A survey reveals a drop of half a degree in average ground temperatures in the Northern Hemisphere between 1945 and 1968 (...) satellite photos indicated a sudden, large increase in Northern Hemisphere snow cover in the winter of 1971-72. And a study (...) notes that the amount of sunshine reaching the ground in the continental U.S. diminished by 1.3% between 1964 and 1972.
http://www.globalclimate.org/Newsweek.h
In the 1970s there was increasing awareness that estimates of global temperatures showed cooling since 1945.
http://en.wikipedia.org/wiki/Global_coo