Modern people aren’t the only ones who’ve polluted the atmosphere. Two thousand years ago, the Romans smelted precious ores in clay furnaces, extracting silver and belching lead into the sky. Some of that lead settled on Greenland’s icecap and mixed in with ever-accumulating layers of ice. Now, scientists studying annual deposits of those ice layers have found that spikes and dips in lead pollution during the Roman era mirror the timing of many historical events, including wars fought by Julius Caesar.
The level of detail is “astounding,” says Dennis Kehoe, a scholar of Roman economic history and law at Tulane University in New Orleans, Louisiana, who wasn’t involved in the work. What really impressed him was how closely the lead pollution numbers tracked what ancient historians know about the expansion and collapse of the Roman economy—a system built on silver coinage known as denarius. “It’s really the rise and fall of a monetary system based on silver,” he says. “Prices were reckoned in silver, so they had to have silver.”
Scientists have known about the Roman-era spike in lead pollution since the 1990s. Back then, researchers measured lead levels at a few places along the length of cores extracted from Greenland’s icecap—with each measurement representing a 2-year period. Later studies confirmed the same pattern in soil samples from peat bogs in Spain, Scotland, and the Faroe Islands. But those studies couldn’t show how lead pollution changed year by year.
So Andrew Wilson, an archaeologist at the University of Oxford in the United Kingdom and an expert on the Roman era, teamed up with ice-core experts to get a more complete picture. The team measured lead levels along a roughly 400-meter cross-section of Greenland ice, representing layers that froze between 1100 B.C.E. and 800 C.E. They melted the ice bit by bit, from one end to the other, and siphoned off the ice melt for analysis—obtaining around 12 measurements per year during the Roman era. Not all the lead came from pollution related to ore-smelting; some came from naturally-occurring dust and volcanic emissions, which researchers estimated and subtracted from the total lead count.
The result: An incredibly detailed 1900-year timeline of Roman lead pollution, published today in the Proceedings of the National Academy of Sciences. Lead pollution was greatest at the height of the Roman empire—during the first century C.E.—at levels roughly six times higher than during the 11th century B.C.E. But after the Antonine Plague hit in 165 C.E., likely killing millions, lead pollution suddenly dropped back down to pre-Roman levels and remained that way for 500 years. Dips in lead pollution also occurred in the middle of the Roman era, particularly when wars erupted in Spain—a hotspot for lead-silver smelting—during the last few centuries B.C.E.
Based on air circulation patterns, the team thinks that the Roman-era pollution, which peaked annually at just under a millionth of a gram of lead deposited per square-meter, came mostly from the western half of the Roman empire, in western and northern Europe. By comparison, the amount of lead that fell on Greenland is roughly 50 times lower than levels in the 1900s, says Joe McConnell, an environmental scientist at the Desert Research Institute in Reno, Nevada, and lead author on the study.
The work “raises all kinds of interesting questions,” says Kevin Butcher, an ancient historian at the University of Warwick in Coventry, U.K. He says there are a few puzzling points where there is a mismatch between peaks in lead pollution and silver coin production, making him wonder if the Romans were smelting and stockpiling silver—but not turning it into coins right away. The data, he says, are “food for thought.”