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One of the reasons that reef-build corals are so successful is the symbiotic relationship that it forms with algae. However, extreme changes in the environment can disrupt this association harming, or even killing, the coral.

Swept Away by Global Warming

Global warming is transforming our environment. The temperature, acidity and water level of the ocean is rising. These changes are increasing in speed and magnitude and their effects will last for centuries. Corals are among those organisms hit hardest by global warming. The rate our climate changes will determine whether coral can survive or not.

 Coral reefs under threat

Over 30% of coral reefs are seriously damaged, and this is likely to rise to 60% by 2030. One major cause of this damage is global warming. Global warming and other sources of disturbance, like pollution and overfishing, work together to harm the coral reef ecosystem.

Some types of disturbance are relatively easy to control because they impact ecosystems within a small area. But global warming acts on a global scale with no one source of disturbance.

In next 50 years, temperature and carbon dioxide (CO2) levels are likely to be higher than they have been in the last 500 000 years. This will change the climate dramatically, and these changes will probably happen very quickly.

Global warming damage coral reef ecosystems in 3 main ways, by:

  • warming seawater
  • making the ocean more acidic
  • causing sea levels to rise
 Coral reefs and warming seawater

Warming the average temperature of the ocean by just 2-3oC is enough to kill many corals. High water temperatures harm coral in 3 ways, by:

  • interfering with coral and zooxanthellae algae symbiosis
  • increasing coral disease
  • increasing the rate and strength of tropical storms

Reef-building coral shelter zooxanthellae algae in exchange for food. Warm water temperatures stop zooxanthellae from making food.  Algae leave the coral and it turns white without algae to color it. This process is called coral bleaching.  Coral can survive for short periods without zooxanthellae, but will die unless it gets more (see "Coral bleaching" for more information).

Rising water temperatures also increase the incidence of coral disease. Unfortunately, disease spreads more easily on densely packed reefs, and can hit a healthy reef harder than a reef that has already lost coral cover.

The ocean helps drive weather patterns, and warming seawaters will increase the frequency and strength of tropical storms. The health of coral reefs will erode if storms happen too frequently because they won’t have time to repair damage from one storm before the next hits. Coral reefs protect coastal property from erosion (see "How reefs protect the land" for more information). Tropical storms will have a much greater impact on coastlines and coastal property if coral reefs are lost.

 Coral reefs and acidic oceans

The ocean absorbs carbon dioxide (CO2) from the atmosphere, and some of this CO2 turns into carbonic acid. The ocean has absorbed so much extra CO2 from industrial sources that it is becoming more acidic.

An acidifying ocean weakens reefs by dissolving the skeleton of coral polyps (see "What is a coral?" for more information). By 2050, the sea may be too acidic for coral polyps to produce a skeleton at all.

 Coral reefs and rising water levels

Average sea levels are rising, and this rate is probably increasing. The average sea level is projected to rise from between ¼ to 3 feet (8cm–1m) by 2100. Rising sea levels will increase wave action and erosion. This increased erosion will need to be countered by growth.

 Is there any hope for coral reefs?

Coral reefs appeared about 500 000 years ago, and have been adapting to changing climates ever since. But we are rapidly approaching temperatures and CO2 levels higher than corals have ever experienced.

Several factors will determine whether coral adapt to the present climate change caused by global warming, including:

  • rate of change
  • local adaptation
  • rate of growth
  • reef diversity
  • zooxanthellae algae

Rate of change
This is probably the most important factor in determining whether coral will adapt in time, because if change is slow enough the coral will have more chance to adapt. Coral can adapt to live in warmer temperatures by adapting physiologically and/or by expanding into waters that are presently too cold for coral.

Local adaptation
Coral that have already experienced elevated temperatures are more likely to survive than those that are used to cooler waters. Corals in the Arabian Sea live in temperatures 10oC higher on average than corals living along the same latitude. It takes much higher temperatures to bleach Arabian Sea corals because they are already partially adapted to warmer temperatures. Additionally, corals in a shallow Ofu Island lagoon in American Samoa experience highly variable hot water temperatures. Coral in this lagoon thrive in conditions that would kill coral that have not adapted to the heat (see the "Research" section for 4 microdocs that talk about the Ofu corals).

Rate of growth
Growth of the organisms within an ecosystem is the mechanisms that ecosystems repair damage. Healthy reefs have a better rate of growth and can repair the ecosystem damage caused by climate change better than unhealthy reefs (see "Productivity" for more details).

Reef diversity
Intact ecosystems are generally more resilient than one that has lost diversity (see "Diversity" for more details).

