Drafting Authors: Neil Adger, Pramod Aggarwal, Shardul Agrawala, Joseph Alcamo, Abdelkader Allali, Oleg Anisimov, Nigel Arnell, Michel Boko, Osvaldo Canziani, Timothy Carter, Gino Casassa, Ulisses Confalonieri, Rex Victor Cruz, Edmundo de Alba Alcaraz, William Easterling, Christopher Field, Andreas Fischlin, Blair Fitzharris, Carlos Gay García, Clair Hanson, Hideo Harasawa, Kevin Hennessy, Saleemul Huq, Roger Jones, Lucka Kajfež Bogataj, David Karoly, Richard Klein, Zbigniew Kundzewicz, Murari Lal, Rodel Lasco, Geoff Love, Xianfu Lu, Graciela Magrín, Luis José Mata, Roger McLean, Bettina Menne, Guy Midgley, Nobuo Mimura, Monirul Qader Mirza, José Moreno, Linda Mortsch, Isabelle Niang-Diop, Robert Nicholls, Béla Nováky, Leonard Nurse, Anthony Nyong, Michael Oppenheimer, Jean Palutikof, Martin Parry, Anand Patwardhan, Patricia Romero Lankao, Cynthia Rosenzweig, Stephen Schneider, Serguei Semenov, Joel Smith, John Stone, Jean-Pascal van Ypersele, David Vaughan, Coleen Vogel, Thomas Wilbanks, Poh Poh Wong, Shaohong Wu, Gary Yohe

Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change

Sea temperatures in many tropical regions have increased by almost 1 degrees C over the past 100 years, and are currently increasing at similar to 1-2 degrees C per century. Coral bleaching occurs when the thermal tolerance of corals and their photosynthetic symbionts (zooxanthellae) is exceeded. Mass coral bleaching has occurred in association with episodes of elevated sea temperatures over the past 20 years and involves the loss of the zooxanthellae following chronic photoinhibition. Mass bleaching has resulted in significant losses of live coral in many parts of the world. This paper considers the biochemical, physiological and ecological perspectives of coral bleaching. It also uses the outputs of four runs from three models of global climate change which simulate changes in sea temperature and hence how the frequency and intensity of bleaching events will change over the next 100 years. The results suggest that the thermal tolerances of reef-building corals are likely to be exceeded every year within the next few decades. Events as severe as the 1998 event, the worst on record, are likely to become commonplace within 20 years. Most information suggests that the capacity for acclimation by corals has already been exceeded, and that adaptation will be too slow to avert a decline in the quality of the world's reefs. The rapidity of the changes that are predicted indicates a major problem for tropical marine ecosystems and suggests that unrestrained warming cannot occur without the loss and degradation of coral reefs on a global scale.

/pdf/climate-change-coral-bleaching-and-the-future-of-the-world-s-1z4g0rwyaf.pdf

Climate change, coral bleaching and the future of the world's coral reefs

The effects of observed and future changes in climate are spatially and socially differentiated. The impacts of future changes will be felt particularly by resource-dependent communities through a multitude of primary and secondary effects cascading through natural and social systems. Given that the world is increasingly faced with risks of climate change that are at the boundaries of human experience3, there is an urgent need to learn from past and present adaptation strategies to understand both the processes by which adaptation takes place and the limitations of the various agents of change – states, markets, and civil society – in these processes. Societies have inherent capacities to adapt to climate change. In this article, I argue that these capacities are bound up in their ability to act collectively.

