To the list of global problems the world's oceans are facing, you can add another: They're losing oxygen.
The Pacific Ocean off the U.S. West Coast, from central California to Alaska, is one of the hardest-hit areas.
Whether you're looking at an ocean or a glass of beer, the same fundamental chemistry holds true.
"When you warm up the water, it holds less gas," University of Washington oceanographer Curtis Deutsch said.
We land-dwelling humans might understandably focus on air temperature as an indicator of climate change — we feel it every day on our skin. Yet very little of the heat trapped by our greenhouse gas pollution stays in the atmosphere.
Over the past half-century, more than 90 percent of the extra heat has gone into the oceans.
Deutsch is part of a team of oceanographers that has documented how climate change is squeezing the air out of oceans in the journal Geophysical Research Letters. Deutsch calls the temperate North Pacific – from the Russian Far East to the U.S. West Coast – a hotspot for the loss of ocean oxygen.
"That whole swath of ocean seems to be declining in oxygen," he said. "Quite likely, we're sitting on one of the areas of the world's coastline that is going to experience relatively strong depletions of oxygen in this century."
Oxygen levels in the sea vary for lots of reasons, including biological activity, coastal pollution and short-term cycles like El Niño. The scientists detected a "widespread negative O2 trend" apart from any year-to-year variations over the past half-century.
The seas' dissolved oxygen content increased slightly until the mid-1980s, then declined afterward.
A recent study by German researchers found that the world's oceans have lost about 2 percent of their oxygen over the past 50 years. Nearly a fifth of all the lost oxygen has gone missing from the North Pacific Ocean, according to the team from Germany's Helmholtz Centre for Ocean Research.
"Far-reaching implications for marine ecosystems and fisheries can be expected," the German team concluded in the journal Nature in February.
Oceans are shedding oxygen more than twice as fast as warming water alone can explain.
Why they are losing oxygen so fast isn't entirely clear. A slowdown in the currents that stir up the oceans is a likely factor. Oceanographers expect vertical mixing to diminish as climate change warms surface waters more than colder, denser waters beneath them.
Other factors, including an increase in fishes' and other animals' demand for oxygen under water, could also be at play. In warmer water, metabolisms speed up. So ocean creatures need to breathe more oxygen to survive, even as their oxygen supply is slowly being choked off.
"The loss of oxygen should be right at the top of the list of global ocean ecosystem concerns," Deutsch said. "The changes in oxygen are big in many places."
In a bit of a vicious cycle, robbing the seas of oxygen could worsen global warming by increasing the oceans' production of nitrous oxide. The "laughing gas" anesthetic is a byproduct of bacterial activity in low-oxygen settings. Nitrous oxide is also a potent greenhouse gas, able to trap nearly 300 times as much heat, molecule for molecule, as carbon dioxide, the main greenhouse gas.
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