In the past 550 million years, the Earth has endured five mass extinction events, which caused whole families, orders and even classes of creatures to disappear from the earth in a short period of time. Now, experts predict that the mankind is facing the sixth great extinction because the rate of extinction for species are thousands of times faster than the past due to human activities leading to damage to the plants growing environment, excessive exploitation of natural resources, pollution and climate change.
Study reveals that each extinction event involved processes that upended the normal cycling of carbon through the atmosphere and oceans. These globally fatal perturbations in carbon each unfolded over thousands to millions of years, and are coincident with the widespread extermination of marine species around the world. Until fairly recently, scientists found no answer to when the sixth great extinction would happen, mainly because they had not identify the connection between massive carbon anomalies in ancient times and carbon changes over the past century.
Recently, the answer seems to have surfaced - 2100. Daniel Rothman, professor of geophysics in the MIT Department of Earth, Atmospheric and Planetary Sciences has analyzed significant changes in the carbon cycle over the last 550 million years, including the five mass extinction events. He has identified “thresholds of catastrophe” in the carbon cycle. He then compared his prediction to the total amount as projected in a report of the Intergovernmental Panel on Climate Change (IPCC), and found that the carbon emissions to the oceans would either be close to or well beyond the threshold for catastrophe by 2100, leading to the sixth mass extinction.
Threshold for catastrophe
Rothman had previously done work on the end-Permian extinction, the most severe extinction in Earth’s history, in which a massive pulse of carbon through the Earth’s system was involved in wiping out more than 95 percent of marine species worldwide. Since then, he decided to study the likelihood of a sixth extinction in depth.
Rothman proposes that mass extinction occurs if one of two thresholds is crossed. For changes in the carbon cycle that occur over long timescales, the change rate in carbon cycling may be used as a reference, and extinctions will follow if those changes occur at rates faster than global ecosystems can adapt. For 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.
He eventually 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 hypothesized that this formula should predict whether mass extinction, or some other sort of global catastrophe, should occur.
Rothman then asked whether history followed his hypothesis. By searching through hundreds of published geochemistry papers, he 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 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 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.
He observed a common threshold that most of the 31 events appeared to stay under. While these events involved significant changes in carbon, they were relatively benign — not enough to destabilize the system toward catastrophe. In contrast, four of the five mass extinction events lay over the threshold, with the most severe end-Permian extinction being the farthest over the line.
2100’s the limit
Rothman calculated the critical mass of carbon for the modern day to be about 310 gigatons. He then compared his prediction to the total amount of carbon added to the Earth’s oceans by the year 2100, as projected by IPCC. The most recent report of IPCC projects that best-case scenario projects that humans will add 300 gigatons of carbon to the oceans by 2100, while more than 500 gigatons will be added under the worst-case scenario, far exceeding the critical threshold of 310 gigatons.
Does this mean the mankind is to see the sixth mass extinction of species at the run of the 21st century? “It’s 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. In the geologic past, this type of behavior is associated with mass extinction,” Rothman says. ”
The world is likely to enter an unknown realm in 2100.