystems sensitive to higher temperatures.

Goreau, who is president of the Global Coral Reef Alliance, and his colleague, Raymond Hayes, former chairman of marine sciences at the University of the Virgin Islands, discerned a rising tide of coral bleaching between 1983 and 1991. They compared its incidence with satellite data on average ocean temperatures (Ambio, vol 23, p 176).

In 1983, coral was bleached throughout the Pacific, from the Java Sea to Costa Rica. Bleaching was widespread in the Caribbean in 1987, and recurred every year until 1990 – a year of record temperatures and bleaching in the Gulf region of Oman. Bleaching was again widespread in the Pacific in 1991, from Thailand to Polynesia.

Bleaching occurs in all waters, from the warmest that can sustain coral reefs to the coldest. Goreau and Hayes found that the effect is triggered not by any specific temperature, but by anomalous warming locally. Bleaching invariably followed the warmest period ever recorded in an area. The threshold appeared to be a monthly mean water temperature more than 1 degree C higher than the longer-term average. ‘Above (that temperature) bleaching always took place,’ say the researchers, ‘and below it did not.’

Most bleaching in the Pacific occurred in 1983, 1987 and 1991, when changes in ocean circulation – known as El Nino events – caused warmer water than usual to spread across the tropical Pacific. However, El Nino is unlikely to have caused the bleaching in the Caribbean, where the two researchers say that a strong warming trend persisted throughout the 1980s.

Until this study, no one had established a global correlation between bleaching and temperature anomalies. Goreau points out that new cases of bleaching ceased abruptly in 1992, probably because of the cooling caused by debris which was injected into the upper atmosphere from Mount Pinatubo in June 1991 in the Philippines.

Goreau and Hayes describe bleaching as ‘a harbinger of reef ecosystem stress from ocean warming’ and warn that ‘the health of coral-reef ecosystems could be compromised if hot spots become more frequent and intense’, as global warming occurs.

Coral reefs are both massive reserves of biological diversity – often dubbed ‘marine rainforests’ – and the main protection for coral islands against storms and rising sea levels. ‘Continued or increased mass coral bleaching would seriously impact marine biodiversity, fisheries, tourism, shore protection and the ability to adapt to rising sea levels in over 100 countries,’ say Goreau and Hayes.