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You are at - Home | Research | Climate change and impact ________________________________________________________________________
Tropical marine environments such as coral reefs and mangrove forests around the world are under unprecedented pressure due to climate change, changes in water quality from terrestrial runoff and overexploitation. Coral reefs are iconic tropical ecosystems represented by Australia’s irreplaceable Great Barrier Reef (GBR) and the less explored reefs off Western Australia. Corals thrive in locations which also happen to be near their physiological limits, making them sensitive to stresses caused by rising sea surface temperature and an increase in ocean acidity linked to rising carbon dioxide (CO2) in the atmosphere. Coral reefs and mangrove forests contribute greatly to tropical coastal productivity and provide habitat for myriad fish and other species. AIMS contributes to understanding the implications of a changing climate by monitoring and modelling ocean climate changes, assessing impacts of climate change on coral reef and other organisms, identifying potential adaptation mechanisms and identifying characteristics and locations that may provide refuge for marine species in a rapidly changing world. What we know
The long-term average temperature for the waters of the Great Barrier Reef has increased by about 0.4oC since the 19th century and the Reef system has experienced two mass coral bleaching events (1998 and 2002) caused by long periods of coral exposure to unusually warm seawater. During the 1998 coral bleaching event, 42 per cent of shallow water coral reefs on the GBR bleached and an estimated 2 per cent died that year. This equates to approximately 400km2 of reef area. In 2002, the largest bleaching event on record, an even greater proportion of the Reef bleached (55 per cent) and an estimated 5 per cent died. This equates to approximately 1000km2 of reef area While these percentages may seem small, they can be localised and severe events. Some local extinctions of coral species in several parts of the Great Barrier Reef have been observed and appear to be linked to higher sea surface temperatures causing coral bleaching. Coral bleaching was again observed in the 2006 summer, particularly in the southern GBR, where local water temperatures reached around 1-2oC above the seasonal average. Coral reefs may take 10 to 20 years to recover from serious bleaching events that cause coral death. The amount of CO2 in the atmosphere is known to be increasing and the extra CO2 does not stay just in the atmosphere with a significant amount dissolving into the ocean. The pathways it then follow under different conditions and the consequences of its accumulation in different environments is under-researched. Some scientists have proposed that the large portion of CO2 that is entering the ocean from the atmosphere is causing a shift downwards in seawater pH, making it more acidic. A growing body of experimental evidence is showing that seawater acidified to mimic potential future scenarios significantly impacts upon the health of some fish and coral species. There are many millions of species in the ocean and each will have different sensitivities to acidification and respond in different ways. No single species lives in isolation and how the effects seen at an individual species level translate to an ecosystem response is not understood. It has been speculated that acidified seawater may alter the makeup of marine ecosystems and weaken coral reef structures. It is known that heat stress causes corals to expel the symbiotic algae they host in their tissues. What is not sufficiently understood are the numerous mechanisms that may enable corals to adapt to new, warmer and potentially acidic conditions. Coral reef ecosystems are dependent upon microbial interactions with each other and with their hosts, such as the central relationship between coral and its symbiotic algae and bacteria. Forecast climate change scenarios depend to a large extent on the action of microbes, since microbes are centrally involved in the physical processes concerned with phenomena such as the carbon cycle, where they convert CO2 into nutrients. Based on observations of an increase in hurricane and cyclone events in recent decades, even more severe storms and cyclones have been proposed to occur as our climate changes, though this remains a topic of debate. The El Niño-Southern Oscillation (ENSO) that influences climate across half the planet and has a major influence on Australian weather is a significant source of variability. It is recognised that the data from the tropics are less robust than from temperate and polar regions, which