Unbleached and bleached coral
Any rise in the sea level due to climate change would effectively ask coral to grow faster to keep up. Also, water temperature changes can be very disturbing to the coral. This was seen during the 1998 and 2004 El Niño weather phenomena, in which sea surface temperatures rose well above normal, bleaching or killing many coral reefs.
Above: Bleached Euphillia sp.
High seas surface temperature (SSTs) coupled with high irradiance (light intensity), triggers the loss of zooxanthellae, a symbiotic algae, and its dinoflagellate pigmentation in corals causing coral bleaching. Zooxanthellae provides up to 90% of the energy to the coral host. Reefs can often recover from bleaching if they are healthy to begin with and water temperatures cool. However, recovery may not be possible if CO2 levels rise to 500 ppm because there may not be enough carbonate ions present. Warming may also be the basis of a new emerging problem: increasing coral diseases. Warming, thought to be the main cause of coral bleaching, weakens corals. In their weakened state, coral is much more prone to diseases including black band disease, white band disease and skeletal eroding band. If global temperatures increase by 2 °C, coral may not be able to adapt quickly enough physiologically or genetically. It has been estimated that, to counter the threat of ocean acidification through global warming, a reduction of up to 40% of current emissions is needed, and up to 95% by 2050. This requires emission reductions larger than the reductions currently proposed for these dates by the EU.