récifs corallienl et changement climatique

Ref ID

23053

Author

Vieux, C., A. Aubanel, J. Axford, Y. Chancerelle, D. Fisk, P. Holland, M. Juncker, T. Kirata, M. Kronen, C. Osenberg, B. Pasisi, M. Power, B. Salvat, J. Shima and V. Vavia

Year

2004

Title

A Century Of Change In Coral Reef Status In Southeast And Central Pacific: Polynesia Mana Node, Cook Islands, French Polynesia, Kiribati, Niue, Tokelau, Tonga, Wallis and Futuna.

Source

p: 363-380. in C. Wilkinson (

Keywords

coral reef monitoring, management, status reports

Caption

Abstract

The Polynesia Mana Node of the southeast and central Paciﬁc contains 7 independent or autonomous countries or territories with only 6,000 km2 of land on 347 islands, but surrounded by 12 million km2 of EEZ. These seas contain 13,000 km2 of coral reefs as the main natural ecosystem providing food resources and opportunities for development, especially for tourism and pearl culture for 500,000 inhabitants. During the 19th and ﬁrst half of the 20th centuries, there was major exploitation by the colonial powers of mother-of-pearl oysters for the button industry, as well as guano, sandalwood and trepang. The Polynesian people were largely involved in a subsistence economy and all coral reefs and lagoons were healthy. During the last two decades of the 20th, all countries experienced rapid development and urbanization, rising populations, and some increased agriculture. These developments were limited to a few islands of each country (i.e. 15 islands amongst the 347) with resulting degradation of the coral reefs around these sites. The other islands remained mostly uninhabited and pristine, and continued with a subsistence economy. Generally, there was more damage to the coral reefs through natural events such as cyclones and coral bleaching, than by human activities. There is however, an urgent need to combat the threats on some islands from increased sedimentation, over-ﬁshing, dredging and nutrient pollution. The coral reefs of Polynesia Mana are predominantly healthy and at low threat risk in the immediate future. These coral reefs are probably the least degraded and endangered in the world as they are remote from continents and in the middle of the largest ocean. The largest threat is still controversial; predicted global climate change threats of more frequent and intense cyclones, and rising sea surface temperatures causing more coral bleaching mortality. Monitoring programs are developing in each country, with some having regular programs running for decades whereas others are just starting. The Node is now a functional reality with countries signing on as partners and authors contributing to status reports. The optimistic predictions are based on increased involvement of national authorities in coral reef protection and resource management. There are many MPAs currently being planned as awareness is raised and monitoring progresses. Local populations are participating more and reviving their culture and traditions as a basis for sustainable reef management. The pessimistic predictions will apply if governments fail in their efforts towards coral reef resource management for these coastal communities, and if they do not remedy the causes of human stresses to the reefs around the heavily populated islands.

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Status of Caribbean coral reefs after bleaching and hurricanes in 2005 : R. BOUGHRIET pour Actu-environnement

Les récifs coralliens sont menacés de disparition, notamment dans les Caraïbes, en raison du changement climatique, des ouragans, de la pollution ou de la surpêche, a prévenu l'Organisation des Nations unies pour l'éducation, la science et la culture (UNESCO).
Près de 500 millions de personnes au niveau mondial dépendraient de récifs coralliens en bonne santé pour leur subsistance, la protection des côtes, les ressources renouvelables et le tourisme. Les récifs constituent des barrières naturelles protégeant les côtes des cyclones et des tsunamis comme l'avait démontré l'Union Internationale de Conservation de la Nature (UICN), suite au tsunamis de l'Océan Indien de 2004. Environ 30 millions de personnes - parmi les plus pauvres du monde - dépendent entièrement des récifs pour leur nourriture, rappelle l'UNESCO.
Près des deux tiers des récifs coralliens, qui sont des écosystèmes fragiles, seraient menacés par l'impact du développement économique et du climat. Ils sont en effet exposés aux pressions exercées par l'activité humaine telles que la surpêche, les sédiments liés à la déforestation, les rejets d'engrais et de produits chimiques.
Le blanchissement du corail lié au réchauffement climatique et l’acidité des mers due à l'augmentation des émissions de CO2 sont également responsables de leur disparition. C'est ce que démontre le rapport intitulé ''Le statut des récifs coralliens des Caraïbes après le blanchiment et les ouragans de 2005'' présenté par l'UNESCO. Ce rapport, soutenu par la Commission océanographique intergouvernementale (COI) de l'organisation, a été réalisé par 80 scientifiques et gestionnaires de récifs coralliens, et doit être publié le 4 février.
Le blanchissement se produit quand les coraux, stressés par l'augmentation de la température de l'eau, expulsent les algues microscopiques avec lesquelles ils vivent en symbiose, les zooxanthelles. Ce sont elles qui fournissent aux coraux leur nourriture et donnent leurs couleurs à leur squelette calcaire. Si les zooxanthelles ne retournent pas dans le tissu corallien, le corail meurt.
D'après ce rapport, l'année 2005, qui fut l'une des années les plus chaudes enregistrées depuis 1880, a été marquée par 26 tempêtes majeures dont 13 ouragans responsables d'un blanchiment corallien très important. Et ce jusqu'à 95% dans plusieurs îles comme les Iles Caïmans, la Jamaïque, Cuba et les Antilles françaises, souligne l'UNESCO. Rappelons que sur l'ensemble des DOM-TOM, les surfaces récifo-lagonaires couvrent environ 55.000 km2 soit 10% de la surface mondiale de récifs. La Polynésie regroupe près de 20% des atolls coralliens du monde et la Nouvelle-Calédonie possède la seconde plus grande barrière récifale.

