Michael Mann, Professor of Atmospheric Science and Director of the Earth System Science Center at Penn State University. 400 parts per million, what does it mean? It’s the number of molecules of CO2 for every million molecules of air; 400 of them are now CO2. Just two centuries ago, that number was only 280 parts per million. We have to go several million years back in time to find a point in earth’s history where CO2 was as high as it is now. We believe that with that amount of CO2 in the atmosphere, we commit to what can truly be described as dangerous and irreversible changes in our climate.