According to the “Impact of Global Warming on Coral Reefs” 60 major episodes of coral bleaching have occurred between 1979 and 1990. However, the 2016 coral bleaching epidemic has been the longest in recorded history. Other episodes tend to correspond with El Nino years. The first widespread bleaching occurred during the 1982-83 El Nino, which was followed by a strong La Nina, which brought warmer waters to the Pacific.
According to the Australian government’s Northern Great Barrier Reef’s Response Plan, this year’s coral bleaching was “the worst ever” and largely due to climate change and resulting rising ocean temperatures. Ocean temperatures are not the only culprit, however. Bright sunlight, pollution, and extremely low tides can also cause coral bleaching. Large corals such as Porites are able to withstand the temperature changes, however branching corals such as Acropora are less likely to survive a temperature change. Scientists determined that there was 67% mortality in the northern third of the reef and approximately 22% mortality overall.
Coral bleaching refers to when corals become stressed by their environments and eject the zooxanthellae living in their tissues. These algae are what provide the coral with their vibrant colors, and their expulsion leaves the coral white. Stony corals have calcium carbonate skeletons and transparent tissues, which provide the coral with its white color. However, the loss of algae doesn’t only impact the coral’s color, it also affects its diet. Algae is traditionally in a symbiotic relationship with coral, providing carbohydrates in exchange for shelter from potential predators.
Although bleaching doesn’t automatically result in death, it does make the coral more susceptible to outside dangers. However, coral cannot survive long term without reabsorbing the algae. Without zooxanthellae, the coral will lack nutrients and will be overtaken by brown algae and other predators. Thankfully, once favorable conditions return, the zooxanthellae can rejoin the coral as long as the coral has not completely died yet.
Reefs only compose 1% of world’s marine environment but they give shelter to almost a quarter of marine species. Some of these species are found exclusively at corals and are at risk of also disappearing when corals disappear. Birds that eat fish will lose their energy source and island plants that depend on bird droppings will also suffer. Losing corals would not only impact marine biodiversity, but also economies. Coral reefs help ecotourism, commercial fishing, and help protect coastlines from flooding. Unfortunately, even if the larger effects of climate change are avoided, ocean temperatures are on track to keep rising. Corals are not able to adjust to the increasing temperatures and are at great risk if current trends continue.