Les Caraïbes qui comptent 10,3 % des récifs de la planète, ont été fortement touchées en 2005. Au cours des 50 dernières années, de nombreux récifs des Caraïbes ont perdu jusqu’à 80 % de leur couverture corallienne, souligne l'UNESCO. Une perte qui pourrait coûter chaque année 140 à 420 millions de dollars à la région, selon l'Institut mondial de ressources Reefs@Risk.
D'après ce rapport, la seule façon de maintenir du corail vivant dans les récifs du monde serait de contrôler le réchauffement en réduisant de manière draconienne les émissions de gaz à effet de serre dans les 20 prochaines années et de gérer soigneusement les agressions directes comme la pollution, la pêche et les développements côtiers dommageables. Rappelons que le tourisme et les produits de la pêche liés aux récifs coralliens des Caraïbes génèrent chaque année un montant de 4,6 milliards de dollars.
Selon les estimations des scientifiques, la planète aurait déjà perdu près de 30 % de ses récifs de coraux. D'après le GIEC (groupe d’experts intergouvernemental sur l’évolution du climat), une augmentation de la température moyenne du globe de 2° C aboutirait à un blanchissement généralisé des coraux au niveau mondial.

R. BOUGHRIET pour Actu-environnement

Résumé sur le site de reefbase : Accelerating Impacts Of Temperature-Induced Coral Bleaching In The Caribbean

Coral bleaching is a stress-related response that can be triggered by elevated sea surface temperatures (SST). Recent increases in the frequency of coral bleaching have led to concerns that increases in marine temperatures may threaten entire coral reef regions. We report exponential increases in the geographical extent and intensity of coral bleaching in the Caribbean with increasing SST anomalies. A rise in regional SST of 0.1°C results in 35% and 42% increases in the geographic extent and intensity of coral bleaching, respectively. Maximum bleaching extent and intensity are predicted to occur at regional SST anomalies of less than +1°C, which coincides with the most conservative projections for warming in the Caribbean by the end of the 21st century. Coral bleaching is therefore likely to become a chronic source of stress for Caribbean reefs in the near future.

Résumé sur le site de reefbase : Addressing climate change and sea level rise in the Pacific Islands

Climate change and sea level rise are no longer a future phenomenon, they are taking place now and require more concerted effort. The situation in the Pacific Islands is even more serious because even though these small islands have done little to cause the problem and can do little to address it, they will be the first victims. Furthermore, the options for these islands are restricted by their small sizes and lack of resources. However, Pacific Islanders have extensive experience living in these small islands for generations and can offer worthwhile lessons on how to address climate change and sea level rise. In this paper, we examine some options for addressing the phenomenon in the Pacific Islands. A strategy for addressing the challenges of living in a world affected by climate change and sea level rise will be unveiled. As always, innovation and good plans and strategies will influence in the ability of Pacific Islands to address this problem. Addressing climate change and sea level rise in the Pacific Islands, has to be appropriate for these islands, which means that the solution has to be found from within the small islands. Some areas where changes can be considered include appropriate coastal protection, adaptation in land use and living practices and new options such as aquaculture, sustainable living at community level and new crops and varieties.

