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Coral reef (changes)

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Coral reef bleaching

Global warming has been causing the "bleaching" of coral reefs. A bleached coral reef has lost its photosynthesizing symbiotic organisms, called zooxanthellae. It may look white as a ghost — as in the picture above — but it is not yet dead. If the zooxanthellae come back, the reef can recover.

In 2010, due apparently to the record high temperatures, many coral reefs are actually dying:

International marine scientists say that a huge coral death which has struck Southeast Asian and Indian Ocean reefs over recent months has highlighted the urgency of controlling global carbon emissions. Many reefs are dead or dying across the Indian Ocean and into the Coral Triangle following a bleaching event that extends from the Seychelles in the west to Sulawesi and the Philippines in the east and include reefs in Sri Lanka, Burma, Thailand, Malaysia, Singapore, and many sites in western and eastern Indonesia. “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,” says Dr Andrew Baird of the ARC Centre of Excellence for Coral Reef Studies and James Cook Universities. “So far around 80 percent of Acropora colonies and 50 per cent of colonies from other species have died since the outbreak began in May this year.” This means coral cover in the region could drop from an average of 50% to around 10%, and the spatial scale of the event could mean it will take years to recover, striking at local fishing and regional tourism industries, he says.
The National Oceanic and Atmospheric Administration released its 2009 outlook for the world's coral reefs this week, and the results are disturbing. The temperature patterns and heat stress that scientists are seeing, particularly in the Caribbean, are reminiscent of 2005. That year set records for coral deaths. Across the Caribbean, 25 to 95 percent of the coral colonies were affected. In the U.S. Virgin Islands, nearly 52 percent of the corals died. In Trinidad and Tobago, 73 percent of all Colpophyllia and Diploria brain coral colonies were wiped out. The damage goes beyond the corals themselves. Reefs provide habitats and ecosystems for tens of thousands of organisms, and they support the fisheries and tourism that some 100 million people worldwide depend on for their livelihoods. "There’s a lot of similarly between what we’re seeing now and what hindcasts of 2005 showed," said C. Mark Eakin, coordinator of NOAA’s Coral Reef Watch. "We can’t say whether it’s going to be worse or how they’re going to compare, but we’re looking at the potential for conditions that could lead to coral bleaching." In 2005, parts of the Caribbean were hit by a double-whammy: high surface temperatures and a lack of tropic storm activity that could have cooled the water. This year, the global ocean surface temperature reached a record 0.62 degrees Celsius above the 20th century average in July, and the warmer water is accompanied by a storm forecast for the Caribbean that promises little rain or cloud cover.
This year many corals are already bleached and dying in the southern Caribbean Sea, especially in the Lesser Antilles, according to Mark Eakin, coordinator of Coral Reef Watch at the U.S. National Oceanographic and Atmospheric Administration (NOAA). The waters are warmer than they were in 2005 when a severe bleaching occurred across much of the Caribbean. More than 60 percent of corals died in the U.S. Virgin Islands, Eakin told Tierramérica. [...] Prior to the 1980s only one large-scale bleaching event had ever been recorded. An increase in water temperature of just one or two degrees Celsius above the average summer peak period is enough for bleaching to begin. [...] In Southeast Asia, ocean temperatures were 4.0 degrees above normal in May. Sixty to 80 percent of corals in various regions near Indonesia, Vietnam, Sri Lanka, Thailand, and Malaysia bleached, and some died, according to surveys done by the Wildlife Conservation Society, based in Indonesia. It is expected that 2010 will be worse than the 1998 bleaching that killed 30 percent of reefs in the Indian and the western and central Pacific Ocean, they reported. That year, 16 percent of the world's corals died due to bleaching. Until the last decade or so, overfishing, pollution and coastal development were the biggest killers of coral. Those threats still exist, although there have been attempts to protect corals in marine protected areas (MPAs) and "no-take" fishery reserves.

Coral reefs and ocean acidification

Not only the warming but also the acidification of oceans are hurting them. Indeed, seawater is reaching the point where aragonite, the mineral from which corals are made, becomes more soluble in water.

