Timeline of a mass extinction - MIT News Office:
“'The rate of injection of CO2 into the late Permian system is probably similar to the anthropogenic rate of injection of CO2 now,' Rothman says. 'It’s just that it went on for … 10,000 years.

"Rothman says the total amount of CO2 pumped into Earth over this time period was so immense that it’s not immediately clear where it all came from.

"'It’s just not easy to imagine,' Rothman says. 'Even if you put all the world’s known coal deposits on top of a volcano, you still wouldn’t come close. So something unusual was going on.'”