Climate change could cause a massive extinction of the oceans

The climate and oceans are warming due to anthropogenic greenhouse gas emissions. Scientists have now modeled the effects on ocean life. As warmer water can store less oxygen, many species are projected to be threatened with extinction by the end of this century. If global warming is not stopped, there could be a mass extinction that would be comparable to the greatest mass extinction in Earth’s history. However, if we manage to limit global warming to less than two degrees Celsius above pre-industrial levels, the model says the risk of extinction would drop by 70 percent compared to the worst-case scenario.

In 2021, the average temperature of the world’s oceans was the highest since the beginning of breaking records. But the higher the temperature, the less oxygen the water can store. At the same time, heat increases the metabolic rate of sea creatures, so they need more oxygen. If the oxygen supply is no longer sufficient to meet the needs of the species, they can no longer live in a suitable habitat. Climate change is therefore a direct threat to many marine organisms, but the extent to which ocean biodiversity is at risk has not been clear so far.

Threat of mass extinction at sea

Justin Penn and Curtis Deutsch of the University of Washington in Seattle modeled marine biodiversity in different climate scenarios. For this purpose, they took into account how much warming and lack of oxygen can tolerate different species and when the habitat becomes uninhabitable for them. They compared this with climate models that predict the development of global temperatures as a function of human greenhouse gas emissions. Additionally, they outlined parallels to previous mass extinctions documented by fossils.

Result: ‘In a high-emission scenario, global species loss could reach levels comparable to the worst mass extinctions in the past,’ the researchers say. According to the forecast, the greatest loss of biodiversity is to be expected in tropical waters. In their warm water, the oxygen content is already low. If temperatures continued to rise, these regions would become uninhabitable for most species. It would also have an impact on human nutrition: “Regions particularly at risk contain highly productive ecosystems,” write Penn and Deutsch. “These regions are also home to some of the world’s most productive fisheries, providing about 17% of the protein in humanity’s diet.”

Emigration from the tropics, polar extinction

According to the researchers, tropical species may have a chance of avoiding extinction if they migrate to regions further from the equator where water temperature and oxygen levels would be tolerated for them. Species from colder waters do not have this possibility. If their environment becomes too warm, there are no more regions to escape to. “Polar species are therefore at the highest risk of extinction anywhere in the world,” explain Penn and Deutsch.

They had shown a similar pattern in earlier work in which they dealt with the largest known mass extinction in the history of the Earth, the “great dying out” at the end of the Permian some 250 million years ago. As the fossils show, more than two-thirds of all sea creatures died out during this time. ‘Similar environmental changes seen at the end of the Permian, including rising temperatures and falling oxygen levels in the oceans, are now visible in the Anthropocene,’ the researchers write.

reduce emissions

In a worst-case scenario, the authors assumed that CO2 emissions would continue to rise in an uncontrolled manner, leading to global warming 4.9 degrees Celsius above pre-industrial levels by 2100. “But it is not too late to reduce GHG emissions to prevent mass extinctions, ‘the researchers said. “The extent of the extinction we discovered depends a lot on how much CO2 we emit in the future.” According to the analysis, limiting warming to a maximum of two degrees Celsius would reduce the extent of the extinction by more than 70 percent. “Rapid reductions in greenhouse gas emissions are critical to avoid a massive extinction of marine species,” said Deutsch.

Source: Justin Penn and Curtis Deutsch (Washington University, Seattle, Washington), Science, doi: 10.1126 / science.abe9039

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