oxygen in the ocean

Are We Depleting the Amount of Oxygen in the Ocean?

Jane Marsh - March 25, 2019

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The level of oxygen in the ocean is falling, an issue scientists say calls for urgent attention. Decreasing oxygen levels could cause substantial harm to the health of the ocean and the life that depends on it.

Across the planet, oxygen levels have fallen by 2 percent in the last 50 years, according to research from GEOMAR Helmholtz Centre for Ocean Research Kiel in Kiel, Germany. If the problem goes unchecked, global ocean oxygen levels could fall by an average of 7 percent by 2100. Another study found that in some tropical regions, oxygen levels declined by as much as 40 percent. For every degree the ocean warms, oxygen concentration decreases by 2 percent.

Why Are Oxygen Levels Falling?

Scientists have attributed ocean oxygen depletion to global warming. Water has a higher heat capacity than air, so it takes up a large portion of the heating that occurs as the climate warms.

As the water warms, it expands, absorbs less oxygen and becomes less dense. This decreased density reduces the amount of mixing that occurs between the ocean surface and the deep ocean. As a result, oxygen that enters the ocean’s surface layers won’t circulate down to the deeper parts of the water, which means the water column becomes more layered. Warming water may also lead to other kinds of circulation changes, such as slower circulation and increased upwelling of deep water with low oxygen levels.

Impacts of Oxygen Depletion

Declining oxygen levels in the ocean will cause marine life to struggle to survive.

Fish will grow more slowly, not grow as large and produce fewer offspring, the Kiel researchers noted. As populations decline, overfishing will become a more pressing issue.

Conditions will be especially difficult for larger fish, which need higher amounts of oxygen. Large fish such as sharks, swordfish and tuna will get forced out of their habitats into areas with more oxygen. As fish get forced into smaller areas, there will be more competition for food sources. Higher temperatures also cause marine animals’ metabolisms to increase and their need for oxygen to rise.

You can see an extreme example of ocean deoxygenation by looking at the dead zones currently found in oceans across the globe. These dead zones are parts of the ocean in which oxygen levels are so low that no animal life can survive there. These zones can occur for several reasons, but a major cause is nutrient pollution. When nutrients enter the water in large quantities through wastewater or agricultural runoff, they cause excessive algae growth. The algae sinks and decomposes, consuming much of the oxygen that marine life depends on.

Other Problems Facing the Ocean

Of course, falling oxygen levels are not the only problem affecting our oceans. They also suffer from the effects of acidification, pollution, overfishing and other issues.

Emissions of greenhouse gases, such as carbon dioxide, are causing climate change. When we emit carbon dioxide into the atmosphere by burning fossil fuels and conducting other activities, the ocean absorbs about 30 percent of it. The ocean acts as something of a buffer to global warming, but the increasing carbon dioxide level also makes the water more acidic. The increased acidity makes it harder for marine creatures to build their shells and may cause reproductive disorders in some fish.

A lot of trash ends up in the ocean each year. Plastic waste is one of the biggest offenders, with more than 4.8 million metric tons of the stuff entering the oceans every year. Marine life can get stuck in plastic waste or choke on it, making plastic a significant threat to the health of sea creatures.

What Can We Do?

So, what can we do to stop the level of oxygen in the ocean from decreasing?

One crucial step is to continue researching ocean deoxygenation and improving our research methods. For example, scientists currently use Argo floats, autonomous robots that collect ocean oxygen data and send it to satellites. Increasing the number of these bots could improve the accuracy of researchers’ predictions.

It’s also important to increase awareness of ocean deoxygenation and its effects. It’s one of the lesser-known impacts of climate change, but it’s a vital issue. Increasing awareness of oxygen depletion could lead to policies that could help address it and increased funding for research.

Of course, we also need to get to the root of the problem and reduce our greenhouse gas emissions to prevent further global warming. We’re making progress on this front through decreasing our fossil fuel use and taking other actions, but there is still a long way to go. While this is certainly a massive undertaking, it may be one of the most important things we could do.

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About the author

Jane Marsh

Starting from an early age, Jane Marsh loved all animals and became a budding environmentalist. Now, Jane works as the Editor-in-Chief of Environment.co where she covers topics related to climate policy, renewable energy, the food industry, and more.