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Dead Zones: Warming Oceans Hold Less Oxygen

By Climate Central

June 8th is World Oceans Day, a day to raise awareness of the ocean’s importance to the planet. 93 percent of the excess heat absorbed by the climate system goes into our oceans, creating major consequences. While more extreme storms and rising sea levels are some of the impacts of warmer oceans, rising CO2 levels and the resulting warmer oceans are impacting ocean health itself. The most well­known effects are coral bleaching and ocean acidification, but an emerging issue is the decreasing oxygen levels in the warming waters.

Oxygen enters the ocean through two ways: interactions between its surface and the atmosphere, and as a photosynthesis byproduct from phytoplankton in upper layers of the ocean, much in the same way plants on land produce oxygen. As the ocean waters warm, they aren't able to hold as much oxygen (decreased solubility). Normally, oxygen­rich water mixes with deeper layers of the ocean that tend to be oxygen­poor. But warmer water creates an ocean that is more intensely stratified, meaning it is less likely to mix with the colder water found below. As a result, not only is less oxygen dissolved in the oceans, but it’s harder for it to be mixed to deeper areas.

When water loses most of its oxygen, it creates areas referred to as “dead zones,” where most marine plants and animals cannot survive. While these dead zones are often caused by fertilizer runoff currently, it seems the decreasing ability of the oceans to hold dissolved oxygen with continued warming will increasingly become a problem for marine ecosystems and fisheries in the coming decades.

A recent study found that 15 percent of the observed oxygen loss in oceans since 1960 could be attributed to the effects of warming water. Since 1960, the researchers found that dissolved oxygen content in oceans has declined by 2 percent on average. This percentage is not evenly distributed, and some oceans have lost significantly more. Another recent study projects that with unabated greenhouse gas emissions, oxygen lows will fall below their current range by midcentury. Marine ecosystems will thus either need to adapt or move in order to survive.

Warmer oceans also lead to coral bleaching. Coral reefs, the “rainforests of the sea,” are important ocean ecosystems that protect coastlines, support fisheries, and draw in a lot of tourism around the world. When ocean waters get too hot, corals expel the algae that live within them, losing their source of nutrients, turning white and sometimes dying.

In addition, the more carbon dioxide in the atmosphere, the more is taken up by the oceans leading to ocean acidification. When carbon dioxide dissolves in the ocean, it reacts with water to produce carbonic acid, a process that depletes the ocean of carbonate ions, which corals and many other animals use to build shells and reef structures. Some of these animals may be small, such as phytoplankton, but they form the base of the ocean food chain and are critically important for overall ocean health.

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