Climate change is thought to be occurring and affecting the oceans temperatures. The reasons for this are unclear; it is thought to be an increase in CO2 in the atmosphere. The Earth’s temperature has warmed by around 0.7°C in the last 100 years (Walther et al. 2002) effecting ecosystems all over the world from polar regions to deep sea communities and tropical reefs. Although it seems like a relatively small temperature increase it is enough to cause concern for delicate reef systems and it is believed that corals are reaching their thermal upper limits (Wilkinson & Souter 2008)
The warming of the sea is causing sea level rise through thermal expansion of the particles (Church et al. 1991) making seas deeper decreasing the amount of light available to corals.
The coral shows signs of stress with sea temperature rise as they expel the zooxanthellae that are living within them. This at a certain level is natural and occurs regularly but when too much is expelled under the stressful conditions the coral can no longer survive; this is called bleaching. The bleaching may also be due to higher UV radiation (Glynn PW 1996)