Decreased solar activity during the 1938 to around 1970 would create the 'cooling' condition for that time period.
Many people forget that the earth is not warmed by 'sunlight' (the visible spectrum) but 'gamma' and 'infrared' radiation which penetrates the earths' crust, including the 'ice' packs sitting on it's surface.
A reduction in solar cycles (flares) also causes a reduction in these forms of radiation reaching the earth and leads to a reduction in 'heat'.
Remember this?
"In what political watchers are calling possibly the biggest gaffe in years, former Vice President Al Gore is set to give a speech tomorrow on the perils of global warming -- on what is expected to be the coldest day in New England in nearly half a century!"
Sorry to jump in the thread but after reading it I couldn't resist.