Issues Coral reefs and climate change February 24, 2005 The health of coral reefs in many parts of the world is declining due to a variety of direct, local human pressures (such as overfishing, land-based activities affecting water quality; see Wilkinson 2004). Coral reefs are now subject to an additional global-scale threat to their long-term wellbeing due to the enhanced Greenhouse effect. The two most important consequences of the enhanced Greenhouse effect for coral reefs are warming of the oceans and changes in ocean chemistry. Rising sea temperatures increase the frequency of mass coral bleaching events. Corals live only 1-2oC below their upper thermal limit and sustained periods of water temperatures above this threshold stresses the coral and the symbiotic algae (the essential partner for reef-building corals) are expelled. The host coral may die, partially die or recover though coral growth and reproduction can be affected in surviving corals. 16% of the world’s reefs were seriously damaged during the 1998 bleaching event – probably the warmest year experienced by modern corals. Although some affected reefs have recovered it is clear that healthy (more resilient) coral reefs recover better than reefs degraded by other human pressures. The Great Barrier Reef (GBR) is probably the best managed and protected reef (because of GBRMPAs Representative Areas Program, zoning and permitting systems, the July 2004 declaration of 30% No-Take Areas and the Reef Water Quality Protection Plan) in the world yet major bleaching occurred in 1998 and 2002 as a consequence of the relatively modest warming of GBR waters (<0.5oC) since the end of the 19th century. Current projections suggest that average tropical ocean temperatures could warm 1-3oC by the end of this century. There is general scientific consensus that global warming and consequent coral bleaching are a significant threat to the maintenance of coral reef communities as they presently exist and that healthy coral reefs (more ecologically intact and less exploited) will be more resilient than those degraded by other human pressures. There is some evidence emerging that corals may be able to alter their symbiotic algae to more thermally tolerant partners though this may be at the expense of growth rates. This change may, however, only occur in a few species and not be sufficiently rapid to keep pace with temperature rises. Current research at AIMS focuses on these possible adaptive changes in corals and their effects on coral growth. Increasing atmospheric carbon dioxide (CO2, the principal greenhouse gas) is changing the chemistry of the oceans. About 30% of the CO2 released into the atmosphere by human activities since the Industrial Revolution has been absorbed by the oceans. This changes the chemistry of the oceans, which become more acidic (lower pH) and these change the concentrations of carbonate and bicarbonate ions. Many marine organisms (corals, calcareous algae, shells, benthic and planktonic organisms such as foramanifera and coccolithophores) use calcium and carbonate ions from seawater to secrete calcium carbonate skeletons. Changing the ocean chemistry essentially shifts the geochemical equation by which these organisms "calcify". The implication of continued change in ocean chemistry due to rising CO2 is that these organisms will not calcify as well as they did in pre-industrial times and thus produce weaker skeletons and grow more slowly. For coral reefs weaker structures would reduce their resilience to the natural forces of erosion and slower growth will set back the rate of recovery after bleaching and other disturbances. Also, changing ocean chemistry will alter the deep ocean depths at which dissolution of calcium carbonate skeletons of different mineralogies occurs. Modelling and experimental (eg Biosphere 2 mesocosm) studies have demonstrated that increased CO2 reduces coral calcification rates. A recent media report, based on respected coral reef researcher Jonathan Erez’s work in Israel, provides the first observational evidence from the field that lower pH is associated with reduced coral growth. Calcification rate also depends on water temperature and AIMS provided evidence (Lough & Barnes 2000) that several long-lived massive Porites on the GBR had increased their calcification rate towards the end of the 20th century (up to ~1980 when cores were collected) which matched the observed rise in GBR water temperatures (AIMS is currently examining more recent coral growth rates from short coral cores). This finding generated some controversy, as it did not match the model or experimental findings. The conclusion from this work was that, at least initially, some corals might respond more to rising water temperatures than to changes in ocean chemistry. More recently scientists from UNSW, CSIRO and AIMS (McNeil et al., 2004) published model results suggesting that the warming effect on coral calcification (in one coral species) outweighs the change in ocean chemistry and that coral calcification will increase with global warming. These controversial findings are currently being debated in the scientific literature. These studies focused on the most heat resistant type of coral and did not consider the overall effects on reef calcification rates of the widespread death of the majority of corals that are less heat resistant. How much ocean warming reefs can withstand will, however, be limited by when temperature thresholds for coral bleaching are regularly exceeded. The general scientific consensus is that changes in ocean chemistry due to rising CO2 has serious implications for coral reefs and other calcifying marine organisms of the open ocean and could well alter the makeup of marine ecosystems and weaken coral reef structures. There is clearly much more we need to learn about the effects of rising CO2 and marine calcification and the importance of this problem and its impacts on marine ecosystems is recognized by a planned international Workshop on the Impacts of Increasing Atmospheric CO2 on Coral Reefs and Other Marine Calcifiers, 18-20 April 2005, St Petersburg, Florida sponsored by NSF/NOAA/USGS (Lough from AIMS will be attending). Coral reefs of the world are under threat from both local and global-scale stresses. The enhanced Greenhouse effect (through bleaching and ocean chemistry changes) is likely to alter the community structure of reefs, including the world’s best-managed reefs of Australia. There is a clear scientific consensus (eg Wilkinson 2004) that reducing and reversing local human pressures on coral reefs has to be accompanied by reduction in greenhouse gas emissions if coral reefs are to survive.   References Erez, J (2005) As reported by BBC http://news.bbc.co.uk/2/hi/science/nature/4226917.stm McNeil BI, RJ Matear & DJ Barnes (2004) Coral reef calcification and climate change: the effect of ocean warming. Geophys Res Lett 31, L22309 Lough, JM & DJ Barnes (2000). Environmental controls on growth of the massive coral Porites. J Exp Mar Biol Ecol 245: 225-243. Wilkinson, C (2004). Status of Coral Reefs of the World: 2004. GCRMN, ICRI, AIMS