Original Article link:
http://www.aims.gov.au/pages/about/communications/issues/coral-reefs-and-climate-change-2007.html

Coral reefs and climate change 2007
(AIMS briefing/position paper)

February 2007

The Great Barrier Reef

"A Reef such a one as I now speak of is a thing scarcely known in Europe or indeed any where but in these seas: it is a wall of Coral rock rising almost perpendicularly out of the unfathomable ocean."
Joseph Banks - August,1770

"We had wheat sheaves, mushrooms, stags horns, cabbage leaves, and a variety of other forms, glowing under water with vivid tints of every shade betwixt green, purple, brown, and white; equalling in beauty and excelling in grandeur the most favourite parterre of the curious florist."
Matthew Flinders - October, 1802

"In among the branches of the corals, like birds among trees, floated many beautiful fish, radiant with metallic greens or crimsons, or fantastically banded with black and yellow stripes."
J. Beete Jukes 1842-46

The scale, biodiversity and beauty of Australia’s Great Barrier Reef (GBR) continue to astound us hundreds of years after these first written observations. Extending for over 2,000 km along the northeast coast of Australia, the GBR covers 35 million hectares – an area larger than England. Listed as a World Heritage Area (WHA) in 1981, in recognition of its outstanding natural values, the GBRWHA is the largest marine protected area in the world. The GBR is the best-managed and protected coral reef ecosystem in the world. The impact of climate change on coral reefs is receiving ongoing national and international media attention. Consequences for Australia’s coral reefs could be severe despite our significant efforts in protection and management, if key issues are not addressed urgently. The Australian Institute of Marine Science (AIMS) maintains the following position on climate change and coral reefs:

What is known

Global climate is changing rapidly due to human activities and will result in continued rising temperatures both on land and in the sea.
Climate change due to the enhanced greenhouse effect has significant consequences for coral reefs. There is a direct link between unusually warm seawater temperature and bleaching of reef-building corals around the world.
Changing ocean chemistry due to rising CO2 may also have serious implications for coral reefs and other marine calcifying organisms and is likely to alter the makeup of marine ecosystems and weaken coral reef structures.
Increased mass bleaching events on the GBR and elsewhere since the mid-1970s are linked to global warming.
Well-protected and well-managed reefs are more resilient to stresses but are not protected from the global-scale effects of rising water temperatures and changing ocean chemistry.
The Great Barrier Reef (GBR) has warmed ~0.4oC since the 19th century (global warming ~0.7oC) and has experienced 2 major coral bleaching events (1998 and 2002).
During the 1998 coral bleaching event 42% of shallow water corals reefs on the GBR bleached and an estimated 2% died that year.
In 2002, the largest event on record, an even greater proportion of the Reef bleached (55%) and an estimated 5% died.
Coral bleaching was again observed in the 2006 summer, particularly in the southern GBR, where local water temperatures reached ~1-2oC above the seasonal average.
Healthy reefs (more ecologically intact and less exploited) recover better from bleaching than highly stressed reefs.
AIMS research is monitoring & modelling ocean climate changes, assessing impacts of climate change for coral reef organisms, identifying potential adaptation mechanisms, and identifying characteristics and locations which may provide refuge for marine species in a rapidly changing world.
The Consequences

The pace of warming is of major concern as it gives organisms little time to respond or adapt to the changed climate conditions. The GBR could be 1-3oC warmer by the end of this century and, as it warms, conditions conducive to bleaching could occur annually within ~20-30 years.
There is a limit to what can be done locally to protect natural ecosystems such as the GBR. Global leadership is required to commit leading world economies to drastic reduction in greenhouse gas emissions. There is only a small window of time for action before changes are irreversible.
Suggestions that rising sea levels and increasing temperatures will be good for coral reefs and even allow the GBR to expand southwards are unlikely as there is a lack of suitable substrate for coral reefs south of the current GBR and also the rapidity with which such changes would have to occur – tens of years compared with the 100s-1000s of years required for intact ecosystems to migrate.
The impacts of bleaching on coral reefs are expected to affect large numbers of other reef organisms given that coral provides the habitat and food for tens of thousands of other organisms.
The Science in Detail

Human Induced Climate Change Will Alter Life on Coral Reefs

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 and 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 during the summer stress the coral and their symbiotic algae (the essential partner for reef-building corals) which are expelled when the coral is stressed. The host coral may die, partially die, or recover, though coral growth and reproduction can be affected in surviving corals. Approximately 16% of the world’s reefs were seriously damaged during the 1998 bleaching event – probably the warmest year experienced by modern corals. Based on the recovery of some affected reefs, it is clear that healthy (more resilient) coral reefs recover better than reefs degraded by other human pressures.

The Great Barrier Reef (GBR) is the best managed and protected reef in the world (because of Great Barrier Reef Marine Park Authority’s Representative Areas Program, zoning and permitting systems, the July 2004 declaration of 33% No-Take Areas and the Reef Water Quality Protection Plan), yet major bleaching occurred in 1998 and 2002 as a consequence of the relatively modest warming of GBR waters (~0.4oC) 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 adapt to climate change by altering their symbiotic algae to more thermally tolerant partners, though this may be at the expense of growth rates. This capability 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; global ocean pH has already dropped by 0.1 and could be 0.4-0.5 lower by the end of this century) thus altering 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 ocean depths at which dissolution of calcium carbonate skeletons of different mineralogies occurs. Modelling and experimental studies (e.g. Biosphere 2 mesocosm) have demonstrated that increased CO2 reduces coral calcification rates (Kelypas et al., 2006).

