1. If present levels of greenhouse gas emissions are not reduced dramatically we will see further rises in:
Ocean temperatures
Over the last 50 years, mean global sea temperature has been increasing2
, alongside a dramatic increase in coral mortality and
reef degradation.3
The most recent 2007 IPCC assessment concluded that a 1.8-4.0°C increase in the average near-surface ocean
temperature is likely by the end of this century4
. Accelerating trends argue that the risk of exceeding these levels is increasing.
Bleaching events: In 1998 corals in over 60 nations experienced mass bleaching, ultimately resulting in the loss of 16% of the world’s reefs.6
In the last 10 years, the Great Barrier Reef has witnessed two major
bleaching events, damaging 50-60% in each event.2
With increases in sea surface temperature (SSTs) of 1-3°C, more frequent and severe coral bleaching episodes and widespread mortality are predicted.3,5
Most of
the Caribbean is expected to experience conditions that currently lead to coral bleaching every two years or more within the next 20 to 50 years, with temperatures increasing 1-2°C above the average warmest month
for periods of over a month.7,8,9
Increased disease: Elevated SSTs have been shown to accelerate the growth rate and disease activity of pathogens. Increased temperatures also reduce the host efficacy against disease10
.
Reduced coral biodiversity: Major changes in coral species diversity will occur with temperature-tolerant species being more viable than temperature sensitive species.11
Changes in aragonite saturation predicted as atmospheric CO2
concentration (ppm)
increases over shallow-water coral reef locations as pink dots. Before industrial revolution
(280 ppm) nearly all shallow-water coral reefs had aragonite saturation of > 3.25 (blue
regions in the figure).33
Sea levels
Coral reefs, especially those growing at their deepest limit, will be affected by rising water levels; reducing essential light.12
Major sea level rise scenarios
associated with loss of Greenland and WestAntarctic ice sheets may have great impacts on coral viability.As sea-levels rise and higher intensity tropical storms
batter already vulnerable coasts13
, increased terrestrial erosion will exacerbate coastal sedimentation. This will further damage the protective breakwater
normally offered by healthy reefs and result in a negative feedback loop.13
Frequency of extreme weather events
Extreme weather events including hurricanes and storm surges will increase in frequency and intensity. 2005 was the hottest recorded year in the Northern
Hemisphere. This coincided with a new maximum of recorded storms and hurricanes (26 and 13 respectively) and a major bleaching event. Across the
Caribbean islands, bleaching affected between 50% and 95% of coral communities. In the U.S. Virgin Islands 51.5% of corals declined due to these events
and subsequent outbreaks of disease.14
Ocean acidification
As rising CO2
dissolves, carbonic acid is formed, especially in surface waters and cold waters. The IPCC 2007 predicts that ocean pH will drop from 8.2 to
7.8 within this century, corresponding to around 150% increase in acidity4
In 2008, measurements of ocean pH taken from temperate coastal zones over a
period of 8 years, showed the decline in pH was between -0.039 and -0.05415, a rate significantly HIGHER than the 0.1 unit decrease since industrialisation
reported in 2005.16,20
Most corals are unlikely to survive in waters more acid than pH 7.6.35
Immediate and primary effects of acidification on corals are: reduced
ability to secrete calcium carbonate and reduced extension rates, dissolution and weakening of existing carbonate skeleton and so increased susceptibility
to weathering. Secondary changes that may result include: lowered reproductive success and reduced survival rates, and reduced variability in community
structure with potential selective advantage for non-calcifying reef organisms.17, 21
In 2007 it was widely believed that all reefs of the world would be under
significant acidification stress by 2050.2
Accelerating CO2
emissions are shortening this timeframe alarmingly.
Critical threshold levels
Determining critical non-viability thresholds to specific greenhouse gas concentrations will always be difficult, with understood
threshold levels becoming lower and more imminent as further research is carried out. We must stay well below these environmental
tipping points to limit the risk of ecosystem collapse and all the resultant impacts on human wellbeing.
