An algal bloom killed these sardines in Chile in 2016. (Image: AP)

You are probably aware that global temperatures are rising thanks to human-made greenhouse gases emissions. You might not be aware of some of the many associated side effects, for instance, the fact that our oceans have been losing oxygen over the past few decades.

Past studies have observed or modeled this effect, but a new five-decade data review from a team of German scientists found that the world’s oceans have lost around two percent of their oxygen on average in recent history, with different regions experiencing different levels of loss. Global temperatures rose one degree Celsius on average during the same period, according to NASA data. Our favorite life-sustaining breath molecule also sustains life below the ocean’s surface, so add this to your stack of reasons we should be doing something, anything at all, about climate change.

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Ocean dwellers, like us humans, require oxygen molecules to stay alive. Fertilizers running into the ocean can cause algal blooms, which gobble up oxygen to form dead zones where fish can’t thrive, but that’s not the only thing stripping the ocean of its O2. Oxygen in the ocean is dissolved in water, and when water warms, its ability to hold onto trapped gasses decreases. As we’ve previously reported, not only does oxygen escape from the surface, but rising temperatures decrease the density of surface water, making it less likely to sink and transport fresh oxygen to the deeper ocean.

For the new study, which was published today in Nature, the researchers pulled data on ocean salinity, temperature, depth, and oxygen since 1960 from several databases, and mapped it around the world. They found an overall two percent decrease in the average oxygen concentration of the planet’s oceans, equaling around 5 petamoles, or 80 billion metric tons, of oxygen. Losses were especially notable in the northern Pacific Ocean and southern Atlantic. Surprisingly, scientists only attributed around 15 percent of the loss to the solubility decrease from warming. The rest was due to more complex processes, including new oxygen not circulating all the way to the ocean’s depths. Worse, the amount of ocean without any oxygen at all has quadrupled, creating new dead zones.

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Some experts found the seemingly slight decrease an immediate cause for concern. Ocean climate researcher Denis Gilbert at the Maurice Lamontagne Institute of Fisheries and Oceans Canada wrote the following in a response in the journal Nature:

Most marine organisms require oxygen to survive. A two percent decrease of ocean oxygen content may not sound like much, but the implications of this for marine ecosystems could be severe in parts of the ocean where oxygen is already low, such as oxygen minimum zones (a region 200 to 1000 meters below the surface with especially low oxygen), because critical survival thresholds may be crossed. Moreover, when other stress factors associated with global warming — such as increased CO2 and warmer waters — combine with lower oxygen levels, the cumulative effects on marine life can be even worse.

Other scientists agree. “Indeed, there is broad agreement across models that indicates we should expect precipitous declines in dissolved oxygen to become evident about now,” Matthew Long, oceanographer at the National Center for Atmospheric Research, told Gizmodo in an email. “It is alarming to see this signal begin to emerge clearly in the observational data. Since ocean deoxygenation is intrinsically linked to climate warming, substantial action to mitigate climate change will be required to reverse the trend of oxygen loss.”

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So, all that is to say that climate change is having more impacts than just melting Arctic ice caps. It’s influencing the breathability of the ecosystems covering 70 percent of our planet.

[Nature]