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'The next great casualty of climate change' could be global fishing stocks

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In a year that has seen the disastrous effects of climate change unfold with frightening speed — from drought and famine to heat domes, wildfires and deadly flash flooding — another potential catastrophe has come into view: depleted oxygen levels in the world's oceans and lakes that threaten marine life. 

"As ocean and atmospheric scientists focus on climate, we believe that oceanic oxygen levels are the next big casualty of global warming," researchers Julie Pullen and Nathalie Goodkin wrote in an opinion piece published Tuesday in Scientific American

In part due to the effects of rising global temperatures, growing portions of the oceans have lost "10-40 percent of their oxygen," and that figure is forecast to continue growing due to climate change. Rising water temperatures and depleted oxygen, which pollution and nutrient runoff also make worse, have been blamed for mass die-offs of fish this year in states like Florida, California, Oregon, Montana, Louisiana, Virginia, Pennsylvania, Missouri, Washington, Idaho, Delaware and Minnesota. While climate change is not the only cause for the fish kills, it is, researchers say, a contributing factor. 

"As the amount of CO2 increases in the atmosphere, not only does it warm air by trapping radiation, it warms water. The interplay between oceans and the atmosphere is complex and interwoven, but simply, oceans have taken up about 90 percent of the excess heat created by climate change," the authors wrote.

Thousands of dead fish float in the Boca Ciega Bay located near the mouth of Madeira Beach in Madeira Beach, Fla., in July. Red tide, which is formed by a type of bacteria, has killed several tons of marine life in Florida so far this year.
Thousands of dead fish float in Boca Ciega Bay in Florida in July. (Octavio Jones/Getty Images)

When a heat dome covered much of the Pacific Northwest this summer, rising temperatures in streams and rivers resulted in mass die-offs of salmon and trout. An estimated 1 billion marine animals along the coast of Canada were also killed as a result of that heat wave. 

That grim reality has brought the urgency of global warming home for even conservative residents of his home state, Sen. Jeff Merkley, D-Ore., told Yahoo News earlier this month at the U.N. Climate Change Conference in Glasgow, Scotland. 

"Fifteen years ago in rural parts of the state, people would say, 'Oh, this is just some Ivy League invention,'" Merkley said. Today those same constituents, many of whom are fishermen, understand that "the trout streams were warmer and smaller, and it affects them." 

Climate change, Pullen and Goodkin wrote, is upsetting the delicate balance that helps provide abundant marine life. 

"Bodies of water can absorb CO2 and O2, but only to a temperature-dependent limit. Gas solubility decreases with warming temperatures; that is, warmer water holds less oxygen. This decrease in oxygen content, coupled with a large-scale die-off of oxygen-generating phytoplankton resulting not just from climate change, but from plastic pollution and industrial run-off, compromises ecosystems, asphyxiating marine life and leading to further die-offs," they wrote. 

With roughly 3 billion people worldwide who depend on fishing to make their living, sustaining oxygen levels in the world's oceans and lakes is certain to be just one more challenge in the era of climate change. 

"Roughly 40 percent of the world depends on the ocean for their livelihoods. If we do not stop marine life from oxygen-starvation, we propagate a further travesty on ourselves," the authors wrote. 

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