Reefs at Risk

5. The effects of climate change

While corals and reef communities can recover from severe episodic disasters such as cyclones and storms, they are vulnerable to many low-level but continuing, chronic stresses created by human populations.

Over the coming century global climate change is expected to lead to:

  • increased air and sea-surface temperatures;

  • rises in sea-level;

  • changes in weather patterns;

  • more frequent storms, droughts, floods and other extremes of weather in some place;

  • possible alterations in the pattern of ocean circulation; and

  • changes in the chemistry of seawater from higher concentrations of carbon dioxide

Admitting the many uncertainties about the mechanisms involved, the intergovernmental Panel on Climate Change (IPCC) has concluded, in its latest (1992) update on climate change, that:

  • the carbon dioxide in the atmosphere will double by the middle of the next century (from 200 years ago, before the industrial revolution greatly increased atmospheric C02concentrations through the burning of fossil fuels);

  • the average temperature across the globe will increase by just under 0.3OC per decade, producing an increase of about 2.5OC by 2100. (The panel cautions that the average could range from 1.6-3.8OC.) Warming will be greater in higher latitudes and greater over land than over the ocean;

  • sea-level will probably rise by about 5cm per decade (with an estimated range of 1-9cm per decade). This means a probable sea-level rise of 45cm by 2100 (perhaps as little as 15cm, or as much as 90cm);

  • extreme weather conditions are likely to be more frequent and more intense. (For coral reefs, two possible changes are important: more of the total rainfall will occur in heavy storms, and the range, frequency and intensity of major storms may increase); and

  • depletion of the ozone in the stratosphere, as a result of

  • chlorofluorocarbon emissions will increase Earth's exposure to ultraviolet light.

How will corals and reefs react?

While corals and reef communities can recover from severe episodic disasters such as cyclones and storms, they are vulnerable to many low-level but continuing, chronic stresses created by human populations.

Many corals appear to live close to their upper temperature limit. Transient increases of only a few degrees above the usual maximum can kill corals. Even small increases can result in bleaching, a stress response which varies within and between species. In today's oceans, rates of growth or calcification as well as species diversity seem to increase with higher average water temperatures.

Changes in light, UV radiation and temperature can make a profound impact on a reef through their effect on plankton.

Coral reefs in the region of the Equator do not have to contend with frequent cyclonic storms. "Hot spots" of storm activity are found outside the equatorial zone at individual sites such as Guam, where storms are frequent and violent and the coral reefs tend to have high diversity but a low profile. By contrast, reefs in Micronesia, French Polynesia, Thailand or the southern Caribbean, for example, fall victim to such storms less than once in 50 years. These rare-storm reefs tend to develop more fragile coral communities of highly complex structure. So changes in the frequency and force of cyclonic storms can be expected to have major effects on the structure and growth rates of coral reefs.

Wave action strongly influences the depth to which reef corals, algae and other key organisms are distributed. Storms provide catastrophic pruning of reefs and substrate renewal. This makes it easier for new corals to recruit from elsewhere and thus influences the way the community is made up or operates. Storms, waves and currents also mobilise sediment and nutrients as well as shaping the coastline, and affect local sea-level. Currents play a major part in transporting pathogens and nutrients and in dispersing larvae that control the distribution of reefs. So changes to these physical parameters could alter the distribution of reefs and reef organisms.

Freshwater runoff or groundwater discharge affect reef development adjacent to land masses. Reef communities need a fairly stable range of salinity. If the salinity drops below 20 parts per thousand (ppt) (normal sea-water is 32ppt) for more than 24 hours, then corals and many other reef organisms die. Salinity variations that damage nearshore reefs could become more frequent if storms, rainfall and flooding increase.

Ultra-violet radiation is known to have detrimental effects on reef organisms.

Most adult corals and shallow water reef animals and plants have developed mechanisms to either block out or avoid this harmful radiation. However, the larval stages of reef organisms are vulnerable, during their planktonic stages, to increased ultraviolet light.

Increased sea-surface temperatures or dissolved nutrients from freshwater run-off could ' affect coral reef plankton and encourage the growth of phytoplankton. Increased phytoplankton, the food of larval crown-of-thorns starfish, may result in increased populations of this harmful species that has devastated many reef systems in the past.

Increased carbon dioxide concentrations affect the acidity of the ocean's surface waters and reduce the amount of dissolved calcium carbonate available to reef-building corals.

The Intergovernmental Panel on Climate Change has produced scenarios of how the global climate could change in the 21st century. This illustration shows the latest revisions (1992).

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March 30, 2010