Sixth mass extinction may begin by 2100: study

Increasing carbon levels in the oceans may lead to the sixth mass extinction in Earth's history by about 2100, MIT scientists have predicted after analysing data from the last 540 million years.

Researchers from Massachusetts Institute of Technology (MIT) in the US analysed significant changes in the carbon cycle over the last 540 million years, including the five mass extinction events.

They identified "thresholds of catastrophe" in the carbon cycle that, if exceeded, would lead to an unstable environment, and ultimately, mass extinction.

The last such event, known as the Cretaceous-Tertiary extinction, occurred about 66 million years ago. It wiped out some three-quarters of the plant and animal species on Earth.

Researchers proposed that mass extinction occurs if one of two thresholds are crossed.

First, the changes in the carbon cycle that occur over long timescales - extinctions will follow if those changes occur at rates faster than global ecosystems can adapt, they said.

Second, carbon perturbations that take place over shorter timescales - the pace of carbon-cycle changes will not matter; instead, the size or magnitude of the change will determine the likelihood of an extinction event, they said.

Researchers predict that given the recent rise in carbon dioxide emissions over a relatively short timescale, a sixth extinction will depend on whether a critical amount of carbon is added to the oceans.

Daniel Rothman, professor at MIT, derived a simple mathematical formula based on basic physical principles that relates the critical rate and magnitude of change in the carbon cycle to the timescale that separates fast from slow change.

He hypothesised that this formula should predict whether mass extinction, or some other sort of global catastrophe, would occur.

Rotham identified 31 events in the last 542 million years in which a significant change occurred in Earth's carbon cycle.

For each event, including the five previous mass extinctions, Rothman noted the change in carbon, expressed in the geochemical record as a change in the relative abundance of two isotopes, carbon-12 and carbon-13. He also noted the duration of time over which the changes occurred.

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.

That amount, calculated, is about 310 gigatons, which the researchers estimate to be roughly equivalent to the amount of carbon that human activities will have added to the world's oceans by the year 2100.

"It would take about 10,000 years for such ecological disasters to play out. However, by 2100 the world may have tipped into 'unknown territory'," said Rothman.

"This is not saying that disaster occurs the next day. It is saying that, 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," Rotham added.

"In the geologic past, this type of behaviour is associated with mass extinction," he added. The study was published in the the journal Science Advances.