claim: the concentration of carbon dioxide in Earth’s atmosphere has climbed to a level last seen more than 3 million years ago — before humans even appeared on the rocky ball we call home
evidence: Last year will go down in history as the year when the planet’s atmosphere broke a startling record: 400 parts per million of carbon dioxide. The last time the planet’s air was so rich in CO2 was millions of years ago, back before early predecessors to humans were likely wielding stone tools; the world was a few degrees hotter back then, and melted ice put sea levels tens of meters higher. [...] The last time the planet had a concentration of 300 to 400 ppm of CO2 in the atmosphere was during the mid-Pliocene, 3 million years ago — recently enough for the planet to be not radically different than it is today. Back then, temperatures were 2 degrees C to 3 degrees C (3.6 to 5.4°F) above pre-industrial temperatures (though more than 10 degrees C hotter in the Arctic), and sea levels were at least 15-25 meters higher. Forest grew in the Canadian north and grasslands abounded worldwide; the Sahara was probably covered in vegetation. Homo habilis (aka "handy man"), the first species in the Homo line and probably the first stone-tool users, got a taste of this climate as they arrived on the scene 2.8 million years ago. (Homo sapiens didn’t show up until 400,000 years ago at the earliest.)