Lack of oxygen hurting corals in world’s oceans

Global warming is making the world’s oceans sicker, depleting them of oxygen and harming delicate coral reefs more often, two studies show.

The lower oxygen levels are making marine life far more vulnerable, the researchers said. Oxygen is crucial for nearly all life in the oceans, except for a few microbes.

“If you can’t breathe, nothing else matters. That pretty much describes it,” said study lead author Denise Breitburg, a marine ecologist at the Smithsonian Environmental Research Center. “As seas are losing oxygen, those areas are no longer habitable by many organisms.”

She was on a team of scientists, convened by the United Nations, who reported that the drop in oxygen levels is getting worse, choking large areas, and is more of a complex problem than previously thought. A second study finds that severe bleaching caused by warmer waters is hitting once-colorful coral reefs four times more often than they used to a few decades ago. Both studies were published in the journal Science.

When put all together, there are more than 32 million square kilometers of ocean with low oxygen levels at a depth of 200 meters, according to the scientists with the Global Ocean Oxygen Network. That amounts to an area bigger than the continents of Africa or North America, an increase of about 16 percent since 1950.

Their report is the most comprehensive look at oxygen deprivation in the world’s seas.

“The low oxygen problem is the biggest unknown climate change consequence out there,” said Lisa Levin, a study coauthor and professor of biological oceanography at the Scripps Institution of Oceanography.

Levin said researchers have seen coastal “dead zones” from fertilizer pollution from farms before, as well as areas of low oxygen in open ocean blamed on warmer waters, but this study shows how the two problems are interconnected with common causes and potential solutions.

“Just off Southern California, we’ve lost 20 to 30 percent of our oxygen off the outer shelf,” Levin said. “That’s a huge loss.”

Some low oxygen levels in the world’s ocean are natural, but not this much, Breitburg said. A combination of changes in winds and currents — likely from climate change — is leaving oxygen on the surface, and not bringing it down lower as usual. On top of that, warmer water simply doesn’t hold as much oxygen and less oxygen dissolves and gets into the water, she said.