Résumé sur le site de reefbase : Biological indicators in the Caribbean coastal zone and their role in integrated coastal management

Caribbean coastal ecosystems are increasingly being threatened by natural and anthropogenic factors. The scale of these factors is at local, national, regional and global levels. Threats include the effects of fisheries and extraction, eutrophication, siltation, and pollution as well as global climate events such as El Niño and global climate change. Integrated coastal management (ICM) should clearly demonstrate the adverse effects of environmental impacts, thus justifying the need for mitigation and should evaluate the success of management efforts. ICM requires robust indicators that gauge the `health' of the coast in relation to environmental, social and economic activities. Biological indicators (bioindicators) offer a signal of the biological condition in an ecosystem. Using bioindicators as an early warning of pollution or degradation in an ecosystem can help sustain critical resources. Biological indicators in the Caribbean are focused around particular ecosystems, especially coral reefs, seagrass beds and mangrove forests and include a range of biological parameters relating to particular species, groups of species and biological processes. The use of these indicators is critically reviewed and the presence or absence of a relevant framework for their use in Caribbean ICM programs is discussed.

Résumé sur le site de reefbase : Le réseau de surveillance corallien à long terme des pentes externes de Polynésie française

Réseau de surveillance des pentes externes de Polynésie française. L'objectif de ce réseau est de surveiller l'état de santé des récifs et les modifications qui peuvent y intervenir sur le long terme à l'échelle de plusieurs décennies. Il a été mis en place dans le contexte d'éventuelles modifications en relation avec les changements globaux météorologiques dus à l'effet de serre et anthropiques dus à la pression croissante des activités humaines. En fonction de cet objectif, c'est la partie la plus importante et la plus vitale de l'écosystème récifal, c'est à dire la pente externe du récif, qui fait l'objet de cette surveillance. Sur 14 îles des 4 archipels de Polynésie française, des stations de références ont été repérées. Leurs peuplements coralliens font l'objet de relevés par la méthode des quadrats par photographie et par la méthode des relevés visuels en apnée tractée. Qu'il s'agisse d'îles hautes volcaniques ou d'atolls, on est en mesure de détecter les modifications qui interviennent sur l'état des récifs. Les données de ce réseau, dont la mise en place a débuté en 1992 et qui se poursuit actuellement, constituent un archivage du milieu naturel polynésien pour le prochain millénaire. Les stations de surveillance sont prospectées tous les 2 ans. Des relevés supplémentaires sont réalisés lorsque survient une catastrophe (e.g. : blanchissement, cyclone) dont on veut établir immédiatement les conséquences.

Résumé sur le site de reefbase : Ocean biodiversity informatics’: a new era in marine biology research and management

Ocean biodiversity informatics (OBI) is the use of computer technologies to manage marine biodiversity information, including data capture, storage, search, retrieval, visualisation, mapping, modelling, analysis and publication. The latest information systems are open-access, making data and/or information publicly available over the Internet. This ranges from primary data on species occurrences, such as in the Ocean Biogeographic Information System (OBIS), to species information pages and identification guides. Using standard data schema and exchange protocols, online systems can become interoperable and, thus, integrate data from different sources. However, insufficient metadata standards, i.e. the terminology to describe data, are available for biology and ecology. Quality assurance needs at least the same rigour as for printed publications, including expert oversight (e.g. Editorial Board), quality-control procedures and peer review. An index of data use is proposed to parallel citation indices for printed journals. Other challenges include data archiving and Internet access in developing countries. Although taxon names are the central, and most unique, element of biodiversity informatics, only about one-third of the names of described marine species are currently available online in authoritative master lists. The scientific community can form alliances that build and maintain biodiversity informatics infrastructures and that address data ownership and commercialisation potential. OBI enables greater access to more data and information faster than ever before, and complements the traditional disciplines of taxonomy, ecology and biogeography. It is urgently needed to help address the global crises in biodiversity loss (including fisheries), climate change and altered marine ecosystems. For OBI to succeed, governments, science-based organisations, scientists and publishers need to insist on online data publication in standard formats that enable interoperability. This change in marine biology culture is already underway.

