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 affects 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 [carbon dioxide], which is a positive feedback to warming.”
Deoxygenation of the ocean can effect life on land as changes in photosynthesis of marine plant life can impact oxygen levels in the atmosphere. About half of the oxygen we breathe comes from land plants, and the other half from ocean plants called phytoplankton.
“Oxygen loss from warming oceans in the atmosphere is something like [a] 10 percent reduction over a century,” Deutsch said. “It’s not trivial but not huge either.”
He added that oxygen loss could be reversed as quickly as it has dropped if world leaders take significant steps to cut carbon emissions.
“As soon as it starts to cool down it will start to reoxygenate. It wouldn’t happen overnight, or in a year or a decade, but it happens on a fairly similar timescale as it did to lose I would think,” Deutsch said.
However, major reductions in populations of marine life are likely, even if they don’t go extinct, according to Deutsch.
Because the impacts from oxygen loss are not widely understood, Levin said oceanography experts are forming a coalition to better communicate the risks.
The Paris-based Intergovernmental Oceanographic Commission is hosting a meeting in San Francisco this weekend to form an ocean-oxygen research network aimed at gathering observations, research and communications, Levin said.
“So the people who study this will be formed into a network bringing together coastal researchers studying dead zones and open ocean researchers [who] study climate change-induced oxygen loss, and we hope that pretty soon the oxygen community will be speaking with a more unified voice,” Levin said.
Despite the Paris agreement not addressing climate change’s effects on the ocean or ways to mitigate that, Levin hopes the new coalition will be able to elevate the issue as world leaders continue to meet every five years to update the treaty.