may increase any uncertainty for global models of warming. Potential changes to ocean currents and circulation have been predicted and these changes may happen at the macro-scale such as with the East Australian Current and Indonesian Through-Flow or at smaller scales. This may affect reef connectivity, but precise forecasts are not available yet. More extreme rainfall and river flow have been predicted and may lead to greater flows of freshwater and terrestrial runoff into reef systems, but exact data are not available yet. Equally, evidence has been put forward that droughts are becoming more extreme when they occur. This may have the secondary effect where loss of vegetation due to the drought may lead to increased terrestrial run-off when droughts break. Severe droughts may also increase the size and density of dust storms which can inject significant amounts of material into oceans. There is some evidence of sea level rise, possibly leading to coastal erosion and storm surges, but the data are not yet robust enough to confidently predict likely future scenarios. Ecosystems operate in a complicated environment and while there is a growing body of data about how marine organisms respond to increased seawater temperature, and increased acidity, far less is known about how these organisms will function and respond when both of these parameters change. Nature is variable and the rapidity of these changes will be important for how able marine organisms and ecosystems can successfully respond and adapt. In many cases, certainty in climate predictions will only increase with the passage of time and the accumulation of more data. AIMS carries out the research that underpins current understandings of how climate change issues such as global warming and an increase in the acidity of the ocean may affect coral reefs. AIMS bases its assessment of current conditions and likely future scenarios on well-researched and verifiable data gained in the field, from a range of laboratory-based experiments, from atmospheric sampling at its Cape Ferguson site and from its unrivalled collection of coral cores dating back hundreds of years. The consequences of increasing atmospheric carbon dioxide The amount of CO2 in the atmosphere has risen to the current level of 383 parts per million (ppm) from about 200 ppm in the days before the Industrial Revolution more than 200 years ago. Measurements of atmospheric CO2 taken from AIMS headquarters outside Townsville show broad agreement with this global figure (see page 1 of this document). Under current IPCC projections and assuming no measures are adopted to reduce CO2 emissions, atmospheric CO2 concentrations are likely to reach 500 ppm in the second half of this century. If that is the case, global temperature averages may increase a further 2oC and possibly more. Coral reefs provide ecosystem services essential to our national identity and wealth. The GBR contributes more than $5 billion annually to the Australian economy. Ningaloo Reef in WA is of growing economic and environmental importance, as are the Kimberley Coast and Oceanic Shoals off WA’s northwest. While Australia’s coral reefs are well managed, they are not isolated from global atmospheric and ocean changes. Coral bleaching Coral bleaching itself is not a new phenomenon – the coral animal is sensitive to a variety of stresses and bleaching is a natural response. What is newly observed is coral bleaching in a co-ordinated manner on a massive scale. Since the first recorded global mass coral bleaching episode in 1998, which was caused by a level of thermal stress many reefs had not previously experienced, the phenomenon has been under intense scientific scrutiny. The stress induced by higher temperatures causes a breakdown in the fundamental relationship that is at the heart of all coral reefs: the mutually beneficial relationship between the coral animal and its symbiotic algae, the zooxanthellae. The fragile equilibrium of this relationship operates within a finely-tuned sea surface temperature range. As sea surface temperatures rise, conditions needed to sustain the symbiotic relationship essential for survival are being compromised. Sea surface temperatures have been increasing over the past century and climate models predict a continuation of this trend. The implications for coral reefs in the long run remain unknown and in need of considerably more research and monitoring. It seems likely, though again unproven, that reefs will continue to exist, but we don’t know in what form. We can speculate that they may suffer a loss of diversity and changes in community compositions. We are likely to see shifts in species to different parts of reefs, depending on their adaptation or otherwise to the changing