To find a time when the planet’s air was consistently above 400 ppm you have to look much farther back to the warm part of the Miocene, some 16 million years ago, or the Early Oligocene, about 25 million years ago, when Earth was a very different place and its climate totally dissimilar from what we might expect today.
There’s a lot of debate about both temperatures and CO2 levels from millions of years ago. But the evidence is much firmer for the last 800,000 years, when ice cores show that CO2 concentrations stayed tight between 180 and 290 ppm, hovering at around 280 ppm for some 10,000 years before the industrial revolution hit.
How the World Passed a Carbon Threshold and Why It Matters
BY NICOLA JONES • JANUARY 26, 2017
Mauna Loa Observatory | Atmospheric CO2 Concentrations
July 9, 2019
July 9, 2018