/pdf/social-capital-collective-action-and-adaptation-to-climate-5enumecpma.pdf

Social Capital, Collective Action, and Adaptation to Climate Change

It has been over 10 years since the phenomenon of extensive coral bleaching was first described. In most cases bleaching has been attributed to elevated temperature, but other instances involving high solar irradiance, and sometimes disease, have also been documented. It is timely, in view of our concern about worldwide reef condition, to review knowledge of physical and biological factors involved in bleaching, the mechanisms of zooxanthellae and pigment loss, and the ecological consequences for coral communities. Here we evaluate recently acquired data on temperature and irradiance-induced bleaching, including long-term data sets which suggest that repeated bleaching events may be the consequence of a steadily rising background sea temperature that will in the future expose corals to an increasingly hostile environment. Cellular mechanisms of bleaching involve a variety of processes that include the degeneration of zooxanthellae in situ, release of zooxanthellae from mesenterial filaments and release of algae within host cells which become detached from the endoderm. Photo-protective defences (particularly carotenoid pigments) in zooxanthellae are likely to play an important role in limiting the bleaching response which is probably elicited by a combination of elevated temperature and irradiance in the field. The ability of corals to respond adaptively to recurrent bleaching episodes is not known, but preliminary evidence suggests that phenotypic responses of both corals and zooxanthellae may be significant.

/pdf/coral-bleaching-causes-and-consequences-5ewp5zv1gs.pdf

Coral bleaching: causes and consequences

Sea surface temperatures were warmer throughout 1998 at Sesoko Island, Japan, than in the 10 preceding years. Temperatures peaked at 2.8 °C above average, resulting in extensive coral bleaching and subsequent coral mortality. Using random quadrat surveys, we quantitatively documented the coral community structure one year before and one year after the bleaching event. The 1998 bleaching event reduced coral species richness by 61% and reduced coral cover by 85%. Colony morphology affected bleaching vulnerability and subsequent coral mortality. Finely branched corals were most susceptible, while massive and encrusting colonies survived. Most heavily impacted were the branched Acropora and pocilloporid corals, some of which showed local extinction. We suggest two hypotheses whose synergistic effect may partially explain observed mortality patterns (i.e. preferential survival of thick-tissued species, and shape-dependent differences in colony mass-transfer efficiency). A community-structural shift occurred on Okinawan reefs, resulting in an increase in the relative abundance of massive and encrusting coral species.

Coral bleaching: the winners and the losers

'It should be clear that the upper temperature limit for life cannot be accurately defined' (Schmidt-Nielsen 1996). The thermal physiology of zooxanthellate reef corals is reviewed in this paper in the context of organismal and biochemical responses occurring during coral bleaching, with emphasis on methods of detection and interpretation of animal and algal symbiont stress. Coral bleaching, as presently defined in the literature, is a highly subjective term used to describe a variety of conditions pertaining to low symbiont densities in the coral–algal complex, including response to thermal stress. Three general types of high-temperature bleaching are defined: physiological bleaching, which may or may not include higher-than-normal temperature responses; algal-stress bleaching, involving dysfunction of symbiotic algae at high light and/or high temperatures; and animal-stress bleaching, where coral cells containing symbiotic algae are shed from the gastrodermal layer of cells. Since none of these methods of bleaching is mutually exclusive, a combination of intrusive and non-intrusive techniques is necessary to determine which mechanisms of symbiont loss are occurring. While quantification of symbiont densities, algal pigments, and coral tissue biomass provide unambiguous evidence of bleaching severity, measurements of physiological and biochemical degradation offer additional correlative evidence of temperature stress. Pulse-amplitude-modulated (PAM) fluorometry has emerged as an easy and relatively inexpensive non-invasive technique for monitoring symbiotic algal function both in situ and in the laboratory, when proper assumptions and interpretations are made. The roles of global warming, water quality, acclimation/adaptation processes, and relation to coral disease and reef heterogeneity are also discussed. A thorough understanding of the organismal responses occurring during bleaching will help explain changes in coral populations and in the coral reef community, and perhaps assist in predicting the future of reef corals and coral reefs during the next century of global climate change.

