In 2018, researchers published a study pointing to a volcano as the likely culprit after analyzing ice cores drilled from glaciers.
In modern times, big volcanoes have affected global weather for a brief (few months) time. Throw enough crap in the air, and solar downwelling radiation does get reduced. It's an observable effect.
Forty years ago people were worried that sulfur dioxide in the atmosphere was causing acid rain and killing the trees and ponds.
Now some people are proposing purposely injecting sulfur dioxide into the atmosphere to cool the planet down, as happened after volcanos released ash, dust, and sulfur-dioxide into the atmosphere.