I've recently read Peter Brannen's book "The Ends of the World" in which he discusses the five big mass extinctions since the Cambrian explosion 541 million years ago. (I highly recommend it to anyone interested in previous mass extinctions by the way.) Often the amount of carbon dioxide in the atmosphere is at least part of the answer to why a certain mass extinction took place. I'm a bit confused about two different sections about the book. I quote the relevant passages. The first one is from the chapter "The End-Permian Mass Extinction".
Today humans emit a staggering 40 gigatons of carbon dioxide a year, perhaps the fastest rate of any period in the last 300 million years of earth history—a period that, you’ll note, includes the End-Permian mass extinction. Burning every last oily drop and anthracite chunk of fossil fuel on earth would release roughly 5,000 gigatons of carbon to the atmosphere. If we do so, the planet will become unrecognizable, with huge swaths rendered uninhabitably hot for mammals like us (to say nothing of the more than 200 feet of sea level rise that would drown much of civilization).
But as exceptional as humans are, estimates of the carbon released in the End-Permian mass extinction range from an utterly catastrophic 10,000 gigatons of carbon—twice as much as we could ever burn—up to a mind-meltingly unfathomable 48,000 gigatons. As a result, temperature estimates for the End-Permian mass extinction and its aftermath strain belief. In the Karoo Desert, as rivers stopped winding, insects stopped buzzing, and mass death swept over the land, the temperature might have jumped as much as 16 degrees Celsius. On Pangaea, 140-degree-Fahrenheit heat waves wouldn’t have been unusual. In the tropics, ocean temperatures skyrocketed from 25 degrees Celsius—similar to today’s oceans—to perhaps upwards of 40 degrees Celsius (104 degrees Fahrenheit). This is the temperature of a hot tub, or as End-Permian expert Paul Wignall puts it, that of “very hot soup.” Multicellular life simply can’t exist in this sort of globe-spanning Jacuzzi. The complex proteins of life denature—that is, they cook. The language of academic papers is typically measured and sober, but even the peer-reviewed science literature describes the early Triassic period that followed this worst mass extinction ever as a “post-apocalyptic greenhouse.”
The second is from the chapter "The Near Future".
If humanity burns through all its fossil fuel reserves, there is the potential to warm the planet by as much as 18 degrees Celsius and raise sea levels by hundreds of feet. This is a warming spike of an even greater magnitude than that so far measured for the End-Permian mass extinction. If the worst-case scenarios come to pass, today’s modestly menacing ocean-climate system will seem quaint. Even warming to one-fourth of that amount would create a planet that would have nothing to do with the one on which humans evolved, or on which civilization has been built. The last time it was 4 degrees warmer there was no ice at either pole and sea level was 260 feet higher than it is today.
Why would the temperature in the near future raise higher if we burned through all fossil fuels and added 5000 gigatons of carbon dioxide to the atmosphere than it did in the end of Permian when 10000-48000 gigatons of carbon dioxide was added?