Mumbai: For decades, vast plumes of smoke have blanketed the Indo-Gangetic Plain between October and December as farmers burn crop residue after the rice harvest. While the overall stubble-burning season in 2025 followed familiar patterns, NASA noted a significant change in the daily timing of fires, with many now being lit hours later than in previous years.

Hiren Jethva, an atmospheric scientist at NASA’s Goddard Space Flight Center, said air quality in Delhi and several other cities deteriorated for nearly a month after fires intensified in the last week of October. However, unlike earlier years when most fires occurred between 1 pm and 2 pm, recent satellite data shows that the majority now take place between 4 pm and 6 pm. “Farmers have changed their behaviour,” Jethva said.

The shift was identified using data from South Korea’s GEO-KOMPSAT-2A geostationary satellite, which provides observations every 10 minutes. Scientists warn that traditional fire-monitoring systems relying on satellites such as MODIS and VIIRS, which pass over locations only once or twice a day, may miss many of these later-burning fires.

Satellite imagery from November 11, 2025, captured dense smoke spreading across Punjab, Haryana, Uttar Pradesh and Madhya Pradesh. News reports said it marked the first of several days when pollution levels crossed 400 on India’s air quality index, the most severe category. The spike forced authorities in some regions to shut schools and tighten restrictions on construction as stagnant weather trapped pollutants close to the ground.

Indian researchers have also observed the same trend. A 2025 Current Science study found that peak fire activity shifted from around 1:30 pm in 2020 to nearly 5 pm by 2024. A multi-satellite analysis released in December 2025 by the International Forum for Environment, Sustainability, & Technology confirmed similar findings.

While the role of stubble burning in Delhi’s air pollution is widely accepted, scientists continue to debate how much it contributes compared to vehicles, industry, domestic fuel use, fireworks and dust storms. NASA air-quality researcher Pawan Gupta said studies estimate crop fires account for 10 to 50 per cent of pollution overall. During peak episodes, the share can rise to 40–70 per cent on a single day, though it falls sharply when averaged over longer periods.

Experts warn that the later timing of fires could worsen overnight pollution, as evening burns often coincide with weaker winds and a shallower atmospheric boundary layer, allowing harmful pollutants to build up more rapidly and linger longer over cities in north India.