And the world didn't end?
Actually it did end for an awful lot of people. The gas and ash from the second volcano killed a lot of people in Europe. The “year without a summer” killed a LOT more.
The scary thing is that two things seem to need to come together to really chill the planet: low sunspot activity (which is actually a symptom of the real cause, a magnetically quiet sun) and a reaaly big volcano or two going “kaboom.”
Short version: Low magnetic activity means more extra-solar cosmic rays reach Earth. Ash from a big volcano means more particles in the atmosphere to become charged by those cosmic rays, which in turn means a LOT more cloud cover. While clouds can hold heat in, too much extra cloud cover blocks so much sunlight that the Earth cools. Voila: Year without a summer.
We have both now.
PS Man’s influence is utterly insignificant compared to either of these forces, let alone both acting together.