Calcification rate also depends on water temperature. AIMS has provided evidence (Lough & Barnes 2000) that several long-lived massive Porites corals 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 changes in ocean chemistry and that coral calcification will increase with global warming. Kleypas et al. (2005) refuted these controversial findings and concluded that they were "based on assumptions that exclude important factors and therefore need to be viewed with caution." These studies focused, however, 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 the point at which temperature thresholds for coral bleaching are regularly exceeded. The general scientific consensus is that changing ocean chemistry due to rising CO2 has serious implications for coral reefs and other calcifying marine organisms of the open ocean. These changes could well alter the makeup of marine ecosystems, alter food webs and weaken coral reef structures. Clearly, there is much more we need to learn about the effects of rising CO2 and marine calcification. The importance of this problem and its impacts on marine ecosystems is recognized in a recent report of the British Royal Society (2005) and the outcomes from an international workshop held in Florida in 2005 (Kleypas et al., 2006; Janice Lough from AIMS was an invited participant in the workshop and a contributing author).

Other impacts of climate change on coral reefs and associated coastal ecosystems will result from changes in air temperatures (2005 was the warmest year on record in Australia), rainfall and river flow, the occurrence and intensity of tropical cyclones, ocean circulation patterns and sea-level rise. Taken together, such climate change impacts threaten the biodiversity of marine ecosystems. In June 2005, the Department of Environment and Heritage supported a workshop National Biodiversity and Climate Change Action Plan, Research and Information Gaps Workshop: Synthesis and Summaries for Four Key Objectives (Janice Lough, AIMS was a co-facilitator with Jo Johnson, GBRMPA of the Climate change and marine, estuarine and coastal ecosystems theme). The GBRMPA is co-ordinating preparation of a book GBR Ecological Vulnerability Assessment which will consider climate change impacts on all aspects of the GBR, not just coral reefs (Several AIMS scientists are chapter lead authors: Janice Lough – climate change scenarios; David Mckinnon – plankton; Nicole Webster- microorganisms, Katharina Fabricius – reefs; and other AIMS staff will be contributing authors).

The Scientific Advice

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. The impacts of bleaching on coral reefs are expected to affect large numbers of other reef organisms, given that coral provides the habitat and food for tens of thousands of other organisms. There is a clear scientific consensus that reducing and reversing local human pressures on coral reefs has to be accompanied by drastic reductions in greenhouse gas emissions to limit the amount of global warming if coral reefs are to survive. Furthermore, there is an urgent need for improved monitoring of the GBR as well as more research into the impacts and response of coral reefs to climate change and climate variability. The impacts of climate change and climate variability is a high priority research area for AIMS.

The Future

Even with rapid global implementation of strategies to stabilize and reduce greenhouse gas concentrations, we are committed to significant rapid climate change and possibly accelerated sea level rise. The urgent scientific challenge is to understand how these rapid environmental changes will affect tropical marine ecosystems such as the GBR and, in particular, how reef communities will respond and/or adapt to the changing physical environment. Climate change and global warming pose significant challenges (in a number of ways) to the GBR – a vast and beautiful ecosystem that we do not, and will probably never, fully understand.

Current understanding suggests that the GBR will not disappear but its appearance and community structure will change from the coral-dominated reef described years ago by Banks, Flinders and Jukes and that we know today. If temperatures rise to a level that is unsustainable for corals, the limestone base structure of the reef will persist. Given the massive size of the GBR, at least a few corals are likely to survive in sheltered locations. Under such a worst-case scenario, however, the ecological goods and services provided by the GBR (including commercial values associated with tourism and fisheries) will dramatically alter as coral communities dwindle and reefs of the GBR shift from being dominated by corals to reefs dominated by algae and filter feeders.

AIMS Research

Ongoing scientific research at AIMS directly addresses key issues associated with the regional impacts of global warming and climate variability. Scientists from AIMS are approaching the issue of climate change using technologies ranging from genetic analysis to monitoring of whole ecosystems. AIMS scientists are:

monitoring detailed changes in weather, climate and circulation on the GBR.
looking back into the past using centuries-old coral cores to detect recent environmental trends and track the growth responses of corals to changing environments.
studying the potential for reef corals to adapt to climate change by focusing on the key relationship between corals and the single-celled algae living within their tissues. Prior research suggests that this relationship is critical in predicting a coral’s ability to withstand varying environmental conditions.
leading the implementation of the GBR Ocean Observing System – a integrated state of the art observing system for the whole of the Great Barrier Reef.
Additional Reading

Kleypas, JA, RW Buddemeier, M Eakin, JP Gattuso, J Guinotte, O Hoegh-Guldberg, R Iglesias-Preito, PL Jokiel, C Langdon, W Skirving & AE Strong (2005). Comment on "Coral reef calcification and climate change: the effect of ocean warming". Geophys Res Lett. 32, L08601

Kleypas JA, RA Feely, VJ Fabry, C Langdon, CL Sabine, & LL Robbins (eds) (2006). Impacts of Increasing Ocean Acidification on Coral Reefs and other Marine Calcifiers. Report from 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, JM & DJ Barnes (2000). Environmental controls on growth of the massive coral Porites. J Exp Mar Biol Ecol 245: 225-243.

Lough J, R Berkelmans, M van Oppen, S Wooldridge & C Steinberg (2006) The Great Barrier Reef and Climate Change. Bulletin Australian Meteorological & Oceanographic Society 19: 53.58.

McNeil BI, RJ Matear & DJ Barnes (2004) Coral reef calcification and climate change: the effect of ocean warming. Geophys Res Lett 31, L22309

The Royal Society (2005). Ocean Acidification due to Increasing Atmospheric Carbon Dioxide. Policy Document 12/05, London UK, (www.royalsoc.ac.uk), 60pp

Wilkinson, C (2004). Status of Coral Reefs of the World: 2004. GCRMN, ICRI, AIMS