How can a volcanic eruption change the weather in other parts of the world?

if it dumps enough ash and sulfuric acid into the stratosphere, it blocks sunlight. this cools the earth. happens fairly often. mt. pinatubo added so much stuff to the atmosphere in 1991, that it cooled the earth by 1 degree for a year. there have been bigger eruptions in iceland and Indonesia within the last two/three hundred years that have seriously damaged crops in europe and india.