Earth has gone through five mass extinctions in the past 540 million years – the most prominent among them in the human imagination being the Ice Age that ended the reign of the dinausaurs some 50 years ago. These mass extinctions involved processes that heavily disrupted the normal cycling of carbon through the atmosphere and oceans.
Now, a new study says the next mass extinction may well occur within the lifetimes of some of us those now alive. Daniel Rothman, professor of geophysics in the Department of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology and the co-director of MIT's Lorenz Center, came up with a study "thresholds of catastrophe" which shows that the earth might face its sixth extinction by 2100.
The carbon emissions that are mainly responsible for such mass extinctions have increased significantly since the 19th century.
In his study, Rothman analysed the five mass extinctions that occurred in the past 540 million years and their carbon cycles.
He proposed his study on the basis of two thresholds - first, if the changes in carbon cycle occur for longer time frames and the rate is faster than global ecosystems can adapt to, and second, the carbon perturbations that take place over shorter time frames. The pace of the carbon-cycle changes will not matter; instead, the size of the change will determine the likelihood of an extinction, he concludes.
Rothaman feels a sixth extinction will depend on whether a critical amount of carbon is added to the oceans. He analysed that 310 gigatonnes carbon is required to lead earth to its 6th mass extinction, which is likely to happen by the year 2100.
"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 behavior is associated with mass extinction," he says.
Rothman also studied the end-Permian extinction, an ecological disaster which wiped out 95 per cent of the marine species due to an increase in the carbon emission. And that's when he decided to start the quest to explore the 6th mass extinction.
"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?" Rothman asks. "So I sat down one summer day and tried to think about how one might go about this systematically."
Rothman derived a simple mathematical formula which helped him come to the conclusion that mass extinctions is on its way.
The formula was based on basic principles that related the critical rate and magnitude of change in the carbon cycle to the timescale that separates fast from slow changes. Rothman hypothesises that the formula should predict whether mass extinction, or some other sort of global catastrophe, should occur.
Rothman searched through hundreds of published geochemistry papers and identified that 31 events in 542 million years witness an unavoidable change in Earth's carbon cycle.
For every event, which also includes the five mass extinctions, Rothman observed the change in carbon along with the duration of time. Rothman expressed in the geochemical record as a change in the relative abundance of two isotopes, carbon-12 and carbon-13.
The mathematical formula helped him convert these quantities into the total mass of carbon that was added to the oceans during each event. 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," Rothman says.
In July, a study published in the Proceedings of the National Academy of Sciences said that at least 50 per cent of the number of animals that once shared the earth with humans are already gone, and that the next two decades would witness more powerful attacks on biodiversity.
"There should be ways of pulling back emissions of carbon dioxide," Rothman says. "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."