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Climate change is extending far beyond the threat of melting polar ice caps -- it's putting a dangerous stranglehold on life in oceans, too.
A new study published in the science journal Nature Wednesday found that the ocean's worldwide oxygen content declined by more than 2% between 1960 and 2010.
Scientists have long warned about the potentially deadly consequences of the ocean's declining oxygen levels on marine life, and its resulting impact on humans.
The study came from the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, where the three co-authors -- Sunke Schmidtko, Lothar Stramma and Martin Visbeck -- pulled data dating back to 1960.
Using information on oxygen, temperature and other factors relating to the world's oceans, they mapped it around the globe and estimated the overall oxygen loss.
“We were able to document the oxygen distribution and its changes for the entire ocean for the first time. These numbers are an essential prerequisite for improving forecasts for the ocean of the future,” wrote Schmidtko.
Where bacteria thrives
While 2% may sound like only a small change, it doesn't take much of a drop to threaten the state of oceans. The only organism in the ocean that thrives with little-to-no oxygen is bacteria.
“Just a little loss of oxygen in coastal waters can lead to a complete change in ecosystems -- a small decrease in oxygen like this can transform from something desirable to very undesirable,” said David Baker, Assistant Professor at the University of Hong Kong's Swire Institute of Marine Sciences.
Oxygen in the world's oceans is not evenly distributed, and the 2% drop represents just an average of all the globe's oceans together.
In some parts of the world, there has been a much steeper decline of oxygen levels over the past five decades -- for example, in the North Pacific, where the largest volume of oxygen was lost. The largest percentage was lost in the Arctic Ocean.
“The oxygen losses in the ocean can have far-reaching consequences because of the uneven distribution. For fisheries and coastal economies this process may have detrimental consequences,” wrote Stramma.
The ocean's oxygen depletion, the study shows, is mostly a result of climate change.
One driving factor, the authors found, is as simple as warming temperatures -- like a warm can of Coke that can't hold fizz, warm ocean water has difficulty holding oxygen. But this only accounts for 15% of the oxygen depletion.
What also causes oxygen depletion, again driven by climate change, is that the ocean is becoming more stratified. This is a result of changing temperature gradients in the Arctic, and the reduction of sea ice.
Oxygen enters the water at the surface, but as that surface layer gets warmer, it's less likely to sink to the oxygen-starved layers below.
“It's almost like the oceans are getting ready for a heart attack,” said Baker. “You're essentially slowing the heartbeat of the ocean, and you're getting less oxygen to the ocean.”
The study finds that the reduction of sea ice has led to more plankton growth -- and with more plankton growth comes more plankton decomposition. Decomposition decreases oxygen levels even further.
So-called “dead zones” -- low-oxygen areas in the ocean's shallow waters -- are also multiplying along the shore, the study finds.
Fish can't thrive there -- a dangerous threat to both the ecosystem and the economy -- but that's not the only problem. These areas are pumping out a harmful greenhouse gas called nitrous oxide – “the evil counterpart to carbon dioxide,” as Baker puts it.
Nitrous oxide is potent. It lasts in the atmosphere a long time and contributes to global warming -- meaning that the effects of climate change on the world's oceans causes more global warming, in turn.
The study's authors end their report on a pessimistic note, writing that “far-reaching implications for marine ecosystems and fisheries can be expected.”
While Baker said that the world is edging closer to solving the carbon dioxide problem, it's barely scratched the surface when it comes to the issue of nitrous oxide.
He said that there's little hope in addressing this in a timely way unless people commit to addressing problems that are quickly becoming cliché, like consuming less and recycling more.
“The oceans are really a mirror of human health -- if they're sick and dying, then that's the future of humanity as well,” said Baker.
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