Every October and November, satellites detect large numbers of crop fires in northwestern India. These small, short-lived blazes are especially common in the states of Punjab and Haryana, where they produce rivers of smoke that often drift east, mix with other sources of air pollution, darken skies in the Indo-Gangetic Plain, and trigger air quality alerts in Delhi and other cities.
The 2024 burning season followed this pattern. In mid-November, New Delhi's air pollution levels rose dozens of times above the limit deemed safe by the World Health Organization, prompting a citywide medical emergency, school closures, and a construction ban. However, remote sensing scientists also noticed something unexpected: the number of fires detected by satellites in Punjab and Haryana plunged even as skies remained as smoky as ever.
Experts are studying the discrepancy. But they caution that untangling the causes of year-to-year variability in the satellite data, as well as the long-term trends in fire activity, is complex and requires careful analysis that can take months and sometimes years.
Before the 1960s, farmers in Punjab and Haryana generally cleared crops by hand. This changed with the Green Revolution, which introduced fertilizers, mechanical harvesting, more frequent crop rotations, and faster-growing crops. By the 1980s, these new ways of farming had significantly increased yields but created a new challenge: mechanical harvesters left a layer of plant residue, or "stubble," after the rice harvest. While baling it into feed or tilling it into the soil were options, burning was often the quickest and cheapest way to ready the fields for winter wheat.
Decades later, the practice persists. Each day in October and November 2024, the MODIS (Moderate Resolution Imaging Spectroradiometer) on NASA's Terra and Aqua satellites and the VIIRS (Visible Infrared Imaging Radiometer Suite) on the Suomi NPP satellite detected hundreds and sometimes thousands of thermal anomalies -- hot areas often associated with fires -- scattered throughout the two states.
The image at the top of the page, captured by VIIRS, shows smoke enveloping Punjab and Haryana and several large cities on November 8, 2024, one of the smokiest days of the burning season. The location of fires are shown with red circles. The false-color image (OLI-2 bands 7-5-4) image from Landsat 9 (above) shows a detailed view of recently burned fields near Faridkot on November 25, 2024. This band combination makes it easier to identify unburned vegetated areas (green) and the recently burned landscape (brown).
Multiple factors likely contributed to the increase. Some research indicates that the total acreage of rice grown has played a key role, with fires increasing in sync with a 25 percent increase in rice production.
Liu and Mickley have examined the possible impact of a 2009 law designed to prevent groundwater loss. The law requires farmers to wait to transplant rice into fields until mid-June, when the monsoon rains arrive, delaying the harvest and burning by two-to-three weeks.
The delay likely took some pressure off groundwater, but it also decreased the time available between the rice harvest and the planting of winter wheat, Mickley explained. This leaves farmers less time for alternative methods of removing crop stubble. "The end result: farmers burn more and later," she said.
Pushing the fires later into November worsened air pollution, the Harvard researchers say, because the fires were more likely to coincide with cooler winter weather. "By November, we see a slackening of winds, less ventilation, and a more compressed boundary layer that traps smoke and pollution near the surface," said Liu, now at the University of British Columbia. In contrast, smoke from fires earlier in the season usually disperses more quickly in the winds.
Hiren Jethva, a Morgan State University atmospheric scientist based at NASA's Goddard Space Flight Center, has tracked another pattern in recent years: sharp declines in the number of fires detected by satellites in 2023 and 2024.
During the 2024 burning season, MODIS observed the lowest number of fires in Punjab and Haryana on record since the two sensors began collecting data in 2000 and 2002, according to Jethva. The VIIRS sensor, which began collecting data in 2012 and can detect even smaller fires than MODIS, also observed record low numbers of fires in 2024. The chart above depicts thermal anomalies detected by VIIRS on Suomi NPP between 2012 and 2024. The past two years were the two lowest years on record; the most active was 2016. (A portion of the fire detection data comes from VIIRS on the NOAA-20 satellite due to a VIIRS data outage from Suomi NPP in November 2024.)
"We are seeing a decrease in fire activity detected by MODIS and VIIRS, but not in the amount of smoke in the air," Jethva said. Based on the intensity and spread of smoke in 2024, it "seems increasingly likely" that farmers are burning later in the day -- after MODIS and VIIRS have made their afternoon pass over the region, he added.
Supporting this idea, Jethva observed a shift in the timing of peak fire activity toward the late afternoon in data collected by GEO-KOMPSAT-2A, a South Korean geostationary satellite that makes observations of the region every 10 minutes.
Multiple researchers who conduct research using fire data from satellites suggest caution when interpreting year-to-year variability in fire observations. "MODIS and VIIRS only collect data twice each day, so we're missing all the fires that happen before and after those passes," said Liu. "There's also great variability in observing conditions that can affect how many fires the sensors detect."
Pawan Gupta, also at NASA Goddard, agrees. "Clouds, fog, thick smoke, certain viewing angles, and various data processing issues can cause MODIS and VIIRS to significantly undercount fires in certain conditions," he said.
For instance, several peak burning days between November 2 and November 9 in 2024 were missing from the Suomi NPP VIIRS record, due to a data processing problem, Gupta said. Also, fog obscured satellite observations of the fires for several days starting on November 12. "The real level of fire activity in 2024 might not be as low as the numbers suggest," he added.
While GEO-KOMPSAT-2 offers a powerful way of observing fires on a frequent basis, this satellite has relatively low-resolution sensors that miss many small fires that MODIS or VIIRS can detect, noted Liu. "There are tradeoffs depending on what sensors and satellites you use," she said.
Nevertheless, satellites offer an unmistakable view of the scope of stubble burning. "People forget how widely this smoke travels. This is an enormous problem," Mickley said. "We're talking about hazardous particles that affect tens of millions of people and work their way deep into people's lungs, where they set up systematic inflammation that exacerbates a whole range of diseases."