https://www.sciencedaily.com/releases/2021/11/211111130325.htm

Large eruptions create a cloud that blocks some sunlight for a year or two. That reduces warming of the land in Asia in the summer and leads to a weaker monsoon and less rainfall, reducing crop harvests.

"We confirmed for the first time that collapses of dynasties in China over the last 2,000 years are more likely in the years after volcanic eruptions," said co-author Alan Robock, a Distinguished Professor in the Department of Environmental Sciences in the School of Environmental and Biological Sciences at Rutgers University-New Brunswick. "But the relationship is complex because if there is ongoing warfare and conflict, dynasties are more susceptible to collapse. The impact of a cooled climate on crops can also make conflict more likely, further increasing the probability of collapse."