Mass extinctions in the oceans could be prevented by stopping fossil fuels

By the end of the Permian period about 252 million years ago, there was one giant continent on Earth. In the ocean the bony fish swim with bony shields and sea scorpions the size of a modern human. Arthropods such as trilobites predominated the deep sea, along with a variety of arpods, which looked like shells but were not, and ammonites, whose spiral shells resembled nautilus, but were actually squid or octopus.

The fact that we still know these animals today is due to their fossil remains. At the end of the Permian period, 90 percent of life in the seas disappeared, due to the largest mass extinction in Earth’s history. Scientists suspect that this is caused by the release of a large amount of carbon dioxide2, possibly due to volcanic activity in an area called the Siberian Steps. A research team showed in 2018 that the most common cause of death at the time was likely physiological stress from rising sea temperatures and low oxygen levels, a byproduct of climate change caused by greenhouse gases.

Two of the scientists who made this discovery, in a paper recently published in the journal Science That warming waters and declining oxygen levels in the oceans could lead to a mass extinction that could rival Earth’s five largest catastrophes if we don’t control greenhouse gas emissions. According to them, the mass deaths would be so extensive that much of the species diversity that has occurred since the extinction at the end of the Cretaceous period, 65 million years ago (leading to the extinction of the dinosaurs), has been negated.

But according to the researchers, we can still turn the tide. If we could end emissions quickly, the risk of species extinction could be reduced by 70 percent. By combining lower greenhouse gas emissions with measures to end, among other things, ocean pollution, overfishing and habitat destruction, marine life is more likely to survive in the long term.

“Even if we reduce our emissions quickly, about five percent of species could still be lost at sea,” said study co-author and climate scientist Curtis Deutsch of Princeton University. With a two-degree increase in temperature, about ten percent of the species may be lost. This results in changes in the species balance that occur in most places. But these are relatively small numbers. Then there will be no mass extinction.

Dennis Breitburg, an ocean oxygen expert at the Smithsonian Environmental Research Center who was not involved in the study, called the findings “shocking, but also significant.” She adds that the research provides “a ground for hope” that “we can conserve a significant portion of marine life”.

“This article outlines the choices we face,” says oceanographer Malin Pinsky of Rutgers University in New Jersey. I participated in writing an opinion article attached to the scientific article. “This appears to be a once-in-a-lifetime opportunity in human history to protect the future of life on this planet.”

Oxygen-deficient water

The crux of the new research from Deutsch and principal investigator Justin Penn, who is also a Princeton University researcher, is that they look not only at the effect of rising temperatures on oxygen levels in the oceans, but also at how life in the sea uses oxygen.

Over the past 15 years, studies have shown that oxygen-poor areas of the oceans are expanding rapidly, but not at the same rate everywhere. As a result, much of the marine life is being pushed toward a shrinking layer of oxygen-rich water near the surface. These hypoxic areas, including the Bay of Bengal, part of the Atlantic Ocean off the coast of West Africa and large parts of the eastern Pacific, have increased by more than four million square kilometers since the 1960s and are getting nearly one meter higher each year. Over the past quarter century, off the coast of Southern California, the oxygen level at a depth of about two hundred meters has decreased by more than thirty percent in some places. Since the middle of the last century, the volume of completely oxygen-deficient areas of the sea has increased fourfold.

In contrast to these “dead zones” along the coast, which occur regularly in the Gulf of Mexico, for example, the lack of oxygen in these areas is not caused by pollution from fertilizers flowing into the sea. It is the result of overheating. As surface waters become warmer, they absorb less oxygen from the layer of air above them. Since warm water is lighter than cold water in the lower layers, there is less mixing of the water, so that oxygen ends up in smaller amounts in the deeper layers.

This phenomenon is already affecting marine species; The habitats of some species are shrinking and thus the prey of other species is more concentrated in certain areas. For example, a sailfish must dive to a depth of less than tens to hundreds of meters to get food. Therefore, like sharks, tuna, cod, herring and mackerel, they spend much more time around the surface of the water, which makes them easy prey for fishermen (or birds or turtles).

