Coral reefs and climate change

While the Arctic is the most climatically vulnerable human-inhabited environment, coral reefs will probably see the most comprehensive destruction in coming decades. According to the IPCC, it is highly likely that they will succumb to a combination of heat and oceanic acidification as temperatures rise in response to greenhouse gas emissions. It is estimated that the last 25 years have seen the loss of 30% of warm-water coral cover. The worst summers so far for coral bleaching have been 1998 and 2002: in which 42% and 54% of all reefs worldwide were affected. As much as 80% of Caribbean coral may already have died.

Coral bleaching occurs when the zooxanthella algae that live in coral tissues die. The report of Working Group II of the IPCC highlights high surface temperatures as “almost certain to increase the frequency and intensity of mass coral bleaching events.” Throughout the Third and Fourth Assessment Reports, coral reefs are highlighted as being especially vulnerable to climatic change, with low adaptive capability.

Oceanic acidification reduces the calcifying ability of corals, by making it more difficult for them to extract calcium from seawater. In cases of extreme acidity, existing structures could begin to dissolve. According to the IPCC “the progressive acidification of oceans is expected to have negative impacts on marine shellforming organisms.” Studies have demonstrated that projected future ocean acidity will reduce coral calcification and weaken coral skeletons.

The Fourth Assessment Report projects that most corals will be bleached by a temperature rise of 1 to 3°C, with increasing coral mortality at higher levels of temperature increase. Between 2.5 and 3.5°C above the pre-industrial mean temperature, a summary table in the WGII report predicts simply “Corals extinct, reefs overgrown by algae.” It warns further that: “It is important to note that these impacts do not take account of ancillary stresses on species due to over-harvesting, habitat destruction, landscape fragmentation, alien species invasions… or pollution.” Given the low probability of keeping further temperature increases below 2°C - even with the advent of relatively stringent new international obligations - it is fair to say that most of the world’s coral is doomed to die. That, in turn, will undermine much of the basis of coral reef ecosystems. This is a further burden to some small island states, as coral reefs can be the habitat for important fish stocks. Reefs are also the most diverse marine ecosystems: home to about 25% of all marine species.

One way to interpret the news is this: if you have always dreamed of SCUBA diving in the natural splendour of a coral reef, make sure you do it fairly soon. Your children might not be able to do it at all. To quote the IPCC once more: “Annual or bi-annual exceedance of bleaching thresholds is projected at the majority of reefs worldwide by 2030 to 2050.”