Climate change and coral bleaching

Publication - 17 June, 2001
This scientific study concludes that if climate change is not stopped, coral bleaching is set to steadily increase in frequency and intensity all over the world until it occurs annually by 2030 - 2070.

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Executive summary: Sea temperatures in the tropics have increased by almost1 degree C over the past 100 years and are currently increasing atthe rate of approximately 1-2degrees C per century. Reef- building corals, which are central to healthy coral reefs,are currently living close to their upper thermal limit.They become stressed if exposed to small slight increases(1-2 degrees C) in water temperature and experience coralbleaching.Coral bleaching occurs when the photosyntheticsymbionts of corals (zooxanthellae) become increasingvulnerable to damage by light at higher than normaltemperatures. The resulting damage leads to the expulsionof these important organisms from the coral host. Coralstend to die in great numbers immediately following coralbleaching events, which may stretch across thousands ofsquare kilometers of ocean. Bleaching events in 1998, theworst on record, saw the complete loss of live coral fromreefs in some parts of the world.This paper reviews our understanding of coral bleachingand demonstrates that the current increase in the intensityand extent of coral bleaching is due to increasing seatemperature. Importantly, this paper uses the output fromfour different runs from two major global climate modelsto project how the frequency and intensity of bleachingevents are likely to change over the next hundred years ifgreenhouse gas emissions are not reduced. The results ofthis analysis are startling and a matter of great concern.Sea temperatures calculated by all model projections showthat the thermal tolerances of reef-building corals arelikely to be exceeded within the next few decades. As aresult of these increases, bleaching events are set toincrease in frequency and intensity. Events as severe as the1998 event could be become commonplace within twentyyears. Bleaching events are very likely to occur annuallyin most tropical oceans by the end of the next 30-50 years.There is little doubt among coral reef biologists that anincrease in the frequency of bleaching events of thismagnitude could have drastic consequences for coral reefseverywhere. Arguments that corals will acclimate topredicted patterns of temperature change areunsubstantiated and evidence suggests that the geneticability of corals to acclimate is already being exceeded.Corals may adapt in evolutionary time, but such changesare expected to take hundreds of years, suggesting that thequality of the world's reefs will decline at rates that arefaster than expected.Every coral reef examined in Southeast Asia, the Pacificand Caribbean showed the same trend. The world's largestcontinuous coral reef system (Australia's Great BarrierReef) was no exception and could face severe bleachingevents every year by the year 2030. Southern and centralsites of the Great Barrier Reef are likely to be severelyaffected by sea temperature rise within the next 20-40years. Northern sites are warming more slowly and areexpected to lag behind changes in the southern end of theGreat Barrier Reef by 20 years. In summary, the rapidityand extent of these projected changes, if realized, spellscatastrophe for tropical marine ecosystems everywhereand suggests that unrestrained warming cannot occurwithout the complete loss of coral reefs on a global scale.

Num. pages: 30

ISBN: 90-73361-52-4

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