The 6th mass extinction hasn't begun yet, study claims, but Earth is barreling toward it

The present extinction rate of life on Earth doesn't qualify as a mass extinction event yet — but current trends show that it eventually will, a new study finds. The number of species that are currently hurtling toward extinction has led many ecologists to argue that we are undergoing a sixth mass extinction — but we are only witnessing the beginning and it will likely get much worse. 

According to a new study, however, the percentage of extinctions caused by global temperature increases from climate change will not reach the same level as a major mass extinction event, at least not in the near future.

There have been five major mass extinctions throughout Earth's 4.5 billion-year history, and scientists look to those cataclysms of the distant past to understand how climate change is now affecting global diversity in ways that may be irreversible. 

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During a mass extinction, a high percentage of global biodiversity is extinguished faster than it can be replaced, and this happens over a relatively short period of time by geologic standards — less than 2.8 million years, according to the Natural History Museum in London. Species may go extinct for a number of reasons, so to understand what a "normal" extinction rate looks like, ecologists measure what is known as the "background rate"' of extinction, said the study's sole author Kunio Kaiho, a professor emeritus at the Department of Earth Science at Tohoku University in Japan. 

According to Kaiho, "5-10% species extinctions in 1 million years corresponds to the background rate." A higher rate, such as "more than 10% species extinction in a short time (e.g., hundreds of years) is a significant event," Kaiho told Live Science in an email.  

However, estimating the background rate of extinction for past epochs can be "really tricky," because fossil records tend to overrepresent larger, more abundant species, said David Storch, a professor in the Department of Ecology at Charles University in Prague who was not involved in the new study. That being said, "the current rate of extinction is about two orders of magnitude higher than the normal rate of extinction," Storch told Live Science. 

Major mass extinctions result in "more than 60% species loss," Kaiho said. However, "minor mass extinction [events] occurred more frequently." In the new study, published July 22 in the journal Biogeosciences, Kaiho argues that changes in climate cause higher extinction rates, but that the present rate cannot yet be considered a mass extinction event by this strict definition. 

The previous five major mass extinction events were the Ordovician-Silurian extinction (about 440 million years ago), the Late Devonian extinction (about 365 million years ago), the Permian-Triassic extinction (about 253 million years ago), the Triassic-Jurassic extinction (about 201 million years ago) and the Cretaceous-Paleogene extinction (about 66 million years ago). These events have also been associated with drastic changes in Earth's climate, such as changes in surface temperature (both warming and cooling), acid rain, ozone depletion, reduced sunlight, desertification, soil erosion and a reduction of oxygen in the ocean, Kaiho reported. But according to Storch, changes in atmospheric and oceanic chemistry played bigger roles in these extinctions than global warming or cooling did. (These changes are linked, as global warming can increase the oceans' acidity as well as the atmosphere's composition, but volcanic activity also played a large role.)

"The climate change detected during these last mass extinctions may not be the [sole] cause of the extinctions, but [the rate of extinction] may be the consequence of the other global changes which happened at that time," Storch said.

Because prior mass extinctions were triggered by volcanic eruptions and, in the case of the Cretaceous event, an asteroid impact, the resultant changes in climate were rapid and drastic. In the study, Kaiho argues that the speed of environmental change is more important than the magnitude of change alone in causing massive extinction rates, because "during slow climate changes, animals can migrate to survive."

In order to meet the definition of a major mass extinction event, scientists would need to observe the extinction of 60% of species and 35% of genera (the plural of genus). However, just because this magnitude of extinctions has not been observed yet, does not mean it is not currently underway. The sixth extinction differs from its predecessors because it is driven by human-made climate change. Kaiho's paper argues that because the pace of such climate change is gradual, rather than abrupt and drastic, we are unlikely to see extinction rates in the near future that meet the definition of a major mass extinction event, but they may well qualify for a minor mass extinction.

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"An increase in average global temperatures of 9 degrees Celsius [16.2 degrees Fahrenheit] is essential for major mass extinctions coinciding with global warming," and such an increase would not happen "at least till 2500 under the worst scenario," Kaiho said. Because the rate of species extinction changes in parallel with global surface temperatures, we will not see an abrupt and massive loss of species, but rather a slow and steady rate of species extinction in the near future, which will not culminate in the loss of 60% of earth's species, Kaiho wrote in the study.

These findings come with an important caveat from many ecologists: the present rate of extinction is only an estimate and may be an inaccurate one. According to a January 2022 study published in the journal Biological Reviews, the number of species extinctions recorded is highly biased toward mammals and birds and overlooks many invertebrates, therefore significantly underestimating the true rate of species extinction. For now, according to David Storch, other human-driven actions such as habitat transformation through deforestation and pollution, as well as overhunting and the introduction of non-native species, play a much bigger role in driving the present rate of species extinction than rising average global temperatures.

Originally published on Live Science.