A ghostly pallor is overtaking the world's coral reefs. This results when heat-stressed corals expel the algae they rely on for food - and which are responsible for their bright and beautiful hues. Death often follows.

Reefs have long been under threat from destructive fishing practices, runoff, coral mining, reckless tourism and coastal development. Now, scientists say, global warming is accelerating the destruction.

One of the worst episodes of coral bleaching began last spring, and affected reefs in virtually all the world's tropical waters.