Diminishing Oxygen Levels in World Oceans

Large areas of the world's oceans are already suffocating from a lack of oxygen.

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We know that climate change is affecting the temperature of the world's oceans and causing them to warm and rise. Acid rain is changing the chemical makeup of ocean waters. And pollution is clogging the oceans with harmful plastic debris. But new research indicates that human activity may be detrimental to marine ecosystems in another way, too - by depriving these biomes of oxygen, affecting all living creatures that make their home in the world's waters.

Scientists have known for years that ocean deoxygenation might become a problem. In 2015, National Geographic found that roughly 1.7 million square miles of the world's oceans had low oxygen levels that were becoming inhospitable to marine life.

But a recent study led by Matthew Long, an oceanographer at the National Center for Atmospheric Research, showed just how big of a problem this environmental issue might be - and how soon it could start affecting marine ecosystems. According to Long, climate change-driven oxygen loss is already happening in certain ocean zones. And it will likely be “widespread” by 2030 or 2040.

For the study, Long and his team used simulations to predict ocean deoxygenation levels through the year 2100. According to their calculations, large sections of the Pacific Ocean, including the areas surrounding Hawaii and off the West Coast of the U.S. mainland will become noticeably devoid of oxygen by 2030 or 2040. Other oceanic zones, such as the coasts of Africa, Australia, and South Asia may have more time, but will likely experience climate-change induced ocean deoxygenation by 2100.

Long's study, which was published in the journal Global Biogeochemical Cycles, paints a grim view of the future of the world's ocean ecosystems.

Why is the Ocean Losing Oxygen?

The deoxygenation of the ocean is occurring as a direct result of climate change. As ocean waters warm, they absorb less water from the atmosphere. Compounding the issue is the fact that the oxygen found in warmer - less dense - water does not circulate as readily into deeper waters. 

“It’s that mixing that’s responsible for sustaining oxygen levels at depth,” Long said in the study. In other words, when ocean waters warm, they don't mix as well and any oxygen that is available stays locked in shallow waters.

How Does Ocean Deoxygenation Affect Marine Ecosystems?​

What would this mean for marine ecosystems and the plants and animals that call them home? A biome devoid of oxygen is a biome devoid of life. Ocean ecosystems that experience oxygen deoxygenation will become uninhabitable for any and all living things.

Some marine animals - like dolphins and whales - might not be directly affected by a lack of oxygen in the ocean, because these animals come up to the surface to breathe. But they would still be indirectly affected by the suffocation of the millions of plants and animals that do draw oxygen directly from ocean waters. Many plants and animals in marine ecosystems rely on oxygen that either enters the water from the atmosphere or is released by phytoplankton via photosynthesis.

“What is very clear is that if the trend of human warming continues — which it seems likely to do given the relative inactivity on curtaining CO2 emissions — oxygen levels in the ocean at depth will continue to decline and there will be significant impacts on marine ecosystems,” Long said. “As oxygen levels decline, more and more of the ocean is going to be uninhabitable by certain organisms. Habitat will become more fragmented, and the ecosystem will become more vulnerable to other stressors.”

From coral bleaching to acidification to rising waters to plastic pollution, the world's oceans are already experiencing their fill of stressors. Long and his team worry that diminishing oxygen levels could be the tipping point that pushes these biomes over the edge and on to a point of no return.