For a quick introduction to this issue, see:

Kirshenbaum writes:

An international team of marine biologists recently traveled to Papua New Guinea where excess CO2 released from volcanic activity has already decreased local ocean pH to the levels that are expected globally by 2100. In this area, they found that more than 90 percent of the region’s coral reef species were lost. The study provided a glimpse of how oceans might one day change around the world and serves as a warning that we must curb carbon emissions as quickly as possible.

The study:

For the underlying physical chemistry, see:

For an overview of the threat to coral reefs, see:

  • O. Hoegh-Guldberg, P. J. Mumby, A. J. Hooten, R. S. Steneck, P. Greenfield, E. Gomez, C. D. Harvell, P. F. Sale, A. J. Edwards, K. Caldeira, N. Knowlton, C. M. Eakin, R. Iglesias-Prieto, N. Muthiga, R. H. Bradbury, A. Dubi and M. E. Hatziolos, Coral reefs under rapid climate change and ocean acidification, Science 318 (14 December 2007), 1737-1742.

Chris Colose has a nice summary of what this paper predicts under three scenarios:

  1. If CO2 is stabilized today, at 380 ppm-like conditions, corals will change a bit but areas will remain coral dominated. Hoegh-Guldberg et al. emphasize the importance of solving regional problems such as fishing pressure, and air/water quality which are human-induced but not directly linked to climate change/ocean acidification.

  2. Increases of CO2 at 450 to 500 ppmv at current >1 ppmv/yr scenario will cause significant declines in coral populations. Natural adaptive shifts to symbionts with a +2°C resistance may delay the demise of some reefs, and this will differ by area. Carbonate-ion concentrations will drop below the 200 µmol kg-1 threshold and coral erosion will outweigh calcification, with significant impacts on marine biodiversity.

  3. In the words of the study, a scenario of >500 ppmv and +2°C sea surface temperatures “will reduce coral reef ecosystems to crumbling frameworks with few calcareous corals”. Due to latitudinally decreasing aragonite concentrations and projected atmospheric CO2 increases adaptation to higher latitudes with areas of more thermal tolerance is unlikely. Coral reefs exist within a narrow band of temperature, light, and aragonite saturation states, and expected rises in SST’s will produce many changes on timescales of decades to centuries (Hoegh-Guldberg 2005). Rising sea levels may also harm reefs which necessitate shallow water conditions. Under business-as-usual to higher range scenarios used by the IPCC, corals will become rare in the tropics, and have huge impacts on biodiversity and the ecosystem services they provide.

Evolutionary history

It’s interesting to look back back at the history of corals — click for more details

Corals have been around for a long time. But the corals we see now are completely different from those that ruled the seas before the Permian-Triassic extinction event 250 million years ago. Those earlier corals, in turn, are completely different from those that dominated before the Ordovician began around 490 million years ago. A major group of corals called the Heliolitida died out in the Late Devonian extinction. And so on.

Saving coral reefs

There are various organizations devoted to understanding and saving coral reefs. This project helps people grow new reefs with the help of artificial ‘reef balls’:

A reef ball is made by pouring concrete into a fiberglass mold containing a central buoy surrounded by various sized inflatable balls to make holes. Molds obtained from the Reef Ball Foundation are generally supplied with spare parts, tool kits, starting concrete additive supplies, and training. Sometimes, additional items such as coral propagation kits are added. Usually, clients only need to supply some plywood to build the base, a source of compressed air, and a fresh water supply. Any type of concrete can generally be used, including end-of-day waste, but additives such as microsilica and a high range water reducer are needed to give the reef balls high strength and to make the concrete suitable for marine life growth.

The following project is an international cooperation of various research institutions:

  • FORCE, Future of Reefs in a Changing Environment.


This report suggests that by 2030, over 90 percent of coral reefs will be threatened. If action isn’t taken soon, nearly all reefs will be threatened by 2050.

  • John Baez,Dying coral reefs

    John Baez, Dying coral reefs, Azimuth.

    , Azimuth.
  • Corals and global warming: the Mediterranean versus the Red Sea, the site of an ERC-funded Advanced grant project; they also obtained the additional “Proof of the concept” ERC grant A photoacoustic instrument for measuring health of corals and aquatic plants (see page 2 of pdf or html)

category: ecology, oceans