One of the most unlikely keepers of humanity's history lies in the frozen Arctic. In a new study, scientists say that particles of lead trapped in ice in the north pole tell the story of commercial and industrial processes dating back to the Middle Ages.

As humanity has burnt fuel, sailed ships, made currency, prepared for war, and any number of countless actions, it has sent emissions into the air. In ancient days, these emissions could stem from mining and smelting iron for Roman coins. Man-made emissions in the modern day are infamous, they have helped create a situation of global warming that has begun to effect the climate disastrously.

Be they from the ancient Roman empire or modern day America, all of these lead particles would catch rides on wind currents that would take them to places like Greenland or the Arctic. Neither Greenland nor the Arctic have a lot of lead naturally, meaning that scientists looking at the lead today can safely assume they rode wind currents to get there. So when there was more trade, particle levels rose. When there was less—like, say, during the Black Plague—the levels fell.

Studying thirteen Arctic ice cores from Greenland and the Russian Arctic, researchers from the Desert Research Institute (DRI), the University of Oxford, the Norwegian Institute for Air Research, the University of Copenhagen, the University of Rochester, and the Alfred Wegener Institute for Polar and Marine Research looked at samples from 500 to 2010 C.E., a span of time covering massive change in humanity's condition. Their work has been published in the Proceedings of the National Academy of Sciences.

Their work expanded on a project some of scientists had done previously, which looked at a single ice core and a smaller time frame.