Currently you have to take a lot of "science" out there with a grain of salt sadly. Here is why.
1. Science by definition means to Isolate,that is highly physically or meta-physical improbable. Everything is effect by more that just one this. That is a universal law.
2. Most scientist only look at the primary calculations they understand, IE Global biologist only look at the earth as a separate entity. Well its not. So when a scientist says the earth was "this way" "back then" and that's "how its supposed to still be" there wrong.
Ex A. 1billion years ago the earth rotated faster, orbited the sun differently, had different species, and the tectonic plates where positioned differently.
So when a scientist can calculate, the amount of body heat given off by all types of species, amount of heat absorbed from the minerals that where on the planet, amount of light absorbed from the volume and type of plants, wind patterns from gravitational rotation, ocean currents from the moons polarity, and even the oxygen to nitrogen % of the planet 1 billion years ago? Then you can believe they have the "facts"
because yes, even then bio thermal heat coming from the dinosaurs had an effect of the environment. And until you can read a good article that explains that, and all variables effecting the planet you can not say you know the planet.
Because one thing is for sure, their is no "tipping point" there is just a pinochle where a lot of life wont be able to survive. So then nature will continue and people plants and animals will adapt. If millions start dying, then that's million not on the road and a instant drop in Co2 emotions, body head, waste, and use of buildings. The planet has a great way of controlling the population. Just make sure you know how to survive when it does.
Can a catastrophe happen "yes" will it kill the entire population? you odds are at leased 100billion possibilities to 1.
Adaptation is your answer
P.S. If it gets to hot your just going to have to consume more salt so retain hydration. It works for horses :p