Around 252 million years ago, life on Earth suffered its most catastrophic blow to date: amass extinction eventknown as the “Great Dying” that wiped out around 90% of life.
What followed has long puzzled scientists. The planet became lethally hot and remained so for 5 million years.
A team of international researchers say they have now figured out why using a vast trove of fossils — and it all revolves around tropical forests.
Theirfindings, published Wednesday in the journal Nature Communications, may help solve a mystery, but they also spell out a dire warning for the future as humans continue to heat up the planet by burning fossil fuels.
The Great Dying was the worst of the five mass extinction events that have punctuated Earth’s history, and it marked the end of the Permian geological period.
It has been attributed to a period of volcanic activity in a region known as the Siberian Traps, which released huge amounts of carbon and other planet-heating gases into the atmosphere, causing intense global warming. Enormous numbers of marine and land-based plants and animals died, ecosystems collapsed and oceans acidified.
What has been less clear, however, is why it got so hot and why “super greenhouse” conditions persisted for so long, even after volcanic activity ceased.
“The level of warming is far beyond any other event,” said Zhen Xu, a study author and a research fellow at the School of Earth and Environment at the University of Leeds.
Some theories revolve around the ocean and the idea that extreme heat wiped out carbon-absorbing plankton, or changed the ocean’s chemical composition to make it less effective at storing carbon.
But scientists from the University of Leeds in England and the China University of Geosciences thought the answer may lie in a climate tipping point: the collapse of tropical forests.
The Great Dying extinction event is unique “because it’s the only one in which the plants all die off,” said Benjamin Mills, a study author and a professor of Earth system evolution at the University of Leeds.
To test the theory, they used an archive of fossil data in China that has been put together over decades by three generations of Chinese geologists.
They analyzed the fossils and rock formations to get clues about climate conditions in the past, allowing them to reconstruct maps of plants and trees living on each part of the planet before, during and after the extinction event. “Nobody’s ever made maps like these before,” Mills told CNN.
The results confirmed their hypothesis, showing that the loss of vegetation during the mass extinction event significantly reduced the planet’s ability to store carbon, meaning very high levels remained in the atmosphere.
Forests are a vital climate buffer as they suck up and store planet-heating carbon. They also play a crucial role in “silicate weathering,” a chemical process involving rocks and rainwater — a key way of removing carbon from the atmosphere. Tree and plant roots help this process by breaking up rock and allowing fresh water and air to reach it.
Once the forests die, “you’re changing the carbon cycle,” Mills said, referring to the way carbon moves around the Earth, between the atmosphere, land, oceans and living organisms.
Michael Benton, a professor of paleontology at the University of Bristol, who was not involved in the study, said the research shows “the absence of forests really impacts the regular oxygen-carbon cycles and suppresses carbon burial and so high levels of CO2 remain in the atmosphere over prolonged periods,” he told CNN.
It highlights “a threshold effect,” he added, where the loss of forests becomes “irreversible on ecological time scales.” Global politics currently revolve around the idea that if carbon dioxide levels can be controlled, damage can be reversed. “But at the threshold, it then becomes hard for life to recover,” Benton said.
This is a key takeaway from the study, Mills said. It shows what might happen if rapid global warming causes the planet’s rainforests to collapse in the future —a tipping pointscientists are very concerned about.
Even if humans stop pumping out planet-heating pollution altogether, the Earth may not cool. In fact, warming could accelerate, he said.
There is a sliver of hope: The rainforests that currently carpet the tropics may be more resilient to high temperatures than those that existed before the Great Dying. This is the question the scientists are tackling next.
This study is still a warning, Mills said. “There is a tipping point there. If you warm tropical forests too much, then we have a very good record of what happens. And it’s extremely bad.”