Relationship with zooxanthellae algae
Much of corals success is because of its symbiosis with zooxanthellae algae. Some zooxanthellae are able to tolerate higher temperatures than others and may help coral to survive (see "Coral bleaching" for more details).

 Global warming and sustainability

The rate that global warming changes the climate will be a big factor in determining whether coral reefs can survive. Coral reefs can adapt to many types of disturbance (and even benefit from a certain amount). But when the disturbance rate becomes too high the reef has no time to recover and its health erodes.

 References

Bates, B. et al. (2008, June). Climate Change and Water. Intergovernmental Panel on Climate Change (IPCC). PDF retrieved on 17 July 2008 from http://www.ipcc.ch/pdf/technical-papers/climate-change-water-en.pdf

Brahic, C. (2008, July 10). Corals join frogs and toads as world's most endangered. New Scientist. Retrieved on 18 July 2008 from http://environment.newscientist.com/channel/earth/dn14295-corals-join-frogs-and-toads-as-worlds-most-endangered.html?feedId=online-news_rss20

Bruno J. et al. (2007, June) Thermal stress and coral cover as drivers of coral disease outbreaks. PLoS Biology. Retrieved on 18 July 2008 from http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0050124

Buddemeier, R., Kleypa, J. & Aronson, R. (2004, February). Coral reefs and global climate change. Pew Center on Global Climate Change. PDF retrieved on 17 July 2008 from http://www.pewclimate.org/docUploads/Coral_Reefs.pdf

Carpenter, K. et al. (2008). One-third of reef-building corals face elevated extinction risk from climate change and local impacts. Science 321: 560-563.

Fine, M & Tchernov, D. (2007, March 30). Scleractinian coral species survive and recover from decalcification. Science 315(5820): 181. Retrieved on 23 July 2008 from http://www.sciencemag.org/cgi/content/full/315/5820/1811

Graham, N. et al. (2006, May 30). Dynamic fragility of oceanic coral reef ecosystems. PNAS 103(22):8425-8429.

Henderson, C. (2006, August 5). Ocean acidification: the other CO2 problem. NewScientist.com. Retrieved on 17 July 2008 from http://environment.newscientist.com/article/mg19125631.200

Hoegh-Guldberg, O. et al. (2007, December 14). Coral reefs under rapid climate change and ocean acidification. Science 318: 1737-1742.

Hughs, T. et al. (2003, August 15). Climate change, human impacts, and the resilience of coral reefs. Science 301:929-933.

Kleypas, J. et al. (2006). Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers: A Guide for Future Research, report of a workshop held 18–20 April 2005, St. Petersburg, FL, sponsored by NSF, NOAA, and the U.S. Geological Survey, 88 pp.

McFarling, U. (2006, August 3). A chemical imbalance. The Los Angeles Times. Retrieved on 26 July 2008 from http://www.latimes.com/news/local/oceans/la-me-ocean3aug03,0,3589668.story

Neeley, L. (2008, July 7). Corals Deliver Acid Shock. James Cook University. Retrieved on 23 July 2008 from http://www.coralcoe.org.au/news_stories/acidshock.html

New Scientist. (2007, May 8). Overcrowding helps spread coral disease. New Scientist. Retrieved on 18 July 2008 from http://environment.newscientist.com/channel/earth/dn11804-overcrowding-helps-spread-coral-disease.html

Pandolfi, J et al. (2005, March 18). Are U.S. Coral Reefs on the Slippery Slope to Slime? Science. Retrieved on 23 July 2008 from http://www.sciencemag.org/cgi/content/full/307/5716/1725

Parmesan , C. (2006, August 24). Ecological and evolutionary responses to recent climate change. The Annual Review of Ecology, Evolution, and Systematics 37:637–69. PDF retrieved on 17 July 2008 from http://arjournals.annualreviews.org/doi/pdf/10.1146/annurev.ecolsys.37.091305.110100

ScienceDaily. (2008, February 12). Global Warming: Sea Level Rise Could Be Twice As High As Current Projections, Greenland Ice Sheet Study Suggests. ScienceDaily. Retrieved on 26 July 2008 http://www.sciencedaily.com/releases/2008/02/080211172517.htm

Wikipedia. (2008, August 23). Sea level rise. Wikipedia. Retrieved on 26 July 2008 from http://en.wikipedia.org/wiki/Sea_level_rise

Wikipedia. (2008, July14). Ocean acidification. Wikipedia. Retrieved on 17 July 2008 from http://en.wikipedia.org/wiki/Ocean_acidification

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