Story at a glance

  • Fish and other marine life need oxygen to survive.
  • But the oceans now hold less oxygen than they once did, and the consequences could be dire.
  • Ocean warming due to climate change and nutrient pollution from industrial agriculture are the two main causes of the decline in oxygen.

The world’s oceans are losing oxygen because of climate change and other human activities. Ocean oxygen levels showed a dangerous decline of roughly 2 percent globally between 1960 and 2010, according to a new report released at the United Nations climate talks in Madrid last week. Without significant global action to curb greenhouse gas emissions, the loss of oxygen will climb to 3 or 4 percent by 2100.

The authors of the report say large fish species like tuna, sharks and marlin that have fast metabolisms are particularly at risk, but that the lack of oxygen could cause entire ecosystems to collapse. 

Dead zones, or areas which lack sufficient oxygen to support marine life, have gotten 400 percent larger over the last 50 years. There were just 45 areas with dangerously low levels of oxygen worldwide in 1960 — now there are 700. 

The loss of oxygen, or deoxygenation, is being caused by climate change and human activities such as intensive agriculture that send harmful quantities of nutrients, often in the form of chemical fertilizers, into the sea, the New York Times reports

Humans are heating up the planet by pumping more and more greenhouse gases into the atmosphere, and the oceans have absorbed 93 percent of that excess heat since 1950. This is a problem for marine life, because warm water can’t hold as much oxygen as cold water. The hotter the oceans get the less oxygen they’re capable of storing. 

The warmer water also isn’t evenly distributed, concentrating its oxygen depleting effects. The warming comes from contact with the air, so much of the hot water stays trapped close to the surface, forming a distinct layer that reduces ocean mixing. Reduced mixing can exacerbate dead zones and low oxygen zones in deeper waters and prevent critical deep water nutrients from reaching creatures that live near the surface. What’s more, animals use more oxygen when their body temperatures are higher, creating even greater demands on the dwindling supply of oxygen in the ocean’s shallower waters. 

Ocean warming is also killing coral reefs via coral bleaching, and, because warm water takes up more room than cold water, it’s also accelerating sea-level rise. The greenhouse gases, such as carbon dioxide, behind all this warming are also being absorbed by the oceans — making them about 26 percent more acidic compared to pre-industrial levels. In some places this increasing acidity is dissolving the shells of creatures like clams and shrimp. 

Industrial agriculture is also removing oxygen from the world’s oceans. When the chemical fertilizers used in industrial agriculture or the manure from livestock wind up in the ocean they can fuel huge blooms of microorganisms. When those microorganisms die, the bacteria decomposing them can use up all the oxygen in the area — which can lead to the creation of a dead zone. Two years ago, a dead zone measuring more than 8,000 square miles appeared in the Gulf of Mexico, directly downstream of many of the largest meat producers in the U.S.

The report’s release during the UN climate conference hopes to inspire greater urgency on the part of world leaders to act on climate change. “To reverse the loss of oxygen, people, industry and governments must urgently cut carbon dioxide emissions and reduce ocean nutrient pollution,” write the authors of the report. 

But others point out that reducing overfishing could also help the oceans remain resilient in the face of climate change. “Ending overfishing would strengthen the ocean, making it more capable of withstanding climate change and restoring marine ecosystems — and it can be done now,” fisheries expert Rashid Sumaila told the Guardian. “The crisis in our fisheries and in our oceans and climate are not mutually exclusive problems to be addressed separately — it is imperative that we move forward with comprehensive solutions to address them.”

Published on Dec 09, 2019