Around 13,000 years ago...Mother Earth suffered global cooling which lasted several thousand years. What caused that cooling hasn't been revealed...but...theorists are looking for a massive hole punched in our world by a rock the size of Mount Everest.
Whatever caused that COOLING could not have been the result of an increase in the amount of TRACE GASES: carbon dioxide or methane since Mother Earth would have instantly corrected that "blip" by eating and absorbing that "increase".
No...folks...whatever it was that precipitated that GLOBAL COOLING came from the outer reaches of space and time. It overwhelmed flora and fauna leaving behind only what was resourceful or genetically able to survive such cataclysm.