A war on global warming is the only way to save the world's coral reefs, researchers have said.
Reducing pollution and curbing overfishing will not prevent the severe bleaching that is killing coral at catastrophic rates, according to a study of Australia's Great Barrier Reef.
Scientists say that local protection of reefs can help damaged coral recover from the stress of rising ocean temperatures.
But the new research shows that such efforts are ultimately futile when it comes to stopping heat-induced bleaching in the first place.
"We don't have any tools to climate-proof corals," said Terry Hughes, director of Australia's ARC Centre of Excellence for Coral Reef Studies and lead author of the study published in the journal Nature.
"That's a bit sobering. We can't stop bleaching locally. We actually have to do something about climate change."
Across the world, scores of brilliantly coloured coral reefs once teeming with life have in recent years become desolate, white graveyards.
Their deaths due to coral bleaching have grown more frequent as ocean temperatures rise, mainly due to increasing greenhouse gases in the atmosphere.
The hot water stresses corals, forcing them to expel the colourful algae living inside them, which leaves the corals vulnerable to disease and death.
Given enough time, bleached coral can recover if the water cools, but if the temperature stays too high for too long, the coral will die.
Preserving coral reefs is crucial, given that humans depend on them for everything from food to medical research and protection from damaging coastal storms.
The researchers conducted aerial and underwater surveys of the Great Barrier Reef, which has experienced three major bleaching events, the worst of which occurred last year.
The scientists found that the severity of bleaching was tightly linked to how warm the water was.
In the north, which experienced the hottest temperatures, hundreds of individual reefs suffered severe bleaching last year, regardless of whether the water quality was good or bad, or whether fishing had been banned.
That means even the most pristine parts of the reef are just as prone to heat stress as those which are are less protected.
Prior exposure to bleaching also did not appear to provide any protective benefit to the coral.
The scientists found the reefs that were highly bleached during the first two events, in 1998 and 2002, did not experience less severe bleaching last year.
Ultimately, the study concluded, saving reefs from the ravages of bleaching requires urgent action to reduce global warming.
"I think it's a wake-up call," Mr Hughes said.
"We've been hoping that local interventions with water quality and fishing would improve the resistance of the corals to bleaching. We found no evidence that that's actually true, at least during a very severe event."
The research also illustrated the gravity of the situation facing the 1,400-mile Great Barrier Reef.
The team found 91% of the reef has been bleached at least once during the three bleaching events.
Even more alarming, Mr Hughes said, is that a fourth bleaching event is already under way.
Corals need years to recover from bleaching, so back-to-back events increase the possibility that the bleached coral will die.