An assessment of the world's coral reef ecosystems has found that almost all face a high risk of collapse in the next 50 years mostly due to warming oceans and heat waves. The International Union for Conservation of Nature's assessment was released Thursday on the sidelines of a U.N. biodiversity conference. The assessment found that 96% of shallow-water reef systems are already critically endangered and could collapse by 2075. The Great Barrier Reef in Australia and most of the world's other famous reef systems are critically endangered. That's the highest level of threat.