Acidification thresholds: Reactions of coral species to increases in ocean pH decreases as predicted in the IPCC scenarios are
heterogeneous22,23,34
, however it appears that calcification rates of tropical-region reef building corals will see a 20-60% decline at
concentrations of CO2
around 560ppm.25, 26, 27
If CO2
levels were to reach three times pre-industrial levels, some species of reef-
building corals such as Galaxea fascicularis would see their calcification rates drop by more than 80%.28
Research29
has shown
that in some situations carbonate accretion approaches or reaches a negative net balance due to aragonite saturation values
when atmospheric CO2
reaches 480 ppm. Even in the absence of all other pressures, these decreases in calcification could prove
to be near fatal to whole communities. The magnitude of current pH change projected by 2100 is likely not to have occurred in
our oceans for more than 20 million years.30
Temperature thresholds: The average SST of the oceans is not as important to coral survival as regional maximum SST and
the frequency and degree of these extreme warming events. Increased average SSTs of less than 2°C above pre-industrial levels
across the year may not push all corals over temperature tolerance thresholds, however higher SSTs increase the likelihood of
extreme warming events which cause mass bleaching and mortality.2, 6
Above a 2°C rise in SSTs (often equated to CO2
equivalence
of >450ppm), substantial adaptation will be required for corals and their symbionts to tolerate the increasingly frequent and severe
warming events. 8, 9
Synergisms
The probability of negative synergisms between decreasing ocean pH levels, increased extreme weather events, increased
temperatures and sea level rise alongside already prevalent anthropogenic pressures on coral species must not be underestimated
and highlights the pressing need to reduce greenhouse gas emissions to a minimum threshold level. Therefore a precautionary
CO2
threshold of ≤400ppm should be considered an urgent priority, in light of current coral viability data. This critical threshold
may have to be lowered significantly further as new data is obtained and the integrity of wider natural systems is factored in.
“Reefs are the oceans canaries. We must heed their call. This call is not just for the reefs
themselves, but for all the great ecosystems of our oceans. These stand behind reefs like a row
of dominoes. If reefs fall, the rest will follow in quick succession. The Sixth Mass Extinction will
be upon us. It will be of our own making, and it will be unstoppable by any means whatsoever.”
(J.E.N Veron, 2008) www.coralreefresearch.org
Scenarios of reef structural changes with increasing temperature and CO2
concentrations.33
Wendy Foden (IUCN), Ove Hoegh-Guldberg (University of Queensland), Pricila Iranah (ZSL),
Aylin McNamara (ZSL), Paul Pearce-Kelly (ZSL), Alex Rogers (ZSL), Charles Sheppard (University
of Warwick), Mary Stafford Smith (Coral Reef Research) and J.E.N (Charlie) Veron (Coral Reef
Research). Contributing researchers: John Atkinson (ZSL), Joanna Corrie (ZSL), Rachel Downey
(ZSL), Holly Wallis-Copley (ZSL)
References
1. Roberts, E. 2003, Marine Scientist 2: 21-23. 2. Veron, J. E. N., 2008, London, Harvard University Press.
3. Pandolfi et al. 2003, Science 301: 955-958. 4. Parry, M., et al. Ed. 2007, WG II in Fourth Assessment Report of the IPCC. Cambridge. 5.
Marshall P.A. et al. 2006. Australia, Great Barrier Reef Marine Park Authority. 6. Glynn, P. W., 2006, Global Change Biology 2(6): 495-509.
7. Wilkinson, C. & Souter, D., 2008, Global Coral Reef Monitoring Network 8. Sheppard, C.R.C., 2003.Nature 425:294-297. 9. Sheppard,
C.R.C. & Rioja-Nieto R. 2005. Marine Environmental Research. 60: 389-396. 10. Porter, J.W. et al. 2001. Hydrobiologica. 460. 1-24 11.
Hoegh-Guldberg, O. 1999, Marine and Freshwater Research. 50. 839–866. 12. Smith & Buddemeier, 1992, Annual Review of Ecology and
Systematics 23: 89-118. 13. Sheppard, C.R.C. et al. 2005. Estuarine, Coastal and Shelf Science 64:223-234. 14. Donner, S. D. et al 2007,
PNAS 104(13): 5483-5488. 15. Wootton, J. T. et al. 2008, PNAS 105(48): 18848-18853. 16. Orr, J. C. et al., 2005, Nature 437 (7059): 681-686.
17. Kleypas, J. A., et al., 1999, Science 284(5411): 118. 18. Carpenter, et al. 2008. Science. 321. 560-563. 19. Donner, S.D, et al. 2005. Global
Change Biology. 11. 2251-2265 20. Sponberg, A. F., 2007, BioScience 57. 21. Kershaw, S., & Cundy, A., 2000, Routledge.288.
22. Anthony, K. R. N et al. 2008. PNAS 105: 17442-17446 23. Marubini, F., C. et al. 2003. Proc. Roy. Soc. Lond. B 270:179–184. 24. Guinotte
J.M. & Fabry V. ICES J. Mar. Sci. 2008; 65: 414-432. 25. Langdon, C. & M.J. Atkinson. 2005. J. Geophys. Res. 110: np. 26. Royal Society.