The symbiosis between reef-building corals and their algal endosymbionts (zooxanthellae of the genus Symbiodinium) is highly sensitive to temperature stress, which makes coral reefs vulnerable to climate change. Thermal tolerance in corals is known to be substantially linked to the type of zooxanthellae they harbour and, when multiple types are present, the relative abundance of types can be experimentally manipulated to increase the thermal limits of individual corals. Although the potential exists for this to translate into substantial thermal acclimatization of coral communities, to date there is no evidence to show that this takes place under natural conditions. In this study, we show field evidence of a dramatic change in the symbiont community of Acropora millepora, a common and widespread Indo-Pacific hard coral species, after a natural bleaching event in early 2006 in the Keppel Islands (Great Barrier Reef). Before bleaching, 93.5% (n=460) of the randomly sampled and tagged colonies predominantly harboured the thermally sensitive Symbiodinium type C2, while the remainder harboured a tolerant Symbiodinium type belonging to clade D or mixtures of C2 and D. After bleaching, 71% of the surviving tagged colonies that were initially C2 predominant changed to D or C1 predominance. Colonies that were originally C2 predominant suffered high mortality (37%) compared with D-predominant colonies (8%). We estimate that just over 18% of the original A. millepora population survived unchanged leaving 29% of the population C2 and 71% D or C1 predominant six months after the bleaching event. This change in the symbiont community structure, while it persists, is likely to have substantially increased the thermal tolerance of this coral population. Understanding the processes that underpin the temporal changes in symbiont communities is key to assessing the acclimatization potential of reef corals.

Coral reefs, the storehouses of much of the world's marine biodiversity and the source of many socio-economic benefits are in decline worldwide. The causes of the ‘coral reef crisis’ are complex but there is general agreement that two broad categories of stress are involved: global-scale climatic changes induced by production of greenhouse gases, and local-scale impacts. The major feature of climate change affecting coral reefs is rising sea temperature, which has caused widespread coral bleaching and is implicated in increased occurrence of coral diseases and reduced rates of calcification. Local impacts on coral reefs stem from natural phenomena, such as storms, and from human populations in coastal areas, which are large and growing. The local human impacts include increased nutrient and sediment loads, habitat modification, destructive fishing and chronic overfishing. The losses of biodiversity, and lost opportunities for coastal communities to earn sustainable incomes from coral reefs, that can result from local human impacts are illustrated by blast, cyanide and muro-ami fishing. These destructive methods reduce the physical complexity and live coral cover of reefs and, because degraded reefs support fewer fish, ultimately remove the basis for long-term fish productivity. In Indonesia alone, blast fishing is estimated to have resulted in a loss of US$3.8 billion over 25 years. Although the corals and algae that build reefs have evolved in the dynamic land-sea-atmosphere interface to cope with environmental variation, the rapid changes occurring to reefs indicate that many corals are being stressed beyond their adaptive capacity. The prevailing view is that continued global climate change will cause further degradation of coral reefs, and that the damage will be greatest where stress from climate change coincides with local human impacts, and where negative synergies occur between multiple local stresses. The time-scales, and multiple actions, involved in reducing emissions of greenhouse gases, notwithstanding the urgent need to do so, have focused attention on ways to enhance and restore the resilience of coral reefs to human and natural impacts on more localized scales. The key recommendations include: protecting the functional groups of animals that build and maintain coral reefs to promote resilience to disturbance; managing reefs as entire ecosystems; and establishing large-scale (global/regional) networks of coral reef marine protected areas and no-take zones. A large body of science shows that these interventions undoubtedly have a role to play in reducing local threats to coral reefs, but our concern is that in developing countries, where two-thirds of the world's coral reefs occur, such bio-physical management strategies are unlikely to be adopted. For developing nations, the imperative is to meet the basic human needs of food security, safety, health and housing. The prevailing biodiversity-conservation perspective tends to regard the people who use reefs as part of the problem, whereas they should be seen as the key to the solution. Unless local interventions designed to counter the coral reef crisis acknowledge the interests of coastal communities to maintain their reliance on reefs as a source of food and income, and involve local users in an integral and effective way in decisions on how best to use their natural resources, both reefs and livelihoods will continue to be lost. The consequences of ineffective action on this issue will be a vicious cycle of misuse of coral reefs by people attempting to meet their basic needs. Within the spectrum of current approaches to conserving biodiversity and reducing poverty, we do not favour placing these two important concerns in separate policy realms. They need to be addressed together. The challenge for the international scientific community and aid agencies is to help developing countries identify and implement an approach for achieving the twin goals of sustaining livelihoods and conserving coral reefs.

