Approximately half of the oxygen on Earth comes from the ocean and in the last 50 years the amount of water in the open ocean with zero oxygen has quadrupled. In bodies of coastal water, including estuaries and seas, low-oxygen sites have increased more than 10-fold since 1950.
Scientists expect oxygen to continue dropping even outside these zones as Earth warms. To halt the decline, the world needs to reduce nutrient pollution and rein in climate change, an international team of scientists asserted in a new paper published on 4 January 2018 in Science.
The study came from a team of scientists from GO2NE (Global Ocean Oxygen Network), a new working group created in 2016 by the United Nation’s Intergovernmental Oceanographic Commission.
“Oxygen is fundamental to life in the oceans,” said Denise Breitburg, lead author and marine ecologist with the Smithsonian Environmental Research Center. “The decline in ocean oxygen ranks among the most serious effects of human activities on the Earth’s environment.”
Climate change is the key culprit in the open ocean. Warming surface waters make it harder for oxygen to reach the ocean interior. Furthermore, as the ocean as a whole gets warmer, it holds less oxygen. In coastal waters, excess nutrient pollution from land creates algal blooms, which drain oxygen as they die and decompose.
“This is a problem we can solve,” Breitburg said. “Halting climate change requires a global effort, but even local actions can help with nutrient-driven oxygen decline.” As proof Breitburg points to the ongoing recovery of Chesapeake Bay, where nitrogen pollution has dropped 24 percent since its peak thanks to better sewage treatment, better farming practices and successful laws like the Clean Air Act. While some low-oxygen zones persist, the area of the Chesapeake with zero oxygen has almost disappeared. “Tackling climate change may seem more daunting,” she added, “but doing it is critical for stemming the decline of oxygen in our oceans, and for nearly every aspect of life on our planet.”
Winning the War: A Three-Pronged Approach
To keep low oxygen in check, the scientists said the world needs to take on the issue from three angles:
Address the causes: nutrient pollution and climate change.
While neither issue is simple or easy, the steps needed to win can benefit people as well as the environment. Better septic systems and sanitation can protect human health and keep pollution out of the water. Cutting fossil fuel emissions not only cuts greenhouse gases and fights climate change, but also slashes dangerous air pollutants like mercury.
Protect vulnerable marine life.
With some low oxygen unavoidable, it is crucial to protect at-risk fisheries from further stress. According to the GO2NE team, this could mean creating marine protected areas or no-catch zones in areas animals use to escape low oxygen, or switching to fish that are not as threatened by falling oxygen levels.
Improve low-oxygen tracking worldwide.
Scientists have a decent grasp of how much oxygen the ocean could lose in the future, but they do not know exactly where those low-oxygen zones will be. Enhanced monitoring, especially in developing countries, and numerical models will help pinpoint which places are most at risk and determine the most effective solutions.
Low-oxygen zones are spreading around the globe. Red dots mark places on the coast where oxygen has plummeted to 2 milligrams per liter or less, and blue areas mark zones with the same low-oxygen levels in the open ocean.