When I started 20 years ago on the research direction that eventually became Cliodynamics, I thought that getting data to test theories about historical dynamics would be tough. Within a couple of years I realized that actually it’s not true. There is an enormous amount of quantitative data about all kinds of aspects of past societies. It’s true that we often don’t have direct measurements of things we want to know about the past. But if one is willing to keep one’s eyes (and mind) open, one constantly encounters quite good data that can serve as a useful “proxy” for a variable of interest.
This week an interdisciplinary team of climatologists and archaeologists published an article, Lead pollution recorded in Greenland ice indicates European emissions tracked plagues, wars, and imperial expansion during antiquity. The main source of lead particles, deposited on Greenland ice during Antiquity, was a result of smelting silver in Iberian silver mines. The amount of lead deposited in Greenland can be resolved down to the year. As a result, Greenland ice contains an excellent quantitative proxy for the intensity of silver production in Western Mediterranean, which in turn traces economic booms and busts in this part of the world.
I downloaded the raw data, smoothed it using kernel regression with bandwidth = 50 years (if anybody cares for such technical details), and here is the trajectory (concentration of lead in Greenland ice) during the period when Rome was one of, or the dominant power in Western Mediterranean:
What we see here is yet another illustration of one of the most pervasive macrohistorical generalizations: all complex societies go through multi-centennial (“secular”) cycles (see our book Secular Cycles). Ancient Rome went through four such cycles of alternating integrative (“good”) periods (indicated with green-colored labels in the figure) and disintegrative (“bad”) phases (indicated with red-colored labels).
Actually, the first cycle probably has more to do with Carthage, which owned Spanish silver mines at the time. In the last cycle (the Dominate), the center of gravity of the Roman Empire shifted east, and that’s probably why the fourth century’s peak is quite modest.
Note that I did the periodization of Roman history into secular cycles before the Greenland ice data were available. It was based on a series of quantitative proxies that are entirely separate from the lead pollution data (and there are many such proxies — in the next post I’ll look into how they correlate with this one). In other words, these new data provide an independent test of the secular cycles theory.
You can read about these Roman cycles in the already mentioned Secular Cycles, and in my popular book War and Peace and War (in particular, Chapter 6, Born to Be Wolves, on the early history of Rome. For the last cycle (the Dominate) there is an article in Cliodynamics by David Baker.
You can also trace the evolution of the Roman polity from the Roman Kingdom (716–509 BCE) to the collapse of the Western Roman Empire (395–476) on the just published beta-version of the Seshat visual data site.