The high precision measurement of the Sr/Ca ratio in corals has the potential for measuring past sea surface temperatures at very high accuracy. However, the veracity of the technique has been questioned on the basis that there is both a spatial and temporal variation in the Sr/Ca ratio of seawater, and that kinetic effects, such as the calcification rate, can affect the Sr/Ca ratio of corals, and produce inaccuracies of the order of 2–4 °C. In the present study, a number of cores of the massive hermatypic scleractinian coral Porites, from the central Great Barrier Reef, have been analyzed for Sr/Ca at weekly to monthly resolution. Results from a 24 year record from Myrmidon Reef show an overall variation from 22.7 °C to 30.4 °C. The record shows a warming/cooling trend with maximum warming centred on the 1986–1987 summer. While some bleaching was reported to have occurred at Myrmidon Reef in 1982, the Sr/Ca record indicates that subsequent summer temperatures were much higher. The 4.5 year record from Stanley Reef shows a maximum SST of 30 °C during the 1997–1998 El Niño event. The calibrations from Myrmidon and Stanley Reefs are in excellent agreement with previously published calibrations from nearby reefs. While corals do not calcify in equilibrium with seawater due to physiological control on the uptake of Sr and Ca into the lattice of coralline aragonite, it can be argued that, provided only a single genus such as Porites sp. is used, and that the coral is sampled along a major vertical growth axis, then the Sr/Ca ratio should vary uniformly with temperature. Similarly, objections based on the spatial and temporal variability of the Sr/Ca activity ratio of seawater can be countered on the basis that in most areas where coral reefs grow there is a uniformity in the Sr/Ca activity ratio, and there does not appear to be a change in this ratio over the growth period of the coral. Evidence from several corals in this study suggest that stress can be a major cause of the breakdown in the Sr/Ca–SST relationship. Thermal stress, resulting from either extremely warm or cool temperatures, can produce anomalously low Sr/Ca derived SSTs as a result of the breakdown of the biological control on Sr/Ca fractionation. It is considered that other stresses, such as increased nutrients and changes in light intensity, can also lead to a breakdown in the Sr/Ca–SST relationship. Two of the main issues affecting the reliability of the Sr/Ca method are the calibration of the Sr/Ca ratio with measured SST and the estimation of tropical last glacial maximum (LGM) palaeotemperatures. Instead of producing a constant calibration, just about every one published so far is different from the others. What is obvious is that for most calibrations while the slope of the calibration equation is similar, the intercepts are not. While the cause for this variation is still unknown, it would appear that corals from different localities around the world are responding to their own particular environment or that certain types of environments exert a control on the corals' physiology. Sr/Ca derived SST estimates for the LGM and deglaciation of 5 °C–6 °C cooler than present are at odds with estimates of 2 °C–3 °C cooling by other climate proxies. The apparent lack of reef growth during the LGM suggests that SSTs were too cold in many parts of the tropics for reefs to develop. This would lend support to the idea that tropical SSTs were much cooler than what the CLIMAP data suggests

The UN Framework Convention on Climate Change calls for the avoidance of “dangerous anthropogenic interference with the climate system”. Among the many plausible choices, dangerous interference with the climate system may be interpreted as anthropogenic radiative forcing causing distinct and widespread climate change impacts such as a widespread demise of coral reefs or a disintegration of the West Antarctic ice sheet. The geological record and numerical models suggest that limiting global warming below critical temperature thresholds significantly reduces the likelihood of these eventualities. Here we analyze economically optimal policies that may ensure this risk-reduction. Reducing the risk of a widespread coral reef demise implies drastic reductions in greenhouse gas emissions within decades. Virtually unchecked greenhouse gas emissions to date (combined with the inertia of the coupled natural and human systems) may have already committed future societies to a widespread demise of coral reefs. Policies to reduce the risk of a West Antarctic ice sheet disintegration allow for a smoother decarbonization of the economy within a century and may well increase consumption in the long run.

