The Great Barrier Reef, the largest coral structure on Earth, is suffering from what may be its worst bleaching event ever recorded.
But according to new research, future bleaching events could be even worse and may disable a natural protection mechanism of the reef's corals - leaving them more defenseless against warmer seas.
The study, published yesterday in Science, was led by Tracy Ainsworth of the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University, with colleagues from other Australian universities and the US National Oceanographic and Atmospheric Administration.
The researchers used satellite records of past ocean temperatures affecting the reef, in combination with genetic studies of coral, to measure how they respond to different types of "thermal stress" events, in which temperatures rise beyond what the organisms are used to.
"The big thing we're able to discover is that there is a mechanism that has been operating in the Great Barrier Reef that has protected the corals," says Mark Eakin, one of the study's authors and head of NOAA's Coral Reef Watch.
"The problem, though, is that in the future, as temperatures warm, that's no longer going to happen."
The study found that in the past 27 years, there have been 372 events that subjected corals to stress due to unusually warm ocean temperatures. It further found that in 75 per cent of these cases, while temperatures rose above the hottest temperatures normally found during the hottest month of the year, they at first "remained below the bleaching threshold", which is defined as 2C higher than those hottest temperatures.
In these cases, temperatures then cooled again before actually crossing the bleaching threshold. And when this cycle occurred - warm, cooler again, then very warm - the researchers found that corals had a mechanism to protect themselves.
Certain genes turned on that helped make the symbiotic algae that inhabit their bodies, and that are expelled during a bleaching event, more tolerant of heat stresses.
"What was really widespread across the reef was this period of time before bleaching, where corals can have this near miss," said Ainsworth.
And this probably means that coral bleaching so far has been less harmful in the Great Barrier Reef than we might otherwise expect.
"When they have these conditions," said Ainsworth, "the bleaching still occurs, but the impact to the animal is not as bad. They keep more of their endosymbiotic algae, and that's really important, that's their chief source of nutrition. The heat shock responses were different, and this accumulated into less cell death and better survival for the coral."
However, the study also found that in the minority of cases in which temperatures marched upward monotonically without an intervening cool down - or, when they crossed the bleaching threshold, dropped down, but then crossed it again - corals lost the protective response. They bleached more, and more of their cells died as a result.
And that's the problem: The study then projected Great Barrier Reef temperatures into the future and found that corals will encounter more stresses that they just can't respond to as well.
"The profile or the shape of the curve, as the temperature goes up, becomes one that is just damaging," said Ainsworth. "We lose a small stress, a relaxation, and a big stress. It just all becomes part of a bigger stress."
In fact, when global sea surface temperatures are just 0.5C higher than present, the study found, most reefs that so far have experienced a warming trajectory that triggers a protective response will instead experience one more likely to overwhelm the corals.
"The way that temperatures on the Great Barrier Reef warm is such that when you add even half a degree of temperature rise, you'll see that they'll be going straight into those bleaching conditions without getting that pre-bleaching pulse," said Eakin.
The key question now appears to be how much this pattern reflects the particular local conditions of the Great Barrier Reef or whether the same principle would apply to coral reefs around the world. The researchers say they'll need to study that next.
Many coral reefs around the globe have been hit hard - and even, in some cases, substantially killed off - by the latest powerful El Nino event, the strength of which has been enhanced by climate change.
The current event is so overwhelming that the corals' normal protective mechanism tends to get overriden anyway, Eakin explained, but that won't always be the case. Not every year is a powerful El Nino year.
"It's kind of like having something that protects you against a category 1 or 2 hurricane. But when that category 5 hurricane hits, you're still going to be nailed," he says. "But on the other hand, there are a lot more of those category 1 and category 2 hurricanes that are going to hit you."
In other words, you might think of it as akin to the situation in which sea-levels rise slowly but surely lessens the storm surge effectiveness of a sea wall, as the wall's elevation above sea level shrinks.
The research matters not only because of this year's extreme coral bleaching around the world, but because some scientists are now saying that stretching coral ecosystems beyond their ability to "adapt naturally" would constitute a dangerous level of climate change.
The term that has a key meaning in international climate policy - basically, it's the kind of climate change that the world has pledged to prevent.
With the bleaching and, in some cases, coral death that has been seen in recent months, it's becoming easier to believe that we're entering such a world.