Gordon,
Thanks for not calling me a name 
The explanation for the cooling seen form 1940 to 1970 is the prominence of sulphate aerosols from pollution. The Clean Air Act helped to get rid of the aerosols. Estimates of aerosols during that time are not very reliable, but the fact that the Northern hemisphere cooled (and the southern hemisphere didn’t, supports this theory (Unlike CO2, sulphate aerosols have a localized effect and are washed out of the atmosphere within a couple of weeks.
The rise in global temperatures in the early part of the century is probably due in part to CO2, but also to an increase in solar irradiation.
As for when to look at trends, 30 years is the standard for climate, but 20 is usually good. Or you could look at the entire temperature record (1850 for surface or 1979 for satellites). One hint that your trend might not be telling you the real story is if every other year around it tells a different story. Start in any other year in the 1990s and the trend is strongly positive. Cherry pick 1998 and it is STILL positive, but not by very much.