Other changes happen as well, sometimes very strange changes. Some crabs and squids see worse when there is less oxygen. Many small zooplankton, which serve as food for large marine creatures, already survive on the minimum oxygen that microorganisms need. When the oxygen level drops further, the animals will have to search for new areas to survive. Low oxygen levels hinder the reproduction of some species of fish and cause more diseases in others.

Other changes happen as well, sometimes very strange changes. Some crabs and squids see worse when there is less oxygen. Many small zooplankton, which serve as food for large marine creatures, already survive on the minimum oxygen that microorganisms need. When the oxygen level drops further, the animals will have to search for new areas to survive. Low oxygen levels hinder the reproduction of some species of fish and cause more diseases in others.

“It’s really concerning,” said Matthew Long, who works as an oceanographer at the National Center for Atmospheric Research. “As a result of increased global warming, we are altering the basic metabolism of the largest ecosystem on Earth.”

An unlikely but useful scenario

Ben and Deutsch have collected metabolic data for dozens of marine species, from crustaceans to sharks, from all oceans, latitudes and depths. They looked at how much oxygen each species needed to survive. They also collected data on pre-existing warming and, using computer simulations, determined the effect on oxygen demand and habitat for each species if temperatures continued to rise.

“There are all kinds of good arguments for why this is a representation of the global status of a wide range of species, while we’re looking at a relatively small number of species,” Deutsch says.

Some species, such as tuna, may move naturally as their habitat dwindles, while less mobile species, such as corals, will not have this ability. Using data from fossils, the pair were also able to determine the maximum amount of habitat a species or local population could lose before it became extinct. They set their models and projections against the changes in the oceans they found in 2018 that led to the extinction at the end of the Permian.

The researchers found that under a higher emissions scenario (where emissions continue to increase, which many scientists now believe is unlikely), ocean warming and lower oxygen levels would have wiped out more species than all other ocean problems (such as overfishing and pollution) together. But the decline of species will not be the same everywhere. Most species disappear into the oceans in the tropics, but many of these species will then migrate to cooler regions. Animals found primarily in higher latitudes, such as the North Pacific, where much of the fish is caught for the US market, will be most vulnerable.

“Tropical fish are more likely to survive because these species have already adapted to the warm, low-oxygen conditions that are increasingly common on Earth,” Benn said. “Animals that love the cold and need a lot of oxygen have nowhere to go.” The same pattern (with a greater risk of extinction for species living around the poles) was also reflected in the fossil records of extinctions at the end of the Permian period.

Ben and Deutsch concluded that if greenhouse gas emissions are not reduced by the end of this century, a mass extinction such as the one that occurred at the end of the Permian is expected around the year 2300. Although this scenario is unlikely (due to the increased use of solar energy and wind energy, although this process is still very slow), may be an important lesson. Even if the future is less bleak, the mechanisms that ended marine life 252 million years ago remain in force. (Other factors have played a role in the other four mass extinctions in Earth’s history, such as global cooling and the effects of meteorites.)

According to biologist Karen Wechner of the University of Rhode Island, the latest research is “impressive,” she says, and the researchers see the “big picture.” But, she adds, ocean life is complex and much remains unknown about how species respond to changing conditions. “There are certain species that have their own way of adapting,” Wechner said.

According to Deutsch, the main conclusion is that it is predictable how many species will be lost. “This process is somewhat linear,” he says. For every half degree of warming, a few percentage points more species die.

This means that some species are bound to go extinct, even if we cut emissions quickly; Global temperatures have already risen by about one degree. But if we can keep this warming below two degrees, as agreed in the 2015 Paris climate agreement, the number of species that have disappeared could be less than ten percent.

If you think there are about 2.2 million species in the ocean, “that’s still a huge number in absolute terms,” ​​says Benn. “But it’s ten times less than it could be.”

This article was originally published in English at

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