Saving the reefs 1. Bleaching: the biggest threat to coral reefs

Posted: 17 November 2000

Author Info: Henrylito D. Tacio is an award-winning environmental journalist and People & the Planet correspondent in the Philippines.

The impact of 'bleaching' on the coral reefs of the world, as sea temperatures rise, was highlighted at the 9th International Coral Reef Symposium recently in Bali, Indonesia, writes Henrylito Tacio.

While coral reefs are in a state of steady decline for a number of reasons, including coral mining, fishing with dynamite and cyanide and haphazard coastal development, mass coral bleaching has now become a major contributing factor in the destruction of the reefs according to Dr Ove Hoegh-Guldberg, founding Professor of Marine Studies and Director of the University of Queensland's Centre for Marine Studies.

"The fact that all major climate models show that the current increases in sea temperature will continue is a source of major concern," he said.

Corals grow in the warm waters, but many of them are near the limits of their tolerance for high temperatures. Bleaching is a breakdown of a "complex biological system" that corals have evolved in order to survive. Each coral formation is a colony of hundreds or thousands of tiny organisms (known as polyps) that jointly build a skeleton that forms the reef. The outside layer of each coral polyp is inhabited by tiny one-celled plants scientists called zooxanthellae. It is these organisms that give the coral its bright colors, and when expelled due to warmer water or some other stress, coral appears bleached (that is, go pale or snowy-white). Without zooxanthellae, the coral cannot survive for long.

"Corals tend to die in great numbers immediately following coral bleaching events, which may stretch across thousands of square kilometers of ocean," explained Dr. Hoegh-Guldberg, who has studied the phenomenon of coral bleaching since the early 1980s.

According to the book, Management of Bleached and Severely Damaged Coral Reefs, coral bleaching can be traced as far back as 1870. However, since the 1980s, bleaching events have become more frequent, widespread and severe. "The massive coral bleaching and mortality event of 1998 devastated large parts coral reefs around the world," says the World Resources Institute (WRI).

The most affected reefs were in the Arabian/Persian Gulf, Kenya, Tanzania, the Seychelles, Maldives, Chagos banks, Sri Lanka and India in the wider Indian Ocean, parts of Southeast Asia, especially Vietnam, the Philippines, Taiwan, southern Japan, and Palau. Many areas reported coral losses of 60-90 per cent over large areas and often down to 30 metres or more.

A report released by Greenpeace cites "warmer than normal temperatures" as the principal cause of the massive coral bleaching and mortality event. "Increased sea temperature is the primary reason why coral bleaching has occurred with increasing intensity and frequency over the past two decades," the report states.

"Sea surface temperatures throughout the tropics have shown dramatic increases over the last two decades - as much as half a degree per decade," said Dr. Al Strong, team leader in the satellite research at the U.S. National Oceanic and Atmospheric Administration. "This is ten times what we are observing globally."

Warmest years

Dr. Thomas J. Goreau of the New York-based Global Coral Reef Alliance (GCRA) traces the recent "warmer than normal temperatures" to global warming. "Global warming is apparent in the worldwide proliferation of coral bleaching," declares World Wide Fund for Nature (WWF) based in Washington, D.C.

The warming of the world is due to the increase of heat being trapped in the atmosphere by greenhouse gases such as carbon dioxide and methane. The five warmest years in history have all been recorded in the last two decades, and there has been an unprecedented increase in the numbers of coral bleaching events during this period.

The worst bleaching episodes have coincided with a strange weather event known as El Niño. El Niños are periodic natural weather cycles that originate in the Pacific but can cause water temperatures to rise and extreme weather (hurricanes, droughts, and floods) around the world.

According to WRI, the recent massive coral bleaching was caused by the combination of extremely calm conditions during the 1997-98 El Niño events, coupled with a steady rising baseline of sea surface temperatures in the tropics. "These drove temperatures in parts of the tropics above records for the past 150 years, and bleaching was indiscriminate; impacts were equally severe on pristine, remote reefs as on reefs already under major human stresses," says the Status of Coral Reefs of the World: 2000, which was released during the 9ICRS.

"It will be several years before we can state that reefs will recover, or whether there will be local losses of species, including some rare endemic species," says Dr. Clive Wilkinson, editor of the 2000 status report. "Reef recovery will depend on few or no repeats of the extreme events of 1997-98, and even then, it will take 20 to 50 years before reefs recover to structures resembling those before the bleaching."

Dr. Goreau said that coral reefs are the first ecosystem to suffer large-scale damage from climate warming. "Coral reefs are uniquely threatened by global warming, because they cannot relocate to more favorable conditions or be replaced by immigrant organisms from warmer zones," he explained.

The Greenpeace report shares the same warning: "Coral bleaching events are projected to steadily increase in frequency and intensity until they occur every year by the 2030 to 2070 if greenhouse gases emissions continue to rise unabated."

Tropical storms

According to Dr. Goreau, bleaching and diseases have caused more coral deaths in the last couple of years than all previous human damage to reefs. "Unless bleaching and disease are reduced, all efforts at reef protection will be futile," he pointed out. "Recovery of coral reefs from mortality caused by severe bleaching and diseases is likely to be very prolonged, if it happens at all."

Coral reefs damaged by severe local stresses such as ship groundings, hurricanes, or predatory pest outbreaks can recover in a few decades as long as surrounding reefs are healthy. "However in recent years we have seen virtually all colonies of the most abundant and rapidly growing branching and plate corals in the Indian Ocean and parts of the Pacific die," Dr. Goreau said.

Warming waters aren't the only threats posed to coral reefs by climate change. Many scientists believe that global warming will herald a new era of extreme and unpredictable weather. Tropical storms may increase and so too would the consequent physical damage to coral reefs. Hurricanes Hugo and Marilyn hit the U.S. Virgin Islands National Park in 1989 and 1995 respectively and did massive damage to coral ecosystems.

Rising sea levels caused by global warming could be an additional problem for some reefs. The governments of small island nations such as the Maldives in the Indian Ocean and Kiribati and Tuvalu in the Pacific have been calling for international action to slow climate change. Their nations are based entirely on coral atolls no more than two meters above sea level; therefore, rising sea water levels could have serious impacts.

This must be the reason why, in the end of his presentation on the 1998 devastating bleaching event in Okinawa, Prof. Yossi Loya of Tel Aviv University made a this call for action: "As a coral reef society, we add our voice to the growing international concern on the issue of global climate change, and call for an effective reduction in greenhouse emissions over the next decade."

The depletion of coral reefs around the world would have dire consequences. "Apart from their beauty, coral reefs have a crucial role in shaping the ecosystems that have inhabited our tropical oceans for the last 250 million years," Prof. Hoegh-Guldberg said.

A huge loss

Coral reefs represent crucial sources of income and resources through their role in tourism, fishing, building materials, coastal protection and providing new drugs and biochemicals.

Globally, many people depend in part or wholly on coral reefs for their livelihood and around 15 percent of the world's population live within 100 kilometers of coral reef ecosystems. The fisheries associated with coral reefs generate significant wealth for countries with coral reef coastlines. Annually, fisheries in coral reef ecosystems yield at least six million tons of fish catch worldwide.

Fisheries in coral reef areas, however, have beyond the mere generation of monetary wealth and are an essential source of protein for many millions of the world's poorer societies. For example, 25 percent of the fish catch in developing countries like the Philippines is provided by coral reef associated fisheries.

Coral reefs protect coastlines from storm damage, erosion and flooding by reducing wave action approaching a coastline. "The cost of losing coral reefs would run into the hundreds of billions of dollars each year," the Greenpeace report claims.