By the dawn of the 20th century, when the first humans set foot on Antarcticaâ€™s seemingly pristine ice cap, pollution had beaten them there. Lead and other heavy metals quickly amassed in the ice in 1888 and beyond, leaving a record of the industrial revolution. Not a subtle one, either. It was like â€śa switch turning on,â€ť said Joseph McConnell, a hydrologist and ice core expert with the Desert Research Institute in Nevada.
On both Greenlandâ€™s and Antarcticaâ€™s ice caps, snow settles in layers, like sediments, recording what was in the atmosphere, year by year. By digging out ice cores hundreds of feet long, scientists can read an environmental history going back thousands of years. Now, with newly precise measurements, McConnell and colleagues announced they can use Greenlandâ€™s ice to read subtle ups and down of lead pollution emanating from the Roman Empire.
Millennia from now, when our civilization is long gone, and our ashes and dust are spread around the earth, scientists and historians could learn much about us by digging into this same ice. They might use written records too, but physical remnants carry a different view of history - a perspective that emphasizes change over time. As with other extinct civilizations, what we leave behind will reveal much more than the history we write down.
Thatâ€™s why historians are getting so excited about the newfound ability to read ancient history in ice. They say most of the lead from the Roman era was released from the process of extracting silver from ore, for use in coins. The periods of heavy lead contamination correspond to the periods the Romans made the most silver coins, and these correspond, roughly, to times of a strong economy.
The idea of looking for lead emissions buried in ice started with a geophysicist named Clair Patterson. He became concerned about lead pollution while attempting to calculate the age of the earth using a natural clock based on the decay of uranium into lead. In a story thatâ€™s laid out in Bill Brysonâ€™s â€śA Short History of Nearly Everything,â€ť Patterson ran into a snag when he discovered massive lead contamination in his lab, on his person, on his colleagues, and pretty much everywhere.
In 1953, after a monumental effort to create a clean laboratory, he correctly calculated the Earthâ€™s age of 4.55 billion years, and soon afterwards started investigating the source of the extra lead, which he realized must be coming from industrial lead and, especially, leaded gasoline. To convince the government to consider regulating lead, Patterson needed to prove that emissions brought levels higher than the natural background. To do that, he found remote spots in Greenland and Antarctica, dug deep into the ice, and found lead levels had ballooned to 200 to 300 times what theyâ€™d been in 1700.
That work revealed other, smaller peaks in lead levels in Greenland, including one during the period corresponding to the Roman Empire. Levels then were only 1/50 what theyâ€™d been during the 20th century, but the rise and fall of Rome was clearly recorded.
McConnellâ€™s team applied more precise measurements than were possible in Pattersonâ€™s time, and for the first time gave us a detailed record. The heavier emissions appear to mark periods of prosperity, such as the peaceful period known as the Pax Romana from 27 BCE to 180 CE. Emissions decreased later during each of several plagues. The researchers used telltale particles from known volcanic eruptions as time markers to match the ice layers to the calendar. McConnell said that researchers can anchor their ice core in time to within an accuracy of a year or two, even going back three millennia.
Itâ€™s not so much a backup to history but a new kind of history, said Joseph Manning, a Yale historian who has collaborated with McConnell on other ice core work. The ice core information tells them about changes over time - and may help historians better understand why things changed.