All around the world, much of the world’s marine biodiversity face threats from activities and events such as

The 2004 edition of Status of Coral Reefs Around the World lists the following top 10 emerging threats (p.19) in these three categories:

Climate change causing global mass coral bleaching

Coral bleaching results in white, dead-looking, coral (top image). Healthy coral, by contrast, is very colorful and rich with marine life. (Images source: Wikipedia

The above-mentioned Status of Coral Reefs Around the World, 2004 also notes (p. 21) that The major emerging threat to coral reefs in the last decade has been coral bleaching and mortality associated with global climate change.

As explained by Rob Painting on the popular Skeptical Science blog, bleaching can occur for a number of reasons such as Ocean acidification

Pollution

Excess nutrients from run-off

High UV radiation levels

Exposure at extremely low tides

Cooling or warming of the waters in which the coral reside Bleaching is not new. Past bleaching has often been localized and mild, allowing coral time to recover. But as Painting also adds, mass coral bleaching on the huge scale being observed certainly appears to be, and represents a whole new level of coral reef decline.

It is believed that almost all species of corals were affected by high sea surface temperatures during 1998 and the El Niño at the time, which resulted in global coral bleaching and mortality.

2002 was then the second worst year for coral bleaching after 1998.

Although there has been bleaching in the past, since 1998 it has become very severe: Global trends in the extent and severity of mass bleaching. The extent and severity of mass coral bleaching events have increased worldwide over the last decade. Prior to 1998 mass coral bleaching had been recorded in most of the main coral reef regions, but many reef systems had not experienced the effects of severe bleaching. Since 1998 coral bleaching has become a common phenomenon around the world. Every region has now experienced severe bleaching, with many areas suffering significant bleaching-induced mortality. Paul Marshall and Heidi Schuttenberg, A Reef Manager’s Guide to Coral Bleaching, Great Barrier Reef Marine Park Authority, November 30, 2006 (p.5) Source: In 2010 scientists observed huge coral death which struck Southeast Asian and Indian Ocean reefs over a period of a few months following a large bleaching event in the region. Dr Andrew Baird of the ARC Centre of Excellence for Coral Reef Studies and James Cook Universities was quoted as saying, It is certainly the worst coral die-off we have seen since 1998. It may prove to be the worst such event known to science.

Scientists have long been pessimistic about the future, with some reefs expected to vanish by 2020.

Additional scientific research, reported by Greenpeace fears climate change will eliminate reefs from many areas:

If climate change is not stopped, coral bleaching is set to steadily increase in frequency and intensity all over the world until it occurs annually by 2030—2070. This would devastate coral reefs globally to such an extent that they could be eliminated from most areas of the world by 2100. Current estimates suggest that reefs could take hundreds of years to recover. The loss of these fragile ecosystems would cost billions of dollars in lost revenue from tourism and fishing industries, as well as damage to coastal regions that are currently protected by the coral reefs that line most tropical coastlines. Climate Change and the World’s Coral Reefs, Greenpeace, 1999

Despite knowing the causes for many years, Australia’s The Great Barrier Reef Marine Park Authority has worried that identifying practicable and effective management responses has proven challenging because traditional management approaches do not work. Coral reef managers are unable to directly mitigate or influence the main cause of mass bleaching: above average water temperatures. This makes mass bleaching a uniquely challenging environmental management problem.

Despite knowing about these issues for many years, conditions have worsened. At the beginning of September, 2009, the Australian agency looking after the Great Barrier Reef released an outlook report warning the Great Barrier Reef is in trouble: Climate change, continued declining water quality from catchment runoff, loss of coastal habitats from coastal development and remaining impacts from fishing and illegal fishing and poaching [are] the priority issues reducing the resilience of the Great Barrier Reef.… [Despite being] one of the most healthy coral reef ecosystems … its condition has declined significantly since European settlement…. While … there are no records of extinctions, some ecologically important species … have declined significantly.… Disease in corals and pest outbreaks … appear to be becoming more frequent and more serious. … Given the strong management of the Great Barrier Reef, it is likely that the ecosystem will survive better … than most reef ecosystems around the world. However … the overall outlook for the Great Barrier Reef is poor and catastrophic damage to the ecosystem may not be averted. Ultimately, if changes in the world’s climate become too severe, no management actions will be able to climate-proof the Great Barrier Reef ecosystem. Great Barrier Reef Outlook Report 2009, Great Barrier Reef Marine Park Authority, Australia, September 2009, (pp. i, ii) But it is not just the Great Barrier Reef at risk. They are all at risk as Charlie Veron, an Australian marine biologist who is widely regarded as the world’s foremost expert on coral reefs, says: The future is horrific. There is no hope of reefs surviving to even mid-century in any form that we now recognize. If, and when, they go, they will take with them about one-third of the world’s marine biodiversity. Then there is a domino effect, as reefs fail so will other ecosystems. This is the path of a mass extinction event, when most life, especially tropical marine life, goes extinct. Charlie Veron, quoted by David Adam, How global warming sealed the fate of the world’s coral reefs, The Guardian, September 2, 2009