Jessica Camille Aguirre writes:

People who make their living catching fish on the open ocean first noticed the strange phenomenon a few decades ago. It occurred in the shadow zones, the spots between the great ocean currents where sea water doesn’t circulate, off the coasts of Peru, West Africa, and California. The fisher people shared the knowledge among them like a common secret, a bounty that had an even stranger explanation: Sometimes, when the conditions were right, fish would swim closer to the surface of the seas. This made them easier to catch, as the shimmering hordes pushed their way upward, where sunlight filtered through the waters and the oxygen was rich. For the fishing trawlers, this was a wild boon. For the fish, it was something else—the shadow zones, low in oxygen, were expanding, and wildlife habitats were shrinking. Swimming upward, the fish were trying to catch their breath.

It wasn’t until the late 2000s that scientists formally identified what was happening. Observing time-series data from a handful of research stations in Hawaii, Bermuda, and the North Pacific, researchers noticed that the world’s oceans had been losing oxygen, probably for half a century. The existence of these shadow zones—where ocean circulation wasn’t robust and marine life sparse, called Oxygen Minimum Zones, or OMZs—was already well documented. But scientists found that these areas were expanding; they also saw that the ocean was deoxygenating on a systematic level, affecting every area of the seas. In addition to providing places where marine wildlife can thrive, oxygen levels are a critical harbinger of the planet’s health—and unlike ocean acidification, another ecological crisis affecting the pH levels of the oceans, deoxygenation is seen as a change to which adaptation is impossible.

Researchers say the world’s oceans lost 2 percent of their oxygen between 1960 and 2010, a rate that would leave the oceans entirely devoid of oxygen in just a few thousand years, making them uninhabitable to most life. The causes of this deoxygenation are myriad, but can mostly be traced back to anthropogenic climate change caused by increasing carbon dioxide in the atmosphere and related global warming trends. These carbon emissions are mostly produced by burning fossil fuels, and even if they were to stop immediately, they have already set in motion processes that will continue to affect the oceans for decades to come. [Continue reading…]