This is probably nothing, but when I was recently looking at the 12 month rate of change in the CO₂ data from Mauna Loa I noticed an irregular periodicity in the resulting 12 month moving average. It may be my coding that’s gone awry or it may actually be some kind of weak periodicity in the series of about 3 to 4 years in the moving average. Sometimes it’s not very well marked or missing but the last four cycles are quite obvious, 2010, 2013, 2016 and 2019. It’s not noticable if you look at graph for the annual rate of change rather than the monthly. I don’t have the necessary statistical knowledge or enthusiasm to take a more serious look, so here’s a look at the latest graph of 12 month changes up to August 2019 for you to make your own mind up.