Rising sea temperatures are increasing the incidences of mass coral bleaching (the dissociation of the coral-algal symbiosis) and coral mortality. In this study, the effects of bleaching (induced by elevated light and temperature) on the condition of symbiotic dinoflagellates (Symbiodinium sp.) within the tissue of the hard coral Stylophora pistillata (Esper) were assessed using a suite of techniques. Bleaching of S. pistillata was accompanied by declines in the maximum potential quantum yield of photosynthesis (Fv/Fm, measured using pulse amplitude modulated [PAM] fluorometry), an increase in the number of Sytox-green-stained algae (indicating compromised algal membrane integrity and cell death), an increase in 2',7'-dichlorodihydrofluroscein diacetate (H2DCFDA)-stained algae (indicating increased oxidative stress), as well as ultrastructural changes (vacuolisation, losses of chlorophyll, and an increase in accumulation bodies). Algae expelled from S. pistillata exhibited a complete disorganisation of cellular contents; expelled cells contained only amorphous material. In situ samples taken during a natural mass coral bleaching event on the Great Barrier Reef in February 2002 also revealed a high number of Sytox-labelled algae cells in symbio. Dinoflagellate degeneration during bleaching seems to be similar to the changes resulting from senescence-phase cell death in cultured algae. These data support a role for oxidative stress in the mechanism of coral bleaching and highlight the importance of algal degeneration during the bleaching of a reef coral.

Elevated ocean temperatures can cause coral bleaching, the loss of colour from reefbuilding corals because of a breakdown of the symbiosis with the dinoflagellate Symbiodinium. Recent studies have warned that global climate change could increase the frequency of coral bleaching and threaten the long-term viability of coral reefs. These assertions are based on projecting the coarse output from atmosphere–ocean general circulation models (GCMs) to the local conditions around representative coral reefs. Here, we conduct the first comprehensive global assessment of coral bleaching under climate change by adapting the NOAA Coral ReefWatch bleaching prediction method to the output of a low- and high-climate sensitivity GCM. First, we develop and test algorithms for predicting mass coral bleaching with GCM-resolution sea surface temperatures for thousands of coral reefs, using a global coral reef map and 1985–2002 bleaching prediction data. We then use the algorithms to determine the frequency of coral bleaching and required thermal adaptation by corals and their endosymbionts under two different emissions scenarios. The results indicate that bleaching could become an annual or biannual event for the vast majority of the world’s coral reefs in the next 30–50 years without an increase in thermal tolerance of 0.2–1.0 1C per decade. The geographic variability in required thermal adaptation found in each model and emissions scenario suggests that coral reefs in some regions, like Micronesia and western Polynesia, may be particularly vulnerable to climate change. Advances in modelling and monitoring will refine the forecast for individual reefs, but this assessment concludes that the global prognosis is unlikely to change without an accelerated effort to stabilize atmospheric greenhouse gas concentrations.

Global warming since the 1950s has been largely due to human activity. The global mean temperature increased by 0.6 degrees Celsius over the 20th Century, seemingly not much but ecosystem changes have already been substantial. • The long residence time of carbon dioxide in the atmosphere makes further temperature increases in excess of the past trend inevitable over the next 20-40 years even if effective action is taken now to reduce emissions. The amount of warming that occurs by the end of the 21st Century will depend heavily on actions taken today. The argument for instant action is undeniable. • Sea temperatures are projected to increase by between 2oC and 6oC over the 21st Century. Mass coral bleaching events are correlated with temperature rises of short duration over summer maxima. The 1998 worldwide mass coral bleaching event was the largest in recorded history, and happened in the warmest year on record. There is little evidence that corals can adapt rapidly enough to match even the lower projected rates of temperature rise. • The Great Barrier Reef is perhaps the best managed Marine Park in the world, but it is not immune from climate change. Increasingly frequent El Niño events superimposed on a rising trend in temperatures will impede the recovery of coral cover from previous bleaching events. We are at risk of seeing the most pristine coral reef system degrade over the next few decades, with macroalgae (seaweed) taking over from the coral, and drastic changes to fish populations and biodiversity generally. This is already happening in large areas of the reef as a result of climate change and other impacts. • Urgent introduction of better coastal management control, and stricter control of already well-managed fisheries, are central to improving the ecological resilience of the Great Barrier Reef to survive the impacts of climate change as a coral-dominated ecosystem. • The other crucial change is to ensure rapid reduction in CO2 emissions worldwide through the replacement of greenhouse gas intensive coal and oil with other energy sources. • The economic base of the regions along the Great Barrier Reef is contributed by mining and mineral processing, tourism (concentrated in Far North Queensland), grazing, cropping and fisheries. ‘Reef-interested’ tourism is estimated at around two-thirds of the total tourism gross product, but higher in the Far North. This base is critical for the well-being of large numbers of Australians. It is currently at risk from climate change and associated coastal human impacts. • Tourism gains further social significance because it is a major labour-intensive industry. Fisheries comprise a relatively small industry but the social importance of commercial and recreational fishing is out of proportion to the economic contribution. The cultural and social values of Indigenous people require a special focus and inclusion in any long-term planning within these industrial sectors. • For the Great Barrier Reef and other coral reef systems to survive, it is essential for global policy-making to become much more ecologically sensitive. This is the inevitable conclusion from the development of four detailed scenario storylines developed in Part 3 of the report.

