Originally Posted by Czin

The answer is Volcanoes, which spew out CO2 in massive amounts. Venus experienced run away Global warming as Volcanoes spewed out CO2 and it's oceans boiled, which became steam and further added to global warming, and then eventually escaped into space. In the late permian era, the Earth was already %40 hotter, but when a massive, 1,000,000 year long volcanic eruption that covered an area of at least 2 million square miles spewed out enough CO2 to raise the Earth's average temperature by 5 degrees, this warmed the earth enough to start melting Methane Hydrate gas en masse, which warmed the earth by another 5 degrees and reacted with the Oxygen in the atmosphere, causing it to drop down to a mere %10 of the atmosphere, allowing Anaerobic bacteria to bloom in number and produce Hydrogen sulfide, killing most of the marine life forms in the sea. %90-%95 of all life died in this event, and if we continue on our path, we could start a second extinction on this scale.