Humans have long left their mark in almost all regions of the world – even on the highest mountain peaks. Now an ice core from the Himalayas shows that the dust and exhaust gases of industrialization were deposited on the “roof of the world” more than 200 years ago. As early as 1780, the Dasuopu Glacier ice showed a significant increase in heavy metals such as cadmium, chromium, nickel or zinc. The researchers attribute this in large part to the entry of exhaust gases and dust from coal combustion in England. With winter west winds, these emissions were blown over thousands of kilometers into the Himalayas.
At the end of the 18th century, a technical revolution began to emerge from England: the era of industrialization began with the burning of coal and the steam engines it powered. “It is an important milestone when it comes to human impact on the global climate system,” said Paolo Gabrielli from Ohio State University and his colleagues. From 1780, coal use dominated primarily in England, and by the middle of the 19th century the technology associated with it had also spread across much of the rest of Europe. At the same time, population density increased in many regions of the world, and associated slash-and-burn and agriculture increased. “Above all, ice cores provide us with clear evidence of past air pollution that is linked to the anthropogenic activities of civilizations on different continents,” the researchers explain. The mining activities of the Spanish conquerors have left their mark on the ice of the Andean glaciers and the Roman era is already evident in the ice of the Alps.
Heavy metal deposits from 1780
However, how far these traces of human activity reached and when which emissions occurred was only partially known. That is why Gabrielli and his team have now looked for evidence in a particularly remote area: on the 7200 meter high Dasuopu Glacier in the central Himalayas. In 1997 an ice core was extracted from the glacier, which is still the highest located climate archive of this kind. “This location is therefore well suited to not only preserving atmospheric contaminants of regional origin, but also widely used chemicals from the entire northern hemisphere,” the researchers explain. In addition, the clearly defined annual layers of the Dasuopu glacier enable a particularly fine temporal resolution. For their study, the scientists therefore examined the different layers of the ice core from the Dasuopu for the content of 23 different trace metals. The ice was from 1499 to 1992.
The analyzes showed that the contents of several heavy metals in the ice of the Himalayan glacier rose significantly from around 1780. Cadmium, chromium, molybdenum, antimony, nickel and zinc were particularly well represented among the pollutants detected, as the researchers report. Because these heavy metals are typically produced when coal is burned and these fossil fuels were not used in Asia at the time, they see a clear connection with the beginning of industrialization in Europe. From additional analyzes of weather data, Gabrielli and his team also conclude that there was a climate at the end of the 18th and early 19th centuries that favored wintry westerly winds and heavy snowfalls. With this atmospheric west current, heavy metals, soot and ash particles probably also reached Europe from the Himalayas.
Slash-and-burn also left its mark
But coal emissions from the beginning of industrialization were probably not the only source of air pollution carried into the Himalayas. From the composition of the trace metals found in the ice core and above all the increased zinc values, the scientists conclude that the combustion of biomass in the form of forest fires could also have contributed to the deposited emissions. Even if it is not clear from the deposits alone whether the fires were of natural or anthropogenic origin, they suspect that in the 19th century slash and burn to gain new agricultural land was the source of this air pollution. “At the time, the population grew strongly parallel to the industrial revolution,” explains Gabrielli. “There was a greater need for acreage, and typically you got it by burning forests.”
In summary, this study confirms that humans left clear traces in the Earth’s atmosphere a good 200 years ago. The emissions from his activities – from coal burning and the first industries to slash-and-burn – were so extensive that they were reflected in phases of correspondingly favorable wind currents, even in regions as distant as the Himalayas.
Source: Paolo Gabrielli (Ohio State University, Columbus) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.1910485117