That was due to our energy production at the time. The sun did only effect this by about 0.01%
At the time (until the 1970's), most of the electric energy production was made rather inefficiently from very dirty coal. As a result they spewed tons of aerosols into the skies every year, especially sulfur. At the ground, these can affect our lungs and make serious diseases and cause cancer and was a major source of acidic rain and the yellowish, hard to breathe smog around cities with them, but high up in the air, they can reflect sunlight before it gets to heat up the atmosphere.
Once these got slowly phased out for other power plants (cleaner coal, petrol, natural gas and nuclear) during the 50's through 70's, the amount of aerosols in the higher atmosphere dropped down and with them their reflective power.
This is the reason why geoengineering is a thing btw, trying to replicate the effect by shooting aerosols high up above our weather systems exactly for this reflective effect. Volcanoes do it by themselves if they are strong enough, but that's too rare an occasion (last big one with a VEI of 6 or more was the Pinatubo eruption in the Philippines in 1991) to bank upon that.
It has a drawback, too. At the time, there were very few measuring stations in third world countries. But in those, the heating were mostly not interrupted since unlike with volcanoes who throw the aerosols very high and many more of them, the effect from coal power plants and other particle producing processes is mostly local or regional, not global. As a result any solar radiation management would need to be a concerted global effort to really work as intended.
Thanks man nice reading. I once read a book called freakenomics (pretty famous) that discussed the possibility to use machines to throw in the atmosphere particules to replicate the effect of a volcano. Its pretty crazy, like huge "hoses" hung by air ballons that would be constantly shooting a particle (cant remember which one) into the atmosphere. It would require only a few in the world and the cost would be of dozens of billions which would be very small.
Of course crazy idea with a huge change to go wrong, but interesting anyway. As I said, I'm a moderate in the subject, I believe in global warming caused by us, Im not entirely sure what if our % participation in it. But I think we should start to prepare ourelved to adapt to the new reality or how to maybe reverse the process through technology.