You are correct that volcanoes are constantly erupting around the world, but the vast majority of these "background eruptions" are relatively small. So if you were to measure any apparent correlation between these background eruptions and social unrest in ancient societies then you will not have anything of significant meaning. As you say there is just far too much noise. But the point is that the periods of social unrest appears to coincide with infrequently large volcanic events, which are much rarer than smaller eruptions, and cause much more of climatic and environmental impact. Indeed, the background of frequent small eruptions have minimal effect upon the climate, as very little volcanic aerosols penetrate into the stratosphere, compared to the more infrequent very large explosive eruptions which have the energy to penetrate the tropospheric ceiling and contaminate the stratosphere.

The difference between tropospheric and stratospheric contamination is that aerosols in the troposphere soon wash out, minimising the climatic effects, whereas stratospheric aerosols can remain aloft for a few years to alter the radiation balance of the planet. For example sulphuric aerosols have a two fold effect in the stratosphere; they reflect some wavelengths of sunlight, causing surface cooling, but they absorb other wavelengths causing stratospheric heating. This can lead to thermal latitudinal gradients which can effect the jet streams and polar vortexes, resulting in sudden climate shifts.

To put this into context, the large eruption that you refer to in your quote is the volcanic event that occurred around 44 BC. This event caused a dust veil event that was recorded in Europe and in China, and caused famines for the Chinese. The Chinese record that three years of failed harvests between 44 and 42 BC, and tree rings record global cooling and sudden dramatic climate shifts during, and following this period. So, indeed, we can ask why is it that the Chinese were badly effected by the event, but Egypt was not, and one answer could be that Cleopatra may have been more organised.

Jonny

The path to good scholarship is paved with imagined patterns. - David M Raup