"Oxygen is fundamental to life in the oceans", said lead author Dr Denise Breitburg, a marine ecologist with the Smithsonian Environmental Research Centre. They say the problem is worsening and more complex than previously thought. Without oxygen in the oceans, marine life will die off or relocate.
The average time between severe "bleachings", when heat makes the stony-bodied creatures that make up coral reefs expel colorful algae, shortened to six years in 2016 from 25-30 years in the early 1980s, the Australian-led team wrote.
Ocean dead zones with zero oxygen have quadrupled in size since 1950, scientists have warned, while the number of very low oxygen sites near coasts have multiplied tenfold.
Since then, growth of industrial and agricultural activity has disrupted the ocean's chemical balance, with regions in many areas worldwide becoming infused with pollutants and nutrients that starved the water of oxygen.
The main problem for protecting reefs was "weak commitments for reductions in emissions from individual countries like Australia and the U.S.", he told Reuters in an e-mail. The study said in order to halt the decline, the world needs to rein in both climate change and nutrient pollution. In coastal areas and seas that are semi-enclosed, once low-oxygen conditions are established, they can persist for thousands of years, according to the study.
Increased runoff of fertilizers have further exacerbated low-oxygen levels by triggering algal blooms. 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.
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The study highlights the Great Barrier Reef, which has bleached four times since 1998, including unprecedented back-to-back events in 2016 and 2017.
A "1.5 or 2 degrees Celsius warming above pre-industrial conditions will inevitably contribute to further degradation of the world's reefs", the report said.
This July 2010 photo provided by NOAA shows bleached corals in Thailand.
In the last few years, 30 percent of bleaching episodes could be described as "severe", extending tens to hundreds of miles, or kilometers. As of 2016, they now are happening just under once every six years, the study found.
New global research reveals that mass coral bleaching events are not only getting worse, but are also occurring more frequently.
The risk of severe bleaching has risen about four percent per year since 1980.
Bleaching happens when the normally endosymbiotic relationship between coral polyps and algae falls out of whack. The Florida Keys, Puerto Rico and Cuba have been hit seven times. Already stressed and bleaching corals, caused by increased sea surface temperatures, can be harmed by a lack of oxygen too.
"Now that the scope of the problem is in full view, we need ... to pivot to finding science-based solutions in an all-hands-on-deck, all-of-the-above approach", said Kim Cobb, an expert in the long-term climate history of coral reefs at the Georgia Institute of Technology who was not involved in the study. Now, reefs are bleaching even in the absence of these winds, as global warming pushes up temperatures without El Nino's help.