by Lorena Lyon
figures by Rebecca Senft

Today, the discussion of climate change generally relates to human impact on the environment since the Industrial Revolution (1760 to mid-1800s). But, how have humans been impacting the planet before then? And how can we find out? It turns out a type of climate science using something called ice cores can give us detailed information on how past human civilizations have been polluting the Earth. These ice cores can also show us how civilizations in turn are affected by small changes in Earth’s climate.

What is ice core science and why do scientists pursue it?

Through wars, famine, and plague, the Earth keeps spinning, the seasons keep changing, and the snow keeps falling. When snowflakes fall on top of one another, they trap gas and any other air particles in the spaces between their ice crystals, creating a snapshot of atmospheric conditions from that time. In cold regions of the world like Greenland and Antarctica where winter snow doesn’t melt away in the summer, annual snowfall compresses into layers of ice, allowing scientists to study the gasses trapped within these layers many years after their formation (Figure 1). Analyzing this frozen record can tell us about climate conditions when the ice formed, such as the temperature and the composition of the atmosphere. By drilling down into these ice sheets and collecting very long cylinders of ice called cores, researchers can analyze these naturally created records and cross reference their findings with other climate and historical data.

Figure 1: Tracking air composition using ice cores. Lead (Pb) particles from pollution in the atmosphere become entrapped in snow as it falls through the air 2. Snow lands on the ground and forms layers of ice. 3. Scientists can drill down into the ice and extract a long cylinder of ice called a core. 4. Deeper ice layers are older, as more and more ice accumulates year after year. 5. Heavy metals like lead (Pb) and gases like carbon dioxide (CO2) are trapped between water molecules in the ice. Their concentrations can then be analyzed in the lab.

Ice cores: a historical perspective

In the early 1950s, scientists drilled the first ice cores to investigate glaciers and ice formation. They were initially interested in the annual differences in snowfall and physical properties of the ice, such as grain size, density, and crystal structure. These properties were important for establishing a history of snowfall and glacier formation in the Arctic, including how much snow fell and how the snow compacted into ice. 

Ice core drilling picked up in 1957, when worldwide excitement for polar expeditions surged during the International Geophysical Year (IGY). During the IGY, new technologies that emerged in WWII for military use were adapted for peacetime scientific endeavors. Ice core drilling sites were established in Greenland and Antarctica, with over 12 nations participating in these Antarctic and Arctic glaciology studies. During the IGY, scientists from the U.S. recovered an approximately 300 meter long, 900-year-old ice core from northwestern Greenland. Chemists analyzed samples of this ice core to estimate annual precipitation and the composition of molecules trapped in the snow that fell. They also analyzed stable oxygen isotope ratios, a proxy for temperature, which meant they could estimate temperatures from over 700 years ago. This investigation was considered the first state-of-the-art analysis of a deep ice core, and paved the way for future funding and more ambitious expeditions. 

Continued ice core research has been incredibly important for climate scientists. Today, ice cores are most widely used to estimate past levels of atmospheric gasses such as carbon dioxide, the primary greenhouse gas responsible for global warming. Ice cores have entrapped records of carbon dioxide levels for the past two million years showing  that today’s carbon dioxide levels are rising at an unprecedented rate.

But, that’s not all the ice cores can tell us. Recently, the science of ice cores has helped historians understand the relationship between humans and climate in the Roman Empire and the Middle Ages. Scientists collaborated with historians and learned something new about lead pollution through this period of time.

Lead pollution in Rome and the Middle Ages

It is well-known among historians that the Roman Empire (27 BCE to 1453 CE) used  lead in much of their infrastructure, including water pipes, weights, and construction materials. Additionally, the Romans smelted silver, a process that releases lead particles into the atmosphere, to mint currency. However, it remained unappreciated just how much lead the Roman Empire released into the atmosphere. In fact, most researchers previously assumed that atmospheric lead levels only began to increase at the onset of the Industrial Revolution, and that prior human lead pollution was minimal. By studying ice cores, researchers found that lead levels have been elevated since the time European civilizations began using lead, long before the Industrial Revolution (Figure 2). Not only that, but these lead level measurements are so detailed, we can pair major historical events with increases and decreases in lead, giving us a proxy for mining and metalworking productivity.

Figure 2: Lead pollution over time. Atmospheric lead levels, measured by lead concentration in ice cores, have been high since ancient times. The arrow indicates the Bubonic Plague.

When released into the air by metalworking, lead particles can be carried hundreds of miles by air currents before being trapped in snowfall and later preserved in layers of ice. Because the process of metalworking creates lead pollution, historians and scientists have been able to align archaeological economic records with data from ice cores. During the Roman Empire, heavy lead pollution corresponded with times of prosperity, such as the Pax Romana (27 BCE to 180 CE), a two hundred year period of relative peace, trade, and imperial stability. Conversely, lead emissions decreased in times of war when areas with silver mines were affected. Even the outcomes of battle conquests were recorded in the cores: a sharp increase in ice core lead levels corresponds with an uptick in the productivity of Carthaginian mines following the Roman occupation during the Second Punic War (206 BCE).

The ice cores also reveal the converse: how climate affects human productivity. Researchers found a small dip in the ice core lead levels at the beginning of the 14th century. This dip corresponds to a climatic period called the Little Ice Age, when temperatures dropped 1.5 degrees on average. This small change in temperature was enough to disrupt the landscape of medieval Europe, causing crop failure, famine, and decreased productivity. Thus, lead pollution levels measurably dropped. 

The largest recorded dip in ice core lead levels, from 1349 to 1352, was due to the bubonic plague (Figure 2). In 1347, the plague arrived in Sicily and quickly spread across the continent, reaching Russia by 1351. The plague decimated Europe, wiping out between 30-60% of the population. This unprecedented scale of human loss diminished the labor pool and shattered demand for metal production. Even the progression of the plague throughout Europe is evident in the ice: the arrival of the plague in the most productive mining regions of Great Britain caused the steepest drop in ice core lead levels.

Ice core research has allowed scientists to dig deeper into the interactions between humans and climate. By recording the interplay of climate, plague, war, and industrialization on past societies, ice cores are contextualizing the historical record through a new lens. As ice core drilling advances steadily back throughout history, scientists may discover clues about how our planet’s climate changes to help us analyze the challenges that we are facing today. 


Lorena Lyon was formerly in the Silver Lab in the Department of Systems Biology at Harvard Medical School. 

Rebecca Senft is a fifth-year Program in Neuroscience Ph.D. student at Harvard University who studies the circuitry and function of serotonin neurons in the mouse.

Featured image: “Ice core drilling” by Bjerknessenteret for klimaforskning is licensed under CC BY-NC-SA 2.0 

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