2005. Policy Document 12/05. 27. Kleypas, J.A. et al. 2006. St. Petersburg. 28. Marshall, A.T. & P.L. Clode. 2002. J. Exp. Biol. 205: 2107–
2113. 29. Hoegh-Guldberg, O. et al. 2007. Science 318: 1737–1742. 30. Feely, R.A. et al. 2004. Science 305: 362–366. 31. Baird, A., &
Maynard, J.A. 2008. Science .320. pp315. 32. Buddemeier et al. 2004, Pew Centre. 33. Hoegh-Guldberg et al, 2008, Science, 318: 1737-1742.
34. Sheppard, C., 2008, Marine Ecology Progress Series 362: 109-117. 35. Hall-Spencer et al. 2008. Nature 454: 96-99. 36. Foden et al, 2008,
IUCN Red List
Acknowledgments
Many thanks to Phil Holmes
Coral ecosystems hold more than 25% of all marine species1
and are of immense economic importance. Climate change
is advancing more rapidly than previously forecast, beyond any of the IPCC predictions.
Continuing atmospheric CO2
build-up will tip marine ecosystems into an altered environmental state, where corals reefs,
their inhabitants and many other marine organisms will no longer be viable. Deep emission cuts must be targeted now
to avoid the global loss of these priceless marine ecosystems.
The IUCN 2008 Red List Climate Change susceptibility assessment
Warm-water reef-building corals have been assessed by the IUCN to
see how many have intrinsic traits that make them vulnerable to climate
change. Traits include; temperature sensitivity by adults and larvae,
sedimentation, cyclones and dispersal ability. The assessment at this
stage does not factor in climate change scenarios which will likely show
even further increases of extinction risk in certain areas.
A total of 566 species out of 799 (71%), were found to have traits that
madethemsusceptibletoclimatechange.36
Theseresultsdonotinclude
the impacts of acidification on coral species. Many species (51%)
previously classified as non-threatened in the IUCN Red List were found
to be susceptible to climate change.18
Coral adaptation potential
Physiological acclimation and/or evolutionary mechanisms could delay the predicted impacts.18,31
Corals that form symbioses with
more than one symbiont can shift their populations so that they are dominated by their more thermally-tolerant symbiont. However,
these short-lived changes have not yet resulted in novel host-symbiont combinations that will survive the predicted changes in
temperature.2,18
Reduced coral and symbiont species biodiversity, as predicted by future temperature increases, will mean that
coral reef ecosystems are less resilient to other survival pressures.18
The change to high-tolerance symbionts, will mean that coral
will be less vigorous in growth as higher-tolerance species are less efficient.19
Long generation times and low genetic diversity
of coral suggest a slow adaptation rate.3
Repeated bleaching episodes have suggested that corals have exhausted their genetic
capacity to adapt to rising SSTs.5,7
Managing the unavoidable
For coral reefs to tolerate or adapt to the already unavoidable climate change impacts,
they need to be healthy and unstressed to maximise their resilience.32
Limiting pollution
and physical stresses, enhancing mangrove vegetation cover and maintaining balanced
fish communities are essential measures. The longer time frame of natural increases
in CO2
allows mixing and buffering of surface layers by deep ocean sinks as well as
adaptation to new environmental conditions by marine organisms. In contrast, when
atmospheric CO2
increases abruptly, its effects are intensified in shallow waters owing
to a lack of mixing, giving marine life little time to adapt.
Avoiding the unmanageable
If critical thresholds are exceeded there are no realistic response options capable of contending with the resultant altered states.
Levels of CO2
and other contributing GHG emissions mean that current ‘CO2
equivalence’ levels stand at an alarming 435ppm.
CO2
levels on its own being at 385ppm, much higher then pre-industrial levels of 278ppm. Whilst geo-engineering options offer
some temperature-mitigation potential, there is no realistic mitigation measure available to address the acidification threat under
present policy measures. Effective engagement with public and policy makers is the most urgent mitigation priority. How this
might be realised will depend on how effective we are at conveying the profound consequences (not least for human survival)
associated with such an altered state marine environment.
Dec 08 poster / v1
The 25,000km² of coral reef systems of the Chagos Archipelago provide an
opportunity for studying climate change impacts in the near absence of other
anthropogenic stressors. Although Chagos was affected by the 1998 mass
bleaching,colonynumbersandjuvenilerecruitmenthasbeenrecoveringinrecent
years34
. This offers some hope that if other human impacts are minimised we can
increasecoralreefscapacitytorecoverfromextremedieoff
events–providingcriticalimpact
thresholds are avoided.