Global change and coral reefs
Coral reefs worldwide are threatened by a number of stresses related to global change, ranging from increasing sea surface temperatures through to changes in ocean chemistry. Furthermore, climate change impacts will be compounded by the rapid escalation in environmental stresses arising from population growth and increasing tourism.
Coral bleaching and algal invasions are now frequently observed in many reef areas, but the extent to which they are natural or human induced is uncertain, and varies with region. Global change is creating a number of stress factors likely to influence the health of the worlds coral reefs:
Coral bleaching occurs when reef-building corals lose their symbiotic algae, and is signalled by loss of colour. It occurs when sea surface temperatures are unusually high, often associated with secondary effects due to low salinity and high solar radiation levels. Frequently, corals recover from bleaching, but death may result if the bleaching is extreme or prolonged. Where death of coral occurs, affected reefs remain unattractive for some years and there can be major impacts on tourism and fishing. Coupled with predicted rises in sea level and storminess, bleaching-induced coral death could also weaken the effectiveness of the reefs in protecting coasts and islands.
Bleaching often occurs when sea surface temperatures rise to a regional threshold some 1 to 2 degrees Celsius above seasonal average summer temperatures. In the Pacific, this is often associated with El Niño episodes. The frequency of high temperature episodes will increase as average temperatures rise due to the enhanced greenhouse effect. This is expected to result in more frequent and widespread damage to corals, especially to those remote from a supply of larvae of reef species, or those stressed through exposure to local climatic and/or human impacts such as riverine runoff, high solar irradiance (especially ultraviolet wavelengths) and pollution. The effects of temperature rise will also depend on latitude, coral height in relation to sea level, and the prevailing wave climate.
Mass bleaching has occurred on several occasions in Australias Great Barrier Reef and elsewhere since the 1970s. Particularly widespread bleaching occurred globally in 1997-98 associated with a major El Niño event. It was severe on the inner Great Barrier Reef, but less severe on the outer reef. This episode was associated with generally record high sea surface temperatures over most of the Great Barrier Reef region, which could be related to global warming trends due to the enhanced greenhouse effect.
Three independent data bases support the view that 1997-98 sea surface temperature anomalies were the most extreme in the past 95 years, and that sea surface temperatures off the northeast coast of Australia have significantly increased from 1903 to 1994. Lowered sea water salinity due to the flooding of major rivers between Ayr and Cooktown early in 1998 is also believed to have been a major factor in exacerbating effects in the inshore Great Barrier Reef.
Global average and regional warming trends have been well-documented by the Intergovernmental Panel on Climate Change with global and regional warmings mostly between 0.5 and 1 degrees Celsius over the last 100 years. These warmings are statistically significant, with a steep rise since the 1960s, and are consistent with the trends found in the Australian region. Current projections of global average warming in the 21st century are generally in the range 1 to 4 degrees Celsius. Warming in coral reef regions is expected on average to be slightly less than the global average, although there will be regional variations, notably those associated with a likely trend towards a more El Niño-like average state (with greater warming in the eastern tropical Pacific and less in the western tropical Pacific). Unless coral reefs can adapt quickly to these higher temperatures, this means that they will experience temperatures above the present bleaching thresholds almost every year well before the end of the 21st century.
Adaptation of coral reefs to increasing sea surface temperatures may well be too slow to be significant in preventing increased bleaching events over the next several decades. This is a crucial, but perhaps contentious, point. It should be noted that the estimated thresholds for bleaching vary between locations that have different average temperatures, with most reefs presently surviving in waters only some 1 to 2 degrees Celsius below the regional threshold. Indeed, some reefs live in very warm waters, such as in the Red Sea. This implies a long-term capability for coral reefs to survive in waters considerably warmer than exist at present in most regions where reefs occur.
In some cases, recent warming rates may be slow enough for the coral reef biota to adapt, at least initially, through changes in their symbiotic partnerships, as better adapted species settle and survive. At the scale of whole coral reefs and their communities, migration to higher latitudes may be a possibility. However, there is a host of historical, hydrodynamic and ecological factors that determine current distributions of coral reef biota. It may be centuries before substantial changes to species composition occur, and increases in diversity are detected, at the more poleward, cooler limits to the existing range of occurrence of coral reefs.
The real question is thus whether present reef ecosystems can adapt and survive in much warmer waters (while remaining coral reefs as we know them), and if so, over what timescale. There is some evidence to suggest that regional thresholds for bleaching have not increased in recent decades. This indicates that adaptation takes longer than a few decades. The geological evidence suggests that adaptation takes place on timescales of hundreds to thousands of years. If this is the appropriate timescale, mass coral bleaching may become a more frequent event in coming decades, and could well occur in many parts of the world.
Widespread bleaching is likely to lead to colonization by seaweeds and other biota, with increasing erosion of reefs, a failure to keep up with sea-level rise, and substantial changes in associated marine ecosystems. Recovery may also be inhibited by the recently discovered reduction in calcification rates due to higher atmospheric carbon dioxide concentrations, and many coral reefs will become more susceptible to the combined effects of other stresses.
It has been predicted that, by itself, a rise in sea level might benefit many reefs, since corals on reef tops would have a renewed and/or extended opportunity for vertical growth, unlike the last several thousand years when they have been largely limited to horizontal growth by a stable sea level. The potential effect of sea-level rise over the next 50 years in tidally-limited reef-top coral habitats is thus an increase in living coral cover, and in many cases, an increase in topographic relief. This is likely in sheltered parts of many reefs, assuming that the majority of these reef-top corals are not killed by increased temperature or other causes, such as pollution or burial by sediments if high intensity rainfall events were to increase.
The vertical growth of most shallow corals can keep pace with a rate of sea-level rise of at least 5 cm per decade. However, this does not mean that reef island growth can similarly keep pace with sea-level rise, since the accumulation of coral debris on low islands is dependent on storm and wave activity and consequently is episodic. Moreover, it takes many decades to centuries for soil to form where raw coral debris accumulates. It is, therefore, possible that many low-lying coral atolls will become uninhabitable.
If sea-level rise over the next century were to be at the upper end of the possible range due to the enhanced greenhouse effect, that is around 10 cm per decade, reef tops might not keep pace. There would then be dramatic changes in the reef-top zonation of corals, and in other reef substrata and ecosystems, since these are strongly controlled by the energy of the waves that pass over the reef.
It seems inevitable that coral reefs in most regions will suffer from several greenhouse-related impacts as well as non-climatic stresses such as coral diseases, over-exploitation and increasing pollution and turbidity of coastal waters by sediment loading, fertilizers, pesticides and herbicides. This combination of stresses highlights the fact that coral reefs are amongst the most susceptible ecosystems to damage due to global change. As coral reefs provide many services to human society, including shelter, nurseries for fish, and a prime tourist attraction, this threat to their future is a major cause for concern.
Barrie Pittock, Climate Impact Group, CSIRO Atmospheric Research, Private Bag No. 1, Aspendale, VIC 3195, Australia. Fax: +61-3-92394688. Email: firstname.lastname@example.org. Web: www.dar.csiro.au/res/cm/impact.htm.
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