On more than one occasion over the past 540 million years, the Earth has lost most of its species in a relatively short geologic time period.
These are known as mass extinction events, and they often follow closely in the wake of climate change, whether due to extreme warming or extreme cooling, caused by asteroids or volcanic activity.
When Kunio Kaeo attempted to measure the stability of Earth’s average surface temperature and the planet’s biodiversity, he found a largely linear effect. The greater the temperature change, the greater the extent of extinction.
For global cooling events, the largest mass extinctions occurred when temperatures fell by about 7 °C. But for global warming events, Kaiho found that the largest mass extinctions occurred at about 9°C warming.
This is much higher than previous estimates, which suggested that a temperature of 5.2 degrees Celsius would lead to a major marine mass extinction, on a par with the previous “Big Five”.
To put that in perspective, by the turn of the century, modern global warming is on track to increase surface temperatures by as much as 4.4 degrees Celsius.
Caio predicts that “a global warming of 9°C will not appear in the Anthropocene until at least 2500 under the worst-case scenario.”
Kaiho does not deny that many extinctions on land and at sea are already caused by climate change; He just does not expect the same percentage of losses as before.
However, it is not only the degree of climate change that puts the species at risk. The speed at which it occurs is very important.
The largest mass extinction event on Earth killed 95 percent of known species at the time and occurred more than 60,000 years ago about 250 million years ago. But today’s warming is occurring on a much shorter time scale thanks to human emissions of fossil fuels.
Perhaps more species will die in Earth’s sixth extinction event not because the scale of warming is too great, but because the changes happened so quickly that many species couldn’t adapt.
“Predicting the magnitude of future anthropogenic extinctions using only surface temperature is difficult because the causes of anthropogenic extinctions are different from the causes of mass extinctions in geologic time,” Caillou admits.
No matter how scientists split the data, it’s clear that many species are doomed to extinction unless we can stop climate change.
The exact percentage of losses and the timing of those losses remain under discussion.
The study was published in Biological Sciences.