Global Warming
It is believed that global warming appears to have the greatest impact on reefs, triggering the collapse of reef ecosystems throughout the tropics. It also appears to create tropical storms, which batter the reef - but due to its ribbons, the inner reef withstands these attacks. Coral bleaching also appears to be happening, due to the water's temperature rising as a result of global warming and the El Nino effect. Coral bleaching is when the coral turns white due it expelling its zooxanthellae while under environmental strain. If the water temperatures continue at this high level, the whole Great Barrier Reef faces extinction.