The Oceans Are Losing Oxygen

Two recent publications have shown that the oxygen level in the world's oceans has decreased over the last few decades.

Decline in global oceanic oxygen content during the past five decades, by Sunke Schmidtko, Lothar Stramma and Martin Visbeck, a letter in the scientific journal Nature in February 2017, found the average oxygen content decreased by 2% since 1960 in the zone between 100 and 1000 meters deep. Some areas experienced as much as 4% decrease with the largest decreases being measured in the tropical and North Pacific Ocean. Depths between 1000 and 2000 meters also experienced oxygen loss. While there are multiple reasons for the oxygen level to be decreasing, ocean warming is a main cause. The upper-level of the ocean is typically saturated with oxygen - even supersaturated. But, this saturation level goes down as the temperature goes up. The authors of this paper attribute 50% of the upper-level loss to thermal causes and 25% to thermal causes when the deeper layer is included. Overall, 15% of the total ocean oxygen loss is attributed to global warming, which is consistent with predictions from models. This is also consistent with reports showing the oceans have been taking up the vast majority of global warming heat.

The second paper is Upper ocean O₂ trends: 1958–2015, by Takamitsu Ito, Shoshiro Minobe, Matthew C. Long, and Curtis Deutsch, and appeared in the journal Geophysical Research Letters (GRL) May 2017.They found a decrease in ocean oxygen levels since 1980 with a corresponding increase in the ocean heat content. They find that, while most of the decline can attributed to non-thermally induced changes in solubility, they find the measured changes are still beyond what can be expected from natural fluctuations alone. Their study also reveals a tight relationship between O₂ inventories and the ocean heat content and these findings were consistent with models. They state the trends they have documented are suggestive of the effects of the ocean warming beginning to supersede natural variability and emerge as a recognizable signal.

So, how does a warming ocean lead to a decrease in dissolved oxygen?

As the oceans get warmer, the solubility of gases decreases. The oceans' ability to dissolve both carbon dioxide and oxygen decreases. There is very little good about this. You can point out the acidification of the oceans would decrease if less CO2 is absorbed, and that would be good. However, the acidification will continue because the oceans are not saturated with CO2, even at the higher temperatures. Instead, the amount of CO2 taken up by the oceans will decrease and the rate of increase of atmospheric CO2 will increase. 

But, the more important point is the decrease in oxygen, which the marine life needs to survive. This will impact fisheries, tourism and ocean nutrient cycles. Lower oxygen levels particularly affect larger animals, who find they don't have enough oxygen to engage in high-energy activities like feeding. Fisheries around the world are already stressed by over-fishing and pollution and now have another stress factor to make things worse. According to the United Nations Food and Agricultural Organization, over one billion people worldwide (17% of the population) rely on seafood as their primary source of protein. Over 3 billion people get almost 20% of their protein from fish. So, the problem extents beyond simply warmer air temperature.

The Schmidtko, et al. paper also discusses the outgassing of N2O - nitrous oxide. Nitrous oxide is both a greenhouse gas and attacks the ozone layer, providing a double whammy effect, neither of which is good for us. As a greenhouse gas, it is about 300 times as effective as CO2. Nitrous is formed and destroyed in the ocean by bacteria and is normally made and destroyed at nearly the same rate. Oxygen-depleted zones create an imbalance between formation and breakdown and a surplus of nitrous is the result. Some gets absorbed by the water. The excess then gets released into the atmosphere but that amount increases with decreasing solubility. Fortunately, the concentration of atmospheric nitrous is very low - about a thousanth as dense as CO2 - and ocean production is estimated to be only about 4 million tons per year, which is much lower than worldwide CO2 production. But, these new studies indicate it may be increasing as the ocean loses its oxygen. Considering the size of the oceans, that has the potential to be a very serious problem. And, the studies indicate even a small imbalance in the bacterial formation/destruction cycle can have large effects on the amount of gas being released to the atmosphere.

Overall, these studies indicate another area of concern about global warming and climate change.