By Ben Ladabaum '20Coral reefs are home to one of the most diverse and beautiful ecosystems on our planet, but rising water temperatures pose a threat to their survival. Coral reefs are composed of thousands of coral polyps, which contain a calcium carbonate exoskeleton and a soft tissue internal structure. Algae live in their internal soft tissue, forming a symbiotic relationship and giving the coral its color. However, when the water temperature around the coral rises, the algae becomes toxic, so the coral ejects the algae and turns white, a process known as coral reef bleaching. Although bleached coral is not necessarily dead, it cannot survive long in such a state.A recent report on the study of the Great Barrier Reef system, which compiled data from 171 coral reefs in the Great Barrier Reef system, revealed that coral bleaching in 2015-16 was the worst in history. There have been three periods of mass bleaching in recent decades: 1998, 2010, and 2015-16. Each bleaching period has been more widespread than the previous one, with 43% of the Great Barrier Reef bleached in 1998, 56% in 2010, and 85% in 2016.While there are many factors that could potentially cause bleaching, the key factor identified in the study was the number of degree heating weeks in a year, a unit of measurement that reflects the total temperature exceeding the bleaching threshold. There was a clear positive correlation between the number of degree heating weeks and severity of bleaching. These findings were consistent in areas that controlled water quality and protected against overfishing, suggesting that any effect these factors might have had on bleaching was negligible. Thus, the study concluded that mass bleaching is almost entirely due to climate change, and local efforts to control factors, such as maintaining water quality and ecosystem sustainability, are ineffective in combatting bleaching.Recovery from mass bleaching can take decades, but some scientists worry that coral reefs may never recover completely. In order to mitigate the current bleaching crisis and give the reefs the greatest chance to recuperate, immediate and extensive action must be taken to combat climate change. However, we are currently emitting greenhouse gases into the atmosphere too rapidly for coral to keep up with rising temperatures. While many ecosystems will no doubt be affected by climate change, coral reefs are perhaps the most sensitive to temperature fluctuations, and thus, they are in the most urgent need of help.References:http://www.nature.com/nature/journal/v543/n7645/full/nature21707.htmlhttp://westhawaiitoday.com/news/local-news/biologist-ruth-gates-trying-engineer-bleach-resistant-coralhttps://nyti.ms/2mK0efT