A team of scientists from the Massachusets Institute of Technology (MIT) has predicted that the increase in carbon dioxide emissions since the 19th century may lead to a new mass extinction event, the sixth mass extinction that will coincide with the widespread extermination of the species of living things throughout the world.
In the last 540 million years, the Earth has undergone five mass extinction events ; each one of them with a series of processes that altered the normal cycle of carbon through the atmosphere and the oceans.
Are we heading towards the sixth mass extinction?
It is a question that many scientists ask themselves due to the constant increase in carbon dioxide emissions since the 19th century. But it is not easy to predict such a fatal event, since it is difficult to relate ancient carbon anomalies, which occurred over thousands or millions of years, to today, which have taken place for little more than a century.
To shed some light on this, Daniel Rothman, professor of geophysics in MIT’s Department of Atmospheric and Planetary Sciences and co-director of the MIT Lorenz Center, has analyzed significant changes in the carbon cycle over the past 540 million years, including the five mass extinction events, identifying “catastrophe thresholds” in the carbon cycle that, if exceeded, would lead to an unstable environment and ultimately mass extinction.
For changes in the carbon cycle that occur over long time scales, extinctions will follow if those changes occur at faster rates that global ecosystems can adapt to. On shorter timescales, the pace of carbon cycle changes will not matter; instead, the size or magnitude of the change will determine the probability of a new extinction event.
Based on this reasoning, Rothman predicts that if these thresholds are exceeded, it would lead to an unstable environment and ultimately mass extinction. The limit would be to add 310 gigatons of CO2 dissolved in the oceans, an amount that will be reached, according to the forecasts of the Intergovernmental Panel for Climate Change, in the year 2100.
Mathematical formulas
“This is not to say that the disaster will happen the next day. It tells us that, if left unchecked, the carbon cycle would enter a realm where it will no longer be stable, and that it would behave in a way that would be difficult to predict. In the geological past, this type of behavior is associated with mass extinction, “says Rothman.
Rothman developed a mathematical formula based on physical principles related to the functioning of the carbon cycle that basically depends on the balance between photosynthesis and respiration. The formula relates the critical rate and the magnitude of the change in the carbon cycle to the time scale that separates a fast change from a slow change.
Subsequently, he tested the formula by reviewing the geochemical history of our planet, finding 31 events in the last 542 million years in which there was a significant change in the Earth’s carbon cycle. For each event, including the five mass extinctions, Rothman pointed to the change in carbon, expressed in the geochemical record as a change in the relative abundance of two isotopes, carbon-12 and carbon-13.
“It became apparent that there was a characteristic rate of change that the system basically didn’t like to go through,” says Rothman.
Four of the five mass extinction events on Earth are above that threshold observed by the scientist. Upon further analysis, he discovered that the critical rate for the catastrophe is related to a hidden process within the Earth’s natural carbon cycle. The cycle is essentially a loop between photosynthesis and respiration. Normally, there is a “leak” in the cycle, in which a small amount of organic carbon sinks to the bottom of the ocean and, over time, is buried as sediment and sequestered from the rest of the carbon cycle.
Rothman found that the critical rate was equivalent to the rate of excess carbon dioxide production that would result from plugging the leak. From the critical rate and equilibrium time scale, he calculated that the critical mass of carbon for our current time is approximately 310 gigatons.
According to data from the most recent report of the Intergovernmental Panel on Climate Change, humans are predicted to contribute 300 gigatonelons of carbon to the oceans in the best case scenario by 2100 ( in the worst case more than 500 gigatonelons). ) far exceeding the critical threshold. In all scenarios, by the year 2100, the carbon cycle will be close to or well above the threshold of catastrophe.
Referencia: “Thresholds of catastrophe in the Earth system,” Science Advances (2017). advances.sciencemag.org/content/3/9/e1700906
