There have been many environmental catastrophes over geological time. A major strike in on what is now the Yucatan Peninsula wiped out about 70% of the species in existance at that time. However, at the Permian-Triassic boundry, the Siberian Trapps released enough GHGs, by direct emission of CO2, and extruding on clathrates, that it created a runaway greenhouse effect, and most of the ocean clathrates were released into the atmosphere. The oxygen level dropped to 11%, and 95% of all species died.
That is certainly one theory. It is not the theory that most ascribe to however.
This is the MOST likely cause of the extinction...GLOBAL COOLING
The Siberian Traps
Then of course there is the ever popular impact theory....
Meteorite impacts>
Then of course there is your ever popular climate change...interesting though how you never mention the possibility of cooling.....puzzling.
Permian Extinction - climate change
Then of course it could have been a by-product of the formation of Pangea
Formation of Pangea
And once again we get back to cooling.
Permian Extinction - glaciation
You will notice that in the scientific community COOLING is suggested as a cause directly three times and heating only once. Only in the factually challenged world of wikipedia is heating considered number one.