Coral reefs are not only one of the most beautiful ecosystems we have, but also one the most important. These reefs are home to over 1 million aquatic species, and provide food and income (from tourism) to people in over 100 countries. Furthermore these reefs serve as natural barriers, helping to protect coastal settlements.  At an economic value of over 350 billion dollars, it is an absolute tragedy that these natural gems are under severe threat.
There are many trends that are stressing coral reefs. These trends are both global, such as climate change and increased ocean acidification, and local, like overfishing.  Combined together, they are causing mass bleachings. But what exactly does it mean for a coral reef to be bleached? Well you see, tiny algae living in the tissue of coral enjoy a mutualistic (both benefit) relation with the coral: the coral provides protection, while the algae produces oxygen and removes waste for the coral. However, there is a limited range of conditions at which this relationship can continue. If the temperature, pH, or oxygen level (caused by overfishing) of the water changes too drastically, the algae, which give the coral their bright colors, are expelled. This turns the coral white (hence the term, bleach), and if the algae do not return, the coral eventually starve to death. 
Now, coral bleaching is not a new phenomenon, but what is new is the scale at which it is occurring. Australia for instance, has only 7% of its coral reefs unbleached!  As Charles Veron, a leading reef expert, states "There is no hope of reefs surviving to even mid-century in any form that we recognise."  As a result of this loss of biodiversity, hunger and political turmoil in many communities that depend of these reefs would swiftly ensue. Ultimately, the bleaching of our coral should serve as example of the dangers in delaying action on climate change. It is not hard to imagine even more unpredictable and catastrophic events may follow.