conditions. Ocean acidification Ocean acidification is a predicted consequence of increasing atmospheric CO2, in which large quantities of carbon dioxide from the atmosphere dissolve in the oceans, causing their alkaline/acid balance (their "pH") to shift towards acidic. The coral animal is a calcifying organism – it takes dissolved material from the surrounding sea water and turns it into a calcium carbonate skeleton. Corals are not alone in doing this – many marine plants and animals convert nutrients into calcareous and other inorganic skeletons or shells. There is a simple chemical equation governing the ability of all marine calcifying organisms to undertake this process, linked to the pH of seawater. Some marine scientists have hypothesised that this simple chemistry is being thrown out of kilter by the rise in atmospheric CO2 and its subsequent absorption by the oceans. The oceans have been efficiently absorbing a large proportion of that extra carbon – in fact, if they hadn’t done so we may have experienced more global warming than we have already. Oceans are naturally slightly alkaline and this is conducive to the calcifying process. As seawater heads down the pH scale to become slightly more acidic, a problem with coral growth (calcification) is predicted to arise. If projections are correct that pH could decrease by up to 0.4 pH units by the end of this century, this would be well outside the realms of anything organisms have experienced over hundreds of thousands of years. While 0.4 pH units might not seem like a lot, because pH is measured on a negative log scale this equates to a 2.5-fold increase in the concentration of hydrogen ions. Scientists are still uncertain about what this will mean for coral reefs and how resilient they may be to these changes if they come about. Nor is there any certainty about the effects on the multitude of other organisms that live in and on reefs and other tropical marine environments. Many processes depend upon the pH of the surrounding environment being within a certain range for them to function properly. These may include deposition of marine cements to enable benthic organisms to attach to surfaces, or fertilisation of eggs by sperm which may affect the number of larvae produced by organisms that release their gametes into the ocean. Absorption of CO2 by seawater may not only result in changes to seawater pH, it also changes the composition of the dissolved gases available for animals to breathe through their gills. Absorbing more CO2 into their bodies may have an effect on their health and behaviour and may unnaturally alter the pH of their body fluids. Possible reef responses to climate change Adaptation may be one way that corals cope with higher temperatures and more acidic oceans. If the current rate of climate change continues, corals will have to adapt within the next few decades rather than over millennia. Whether corals can adapt quickly enough to cope with climate change is a subject of much current research. Are corals able to pass genes on to their offspring to enable them to tolerate higher temperatures in a short time span? With conditions changing rapidly, their capacity to do this within only a few generations may be crucial. AIMS is examining whether and how the community of the algae within corals might "shuffle" to be dominated by more heat-tolerant strains. This phenomenon was first observed by AIMS scientists in corals from the Keppel Islands group after the 2006 bleaching event and is being extensively investigated at present. Corals in the waters around the Keppel Islands now have a much higher proportion of two more thermally tolerant strains of symbiotic algae than they did before the bleaching event, and are better able to cope with higher sea surface temperatures. However, this is just one species at one location and it is not known yet whether this can be extrapolated to the rest of the Great Barrier Reef or to other reef systems. The combined effects of seawater warming and acidification are only now starting to be studied. Many studies can be done at naturally occurring sites that exhibit conditions akin to those forecast for future oceans. This includes organisms that live naturally in warm and acidic environments such as those found around undersea volcanic CO2 vents. Controlled experiments in laboratories, however, are technically challenging. The range of marine species that can be cultured in the laboratory is a tiny fraction of the number of species that live in the ocean. Once these creatures are cultured and in such a way that reflects their natural state, we need to be able to manipulate, control and measure the acute and chronic changes that may be wrought by the changed seawater conditions.
