Ove Hoegh-Gulberg, Foundation Professor
Marine Studies, University of Queensland
Sea Temperatures in the tropics have increased by almost 1 degree Centigrade over the past 100 years and are currently increasing at the rate of 1-2 degrees Centigrade 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 Centigrade) in water and experience coral bleaching.
Coral bleaching occurs when the photosynthetic symbionts of corals (zooxanthellae) become increasingly vulnerable to damage from light at higher than normal temperatures. The resulting damage leads to the expulsion of these important organisms from the coral host. Corals tend to die in great numbers following coral bleaching events, which may stretch across thousands of square kilometers of ocean. Bleaching events in 1998, the worst on record, saw the complete loss of live coral from some reefs in some parts of the world.
This paper reviews our understanding of coral bleaching and demonstrates that the current increase in the intensity and extent of coral bleaching is due to the increasing sea temperature. Importantly, this paper uses the output from four different models to project how the frequency and intensity of bleaching events are likely to change over the next hundred years if greenhouse gas emissions are not reduced. The results of this analysis are startling and a matter of great concern. Sea temperatures calculated by all model projections show that the thermal tolerances of reef-building corals are likely to be exceeded within the next few decades. As a result of these increases, bleaching events are set to increase in frequency and intensity. Events as severe as the 1998 event could become commonplace within twenty years. Bleaching events are very likely to occur annually in most tropical oceans by the end of the next 30-50 years.
There is little doubt among coral reef biologists that an increase in the frequency of bleaching events of this magnitude could have drastic consequences for coral reefs everywhere. Arguments that corals will acclimate to predicted patterns of temperature change are unsubstantiated and evidence suggests that the genetic ability of corals to acclimate is already being exceeded. Corals may adapt in evolutionary time, but such changes are expected to take hundreds of years, suggesting that the quality of the world’s reefs will decline at rates that are faster than are expected.
Every coral reef examined in Southeast Asia, the Pacific and Caribbean showed the same trend. The world’s largest continuous reef system (Australia’s Great Barrier reef) was no exception and could face severe bleaching events every year by the year 2030. Southern and central sites of the Great Barrier Reef are likely to be severely affected by sea temperature rise within the next twenty to forty years. Northern sites are warming more slowly and are expected to lag behind changes in the southern end of the Great Barrier Reef by twenty years. In summary, the rapidity and extent of these projected changes, if realized, spells catastrophe for tropical marine ecosystems everywhere and suggests that unrestrained warming cannot occur without the complete loss of coral reefs on a global scale.