Threat from oxygen loss in oceans left off table in Paris talks

Threat from oxygen loss in oceans left off table in Paris talks

 Many know about acidification and rising sea levels from climate change; few realize the threat from oceans’ oxygen loss

While some effects of climate change on the oceans—including acidification and rising sea levels — are well known, its effects on oxygen levels in the seas have largely been left out of the discussion at COP21 in Paris.

“There’s lots of meetings about small island nations, thinking about rising seas, and talking about acidification, but not about deoxygenation of the ocean,” said Lisa Levin, an ocean oxygen expert at the Scripps Institution of Oceanography in San Diego.

Like acidification and rising sea levels, low oxygen zones in the ocean also appear to be expanding as a result of climate change, Levin said.

But as world leaders are wrapping up talks aimed at a treaty to slow global warming, Levin said many of the negotiations failed to address the impacts of climate change on the oceans. The word “oceans” is mentioned once in the preamble of the latest draft of the agreement, Levin said. 

This means climate change’s impact on oceans won’t be addressed by actions agreed to in the global treaty.

In the Pacific Ocean off the coast of southern California, oxygen levels have dropped between 20 to 30 percent in the past 50 years, Levin said. Globally, the losses are around 20 percent. The declines are expected to continue as ocean temperatures increase.

Low-oxygen zones occur a few hundred feet below the well-oxygenated surface, which gets a steady supply of oxygen from mixing with the atmosphere.

The boundaries of low-oxygen zones have been expanding, making more of the ocean uninhabitable to marine life that needs higher levels of oxygen. Such habitat loss could lead to a loss in biodiversity, scientists say.

“It will have pretty profound reverberations on water-breathing animals, and I think it’s likely to be comparable to the effects on ecosystems to ocean acidification,” said Curtis Deutsch, an associate professor in chemical oceanography at the University of Washington in Seattle.

There are multiple mechanisms that lead to low oxygen levels in oceans, scientists say. 

Climate change has warmed up the earth’s atmosphere. In turn, the ocean has absorbed a significant amount of that heat — in some cases around 90 percent, Levin said. Warm water holds less oxygen than cold water, and as the oceans warm it becomes more stratified and the deep and shallow layers mix less.

“If you go swimming in a lake in higher latitudes, in summer it’s nice and warm on the surface but just below the surface it becomes really cold on a sharp gradient,” said Tony Koslow, a fisheries ecologist at Scripps. “Basically, warm water is lighter and it expands and floats on the surface.”

The effect of ocean stratification on mixing is important, Koslow said. Deeper waters get their oxygen mostly from the atmosphere. In the winter, storms help break down the warm-water surface layer and well-oxygenated water gets mixed down with deeper waters.

Unlike surface levels of the ocean, deep waters cannot get oxygen from photosynthesis because there’s not enough light, Koslow said. In fact, deeper waters are usually losing oxygen because bacteria consume oxygen while digesting dead organic material like algae or phytoplankton that has sunk.

That risks undoing the balance of oxygen gained in deeper oceans and oxygen consumed by bacteria, Koslow said, which if left unchecked could have significant consequences.

“The mass extinctions in the past appear to be linked to periods of deoxygenation, that’s why we’re concerned,” said Koslow, who is working with the National Oceanic Atmospheric Administration to examine ecological change in the oceans.

To make matters worse, lower oxygen levels in the ocean can exacerbate climate change through feedback loops that lead to more warming.

“As the ocean warms, its capacity to contain dissolved gases decreases. Thus a warmer ocean will have less dissolved (oxygen),” Bradley Sageman, chair of the Earth and Planetary Sciences department at Northwestern University, said in an email. “It will also have a lower capacity to absorb