The world's coral reefs are located in one of the most thermally stable habitats on the planet today. Not only are seasonal and diurnal fluctuations in tropical sea temperature small, but recent evidence suggests that tropical oceans have varied by less than 2oC over the past 18,000 years. Temperate seas (> 30o latitude) by comparison have experienced far greater changes in sea temperature. There is abundant evidence that the web of interactions making up reef ecosystems are particularly sensitive to slight (1-3oC) changes in temperature. Recent episodes of coral bleaching are evidence of this sensitivity and are instructive as examples of the potential impact that increased sea temperature is likely to have on coral reefs. Current projections of sea temperature change for tropical regions in a global warming world suggest sea temperatures may increase by as much as 3oC by the end of next century. How organisms respond to this change will depends on several aspects of their biology. Firstly, organisms may be able to acclimate to this temperature change by regulating physiological rate processes to new temperature regimes. There is ample evidence that tropical organisms can acclimate to small changes (1-3oC) in temperature. More complex interactions, however, such as those represented by the symbiosis between corals and their singlecelled algae, appear to be less able to acclimate. Secondly, populations of tropical organisms may respond to temperature change via the selection of individuals that are more suited to the new temperature regime (ie. evolve in response to change). This type of natural selection, however, requires many generations over which to operate, and is exacerbated by the long generation times of many important coral reef organisms (e.g. generation of many corals > 20 years). It is therefore extremely unlikely that the majority of coral reef organisms will be able to evolve quickly enough to keep pace with the predicted changes in sea temperature. The current prognosis for coral reef ecosystems in the face of the warming of tropical seas is not favorable. If sea temperatures change at the predicted rates, the tolerance levels of keystone organisms like those of reef-building corals are likely to be exceeded. Given the central role that these organisms play in reef ecosystems, the predicted outcomes of the warming of tropical seas are not attractive. Reduced reef health is likely to lead to reduced productivity, calcification and an exacerbation of other anthropogenic impacts. This in turn will mean reduced coastal stability, fishery yields, water quality and biodiversity. Coral reefs may also collapse completely (as has been reported already in the Eastern Pacific and Caribbean) under the impact of what might seem relatively small changes to tropical sea temperature. The economic and social cost of these changes to reef ecosystems are likely to be catastrophic and should be avoided at all costs.

