'Cause I'm sick of typing this every time someone brings up global cooling, here's the standard response:
Global cooling–Byproduct of sulfur emissions and incomplete understanding of Milankovitch cycles. Sulfur dioxide creates aerosols that reflect solar radiation back out to space, thus cooling the atmosphere. Starting in the 40’s, global temperatures dropped because of the presence of sulfur dioxide in the atmosphere, mostly from burning high-sulfur coal in power plants without scrubbers. Scientists were also coming to accept and find evidence of Milankovitch cycles in climate about the same time, and it was noted that the Holocene (which we are in now) is a few thousand years longer than previous interglacial periods. Understanding the the climate is largely controlled by solar insolation, thinking that at least based on the previous length of interglacials we must be heading into a glacial period soon, and noticing the downtick in global temperatures, some people came to the conclusion we would soon find ourselves in a glacial period. Better simulations of orbital dynamics have subsequently shown the Holocene has lasted longer because of a fortunate confluence of orbital parameters, and has shown that the Holocene, even without Anthropogenic greenhouse gases, would last a few thousand years more. Concerns over acid rain led to controls on sulfur emissions, which then decreased the amount of aerosols in the atmosphere, which is why we now see the impact of CO2 in the instrumental record.
Incidentally, if you don’t think we puny humans can really influence the atmosphere, here’s a good exampe of us actually doing so.