The climate and oceans are warming due to man-made greenhouse gas emissions. Researchers have now modeled the impact on ocean life. Because warmer water can store less oxygen, numerous species would be threatened with extinction before the end of this century, according to the forecast. If global warming continues unchecked, a mass extinction could occur that would be comparable to the largest mass extinctions in the history of the earth. However, if we manage to limit global warming to less than two degrees Celsius above pre-industrial levels, the model says that the risk of extinction would fall by 70 percent compared to the worst-case scenario.
In 2021, the average temperature of the world’s oceans was the highest it has been since records began. But the higher the temperatures, the less oxygen the water can store. At the same time, the heat increases the metabolic rate of sea creatures, so they need more oxygen. If the oxygen supply is no longer sufficient to cover the needs of the species, they can no longer live in the corresponding habitat. Climate change thus poses a direct threat to numerous marine life. However, the extent to which the biodiversity of the oceans is endangered has so far been unclear.
Threatening mass extinctions in the sea
Justin Penn and Curtis Deutsch from the University of Washington in Seattle have now modeled marine biodiversity under different climate scenarios. To do this, they took into account how much warming and lack of oxygen various species can still tolerate and when a habitat becomes uninhabitable for them. They compared this with climate models that predict the development of global temperatures depending on human greenhouse gas emissions. Additionally, they drew parallels to past mass extinctions documented by fossils.
The result: “Under the scenario with high emissions, global species loss could reach a level comparable to the largest mass extinctions in the past,” the researchers say. According to the forecast, the greatest loss of biological diversity would 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. This would also have an impact on human nutrition: “The particularly endangered regions include highly productive ecosystems,” write Penn and Deutsch. “These regions are also home to many of the world’s most productive fisheries, providing about 17 percent of humanity’s dietary protein.”
Emigration from the tropics, extinction at the poles
According to the researchers, the tropical species could have a chance of avoiding extinction if they migrate to regions further away from the equator, where water temperature and oxygen levels would be tolerable for them. Species native to colder waters do not have this option. If their habitat becomes too warm, there are no longer any regions to which they can escape. “Polar species therefore have the highest risk of becoming extinct worldwide,” explain Penn and Deutsch.
They have already shown a similar pattern in earlier works in which they dealt with the largest known mass extinction in the history of the earth, the “Great Dying” at the end of the Permian around 250 million years ago. As fossils show, more than two thirds of all sea creatures died out at that time. “Similar environmental changes seen at the end of the Permian, including rising temperatures and declining ocean oxygen levels, are now evident in the Anthropocene,” the researchers write.
limit emissions
For their worst-case scenario, the authors assumed that CO2 emissions would continue to rise unchecked, which would lead to global warming of 4.9 degrees Celsius above the pre-industrial value by 2100. “But it’s not too late to reduce greenhouse gas emissions to prevent a mass extinction,” the researchers said. “The extent of extinction that we have found depends very much 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 extinction by more than 70 percent . “Rapid reductions in greenhouse gas emissions are critical to avoiding major mass extinctions of marine species,” Deutsch said.
Source: Justin Penn and Curtis Deutsch (Washington University, Seattle, Washington), Science, doi: 10.1126/science.abe9039