The last major marine extinction event occurred 252 million years ago, but the next may be just decades away, according to new research on past extinctions which found that the ocean’s rapid absorption of carbon dioxide could trigger another mass die-off of species by the century’s end.
The study, published on Wednesday in the journal Science Advances, reveals a startling pattern that preceded past mass extinction events: major upward fluctuations of carbon in marine sediment – similar to what scientists are seeing today.
“Once a threshold is breached, the carbon cycle, and the Earth system more generally, are at risk of becoming unstable,” said study author Daniel Rothman, a professor of geophysics at the Massachusetts Institute of Technology. “Looking at past data, crossing the threshold is associated with mass extinction.”
Compared to marine carbon accumulation in the past, Rothman found that marine carbon concentrations are increasing at a much more rapid pace today. He arrived at that conclusion after comparing the amounts of carbonate found at various depths of two deep-sea sediment cores, which contain layers of isotopic carbon. This type of carbon contains a chemical signature that allowed Rothman to calculate when it was deposited on the seafloor.
He said there are two types of thresholds that, when exceeded, appear to trigger mass extinction events: rate for slow carbon increases and magnitude for fast increases. To determine those thresholds, Rothman devised an equation that relates both the rate and amount of carbon change to a geological time scale, allowing him to distinguish fast change from slow.
For the modern era, the “threshold for catastrophic change” is related to the amount of carbon present rather than its rate of increase. Rothman said that based on his research, it appears that today the threshold is 310 gigatons of carbon stored in marine sediments, a trigger he expects to be pulled by 2100. “The rate of modern change is so fast that the actual rate doesn’t matter,” he said.
Such rapid absorption of carbon can wreak havoc on marine habitats as ocean temperatures rise, oxygen levels fall and acidification intensifies.
Once marine carbon levels exceed a certain threshold, ecosystems become unstable, according to Rothman. That’s happened 31 times in the past 540 million years, with five of those fluctuations causing mass die-offs of marine organisms. Rothman said that by pumping enormous amounts of carbon into the environment, humanity is quite possibly setting the stage for a global sixth mass extinction event in the marine – and possibly terrestrial – environment.
In 2014, Rothman wrapped up a research project on the most severe mass extinction – the end-Permian period, which he discovered coincided with a major marine carbon increase that killed off more than 95 percent of marine species 252 million years ago. After he finished that study, he considered how he might be able to mathematically predict the next mass extinction based on historic and present carbon data.
Many scientists have concluded that the planet has already entered a sixth mass extinction. The low-end estimate of present species loss is between 200 and 2,000 species per year, while the high-end estimate is between 10,000 and 100,000 species. According to the World Wide Fund for Nature, these extinction rates are between 1,000 and 10,000 higher than what is considered natural. Some have gone as far as calling the current loss of biodiversity “biological annihilation,” driven by poaching, habitat destruction and climate change.