Wipeout of ocean life in prehistoric age 'shows risks of global warming'
An episode of extreme global warming that left ocean animals unable to breathe caused the biggest mass extinction in Earth's history, research has shown.
The extinction event at the end of the Permian period 252 million years ago wiped out 96% of all marine species and 70% of land-dwelling vertebrates.
Scientists have linked what has become known as the "Great Dying" with a series of massive volcanic eruptions in Siberia that filled the atmosphere with greenhouse gas.
But precisely what made the oceans so inhospitable to life had remained an unanswered question until now. The new study, reported in the journal Science, suggests that as temperatures soared the warmer water could not hold enough oxygen for most marine creatures to survive.
Lessons from the Great Dying have major implications for the fate of today's warming world, say the US scientists.
If greenhouse gas emissions continue unchecked, ocean warming could reach 20% of the level experienced in the late Permian by 2100, they point out.
By the year 2300 it could reach between 35% and 50% of the Great Dying extreme.
Lead researcher Justin Penn, who is a doctoral student at the University of Washington, explained: "This study highlights the potential for a mass extinction arising from a similar mechanism under anthropogenic [human caused] climate change."
Before the Siberian eruptions created a greenhouse-gas planet, the Earth's oceans had temperatures and oxygen levels similar to those of today.
In a series of computer simulations, the scientists raised greenhouse gases to match conditions during the Great Dying, causing surface ocean temperatures to increase by around 10C.
The model triggered dramatic changes in the oceans, which lost around 80% of their oxygen.
Roughly half of the ocean floor, mostly at deeper depths, became completely devoid of the life-sustaining gas.
The researchers studied published data on 61 modern marine species including crustaceans, fish, shellfish, corals and sharks, to see how well they could tolerate such conditions.
These findings were incorporated into the model to produce an extinction "map".
"Very few marine organisms stayed in the same habitats they were living in - it was either flee or perish," said co-author Dr Curtis Deutsch, also from the University of Washington.
The simulation showed that the hardest-hit species were those found far from the tropics and most sensitive to oxygen loss.
Data from the fossil record confirmed that a similar extinction pattern was seen during the Great Dying. Tropical species already adapted to warm, low-oxygen conditions were better able to find a new home elsewhere. But no such escape route existed for those adapted to cold, oxygen-rich environments.
Previously, experts had been undecided about whether lack of oxygen, heat stress, high acidity or poisoning chemicals wiped out life in the oceans at the end of the Permian period.
"This is the first time that we have made a mechanistic prediction about what caused the extinction that can be directly tested with the fossil record, which then allows us to make predictions about the causes of extinction in the future," said Mr Penn.