And I remember reading book from 1906 (not when the books were first published, obviously) from scientists who were convinced that the unavoidable warming due to enhanced industrial carbon dioxide emissions would actually be beneficial, http://en.wikipedia.org/wiki/Svante_Arrhenius#Greenhouse_eff...
So in 1974 there were reasons to suspect that some powerful effect might have reversed the expected human driven rise in global temperatures typical for the first half of the century in spite of continuing strong carbon dioxide emissions. Today we see that these decades were just a temporary deviation from the expected trend, caused mostly by vulcanic ashes and soot in the atmosphere.
And the trend in global temperatures has changed quite dramatically since the mid-70ies http://en.wikipedia.org/wiki/File:Global_Temperature_Anomaly...
So in 1974 there were reasons to suspect that some powerful effect might have reversed the expected human driven rise in global temperatures typical for the first half of the century in spite of continuing strong carbon dioxide emissions. Today we see that these decades were just a temporary deviation from the expected trend, caused mostly by vulcanic ashes and soot in the atmosphere.