Update: This post has changed to correct a mistake I made with the ERA-5 data.
I keep hearing about such-and-such month being the “hottest such-and-such month on record.” October of this year, for example. For data sources like NASA and NOAA in the U.S. and HadCRU in the U.K., the time needed to do the computations (mainly waiting for all the observing stations to report) delayed such announcements until mid-month, but lately I’ve been hearing it early in such-and-such month, often based on announcements from the Copernicus Climate Change group in Europe. I think they base their announcments on the ERA-5 data, a re-analysis data set, which incorporates both observed data and computer simulation.
I looked at the ERA-5 data, which starts in 1979 (probably because that’s when the satellites really kick in). The interesting thing is that it doesn’t just show increase, it shows (statistically significant) acceleration.