Siberian super-volcano, 250 million years ago, was a sea of molten lava, about 10 million square kilometers large.
Therefore, the amount of heat radiating from there, according to Stefan’s law, was about 5 times as much as the entire Earth’s surface normally radiates. Or gets from the Sun, which is the same number. (Roughly 4 times hotter surface radiates roughly 250 times more. Then just the 2% of the surface gives you this number – 5 times as much radiation. Very elementary.)
The majority of the heat went directly to space. But there had to be hot winds, blowing in every direction possible. Today we still have long distance winds, only much, much colder. Still they are hot enough to produce heatwaves, thousands of kilometers from their origin.
So, it’s quite imaginable, that back then it was very hot everywhere on our planet. The Siberian furnace was on, all the time, for a million years.
The hypothesis of a massive CO2 buildup which allegedly caused those high Permian temperatures – isn’t even needed.