Global cooling in general can refer to a cooling of the Earth. More specifically, it refers to a conjecture during the 1970s of imminent cooling of the Earth's surface and atmosphere along with a posited commencement of glaciation. This hypothesis never had significant scientific support, but gained temporary popular attention due to press reports following a better understanding of ice age cycles and a slight downward trend of temperatures from the 1940s to the early 1970s. Earth as a whole has not been cooling in recent decades, but is in a period of global warming.
The cooling period is well reproduced by current (1999 on) Global Climate Models (GCMs) that include the effect of sulphate aerosol cooling, so it (now) seems likely that this was the dominant cause. However, at the time there were two physical mechanisms that were most frequently advanced to cause cooling: aerosols and orbital forcing.
Thirty years later, the concern that the cooler temperatures would continue, and perhaps at a faster rate, can now be observed to have been incorrect. More has to be learned about climate, but the growing records have shown the cooling concerns of 1975 to have been simplistic and not borne out.
As the NAS report indicates, scientific knowledge regarding climate change was more uncertain than it is today. At the time that Rasool and Schneider wrote their 1971 paper, climatologists had not yet recognized the significance of greenhouse gases other than water vapor and carbon dioxide, such as methane, nitrous oxide and chlorofluorocarbons.
