Researchers at the Australian Institute of Marine Science say the reef has lost half of its coral cover over the past 27 years. There are multiple causes, including a destructive kind of starfish, shown here.

The starfish is about 42 percent. Typhoons, big strong storms another 48 percent, and then coral bleaching is the remaining 10 percent which is caused whenever the water gets too hot.

GWEN IFILL: So when you say it's human-caused, do you mean what we do with our waters, what we do with our fishing, what we do with our runoff from agricultural causes?