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Rising sea temperatures and disappearing oxygen are, in fact, closely linked. Global warming is “the likely ultimate cause” of oxygen loss in the open ocean, according to the study. That is because warmer water absorbs less oxygen and also speeds up the metabolism of organisms in the sea, causing them to consume oxygen faster. Another issue is that warmer surface water mixes less readily with the oxygen-rich waters of the deep sea.

Closer to coasts, there are additional challenges. Fertilizer, sewage and other pollutants found in coastal runoff deliver an influx of nutrients that fuel coastal algae blooms and lead to oxygen-free “dead zones.” Since 1950, the area of the ocean at risk of developing dead zones has increased more than 10 times. Many more dead zones may exist in developing countries where monitoring is sparse, the scientists wrote.

Global Ocean Oxygen Network

“There will always be some areas of the sea that have low oxygen – just as there are deserts on land – the problem is when these areas expand and replace more productive ecosystems,” said Breitburg.

Matthew Long, an oceanographer with the National Center for Atmospheric Research who has authored several studies on deoxygenation but was not involved in the new report, said that he believes awareness of the problem is growing.

“I get the sense that there’s some momentum,” he said. “I think the scientific community is becoming more and more aware that we need to do a better job of communicating this issue so policymakers are aware of the potential impeding crisis.”

The report identified feasible steps to address dead zones near coastlines, such as reducing nutrient runoff through measures such as improving septic systems. However, slowing or halting the larger-scale decline of oxygen due to warming is a harder issue that “will take a global effort,” Breitburg said.

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