Climate change may kill off the worlds reefs within just a few decades predicts the latest International Programme on the State of the Ocean. Ove oegh-‐Guldberg, Prof and Dir, Global Change Institute,University of Queensland, prepared the preliminary Case Study 2 which was a collaborative effort of 2 workshops dealing with ocean stress. These were the
International Earth System Expert Workshop on Ocean Stresses and Impacts held on the, 11th–13th April, 2011 and the International Earth System Expert Workshop on Integrated Solutions for Synergistic Ocean Stresses and Impacts, 2nd–4th April, 2012.
The preliminary report reiterated that coral reefs are the most diverse on the planet. And that the reefs are not only beautiful and provide food for millions, they also are coastal barriers to wave stress, protecting humans and infrastructure.
Coral reefs around the world are facing major stress from local factors such as overfishing, pollution, and unsustainable practices along tropical coastlines. Over the past 50 years, these activities have resulted in at least 40% of the world's coral reefs
disappearing. This problem is in itself extremely serious. Recently, however, climate change has also begun to threaten the world's reefs, through the dual effects of ocean warming and acidification.
Scientific projections of how the temperature and chemistry of the world's oceans will change in the future indicate a high risk of major catastrophe. Higher sea temperatures
and altered chemistries of the future are hostile to coral reefs, and are likely to eliminate
these wonderful ecosystems in the next 30-‐50 years if urgent action is not taken.
Marine Pollution Bulletin (pdf) illustrates how our coral reefs will die as a result of climate change as well as other assaults.
Interactive effects among the main climate change factors of warming and ocean acidification and coral diseases.
Warming Induces coral bleaching; bleached corals are more sensitive to diseases and have lowered calcification rates; affects postdisturbance recovery through negative impacts on reproduction, development and recruitment
Warming's extreme temperatures will reduce calcification; Induces coral disease; disease stressed corals are more sensitive to bleaching and have reduced calcification rates; affects postdisturbance recovery through negative impacts on reproduction, development and recruitment and expending of resources to combat infection.
Ocean acidification and reduced carbonate and aragonite concentration. Results in reduced calcification; corals with reduced calcification are more sensitive to bleaching and diseases; affects post disturbance recovery through negative impacts on reproduction, development and recruitment
Ocean acidification and reduced carbonate and aragonite concentration. Results in dissolution of aragonite and calcite skeleton; weakened skeleton is more sensitive to the impact of bioeroders and storms.
Interactive effects of local stress factors on climate change factors and marine diseases.
Sedimentation and turbidity increase coral susceptibility to
bleaching; decrease post bleaching recovery by smothering corals and limiting settlement of coral larvae.
Nutrients: increase coral susceptibility to bleaching through imbalance of nutrients in surrounding water that induces biochemical changes in cells; decreases post bleaching recovery through reduced reproductive output and by promoting growth of competitive algae, coral disease and increase of bioerosion and breakage
Overfishing: resistance to bleaching may decrease due to reduction in biomass and functional diversity in reef fishes; post bleachingrecovery by promoting overdominance of fleshy macroalgae and soft-bodied reef invertebrates, and loss of hard substrates due to intensified bioerosion and expansion of ‘urchin barrens’ associated with loss of keystone predators.
Destructive practices: physical destruction may result in partial
mortality and weakening, increasing susceptibility to bleaching; reduces post bleaching recovery through reduced reproductive potential, development and recruit survival
Sedimentation and turbidity: increase coral susceptibility to diseases; promote growth of disease causing micro-organisms and disease inducing fleshy macroalgae
Nutrients: induce proliferation of disease causing microorganisms and bioeroders; intensify growth of fleshy macroalgae that induce
Overfishing: reduction of keystone predatory fishes promotes population explosion of prey organisms that become vulnerable to marine diseases; reduction of herbivorous organisms promotes overgrowth of fleshy macroalgae that induce coral diseases
Destructive practices: corals suffering from mechanical damage are more sensitive to diseases; damaged corals may have low capacity of post disturbance recovery due to reduced reproductive potential as a result of trade-off between recovery and reproduction.
The Marine Pollution Bulletin concludes it's research with the following.
There is no single ‘most important’ stressor affecting coral reefs in the immediate term, rather different factors assume dominance in different areas and times. Continuing over-use or abuse of reef systems has already led to the demise of an unacceptably high proportion of reefs in all ocean basins, and reduction of many of the local stressors in most reef areas is clearly urgently needed. While it is common to refer to a certain percentage of the world’s or region’s reefs having suffered ‘degradation’ or similar, such statements common in policy documents for example, appear to gloss over the fact that many reefs are already dead and probably an irrecoverable state. Thus, comments like a certain region has suffered a 30% decline in reefs’ may mean that 30% are dead and irrecoverable, not that conditions on all of them have declined by 30%. The difference is critical. While CO2 rise is over-arching, it may be of little consequence to one of the approximately 25% of reefs that are already dead from other factors, the reefs having failed to ‘adapt’ to the stressors existing at those particular sites. Without coordinated action at local, regional and global levels to reduce local stress factors and combat climate change, there will be continued decline of reefs, and of their ability to support human communities. Present rates of deterioration, if continued, mean that most reefs will be lost as effective systems in a few decades. However, even if the local stressors can be averted, reduction of CO2 levels remains of paramount importance for their long term survival. The current global targets of carbon emission reductions, including the targeted limit of a 2 C rise (450 ppm), are unrealistic and definitely not enough for coral reefs to survive, and lower targets should be pursued. Without such action then entirely new and radical conservation strategies may be required to protect remaining coral reefs (e.g. Rau et al., 2012), although in such a scenario survival of these ecosystems is likely to be confined to a few intensively- managed localities. A huge loss in biodiversity, and productivity which is of value to people, is inevitable in such a high CO2 world.