that relationship to become toxic. >> reporter: his team released footage of barren expansions of coral bleached bone white and in some cases turning a drab lifeless brown. look at the before and after contrast of coral gone from healthy to bleached. dr. nancy knowlton, a coral reeve -- reef says it's suffering from heat stroke. >> what's happening is the baseline temperature is getting warmer and warmer and any increase in temperature caused by local weather conditions and adds on top of the global warming and kicks off this bleaching event so the global warming is cautioned by people, and it's providing the conditions that make bleaching happen. >> reporter: the great barrier reef is more than just home to thousands of species of fish, birds, coral, whales and dolphins. it's also a major tourist attraction that earns australia