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Life on Earth, beginning with simple microorganisms that evolved and diversified into the incredible breadth of species living today, has always been turbulent.
Mass extinction events happen seemingly without warning, wiping out countless species and allowing others to take their place.
But it’s not just disasters like supervolcanos and asteroid impacts that cause mass extinction.
New research suggests that around the time of five of the biggest mass extinction events on record, Earth’s chemistry ‘wobbled’.
In hindsight, a warning sign of the change to come.
The new study, published in Nature Communications, shows that Earth’s climate occupied five distinct climate regimes separated by sharp transition periods of rapid environmental changes over the past 539 million years.
During these transition periods, the scientists found, life on Earth was particularly vulnerable to mass extinction, offering a new historical explanation for why some time periods were marked by such extreme loss of biodiversity.
The research describes a metric for overall ‘biosphere vulnerability’ – combining rates of origination and extinction with standing biodiversity – indicating whether or not Earth is in a period where mass extinctions are more likely.
This measure for biotic stress “increases when we have increases in extinction rates and origination rates which also represent a drive to adapt to new and challenging conditions,” study co-author Andrej Spiridonov, a paleontologist and Earth systems scientist at Vilnius University in Lithuania, told ScienceAlert.
“It also increases when diversity decreases, thus also reflecting the contraction of biotic possibilities. It should be noted that vulnerability is an ensemble metric, and when the biospheric state is vulnerable, it doesn’t mean that every single species or even region of the world is ‘stressed’.”
If you want to get into the math, the biosphere vulnerability index itself is a simple formula calculated from the natural logarithm of the total intensity of total turnover – the combined rates at which taxa originate and go extinct, relative to standardized standing diversity.
In short, it compares the rate of ecological change to the overall biodiversity to indicate whether or not an ecosystem is unstable and therefore vulnerable to even more drastic change.
Putting the index to the test, the researchers found that overall vulnerability varied across the five time periods they identified, each lasting 10 to more than 100 million years.
During these time periods – the researchers called them ‘Haggis bins’ because some of their graphs reminded them of the Scottish dish – heightened vulnerability to extinction and biodiversity loss largely correlated with increased temperatures.
Eras with higher temperatures, they found, tended to coincide with greater background biosphere vulnerability.
But biosphere vulnerability skyrocketed whenever one period was ending and another beginning – while the ‘Big Five’ mass extinctions were also associated with vulnerability peaks.
That doesn’t mean that environmental turbulence causes mass extinction, Spiridonov said. For instance, we know that in some of these cases, another event like a massive asteroid impact was involved.
But it shows that these transitions can serve as a warning sign that life on Earth will temporarily be more fragile than normal, and more susceptible to another disaster coming along.
“When the system approaches a critical transition,” he said, that offers information “on possible future impending great change, such as a mass extinction or even a series of them.”
What does that mean for today?
The study offers a historical analysis – it cannot predict what will happen to humanity or other species currently navigating an ongoing climate crisis and period of accelerated extinction.
Related: Earth’s Biggest Mass Extinction Reveals Chilling Parallels to Today’s World
“Our study suggests that the current geological era – the Cenozoic – is exceptionally stable having low vulnerability,” he explained.
“It doesn’t mean that the external impact by geophysical forces or the human impact wouldn’t have any significant effects on the evolution of the biosphere. It just means, that under different background conditions, which prevailed in other geological eras, such a disturbance would have had much higher effect.”
The research has been published in Nature.
This article was fact-checked by Peter Dockrill and edited by Peter Dockrill. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.