Hoegh-Guldberg et al., 2007:

"The vulnerability of coral and the reefs they build to climate change was bought into sharp focus after 1998, when an estimated 16 percent of the world’s coral communities died. Analysing the literature since that time reveals that rapidly rising sea temperatures and increasing levels of acidity in the ocean remain the major threat to coral reefs. Successive studies of the potential impacts of thermal stress on coral reefs have supported the notion that coral dominated reefs are likely to largely disappear with a 2°C rise in sea temperature over the next 100 years. This, coupled with the additional vulnerability of coral reefs to high levels of acidification once the atmosphere reaches 500 parts per million, suggests that coral dominated reefs will be rare or non-existent in the near future. In this regard, we conclude that communities of Australian corals are extremely vulnerable to the effects of ocean warming and ocean acidification. While new assemblages will certainly form in the absence of coral-dominated reefs, the diversity and structure of these communities as well as the types of fishing and tourist industries they might support are completely unknown.

As outlined above, 500 parts per million is the highest CO₂ concentration under which any semblances to the communities of corals we have today can survive. It is also the only scenario in which the climate will eventually stabilise. Above this point (500 parts per million), coral reefs will also change irreversibly and be lost for many thousands of years. To contemplate any higher CO₂ is untenable given the huge likelihood of such catastrophic events as runaway greenhouse effects and the flooding of the planet as the Greenland and Western Antarctic Ice Sheets melt. Even though 500 parts per million is seen as an ambitious greenhouse target, effects on ocean temperature and acidity will mean that coral calcification will decrease to 40 percent of today’s value and major (1998 level) bleaching events will occur every 2 to 4 years. Under these conditions, Australian reefs will have the following characteristics:

• Major increase in the frequency and intensity of coral bleaching, mortality events and recruitment failure with increased incidences and outbreaks of coral disease.
• Coral dominated reefs will contract to less that 20 percent of today’s distribution and corals will be rare on most coral reefs. Benthic microalgae, macroalgae and cyanobacteria communities will dominate these reefs although it is uncertain which species or taxa will dominate.
• Reef carbonate frameworks are likely to slowly disintegrate under vastly reduced calcification (due to elevated temperatures and decreasing pH) and the possible acceleration of bioerosion.
• Reefs will have less structure and hence reduced habitat complexity and holding capacity for reef organisms. It is not known how long these processes will take to have an effect on coral reefs.
• Reduced coral communities and reef structure will lead to a major reduction in reef biodiversity with some coral-dependent species going extinct.
• At longer time frames, negative reef maintenance and growth will mean that sections of the Australian coastline that are currently protected by reef structures like the GBR will gradually become more exposed to ocean wave stress. This may eventually have ramifications for the current distribution of coastal seagrass and mangrove communities.
• Intensified cyclone regime will increase physical impacts on coral communities and will accelerate the shift from high-diversity communities to assemblages dominated by few resistant massive/encrusting species. Reduced vitality of corals will mean that recovery will be compromised; further accelerating the shift of reefs away from coral dominated reefs.
• The increased intensity of flood events along with prolonged drought along east Australia will lead to periods of reduced water quality and flooding (with associated sediment, nutrients, and freshwater impacts) that will affect reefs further offshore."