By the year 2100, about 310 gigatons of carbon will have been added to the oceans – a potential ‘tipping point’ for ecological disaster, according to the study.
When CO2 dissolves into the ocean – as in the case of the End-Permian mass extinction – life on Earth is at risk.
Professor Daniel Rothman from the Massachusetts Institute of Technology came up with a simple mathematical formula to predict when the next mass extinction would take place.
The formula predicts that by the end of the century oceans will hold enough carbon to launch a mass extermination of species in the future.
The model showed the critical extra amount required is about 310 gigatons, which is around the best case scenario projected by the Intergovernmental Panel on Climate Change (IPCC).
And it’s well below the worst of more than 500 gigatons that would far exceed the line.
In all scenarios the study found by the end of the century the carbon cycle will either be close to, or well beyond, the threshold for catastrophe.
It also found although mass extinction won’t soon follow at the turn of the century the world may have tipped into ‘unknown territory.’
Professor Rothman says it would take some time, about 10,000 years, for such ecological disasters to play out.
He said: ‘This is not saying disaster occurs the next day.
‘It’s saying – if left unchecked – the carbon cycle would move into a realm which would be no longer stable and would behave in a way that would be difficult to predict.
‘In the geologic past this type of behaviour is associated with mass extinction.’
In the modern era CO2 emissions have risen steadily since the 19th century, but deciphering whether this could lead to mass extinction has been challenging.
That’s mainly because it’s difficult to relate ancient carbon anomalies, occurring over thousands to millions of years, to today’s disruptions which have taken place over little more than a century.
Professor Rothman’s analysis identified ‘thresholds of catastrophe’ in the carbon cycle that, if exceeded, would lead to an unstable environment, and ultimately, mass extinction.
He had previously done work on the end-Permian extinction, or Great Dying.
Since then conversations with colleagues spurred him to consider the likelihood of a sixth, raising an essential question.
Professor Rothman said: ‘How can you really compare these great events in the geologic past, which occur over such vast timescales, to what’s going on today, which is centuries at the longest?.
‘So I sat down one summer day and tried to think about how one might go about this systematically.’
His unique equation factored in the critical rate and magnitude of change in the carbon cycle to the timescale that separates fast from slow change from 31 events in the last 542 million years in which a significant change occurred in the carbon cycle.
For each, including the five mass extinctions, he noted the change in carbon, expressed in the relative abundance of two telltale chemicals in the geologic records.
He then devised a mathematical transformation to convert these quantities into the total mass of carbon that was added to the oceans during each event.
Finally, he plotted both the mass and timescale of each event.
‘It became evident that there was a characteristic rate of change that the system basically didn’t like to go past,’ said Professor Rothman.
There was a common threshold that most of the 31 events appeared to stay under. They were relatively benign – not enough to destabilise the system toward catastrophe.
In contrast four of the five mass extinctions lay over the threshold – with the ‘Great Dying’ being the farthest over the line.
‘Then it became a question of figuring out what it meant,’ said Professor Rothman.
He added: ‘There should be ways of pulling back emissions of carbon dioxide.
‘But this work points out reasons why we need to be careful, and it gives more reasons for studying the past to inform the present.’
Humans have emitted 1,540 billion tonnes of CO2 since the industrial revolution.
That’s equivalent to burning enough coal to form a square tower 72 feet wide stretching 240,000 miles from Earth to the Moon.
Half of these have remained in the atmosphere causing a rise in levels at least 10 times faster than any known natural increase during Earth’s long history.
Most of the other half has dissolved into the ocean, causing acidification.
The full results of the study were published today in the journal Science Advances.
FIVE GREAT EXTINCTION EVENTS
Five times, a vast majority of the world’s life has been snuffed out in what have been called mass extinctions.
End-Ordovician mass extinction
The first of the traditional big five extinction events, around 540 million years ago, was probably the second most severe. Virtually all life was in the sea at the time and around 85 per cent of these species vanished.
Late Devonian mass extinction
About 375-359 million years ago, major environmental changes caused a drawn-out extinction event that wiped out major fish groups and stopped new coral reefs forming for 100 million years.
End-Permian mass extinction (the Great Dying)
The largest extinction event and the one that affected the Earth’s ecology most profoundly took place 252 million years ago. As much as 97 per cent of species that leave a fossil record disappeared forever.
End-Triassic mass extinction
Dinosaurs first appeared in the Early Triassic, but large amphibians and mammal-like reptiles were the dominant land animals. The rapid mass extinction that occurred 201 million years ago changed that.
End-Cretaceous mass extinction
An asteroid slammed down on Earth 66 million years ago, and is often blamed for ending the reign of the dinosaurs.