Often called "the rainforests of the ocean," coral reefs house the staggering diversity that that phrase implies. Although reefs comprise less than 0.1 percent of the ocean's total area, they house 25 percent of the ocean's fish. Over a million different species reside in reefs, many of which we known little to nothing about. Additionally, coral reefs are the major source of GDP for some countries, especially those that rely on the fishing and tourism industry. Approximately 500 million people's livelihoods depend on those corals. "Medicine cabinet of the future" is another term used to describe coral reefs. Already we obtain medical treatments from corals, such as secosteriods used to treat asthma, and many more treatments may be there awaiting discovery. But coral bleaching poses a major threat to the reefs that we, and millions of other organisms, rely on. To truly understand the danger that coral reefs are in, and the impact their death would have on humans, you have to know a few things about coral.

Firstly, corals are not plants. These huge, complex living structures are composed of hundreds of thousand of tiny animals called polyps. Those polyps have a special relationship with photosynthetic algae known as zooxanthellae. Basically, zooxanthellae provide the coral with food and in exchange the coral provides the zooxanthellae with shelter. This symbiotic relationship not only benefits the coral and the algae, but ever organism that calls the reef home.

The zooxanthellae are very sensitive to change. As the temperatures of the ocean rise, the zooxanthellae start to produce a compound that is toxic to the coral. Consequently, the coral expels the zooxanthellae. Since those algae are what give the corals their beautiful colors, once the zooxanthellae are kicked out, the corals lose their color, giving it a bleached appearance.

That before and after picture were two months apart. The picture on the left is what the reefs of the American Samoa looked like in December 2014. On the right is a picture of the same reefs two months later, in February 2015. The speed at which coral bleaching occurs is terrifying. Some reefs can be bleached within the span of seven weeks.

The difference between the two photos is stark, dramatic, and heartbreaking. While it's not impossible or unheard of for coral reefs to recover from bleaching, it's very difficult. We don't know how to help reefs recover, and there are so many factors that may affect a reef's chance at survival. So, as of now, if a reef is bleached it may very well be a death sentence.