Man-made climate change is already cutting into oxygen levels in some parts of the world's oceans and could start producing new "dead zones" in some parts of the seas by 2030, US researchers warned this week.
Seawater has natural swings in the amount of oxygen it holds.
But warming makes it harder for that water to absorb and distribute oxygen — and if carbon emissions continue on their current path, human-produced warming is likely to lead to falling oxygen concentrations across large areas of the oceans, scientists at the National Center for Atmospheric Research have found.
"It's very difficult to pull out the human driven-trend until that trend becomes so pronounced that it falls outside the envelope of that natural variability," said Matthew Long, an oceanographer at the Colorado-based center and the leader of the new study.
So Long and his colleagues used computer models to calculate the extent of the natural swings and projected when oxygen depletion that was clearly human-made was likely to start happening.
In some environments, like the Arctic — where temperatures are rising roughly twice as fast as they are across the planet as a whole — and the tropical Atlantic Ocean, the process has started already, the study concluded. Much of the Pacific Ocean is likely to start seeing large sections with reduced oxygen by 2030, while other stretches of the Pacific and much of the northern Indian Ocean will see little change by the end of the century.
"The fact that ocean deoxygenation is occurring and will continue to occur, likely at an accelerated rate, is a near-certainty if human-driven warming continues," Long said.
The findings were published this week in the peer-reviewed journal Global Biogeochemical Cycles, put out by the American Geophysical Union.
"I think for a long time that people thought the oceans were unchangeable that we could never have an impact on such large bodies of water," said Keryn Gedan, a marine ecologist at the University of Maryland, College Park. "This paper makes you realize how far our reach is and the impact that climate change is going to have globally."
Warmer tropical oceans, supercharged by the past year's El Niño phenomenon, are already causing a worldwide coral bleaching event that is killing the picturesque reefs of the tropical oceans. Add in a reduction in the concentration of dissolved oxygen, and the effect is likely to pose a threat to some forms of marine life.
"I think it has implications for the health of marine ecosystems," Long said. "And to the extent that we rely on those systems for a variety of services — one of them is a food supply — we're basically pushing the system towards a place where those services may be less reliable."
Another study out this week, published by the British journal Proceedings of the Royal Society B, points to the continental margins off the Pacific coasts of the Americas and the Arabian Sea from Pakistan to southeastern Africa as potential zones where lack of oxygen and ocean acidification — caused by the ocean absorbing more carbon dioxide from the atmosphere — could damage undersea ecosystems at depths of 200 meters (650 feet) or more.
Those are the areas where human activities like fishing and oil and gas exploration are most common, said Lisa Levin, a marine ecologist at the Scripps Institution of Oceanography and co-author of that paper.
"We rely on bottom fisheries in these areas. Our study was essentially on fish food," Levin said. "Species that form structures that provide biodiversity-sustaining habitat on the sea floor might be important to watch for susceptibility to ocean deoxygenation, acidification, and warming. They are also vulnerable to human disturbance."
How big an effect that reduced oxygen levels would have on marine life will depend on several variables, Gedan said.
"If an area has low oxygen to begin with, then any change is going to have fairly significant ecological repercussions," said Gedan, who studies coastal estuaries. Larger fish or other organisms can flee, but smaller animals or fixed species like coral would be in danger.
"We know that the shallow, coastal ocean is warming faster than the open ocean, especially in estuaries that are fairly sheltered," she said. "We're seeing numerous dead zones pop up all around the world, and that's going to become more common."
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