Coral bleaching: interpretation of thermal tolerance limits and thermal thresholds in tropical corals

Individual zooxanthellate corals vary in their susceptibility to bleaching, caused by the loss of symbiotic algae in response to increased temperatures and solar radiation. In 1995 at Phuket, Thailand, the west face of colonies of the massive coral Goniastrea aspera displayed bleaching in January–March, mediated by high solar radiation. By May, when sea temperatures were anomalously high but solar radiation much reduced, the east sides preferentially bleached. Cores from the east faces of corals exposed to elevated temperature under low irradiance in a laboratory experiment also bleached in response to elevated temperature. The algae in G. aspera were Symbiodinium sp., and the rDNA (both ssu and ITS regions) sequences did not differ between the east and west faces, indicating that the bleaching responses cannot be attributed to genetic factors in the symbiotic algae of the type described in other coral species. It is concluded that, in this system, bleaching is shaped by experience, and that the algae may acquire cross-protection against bleaching from solar to temperature stresses. The impact of experience on bleaching susceptibility may become increasingly important in shaping coral responses to temperature and solar radiation stresses in the coming decades.

Experience shapes the susceptibility of a reef coral to bleaching

The shallow-water reef coral Goniastrea aspera Verill 1865 has previously been reported to demonstrate differences in within-colony susceptibility to bleaching at elevated sea tem- peratures; parts of the colony which are exposed to the highest solar radiation are more thermotoler- ant than areas which are less exposed. In this paper, we show that at elevated experimental sea tem- perature the 'high light' surfaces lose fewer symbiotic algae, have lower levels of oxidative stress, higher levels of host antioxidant-enzyme copper zinc superoxidase dismutase (CuZnSOD), and host heat-shock proteins 60 and 70, compared to the less exposed surfaces. In addition, 'high light' sur- faces show less chronic photoinhibition and greater Photosystem II (PS II) recovery potential when exposed to high irradiance at ambient sea temperature. In contrast, no differences were noted in algal defences (e.g. antioxidant enzymes and stress protein production, and xanthophyll cycling) either at elevated or ambient temperatures. These results are noteworthy because they suggest that corals which acclimatise to high irradiance can, as a result, develop increased thermotolerance which may prevent bleaching at high sea temperatures. Importantly, they also demonstrate the significance of the host tissues in maintaining the intact symbiosis of G. aspera under thermal stress.

/pdf/exploring-the-basis-of-thermotolerance-in-the-reef-coral-1s1q1weag0.pdf

Exploring the basis of thermotolerance in the reef coral Goniastrea aspera

Diurnal patterns of photoinhibition have been identified in seven species of shallow water reef corals from the Andaman Sea, off the west coast of Thailand, using pulse amplitude fluorometry. Photochemical efficiency (Fv/Fm) and quantum yield (ΔF/Fm∑) of symbiotic dinoflagellates within the corals declined after dawn to reach a minimum between midday and early afternoon, recovering to former dawn levels by early evening. Parallel studies on the xanthophylls diadinoxanthin (Dn) and diatoxanthin (Dt), and their inter-conversion, also revealed a strong diurnal pattern as well as inverse correlations between the xanthophyll ratio Dt/(Dn+Dt) and Fv/Fm and ΔF/Fm′. These findings suggest a photoprotective function for these pigments.

Diurnal changes in photochemical efficiency and xanthophyll concentrations in shallow water reef corals : evidence for photoinhibition and photoprotection

Coral reefs are under threat from the effects of bleaching, in which symbiotic algae or their photosynthetic pigments are destroyed by increased sea temperatures and solar radiation1,2. Here we show that the bleaching susceptibility of Goniastrea aspera, a shallow-water Indo-Pacific coral, can be predicted from its history of exposure to solar radiation, demonstrating how experience can shape coral bleaching patterns.

R. P. Dunne

Papers

Coral bleaching: interpretation of thermal tolerance limits and thermal thresholds in tropical corals

Experience shapes the susceptibility of a reef coral to bleaching

Exploring the basis of thermotolerance in the reef coral Goniastrea aspera

Diurnal changes in photochemical efficiency and xanthophyll concentrations in shallow water reef corals : evidence for photoinhibition and photoprotection

Bleaching patterns in reef corals.