Frequency of coral bleaching has increased almost fivefold since the 1980s

Dead fish brought out of the sea after water polluted and oxygen scarce

The analysis, published in the journal Science, is the first comprehensive analysis of the areas and states: "Major extinction events in Earth's history have been associated with warm climates and oxygen-deficient oceans". Without oxygen in the oceans, marine life will die off or relocate. "Unfortunately, it will take severe and persistent mortality of fisheries for the seriousness of low oxygen to be realised". "Halting climate change requires a global effort, but even local actions can help with nutrient-driven oxygen decline", said Breitburg.

"Oxygen is fundamental to life in the oceans and this decline ranks among the most serious effects of human activities on the Earth's environment", said Denise Breitburg, of the University of California, San Diego.

The main problem for protecting reefs was "weak commitments for reductions in emissions from individual countries like Australia and the USA", he told Reuters in an e-mail.

Increased runoff of fertilizers have further exacerbated low-oxygen levels by triggering algal blooms.

"Tackling climate change may seem more daunting, but doing it is critical for stemming the decline of oxygen in our oceans, and for almost every aspect of life on our planet", she said.

The open ocean has natural low oxygen areas, usually off the west coast of continents due to the way the rotation of the Earth affects ocean currents.

The only solution, scientists say, is to aggressively address climate change and pollution runoff. One irony is that warmer waters not only hold less oxygen but also mean marine organisms have to breathe faster, using up oxygen more quickly.

The study by 25 researchers, led by Terry Hughes from the Australian Research Council Centre of Excellence for Coral Reef Studies, found 31 percent of reefs examined were at risk of bleaching in 2016 compared with just 4 percent in the 1980s.

This July 2010 photo provided by NOAA shows bleached corals in Thailand. "The climate has warmed rapidly in the past 50 years, first making El Ninos unsafe for corals, and now we're seeing the emergence of bleaching in every hot summer". Recent research has found that 900 miles of the 1,4000-mile-long reef experienced this particularly severe form of bleaching in the a year ago or two. As of 2016, they now are happening just under once every six years, the study found.

"With a fourfold increase over the last 35 years, if you take that forward, it's unfortunately in complete agreement with what the climate models have been saying", said Mark Eakin, one of the study's authors and head of the National Oceanic and Atmospheric Administration's Coral Reef Watch.

The risk of severe bleaching has risen about four percent per year since 1980.

In the past, coral bleaching occurred when warmer winds and El Nino weather conditions heated up sea surfaces. The Florida Keys, Puerto Rico and Cuba have been hit seven times.

The authors point out that in areas traditionally called "dead zones", like those in Chesapeake Bay (in the US) and the Gulf of Mexico, oxygen plummets to levels so low many animals suffocate and die. "The climate has warmed rapidly in the past 50 years, first making El Niños unsafe for corals, and now we're seeing the emergence of bleaching in every hot summer".

Georgia Tech climate scientist Kim Cobb, who studies reefs but wasn't part of this worldwide team, applauded the research and said that as the world warms more there will be "profound and lasting damage on global reefs".



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