In the year of the Lord 1347, the Black Death arrived in Europe. Introduced by merchants coming from Asia, the plague, caused by the bacterium Yersinia pestis, spread quickly. Following trading routes, in just six years this incurable disease killed 25 million people, one-third of the population on the continent.
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Entire villages were wiped out, some cities lost 80% of their citizens. The plague was followed by famine. Thomas Basinus (1412-1491), bishop of Èvreux and later historian, notes that 'many peasants fled or died so that many fields remained uncultivated or there was nobody left to care.'
In the cities, overpopulation and poor hygiene helped to spread the plague, rivers were used to dispose of the many corpses, contaminating the water. Riots of desperate people were common, like in 1323 in Flanders and in 1358 in France. Many believed, as one witness testified, that the end of the world had arrived.
The dramatically reduced population had, however, a surprisingly beneficial effect on the environment. The pollution of the air dropped to a historic low.
Analyzing a 236 feet long ice core recovered from a glacier in the Swiss-Italian Alps, a team of scientists from Harvard University was able to reconstruct the concentration of lead in the air over Europe for the last 2,000 years. The research with the title 'Next-generation ice core technology reveals true minimum natural levels of lead (Pb) in the atmosphere: Insights from the Black Death,' was published in the open access journal GeoHealth.
Atmospheric circulation transported the lead from the lowlands into the Alps, where it was washed out of the atmosphere by rain and snow. The snow, accumulating mostly during winter, partially melts and changes over the summer into ice, forming single layers, as found in a glacier. By analyzing the concentration of elements in the single layers, it is possible to create an annual record of the atmospheric deposition. One significant spike can be found around 1349-1353 when the measured concentration of lead dropped far below the average value of 10^2 nanogram of lead per liter air. Even today, after the introduction of unleaded fuel in the 1980s, the concentration of lead in the air is still 10 times higher at 1350.