By Tim Radford

Some time this century, if humans go on burning fossil fuels at the present rate, severe bleaching will hit 99 percent of coral reefs every year. Coral bleaching happens when the organisms become uncomfortably hot, and reject the algae on which their lives ultimately depend.

Since it takes a reef five years to recover from any one bleaching event, the consequences for some of the world's richest ecosystems could be catastrophic. But catastrophe could be delayed. Drastic cuts in emissions reductions could give reefs an average of another 11 years before they start bleaching every year, according to new research.

Right now, the world's reefs are caught up in the longest global coral bleaching event ever recorded. It began in 2014, and could go on well into 2017, according to the journal Scientific Reports.

Corals are acutely sensitive to ocean temperatures and when the thermometer rises, their symbiotic relationship with a mutual beneficiary, the zooxanthellae, breaks down. Some 90 percent of the Great Barrier Reef off Australia has been affected by the latest episode, and 20 percent of the coral killed.

"Bleaching that takes place every year will invariably cause major changes in the ecological function of coral reef ecosystems," said Ruben van Hooidonk of the U.S. National Oceanic and Atmospheric Administration and the University of Miami.

"Further, annual bleaching will greatly reduce the capacity of coral reefs to provide goods and services, such as fisheries and coastal protection, to human communities."

Acid Trend

The warning supports earlier studies that have already predicted problems for the world's reefs by the century's end. The reefs are being hit by changes in ocean chemistry, as carbon dioxide levels rise and as waters become more acidic.

Changing conditions make reefs vulnerable to new predators. And biologists have warned, again and again, that reefs are home to around a quarter of all marine life, and worth an estimated $375bn a year to humans, as coastal protection, as fishery nurseries, and as a source of tourism.

And even if corals recover from bleaching, and pollution, they could still be vulnerable to drowning as sea levels rise. A new study in the journal Global and Planetary Change has identified a crisis at the Great Barrier Reef 125,000 years ago, when polar ice melted, sea levels rose by perhaps six metres, and the reef's corals all but perished.

"The findings highlight the importance of increasing the reef's resilience now," said Belinda Dechnik of the University of Sydney, who led the research..

"In combination with climate change predictions by the Intergovernmental Panel on Climate Change and in the absence of improvements to reef management and human impacts, sea level pressures could tip the reef over the edge, potentially drowning it for good."

But the latest computer model predictions revealed in Scientific Reports deliver even more urgency, more global detail and more alarm. There are 87 countries or territories that are home to 500 square kilometers or more of reef .On average, these reefs will start to experience annual bleaching by 2043. This will leave the living corals vulnerable to starvation and disease.

About one reef system in 20 will already be hotter and have started bleaching a decade before that. Among the first will be reefs around Taiwan and the Turks and Caicos Islands.

Some 11 percent of reefs will be affected a decade later than average, and these include the corals off Bahrain, Chile and French Polynesia.

If nations adhere to an international agreement made in Paris in December 2015 to limit global warming to 1.5°C, the annual bleaching experience could be delayed by another 11 years.

The low latitude reefs of the South Pacific, India, Florida and the Great Barrier off Australia could be protected for another 25 years. In effect, the research has identified the conservation priorities.

"These predictions are a treasure trove for those who are fighting to protect one of the world's most magnificent and important ecosystems from the ravages of climate change," said Erik Solheim, head of the UN Environment Programme.

"It is imperative that we take these predictions seriously and that, at the very minimum, we meet the targets of the Paris agreement. Doing so will buy time for coral reefs and allow us to plan for the future and adapt to the present."

Reposted with permission from our media associate Climate News Network.