)
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
-
Alongi DM (2008) Mangrove forests: resilience, protection from
tsunamis, and responses to global climate change. Estuarine Coastal and Shelf
Science 76: 1-13.
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Berkelmans RWC, De'ath AG, Kininmonth SJ Skirving WJ (2004) A
comparison of the 1998 and 2002 coral bleaching events on the Great Barrier
Reef: spatial correlation patterns, and predictions. Coral Reefs 23: 74-83
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De'ath AG, Lough JM and Fabricius KE (2009) Declining coral
calcification on the Great Barrier Reef. Science 323: 116-119.
-
Fabricius KE (2008) Theme section on "Ocean Acidification and
Coral Reefs". Coral Reefs 27: 455-457.
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Veron JEN (2008) Mass extinctions and ocean acidification:
biological constraints on geological dilemmas. Coral Reefs 27: 459-472.
-
Jokiel PL, Rodgers KS, Kuffner IB, Andersson AJ, Cox EF,
Mackenzie FT (2008) Ocean acidification and calcifying reef organisms: a
mesocosm investigation. Coral Reefs 27: 473-483
-
Albright R, Mason B, Langdon C (2008) Effect of aragonite
saturation state on settlement and post-settlement growth of Porites
astreoides larvae. Coral Reefs 27: 485-490.
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Marubini F, Ferrier-Pagès C, Furla P, Allemand D (2008) Coral
calcification responds to seawater acidification: a working hypothesis towards
a physiological mechanism. Coral Reefs 27: 491-499.Lough JM (2008) A changing
climate for coral reefs. Journal of Environmental Monitoring 10: 21-29.
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Lough JM (2000) 1997-98: Unprecedented thermal stress to coral
reefs?. Geophysical Research Letters 27: 3901-3904