evidence: The study found CO2 levels of more 1500 parts per million (ppm) in the atmosphere associated with extreme global warmth 50 million years ago, a climate so warm that no ice existed on the poles, the researchers said. [...] For the last one million years, CO2 levels slowly oscillated between 180 ppm and 300 ppm, as Earth’s climate transitioned in and out of the ice ages, with the dawn of agriculture and human settlement only establishing when CO2 and climate stabilized at the end of the last ice age, around 10,000 years ago, Rae said. [...] The last time CO2 was as high as it is today was three million years ago, Rae said, and it drove enough warming to melt all of the ice from Greenland and West Antarctica and raise sea level by around 60 feet.