AIMS actions on climate change While supporting moves to reduce CO2 emissions globally, and making significant cuts to our own energy consumption, AIMS is investing significant public resources in research to understand the significance of climate change for tropical marine ecosystems including the Great Barrier Reef. Since 1993, AIMS has monitored the status and trends of fish and coral populations on 47 reefs representative of the whole GBR, providing the longest continuous record of change in reef communities over such a large scale. As part of this, our scientists monitored the 1998 and 2002 mass coral bleachings. In 2006, a more severe regional disturbance caused bleaching of over 80 per cent of corals at the Keppel Islands in the southern GBR, resulting in 40 per cent coral mortality. In Western Australia, AIMS has also monitored since 1993 the status and trends of fish and coral populations on Scott Reef (just over 400km northwest of Broome in the Indian Ocean). In 1998, when mass bleaching was observed on 70 per cent of reef systems worldwide, severe bleaching conditions killed more than 80 per cent of the corals on Scott Reef. Ongoing monitoring shows that coral and fish communities are still recovering from this major disturbance. AIMS monitoring of the physical ocean environment through satellite observations and moored oceanographic instruments has shown that most coral bleaching is caused by a combination of high light and heat stress (i.e. seawater that is warmer than the average seasonal cycle). Rigorous aquarium and laboratory testing by AIMS scientists has defined an index of bleaching risk based on cumulative heat stress (degree-heating days), making possible the forecast of major bleaching events using ocean monitoring programs including satellite measurements of sea surface temperature. About a quarter of AIMS’ research effort is now directed at marine microbes and symbiosis using advanced genetic and molecular approaches. This reflects the importance of microbial life to all major biogeochemical cycles and disease. The symbioses of sponges, giant clams and corals all involve specialised microbes; and all of these symbioses are disrupted by higher temperatures. More detailed studies of coral symbiosis have shown that one species of coral can host multiple genetic strains of the photosynthetic zooxanthellae symbionts and that different combinations have highly varied heat tolerance. More heat tolerant combinations trade off better survival in warmer water with slower colony growth under average conditions, raising the possibility that reef-building corals can persist in a warming ocean through selection for more heat-tolerant symbioses. Answers to this crucial question are being pursued through a combination of controlled experiments in aquaria and reciprocal transplant experiments in natural populations to assess the viability of different symbioses in different thermal environments. Theoretical models of coral bleaching have been developed that implicate water quality as an extra factor modifying bleaching risk. The models predict that higher levels of dissolved nitrogen in the environment will raise the risk of bleaching for any given combination of light and temperature. This important idea requires testing through controlled experiments and, if supported, will provide a strong rationale for the current attempts to halt and reverse the decline of water quality in coastal seas caused by terrestrial runoff from agricultural landscapes. At the ocean boundary, some locations experience the upwelling of deep water to the surface. These intrusive waters have the potential to both raise and lower the risk of coral bleaching because they contain higher levels of key dissolved nutrients but are also colder coming from deeper levels. The trade off between these influences seems likely to explain some of the spatial variations in bleaching among offshore reefs during seasons of heightened risk. To incorporate such variations, AIMS and CSIRO are building the most accurate three-dimensional model of water circulation yet developed for the continental shelf of Queensland. Once validated, this model will be used to forecast regional risk for different parts of the GBR under different climate scenarios downscaled from Global Climate Models. AIMS scientists have recently detected a decline in calcification rates by massive corals during the past two decades that is a unique variation in the past 100 years. This effect is consistent with that expected in an acidifying ocean but controlled experiments are required to confirm "cause and effect". AIMS’ ability to conduct the required experiments will be boosted by new resources approved in the 2009-10 Commonwealth Budget. Recognising the importance of climate change in the marine environment, the Australian Government has provided $25 million for the construction of an advanced experimental seawater facility, the Australian Tropical Oceans Simulator, at AIMS’ headquarters in Townsville. This large facility will provide controlled environment spaces for multiple investigators and will be available to external scientists by application. The ability to manipulate multiple factors (temperature, salinity, nutrients and pH) independently in complex experimental designs will allow scientists to test the responses of marine organisms to the range of hypothetical environments anticipated from different degrees of climate change and to predict the future for marine systems under various climate scenarios. AIMS is also increasing its investment in monitoring natural marine systems across tropical northern Australia. This has been made possible by new investments by the Commonwealth and Queensland Governments in integrated marine observing systems (http://imos.org.au/). This will include the first direct monitoring of carbonate saturation levels in complex reef systems to understand how water chemistry cycles affect reef construction rates by calcareous organisms. Among the new observing instruments, the most innovative are the wireless sensor networks covering coral reefs adjacent to island research stations in the southern, central and northern GBR. These research stations are magnets for domestic and foreign investigators and will be key platforms for observing the responses of whole systems to climate change. The wireless sensor networks will allow predictions emerging from the small-scale experiments in the Oceans Simulator about biogeochemical processes and physiological responses to be tested in real ecosystems spanning two-thirds of the GBR. All of this new knowledge and process understanding will inform a new generation of spatial and process models predicting areas of relative risks and vulnerabilities from climate change that will in turn inform decision making by natural resource managers, policy formulation and the exploration of potential mitigation strategies. Further reading
December 19, 2009
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