Sea temperatures in the tropics have increased by almost 1oC over the past 100 years and are currently increasing at the rate of approximately 1-2oC per century. Reef-building corals, which are central to healthy coral reefs, are currently living close to their thermal maxima. They become stressed if exposed to small slight increases (1-2oC) in water temperature and experience coral bleaching. Coral bleaching occurs when the photosynthetic symbionts of corals (zooxanthellae) become increasing vulnerable to damage by light. The resulting damage leads to the expulsion of these important organisms from the coral host. Corals tend to die in great numbers immediately following coral bleaching events, which may stretch across thousands of square kilometers of ocean. Bleaching events in 1998, the worst year on record, saw the complete loss of live coral in some parts of the world. This paper reviews our understanding of coral bleaching and demonstrates that the current increase in the intensity and extent of coral bleaching is due to increasing sea temperature. Importantly, this paper uses the output four major global climate change models to predict how the frequency and intensity of bleaching events will change over the next hundred years. The results of this analysis are startling and a matter of great concern. Sea temperatures calculated by all models (ECHAM3/LSG, ECHAM4/OPYC3 with and without aerosols, CSIRO DAR model) and based on the IPCC Scenario A (doubling of carbon dioxide levels by 2100, IPCC 1995) show that the thermal tolerances of reef-building corals will be exceeded within the next few decades. As a result of these increases, bleaching events are set to increase in frequency and intensity. Events as severe as the 1998 event will become commonplace within twenty years and bleaching events will occur annually in most tropical oceans by the end of the next 30-50 years. There is little doubt among coral reef biologists that an increase in the frequency of bleaching events of this magnitude will have drastic consequences for coral reefs everywhere. Arguments that corals will acclimate to predicted patterns of temperature change are unsubstantiated and evidence suggests that the genetic ability of corals to acclimate is already being exceeded. Corals may also adapt in evolutionary time, but such changes are expected to take hundreds of years, suggesting that the quality of the world’s reefs will decline at rates that are faster than expected. Every coral reef examined in Southeast Asia, the Pacific and Caribbean show the same trend. The world’s largest continuous coral reef system (Australia’s Great Barrier Reef) was no exception and will face severe bleaching events every year by the year 2030. Southern and central sites of the Great Barrier Reef will be severely effected by sea temperature rise within the next 20-40 years. Northern sites are warming more slowly and will lag behind changes in the southern end of the Great Barrier Reef by 20 years. In summary, however, the rapidity of this change spells catastrophe for tropical marine ecosystems everywhere and suggests that unrestrained warming cannot occur without the complete loss of coral reefs on a global scale.

Most major climate models project that rapid increases to ocean temperature (1-3°C per century) will continue if atmospheric greenhouse gases continue to accumulate. This scenario, together with the explicit link between coral bleaching, mortality and sea temperature, has led to the bleak prediction of annual mass coral bleaching event by the end of the current century. This paper explores this projection for 12 sites in the Pacific Ocean, and improves the projections by including intensity as well as frequency information. The behaviour of coral reefs over the past twenty years indicates a strong predictive association between the size and length of sea surface temperature anomalies and the intensity of mass coral bleaching (Degree Heating Months, DHM). The DHMs associated with severe bleaching events seen in 1998 in Palau, Okinawa, Seychelles and Scott Reef (Australia) were 3.9, 3.0, 3.1 and 2.6 respectively. By contrast, DHMs for the less affected reefs of Moorea, Cook Islands; and the southern and central sectors of the Great Barrier Reef were 0.9, 0.4, 1.7 and 1.4 respectively. Using the Global Circulation Model, ECHAM4/OPYC3 (forced by the moderate IS92a scenario) sea surface temperatures were projected for the next 100 years at the 12. Pacific sites. In all cases, thermal stress increases so rapidly such that DHM values of greater than 10 (triple current extreme DHM values) are projected to be commonplace by 2080. Data showing that corals are rapidly adapting to these changes remains extremely scant, as is evidence that corals can rapidly swap their symbiotic algal for more thermally tolerant ones. The inescapable conclusion from this study is that the projected changes in thermal stress are likely to result in major negative changes in the distribution and abundance of corals and related organisms as this century progresses.

Cet article présente, de façon pragmatique, le bilan de l’usage de la
télédétection pour l’étude des milieux coralliens, à travers l’analyse
des travaux publiés dans les vingt dernières années. Nous exposons
les échelles de travail potentielles et celles réellement pratiquées, les
principales applications et leurs facteurs limitants. Nous détaillons ces
limites qui sont d’ordre technologique (résolutions spatiales et spectrales
des capteurs), mais aussi bien souvent d’ordre conceptuel (place de la
télédétection dans l’étude), ou méthodologique (traitement des images).
La compréhension de ces limites, confrontées à la réalité du terrain,
permet de cadrer les travaux qui pourront être menés au début du
troisième millénaire.

Abstract
The aim of this paper is to present in a practical fashion the status of the
use of remote sensing for the study of coral reef environments through
the analysis of work published in the years 1980-2000. We review the
scales of the potential work and of that actually conducted, the principal
applications and their limiting factors. We detail these limitations which
are of a technological nature (spatial and spectral resolution of sensors),
as well as often of a conceptual (role of remote sensing in the study) or
methodological (image processing) nature. The understanding of these
limitations, in relation to the reality of the field, allows identification of
work which could be undertaken in the years 2000.

Changement climatique et récifs coralliens

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