- Researchers have found that fires from crop residue burning over northwestern India have risen from 2010 onwards. The resultant air pollution is spreading to east, central and south India as well.
- Blown by northwesterly winds, fine black carbon particles and greenhouse gases released from crop burning, spread to the eastern Indo-Gangetic Plains and to central and southern states reaching as far as Telangana.
- A greater shift towards mechanised harvesting, which leaves behind residues, may be contributing to these trends, the researchers said.
- Subsidies provided by the government can help farmers to purchase machines that can harvest without leaving residues. The residues could be used as a fertiliser or sold for use in different industries instead of being burnt.
Northern India, particularly Delhi, is notorious for its choking smog blanketing the city and nearby areas during the post-monsoon period in October-November. This is the time when farmers in the northern states burn crop residues left over from rice harvesting. The resulting smoke has been attributed as the cause for soaring air pollution in Delhi.
In 2017, the air quality was so poor that authorities were forced to shut schools for several days to protect children from the adverse health consequences of inhaling pollutants in the air.
Now a new study analysing data from multiple sources from 2003 to 2017 finds that it is not just Delhi that is experiencing poor air quality; the fine particulate matter generated from crop residue burning in the post-monsoon season is blowing to the eastern Indo-Gangetic Plains, central states and even southern parts of India, thanks to the prevailing wind patterns during that time.
Fires are rising, which the researchers attribute to an increased shift towards mechanised harvesting.
While most studies have discussed the effects of crop burning over Delhi and surrounding areas, “no comprehensive assessment had been done of the impact of this crop burning in northern India, over other parts of India,” said Sudipta Sarkar, lead author of the study and research scientist at NASA’s Goddard Space Flight Center.
The findings are “really a whistleblower of sorts that point to the impending doom if this issue is not tackled right away,” he added.
Ruth De Fries, a professor at Columbia University in New York, who was not involved in the study, stressed that the study is “a wake-up call for the government to shore up efforts to promote alternative harvesting methods that do not require burning”.
“Mechanised agriculture promotes burning over the traditional practice of collecting and feeding the residue to livestock,” added De Fries, noting that many millions of people in northern India breathe air laden with particulate matter from crop residue burning during the burning season.
Crop burning takes place twice a year after the harvest of wheat in April-May and rice in October-November to prepare the fields for planting the next season’s crop. But it is only during the latter season when air quality deteriorates because of specific meteorological conditions.
“There is a difference in temperature in the summer time and winter time,” said Ramesh Singh, a professor at Chapman University and co-author of the study.
“In summer, when the temperature is very high, the gas disperses very fast and so its effects are not seen,” he explained. In contrast, “during winter time, the temperature is very cold” and depending upon the meteorological conditions such as wind patterns and speed you see dense haze, fog and smoke, he elaborated.
Sarkar, Singh and graduate student Akshansha Chauhan from Sharda University in Uttar Pradesh gathered data from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) satellite to detect active fires from 2003 to 2017 during October-November over northwestern India.
To assess the impact of crop residue burning in the post-monsoon season over India, they tracked the changes in methane emissions (a greenhouse gas), a by-product of incomplete combustion of paddy.
Changes in methane vertical mixing ratios were analysed from the daytime mode of Atmospheric InfraRed Sounder (AIRS) sensor onboard the EOS Aqua platform and the magnitude of methane emissions from different sources was based on Emissions Database for Global Atmospheric Research (EDGAR).
Data on black carbon and dust columns were obtained from NASA’s database MERRA-2. They combined all the data with models and with aerosol ground data from Aerosol Robotic Network (AERONET) stations located in Kanpur and Gandhi College in Balia.
The transport of fine black carbon particles over the rest of India
The team found increasing trends of black carbon particles and greenhouse gases emitted from crop residue burning in Punjab and Haryana from 2010 onwards.
These were spreading to Uttar Pradesh in the eastern Indo-Gangetic Plains and to the states of Madhya Pradesh, Maharashtra, Chhattisgarh, and even down to Odisha and Telangana because of the northwesterly winds during the post-monsoon period. The impact was the worst during the first and second week of November.
Black carbon particles, often referred to as soot, are a subset of PM 2.5 (fine particles with a diameter of 2.5 microns or lower) and can be inhaled into the lungs, posing a threat to health.
“Since black carbons are of finer nature, they remain entrained in the wind for longer distances,” said Sarkar. “Also, since during winter the vertical convective mixing slows down, so these tend to linger further,” he added.
The level of PM 2.5 detected in Aurangabad, Maharashtra peaked in 2016, which was also consistent with a peak in the fire counts detected over Punjab and Haryana during the same period. The numbers of fires have been steadily rising since 2010 particularly in the afternoons when they reach a peak, the team noted.
The role of mechanised harvesting
The researchers believe a greater shift towards mechanised harvesting is to blame for rising trends in crop residue burning. Burning of crop residues began in 1986, when mechanised harvesting for wheat and rice was adopted.
This method leaves behind stalks, stubble and straw in the field, which need to be cleared for sowing the crop for the next season.
About one foot of straw is left in the field and farmers in Punjab found that the easiest and cheapest way to get rid of it is to burn it, said Singh. “Slowly, the whole thing becomes viral and now people from western U.P. have also started burning it,” he elaborated.
Burning of the crop is prevalent everywhere, he added, stating it also occurs in Vietnam, China and Canada. But “our geographical situation is quite different. In the northern part, we have the towering Himalayas. The pollutants do not cross it and the whole thing goes in the Ganga valley.”
How to tackle residues without burning?
In the past, farmers harvested crops manually, which was better, said Singh. “The whole root goes and immediately they ploughed the fields, so everything is mixed up and acts as a fertiliser.”
As this is labour-intensive, there are machines that can harvest crops from the roots. But they are expensive, so government subsidies can help, he said.
The residues leftover by mechanical harvesting can either be used as fertiliser in the ground or as inputs for thermal industries, for instance, said Sarkar. “But some of the machinery and technology needed to do that can be quite costly. So the central and state government must subsidise the farmers or lend their support so that the small farmers can afford all these.”
He also emphasised the importance of educating farmers to make them aware of the monetising opportunities from selling the residue products.
While these measures will take time, “one has to make a start now, before it is too late,” warned Sarkar.
Sarkar, S., Singh, R.P., Chauhan, A. (2018). Crop residue burning in northern India: Increasing threat to greater India. J. Geophys. Res. doi: 10.1029/2018JD028428