Oxygen deficiency in the ocean – looking bleak for polar marine life | Polarjournal
Polar species like the polar cod could literally run out of air if we fail to drastically reduce emissions. Photo: Shawn Harper/University of Alaska, Fairbanks/NOAA

In 2021, the ocean was warmer than at any time since human records had started. For marine life, this is anything but good news. Because when ocean temperatures rise, not only do corals shed their algae and bleach out as a result, or fish migrate northward or southward to cooler waters. A warming ocean also means that oxygen levels are decreasing, and this affects virtually all marine life. In a new study, U.S. researchers have modeled that mass extinctions may occur in the ocean if humanity does not drastically reduce greenhouse gas emissions quickly. Polar species would be most at risk.

The last mass extinction occurred at the end of the Permian, about 250 million years ago. It is the largest extinction in history to date, with 95 percent of marine species disappearing forever. This was caused by a warming of the climate due to high emissions, accompanied by a lack of dissolved oxygen in the ocean – conditions that we are approaching again under modern climate change, should anthropogenic emissions remain high. There could be a new mass extinction on a scale similar to the “Great Dying” 250 million years ago, the study’s authors state.

“Oxygen is a basic requirement, and there is no substitute for it. So the question is how much oxygen is enough and what is the minimum amount that a particular organism needs to survive,” says Curtis Deutsch, one of the two authors, who, like his co-author Justin L. Penn, conducts research at Princeton University’s Department of Earth Sciences and the University of Washington’s School of Oceanography.

Penn and Deutsch created a model using existing data on oxygen demand and availability to predict thermal limits and their effects on species. Their goals included determining whether a particular species is capable of surviving in a resting state with minimal activity, whether it could be active enough to reproduce, and whether it could survive a state of maximum effort.

“When the temperature rises, the ocean has less oxygen, but the marine species need more oxygen,” Penn tells Eos magazine.

Oxygen enters the surface layer from the atmosphere, which is thus rich in oxygen and well mixed. Beneath this is the density thermocline, which acts like a barrier and virtually prevents oxygen-rich surface water from mixing with the water layers below. Only in deep water formation zones in the Arctic and Antarctic does oxygen reach the depths. Inside the ocean, marine organisms consume oxygen. Graphic: World Ocean Review/maribus gGmbH

The two researchers modeled potential species loss under different emissions scenarios. Assuming low emissions and a temperature increase of 1.9 degrees Celsius by the end of the century, the model predicted species loss equal to current levels. “Significantly increased,” on the other hand, would be the number of extinct species under a high-emissions scenario, in which warming could be as high as 4.9 degrees Celsius.

Their calculations showed that polar species are the most endangered. “Polar species are most sensitive to temperatures and oxygen compared to the tropics because species in the tropics are already adapted to living in regions with higher temperatures and lower oxygen levels,” Penn explains.

And while tropical species have the option of migrating to cooler latitudes, polar species have no alternative. Their habitat is already slowly disappearing and there is no refuge for them as temperatures continue to rise. They would become globally extinct.

Species extinctions have usually been much more severe in the ocean than on land, yet marine ecosystems are often overlooked, said Pedro Manuel Monarrez, a postdoctoral fellow in the Department of Geological Sciences at Stanford University who was not involved in the study. “From what we’ve seen about the mechanisms of extinction in the fossil record in the oceans, the main factors are lack of oxygen and warming, which don’t have the same effects on land,” he explains. “However, most conservation efforts focus on terrestrial species because they are easier to spot.”

The loss of species and just a small reduction in biomass in the oceans not only has implications for marine ecosystems themselves, but would also threaten human food supplies worldwide.

According to their modeling, the authors consider the effects of accelerating climate change on marine life to be profound, increasing the risk of extinction for them more than at any time in the past tens of millions of years. Additional drivers for an increase in species loss are ocean acidification and thus declining primary production. Still, they say it’s not too late to reduce emissions to prevent the worst-case scenario.

Julia Hager, PolarJournal

Link to the study: Justin L. Penn, Curtis German. Avoiding ocean mass extinction from climate warming. Science, 2022; 376 (6592): 524. DOI: 10.1126/science.abe9039

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