What I Learned about Climate Change: The Science is not Settled
David Siegel

I know it is not the accept norm but I recommend replacing million with 1,000,000 when discussing parts per million. When people read “an extra 120 parts per million” all they see is 120. 120 out of 400 seems large, 120 parts per 1,000,000 may better allow the reader to realize the scale more readily. 120/1000000 would be even better!