NASA flags shift in stubble-burning timing hindering air quality tracking in north India

Seasonal crop fires across northern India are increasingly being set later in the day, a change that scientists warn could complicate efforts to monitor stubble burning and accurately assess its impact on air quality, according to new findings released by NASA.

For decades, vast plumes of smoke have spread across the Indo-Gangetic Plain between October and December as farmers burn crop residue after the rice harvest. While the overall pattern of the stubble-burning season in 2025 broadly followed expectations, researchers observed a marked shift in the daily timing of fires, with important implications for air pollution tracking.

“In many ways, the seasonal behaviour was typical,” said Hiren Jethva, an atmospheric scientist at NASA’s Goddard Space Flight Center. He noted that air quality deteriorated in Delhi and several other cities for about a month after fires intensified in late October. However, the time of day when most burning occurred has changed significantly.

Jethva, who has tracked stubble burning in India using satellite data for nearly a decade, said that earlier fire seasons were dominated by burns in the early afternoon, usually between 1 pm and 2 pm. “In the past few years, stubble fires have occurred progressively later in the day,” he said, adding that most fires are now detected between 4 pm and 6 pm. “Farmers have changed their behaviour.”

The shift was identified using high-frequency observations from South Korea’s GEO-KOMPSAT-2A satellite, which records data every 10 minutes. By contrast, widely used fire-monitoring instruments such as MODIS and VIIRS pass over the same location only once or twice a day, increasing the risk that later-burning fires may be missed.

Satellite imagery illustrates the scale of the issue. On 11 November 2025, the MODIS instrument aboard NASA’s Aqua satellite captured thick smoke and haze stretching across Punjab, Haryana, Uttar Pradesh and Madhya Pradesh. Media reports said it marked the first of several days that year when pollution levels crossed 400 on India’s air quality index, the most severe category.

As in previous years, the pollution spike led authorities in some regions to close schools and tighten controls on construction. Scientists say that when winds are weak and atmospheric conditions remain stagnant, smoke from crop fires can push pollution levels to several times above World Health Organisation limits.

Despite the timing shift, Jethva’s analysis suggests that overall stubble-burning activity in Punjab and Haryana in 2025 was moderate compared with other recent years. Fire counts were higher than in 2019, 2020 and 2024, but lower than levels recorded in 2021, 2022 and 2023.

Indian researchers have reached similar conclusions. A 2025 study published in Current Science reported that peak fire activity shifted from around 1.30 pm in 2020 to about 5 pm in 2024, based on Meteosat Second Generation satellite data. In December 2025, researchers at the International Forum for Environment, Sustainability & Technology released a multi-satellite analysis that echoed these findings.

While the role of stubble burning in Delhi’s winter smog is widely recognised, scientists continue to debate its precise contribution relative to other sources such as vehicles, industry, domestic fuel use, fireworks and dust storms. Estimates vary widely, ranging from 10 to 50 per cent.

Pawan Gupta, a NASA research scientist specialising in air quality, said crop fires can account for 40 to 70 per cent of pollution on individual peak days, though the share falls to 20 to 30 per cent when averaged over a month and to under 10 per cent on an annual basis. He added that meteorological factors, including low temperatures, weak winds and a shallow boundary layer, further complicate the picture during the burning season.

Scientists say the growing tendency to burn stubble in the evening could worsen overnight pollution build-up, as emissions released later in the day are more likely to be trapped near the surface, underscoring the need for improved monitoring and mitigation strategies.

With IANS inputs