For various reasons Eli has been thinking about aerosols and how the mid-century cooling is attributed to them. Now the Bunny has also been consorting with a bunch of acid rain and regional forcasting types and the thought occurred that maybe we have a case here of the urban cooling effect. To make a long story short, and this is really a WAGNER (wild assed guess, no explanation required) what if the large amounts of SO2 injected into the northern hemisphere atmosphere by WWII and the unrestrained coal burning (see London, smog) produced huge amounts of sulfate aerosol which shadowed and cooled downwind rural measurement sites. Sulfate aerosol gets rained out pretty quick, so the range would not be global. This would mean that the dip between 1940 and 1970 was in a sense an artifact, the UCE.
The figure at the left from GISS shows that the cooling was a northern hemisphere thing. Warming in the tropics and the southern hemisphere has been quite steady, even though there are plenty of aerosols there (although they are different, the major sources in the SH being sea spray and in the tropics sand from the Sahara as well as agricultural burning in Brazil and Africa.