One of the things that made global cooling plausible was the observed temperature record although you have to be a bit wary, when talking about what people thought in the 1970s, because of course the record they had available then wasn't as good as the one we have now. But anyway, as you can see from that pic, the "cooling" from the 1940s to the 1970s was more of a plateau, coupled with a peak during WWII (insofar as it makes any sense to talk about a record in that way).
This was always a bit problematic, but in 2008 quite a bit of it disappeared, when A large discontinuity in the mid-twentieth century in observed global-mean surface temperature (Thompson et al., Nature 453, 646–649 (29 May 2008) doi:10.1038/nature06982) appeared; also known as "Post-World War II cooling a mirage". I noticed this at the time; and JA was characteristically caustic; but other than making the modelling easier it didn't really have much consequence and I don't know quite how it got folded into the records. Not very much, I suppose, since I've just pointed you to a recent record with the same problem in it.
And now, Kevin Cowtan, Robert Rohde, Zeke Hausfather have discovered the same thing. At least, it looks very much like the same thing. Sou notices the new stuff, but doesn't refer to the old. Cowtan's blog explainer doesn't mention "Thompson". The paper itself does: "(Thompson et al. 2008) detected an inhomogeneity in the sea surface temperature record arising from a change in the shipping fleet at the end of World War 2 by comparison of sea surface temperatures to temperatures from coastal weather stations and from climate models".