Air pollution not only impacts health, but can also trigger floods and landslides, study findsPollutants facilitate the formation of larger clouds that remain for a longer period during which more water is accumulated. When it rains, its intensity is therefore higher.
In the last week of March—when wildfires raged across the country, the country was shrouded in haze and harmful smoke.
Nationwide forest fires had taken the air quality to hazardous levels leaving the general public gasping for clean air.
The situation called for a public health emergency.
However, the adverse impact of air pollution and deteriorated air quality is not limited to human health.
The effects of air pollution transcend public health and can lead to massive loss of lives and property by unleashing catastrophic events like floods and landslides, according to the findings of a recent study published in Climate Dynamics, a scientific journal.
Researchers had studied the climatic impact of aerosols—a collection of solid particles or liquid droplets dispersed in the air—on clouds, precipitation, and the freezing temperatures over Himalayan foothills and the mountainous region of Nepal.
“Our findings suggest that atmospheric pollution affects the clouds, rainfall, and the free air freezing temperature,” Pramod Adhikari, co-author of the study, told the Post from the University of Nevada in Reno, United States over the phone. “Aerosols, the tiny particles suspended in the atmosphere, modulate cloud properties and hence the intensity and amount of the rainfall. Such a phenomenon can result in excessive rainfall and can cause natural disasters.”
For one of its kind study in the region, researchers—Adhikari and John Mejia, associated with the Division of Atmospheric Sciences, Desert Research Institute—analysed the long-term satellite data of 16 years from 2002 to 2017, focusing on Nepal and northwestern India, to understand the atmospheric impact of aerosols.
Researchers had then separated polluted days and clean days—280 days as polluted and 275 as clean days through a fixed parameter—to study the level of effects of aerosols on polluted and clean days.
Aerosols include dust, pollutants emitted from sources like vehicles and factories, wildfire, and construction sites, among others.
According to Adhikari, aerosols present in the atmosphere can block sunlight or radiation from reaching the surface and, as a result, increase atmospheric temperature.
An increase in temperature means the 0°C isotherm altitude, the freezing level that represents the altitude in which the temperature is at 0°C or known as the freezing point of water in a free atmosphere, is pushed further up in the atmosphere.
“What aerosols do is push freezing point up from the earth’s surface,” said Adhikari. “Our study has found that the freezing point had gone up in Nepal and the northwestern part of India on polluted days because the radiation from the sun was absorbed and the temperature had increased.”
The study recorded that the freezing point was higher by 136.82 metres on days with extreme levels of pollution in the atmosphere. And the impact of this could be evident on the ground.
“With the elevated level of freezing point, there could be rainfall in areas where there should have been snowfall,” said Adhikari. “More rainfall means melting of the existing snowpack and glaciers.”
With the rising temperatures and climate change, mountains have already witnessed excessive melting of snow, and most of the Himalayan glaciers are rapidly melting and shrinking. Such melting of glaciers leads to a shortage of freshwater stored in mountains.
Also, the rapid shrinking and retreating of glaciers influences the formation and expansion of glacial lakes, multiplying the risk of glacial lake outburst floods—a threat to communities living downstream and infrastructures.
Researchers Adhikari and Mejia also found that the presence of aerosols had a bearing on the intensity and amount of rainfall.
According to Adhikari, heavily polluted days with higher aerosols facilitate the formation of taller and broader clouds over the foothills and mountainous regions of the Nepal Himalayas and northwestern India.
Such a phenomenon increased the intensity and the amount of rainfall, which could lead to catastrophic floods and landslides.
“Besides pushing the temperature up, the aerosol level can affect cloud condensation nuclei which are tiny particles of a cloud droplet. Aerosols can impact the process of cloud formation,” said Adhikari. “These cloud particles remain smaller and will take time in converting into raindrops. Such clouds will hang on in the atmosphere for a long time and can accumulate more water and ultimately lead to excessive rainfall when it rains, as our study found.”
This higher intensity rainfall results in the triggering of floods and landslides in the region, according to the study’s findings.
The study found that on polluted days precipitation went up by 1.28mm on average compared to cleaner days.
