A rise in sea temperatures killed off 90 per cent of the coral reefs near the surface of the Indian Ocean in only one year, while the remaining 10 per cent could die in the next 20 years, devastating fish stocks and tourism vital to coastal economies, research published today says.
The loss of these "rainforests of the ocean" would also lead to increased coastal erosion as the natural breakwaters formed by the live corals were worn down.
The dire warning, by Dr Charles Sheppard of the University of Warwick, follows a gradual rise in maximum sea temperatures, which in 1998 devastated the shallow corals lying from 10 to 40 metres (33ft to 130ft) below the surface. Since then some corals had begun to recover - but the risk continues.
"It's like a forest," said Dr Sheppard. "If you kill off 90 per cent, there might be just enough left to sustain some life around it, such as squirrels and so on. But if you have the same impact again and again there's no clear line as to when it's alive or not as a forest."
Coral reefs support aquatic organisms in complex, linked food chains. But with global warming causing a rise in sea temperatures - to which the organisms that build reefs are sensitive - environmentalists fear they will be destroyed.
In research published today in the science journal Nature, Dr Sheppard uses the computer model for global warming developed in Britain by the Hadley Centre for Forecasting to predict how sea temperatures will affect coral survival. The result is not good.
"Most corals don't mature until five years old and, five years since the 1998 event, most sites have recovered only marginally," he said.
A proportion of corals dies with each annual peak in the sea temperature, which varies seasonally by between 3C in the southern Indian Ocean and 15C in the Gulf region in the north.
As sea temperatures rise globally, more corals will be killed by the annual peak.
"The warming trend is only a fraction of a degree each year, which is swamped by the annual change," Dr Sheppard said. "But south of the Equator the probability of the temperature rising enough to kill all the corals means they could be wiped out as soon as 2020." Scientists think between 20 and 30 per cent of the world's coral reefs have been wiped out and because the Indian Ocean is home to 16 per cent of all reefs, their survival is all the more important.
Reef deathwould be calamitous on local economies. Some areas have already begun to limit fishing off reefs, despite pressures from people to exploit them: the Gulf of Mannar, at the south-eastern tip of India, is to limit fishing on its reef, reckoned to have been 65 per cent destroyed.
With global temperatures expected to rise another 2C to 2.5C this century, many scientists believe global warming is a greater threat than development, pollution or any other risk factor the reefs face.
Steven Miller, director of the National Undersea Research Centre at the University of North Carolina, is not optimistic. "The extra degree or two we'll see towards 2050 will push us over the edge," he said.
From NATURE, Sept. 18, 2003
More than 100 million people depend on coral reefs for their livelihood in the Indian Ocean region, but these reefs were heavily damaged, and most shallow corals killed, by the sea warming that followed the 1998 El Niño event. The rate of recovery of the "bleached" corals, and the risk of further episodes of ocean-warming in the future, are of vital importance to the populations in the region, but are notoriously difficult to predict. The best estimates had predicted that repeated lethal episodes are likely to come in a few decades, but a new approach that seamlessly combines historical data on sea surface temperatures with numerical models has come up with more disturbing predictions. The critical time for many sites is in fact just 1025 years, and a geographical pattern emerges that suggests that several of the world's poorest countries will be affected soonest. This new modelling approach includes the varying abilities of corals to adapt to temperature changes, and thus shows that it is not necessarily those sites that will experience the greatest temperature increases that will suffer the greatest coral mortalities in the near future.