They've found evidence of periods of apparent high atmospheric SO2 in some ice cores. There are multiple reasons, outside of volcanoes, this could have occurred.

The fact that vulcanologists have no evidence of this eruption doesn't seem to trouble anyone on the "climate science" side. So much so, they _insist_ the volcanologists are just "missing" it. They also need it to exist, because it is their only explanation for widespread temperature anomalies around this time period. Whenever the model fails to predict something, they go searching for a volcano, seemingly. It helps because it has high levels of atmospheric distribution, other sources of SO2, obviously, do not.

And, on the back of this wholly incomplete work, they would like to purposefully put high levels of man made SO2 in the atmosphere to try to interfere with the climate. We should do our best to positively impact the climate. I have strong doubts about all of these radical SO2 ideas.