“The 1.28mm surge in rainfall could seem trivial as it was the average for the entire period [of 280 days] we analysed. However, on some days, the surge was much higher and it can be termed significant,” said Adhikari. “Other studies in Arunachal Pradesh and Himachal Pradesh in India too have shown that one or two days before the extreme events took place, there was a higher level of aerosols or pollution in the atmosphere.”
Nepal, as a natural disaster-prone country, is annually battered by various disasters, mainly monsoon-related calamities like floods, landslides and thunder strikes. The country suffers a massive loss of life and property due to monsoon-induced disasters.
Changing weather patterns, attributed to climate change, have been unleashing erratic and intense rainfall in recent years. The last monsoon was considered the deadliest one in a decade in terms of the loss of lives and properties.
The upcoming monsoon season, which is predicted to receive ‘above normal’ rainfall, is likely to bring similar disasters, meteorologists have already warned.
“An above-normal monsoon could prove to be a disaster if the response is not timely,” Indira Kadel, senior divisional meteorologist and chief of the climate analysis section at the Department of Hydrology and Meteorology, told the Post last week. “It’s sensitive. You need proper planning so that risks are minimised.”
Besides water-induced extreme events, air pollution has been a regular problem for the country for several years.
Every year, the air quality deteriorates severely in the dry season before improving once again in the monsoon season.
However, this study has even come up with findings that the pollution levels have a detrimental impact during the monsoon too.
The researchers applied Aerosols Optical Depth (AOD), a measurement of particles in the atmosphere which calculates how much direct sunlight is prevented from reaching the ground by these aerosol particles, and discovered that pollution level was higher even during the monsoon.
The region recorded an average 0.41 Aerosols Optical Depth value during the monsoon season.
A value of 0.01 spells an extremely clean atmosphere whereas a value of 0.4 would correspond to a very hazy condition. An average aerosol optical depth for the US is 0.11 to 0.15.
“We [erroneously] tend to believe that with frequent rainfall, pollutants settle down and the atmosphere is relatively cleaner,” said Adhikari. “However, the pollution level was still high in the higher reaches of the atmosphere. On polluted days, the Aerosol Optical Depth value had even clocked to 0.9.”
Nepal and its Capital Kathmandu are constantly ranked as one of the most polluted countries and cities in the world. A 2020 study concluded that more than 42,000 deaths are attributed to air pollution in the country.
The country’s geographic location and its topography, which keeps pollutants trapped, further worsen pollution.
According to Adhikari, in addition to locally emitted aerosols, the aerosols are lifted from the Ganga basin, one of the world’s most polluted regions, and transported towards the Himalayas with the monsoonal flow.
“When the monsoonal flow enters the country, it brings that pollution to mountains from the Indo-Gangetic plains,” said Adhikari. “Moreover, pollutants are also deposited on the snowpack and this accelerates the melting of glaciers in the region.”
Nepal’s vulnerability to climate change, already evident in various sectors, is likely to exacerbate the impact of air pollution, as pointed out in this study. Besides, other studies have already warned that climate change will increase natural disasters in Nepal.
The study concluded that freezing line altitude moving higher by 136.82 meters during the heavily polluted days than relatively cleaner environments can also be critical in changing the snowline and melting of glaciers which in turn would impact the hydroclimate of the Himalayas.
“Freezing level going up means snowline, the line that demarcates snow-covered and snow-free areas, will move up too. An area that witnessed snowing earlier would not receive snowfall, but the only area above,” said Adhikari. “When we receive less snowfall, glaciers will melt quicker and water will not be held in the mountain regions where it had been deposited in the snowpack. The water crisis will loom large as mountains store fresh drinking water in the form of the snowpack.”
Researchers like Adhikari feel the effects of air pollution should be discussed beyond its immediate impact on human health as it affects the climate and everyday weather potentially leading to loss of lives and property.
“Aerosols emanating from vehicles and factories, dust, construction sites, or even wildfires impact human health. Besides, they can linger in the atmosphere for days and even several weeks and travel far, impacting cloud formation and precipitation elsewhere,” said Adhikari.
“These pollutants have a direct impact on health but can also alter rainfall patterns, increase snowpack meltings as cascading effects. For these reasons too, emission of pollutants should be controlled at their sources. Measures must be adopted for minimising pollution sources, if not fully control them.”
“Otherwise, the results will be catastrophic.”