Bengaluru: In the 18th and 19th century, Britain was abuzz with cranking steam engines, rattling power looms, and clattering machines. Amidst this daily ding, the world was witnessing a defining movement in human history—the Industrial Revolution—that soon spread to the rest of Western Europe. Powered by coal, the production of most things transitioned from hand to machine, spurring a rise in population and air pollution. For the next two centuries, London became infamous for its soot and smog, which turned fatal for about 12,000 people.
Now, a new study has shown that this mal air has left its trace in the lofty Himalayan glaciers, thousands of kilometres away from Europe. The study, by researchers at the Ohio State University, USA, shows evidence of several toxic metals in the Dasuopu glacier in the central Himalayas purportedly deposited since the Industrial Revolution. Besides, massive deforestation and biomass burning in parts of the Northern Hemisphere seems to have added to these deposits. The researchers point out that atmospheric currents and wind patterns are responsible for bringing these metals to the snow-covered Himalayas.
“The main motivation of our study was to investigate when humanity started to contaminate the highest layers of the atmosphere,” says Dr Paolo Gabrielli, the corresponding author of the study. He is the Principal Investigator at the Byrd Polar and Climate Research Center at OSU.
Their analysis, he says, also helps to determine the onset of the Anthropocene—a geological period dominated by human activities. The findings of the study were published in the Proceedings of the National Academy of Sciences.
Ice cores as chronicles
Glaciers receive precipitation from the atmosphere, which brings down some part of the suspended particles. , glaciers are therefore natural repositories of atmospheric composition. Ice cores, which are ice cylinders of about 10 cm in diameter drilled from glaciers, have provided evidence of past atmospheric pollutants due to human activities since ancient civilisations.
“As the Himalaya is the most elevated mountain range of the world, its glaciers accumulate snow by layer by layer over time. These layers of snow act as a chronicle, detailing the chemical content of the past atmosphere, that can be read by extraditing ice cores,” explains Dr Gabrielli.
Ashes and fly ash from burning coal and plants contain high concentrations of trace metals, which are released into the atmosphere. “For the same reasons as snow captures particles from the atmosphere, toxic elements accumulate at the surface of, and over time within, glaciers,” says Dr Denis Samyn, a glaciologist with ICIMOD, who was not involved in the study. These elements are studied by scientists to precisely assess the impact, at various levels, of anthropogenic activities on the environment. When the trace metal concentrations from the ice cores are compared to those emitted by burning coal and biomass, we can determine the source of these metals.
The researchers of the study did exactly that. They analysed the composition of the snow on Dasuopu, during the nearly 500-year period between 1499 and 1992, by drilling an ice core at 7,200 m altitude. They accounted for the annual and decadal variations in the concentrations of trace metals and their concentration in the local soil to pinpoint the source.
The study found a variable concentration of twenty-three trace metals, with aluminium and iron being in abundance as they are also found in the soil. The concentration of others, like cadmium, bismuth and zinc, were found to be remarkably higher in the ice than in the soil of the region, during the end of the 18th century and beginning of the 19th century, coinciding with the Industrial Revolution and massive burning of biomass.
But how did these toxic metals find their way to the far away Himalayas? “Atmospheric circulation and the winds blowing from west to east were primarily responsible for transporting these trace metals over long distances,” says Dr Gabrielli. Besides, the unusually strong winter westerlies during 1810-1880 also resulted in exceptional accumulation of snow. “It also contributed to the increase in the transport of particles such as fly ash from coal combustion and forest fires to this Himalayan glacier,” he adds.
Interestingly, the study found lower than expected concentration of toxic metals, like lead, during the 20th century, indicating a turn in the atmospheric currents. “During this period, the atmospheric circulation changed and winters became dryer, probably limiting the deposition of contaminants during snow events,” reasons Dr Gabrielli. However, it could also be possible that the analysis of the recent interval, between 1950-1992, may not completely capture the variations in the concentration of trace metals, say the researchers.
An uncertain future for the mountains?
The pristine Himalayas, also called the world’s third pole, are witnessing unprecedented warming and melting of glaciers due to climate change. The findings of the study reiterate the fact that these mountains are sensitive to atmospheric contamination. Black carbon particles, with a core composed of carbon, are released during the incomplete combustion of fossil fuels and biomass. “Because of their dark optical properties, these carbonaceous aerosols have the particularity of capturing and absorbing sunlight,” explains Dr Samyn.
“Once deposited on the surface of the glaciers, dark contaminants such as soot and black carbon would absorb more solar radiation, contributing to the melting of high altitude glaciers,” warns Dr Gabrielli.
Glaciers in the Himalayas are the source for India’s perennial rivers like the Ganga, with the meltwater contributing to these water bodies. Could the occurrence of toxic elements in the pristine snow put us at risk?
“Glaciers are melting at an increasing rate, which leads to the release of atmosphere-derived toxic elements in the environment through glacial streams,” says Dr Samyn.
Recent studies in the Tibetan Plateau have shown that trace contaminants could endanger ecosystems and human health in the region if atmospheric pollution continues unabated. As of today, the concentrations of trace metals are too low to cause any danger, opines Dr Gabrielli. However, over time, these metals could accumulate in the cells of plants and animals, posing a threat.
“In some cases, the concentration of toxic metals may be detrimental for the ecosystems in the future,” he says.
Paleoclimatic studies that decipher the climate and atmospheric composition of the past, such as the current one, allow us to better understand the current climate and pollution dynamics and to predict the changes in the future using powerful tools.
“This new knowledge can then be used by stakeholders, city planners and politicians, to provide solutions to mitigate the severe challenges faced by countries like India and China due to the sprawling urban pollution,” explains Dr Samyn, about the implications of such studies.
This article was originally published in Research Matters.
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