Coral reefs around the world are rapidly being degraded by a number of human activities including over-fishing, coastal development, and the introduction of sewage fertiliser and sediment. Climate change is also a major cause of coral reef destruction.
Coral reefs are often described as “the rainforests of the sea”, as they are home to over 25 per cent of all marine species. However, corals are also highly sensitive to changes in the oceans’ temperature. Consequently, coral reefs are also sometimes described as “the ocean’s canaries”.
Increasing atmospheric carbon dioxide levels are threatening corals in two major ways:
The oceans are a major carbon sink, absorbing between 30 and 50 percent of all anthropogenic CO2 emissions. The pH of the ocean has been slightly alkaline – around 8.2 – for most pre-industrial history, but has dropped by 0.1 in the last century. The IPCC predicts that it will drop to 7.8 in the next century, which would be the largest such change in more than 20 million years. Several studies have suggested that reef-forming coral cannot survive at pH levels less than 7.6.
Increased environmental stress often causes corals to expel their symbiotic zooxanthellae, which removes their color and results in “bleaching.” Bleached corals are weaker and more prone to disease. The combination of increased sea surface temperatures and ocean acidification has led to mass bleachings, even with temperature increases as slight as 1°C; with increases of 2°C-3°C, the corals may die.
(Source: World Resources Institute)
The World Resources Institute also recently published this map, showing how coastal development threatens coral reefs: