A study of 100 reefs, published in Science Magazine, shows the interval between bleaching events in recent decades has shortened dramatically.
It has gone from once every 25-30 years in the early 1980s to an average of just once every six years today.
Bleaching is caused by anomalously warm water, which prompts coral polyps to eject their symbiotic algae.
This drains the corals of their colour and is fatal unless conditions are reversed in a reasonably short time.
But even if temperatures fall back quickly, it can still take many years for damaged reefs to fully recover.
"If you go into the ring with a heavyweight boxer, you could probably stand up for one round, but once that second round comes - you're going down," said Dr Mark Eakin from the US National Oceanic and Atmospheric Administration (Noaa).