Climate change is posing three deadly threats for marine life: acidification (lowering pH), rising temperatures and decreased oxygen levels. NOAA just reported that ocean acidification is spreading rapidly in the Arctic. Mass bleaching is hitting the world’s coral reefs due to warming temperatures. Deep sea life will face starvation in the next few decades.

Oxygen depletion happens naturally, mostly around the world’s coasts and shallow waters, but a new study published in the journal Nature found a eye popping 2% decline in ocean oxygen in just 50 years. It will plummet up to a 7% loss before the end of this century. The recent study confirms a previous study published 2016 in Global Biogeochemical Cycles, a journal of the American Geophysical Union. “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”.

The Guardian reports:

Scientists have long predicted ocean deoxygenation due to climate change, but confirmation on this global scale, and at deep sea level, is concerning them. Last year, Matthew Long, an oceanographer at the National Center for Atmospheric Research in Colorado, predicted that oxygen loss would become evident “across large regions of the oceans” between 2030 and 2040. Reacting to the German findings, Long said it was “alarming to see this signal begin to emerge clearly in the observational data”, while Roberts said, “We now have a measurable change which is attributable to global warming.” The report explains that the ocean’s oxygen supply is threatened by global warming in two ways. Warmer water is less able to contain oxygen than cold, so as the oceans warm, oxygen is reduced. Warmer water is also less dense, so the oxygen-rich surface layer cannot easily sink and circulate.

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Fish that rely on dissolved oxygen will grow more slowly, peak at a smaller body size, and produce fewer offspring. And, Roberts pointed out, larger fish such as tuna, swordfish and sharks will be badly affected given their greater dependence on larger amounts of oxygen – they will be driven into ever narrower bands of oxygen-rich water near the surface, as will much of their prey, leading to more competition for food sources and other changed behaviour. One knock-on effect is likely to be an increase to overfishing: “The eastern Pacific has huge tuna fisheries already,” he pointed out. “If the tuna can’t dive down where it is uninhabitable, as oxygen deficient areas expand, they have less space at the surface, they’re squeezed into ever tighter spaces and they’re more vulnerable to being caught.”

It is becoming evident that the oceans are taking a major hit from the burning of fossil fuels as they absorb 30% of the carbon that we produce worldwide.

AGU reports:

The entire ocean—from the depths to the shallows—gets its oxygen supply from the surface, either directly from the atmosphere or from phytoplankton, which release oxygen into the water through photosynthesis. Warming surface waters, however, absorb less oxygen. And in a double whammy, the oxygen that is absorbed has a more difficult time traveling deeper into the ocean. That’s because as water heats up, it expands, becoming lighter than the water below it and less likely to sink. Thanks to natural warming and cooling, oxygen concentrations at the sea surface are constantly changing—and those changes can linger for years or even decades deeper in the ocean. For example, an exceptionally cold winter in the North Pacific would allow the ocean surface to soak up a large amount of oxygen. Thanks to the natural circulation pattern, that oxygen would then be carried deeper into the ocean interior, where it might still be detectable years later as it travels along its flow path. On the flip side, unusually hot weather could lead to natural “dead zones” in the ocean, where fish and other marine life cannot survive.

CNN reports that the loss of oxygen is not the only problem that the study identified.