When it comes to climate change, there is bad news and there is even worse news. Into the latter category falls a new study in which scientists predict that more than two-thirds of the planet’s oceans will be deprived of enough oxygen by 2080 as a result of warming temperatures.
That lack of oxygen will decimate fish stocks worldwide, likely leading to food shortages in many parts of the world. The trend is already being felt as the increased loss oxygen driven by planetary warming passed a critical threshold last year.
Under scenarios devised by Chinese scientists in research that is the first to use climate models to predict how deoxygenation (the reduction of dissolved oxygen content in ocean water worldwide outside its natural variability), “more than 72% of the global ocean is projected to experience an emergence of deoxygenation before 2080 for all three vertical zones,” the researchers explain.
“Regionally, the emergence of deoxygenation is projected to be widespread below the epipelagic zone of the western North Pacific, North Atlantic, and Southern Oceans before 2080,” they add.
As water temperatures rise, ocean water retains less dissolved oxygen, which creates less circulation between ocean layers. For their study, the researchers charted the expected rates of deoxygenation in three ocean depth zones (shallow, middle and deep) through examining when the loss of dissolved oxygen in water will exceed natural fluctuations as the climate warms in coming decades.
“The middle layer of the ocean is particularly vulnerable to deoxygenation because it is not enriched with oxygen by the atmosphere and photosynthesis like the top layer, and the most decomposition of algae — a process that consumes oxygen — occurs in this layer,” the scientists note.
That is why this so-called mesopelagic zones, which range from depths of about 200 meters to 1,000 meters, will be the first to lose significant amounts of oxygen in both of their climate models with different rates of carbon emissions in coming decades.
The finding is a troubling because “this zone is actually very important to us because a lot of commercial fish live in this zone,” explains Yuntao Zhou, an oceanographer at Shanghai Jiao Tong University who was the study’s lead author. “Deoxygenation affects other marine resources as well, but fisheries [are] maybe most related to our daily life.”
In another finding, the researchers discovered that the parts of the oceans closer to the poles such as the west and north Pacific as well as the southern seas are particularly vulnerable to deoxygenation, although the reason for this will need to be further examined.
Meanwhile, tropical oxygen minimum zones, which are areas with already low levels of dissolved oxygen, appear to be spreading. “The oxygen minimum zones actually are spreading into high latitude areas, both to the north and the south,” Zhou says.
“Even if global warming were to reverse, allowing concentrations of dissolved oxygen to increase, whether dissolved oxygen would return to pre-industrial levels remains unknown,” the scientists say.
Importantly, the new findings highlight the urgent need for effective climate mitigation policies.
“Humanity is currently changing the metabolic state of the largest ecosystem on the planet, with really unknown consequences for marine ecosystems,” stresses Matthew Long, an oceanographer at the National Center for Atmospheric Research who was not involved in the research. “That may manifest in significant impacts on the ocean’s ability to sustain important fisheries.”
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