The Year Without a Summer
Mount Tambora in Indonesia produced the largest volcanic eruption in recorded history, killing perhaps 90,000 and throwing enough sulphur into the stratosphere to dim the sun worldwide. The following year had no summer: snow fell in New England in June, harvests failed across Europe and China, food prices doubled, and famine and typhus followed. Nobody at the time connected the weather to a mountain on the other side of the planet. It is the clearest demonstration that a single geological event can reach into every economy on earth within a year.
An environmental shift (drought, cooling) strains food and water systems, amplifying every other tension at once.