Paddy stubble burning, when farmers in Punjab and Haryana prepare fields for wheat planting, by setting fire to stubble and loose straw, will aggravate pollution this year more than in earlier years because of the delayed onset of an equatorial Pacific Ocean phenomenon called La Nina, and the extended monsoon season. Wheat planting is done from the last week of October. The best time is the first two weeks of November.
In an article in the Indian Express, Gulfran Beig writes that the late retreat of monsoons has resulted in a longer stretch of high humidity and calm winds. This combined with anti-cyclonic circulation in the post-monsoon period would reduce the mixing of air in the atmosphere and dispersal of pollutants. Beig is a professor at the National Institute of Advanced Studies in Bengaluru. He developed Safar, an air quality monitoring system.
While delayed monsoon retreat will cause higher humidity, the late onset of La Nina will result in stagnant surface winds, which will aggravate pollution in India during autumn and early winter. But La Nina also causes longer and more severe winters. This will lower the inversion layer — the part of the atmosphere that traps pollutants, and limit vertical mixing. So the benefit of stronger winds caused by La Nina will be restricted.
La Nina means Little Girl in Spanish. It has the opposite effect of El Nino. El Nino occurs when sea surface temperature over a six million sq. km area in the equatorial Pacific, just west of the International Dateline is 0.5 degrees centigrade or more higher than the average of the previous three decades. It began in June last year and peaked between November 2023 and January 2024. El Nino causes the low-level surface winds, which normally blow from east to west along the equator (easterly winds), to weaken or, in some cases, start blowing in the other direction (from west to east (westerly winds). El Nino is associated with poor monsoons in India.
The US Climate Prediction Center (CPC) has said there was a 71 percent chance of La Niña emerging between September and November 2024, and persisting through January to March 2025. It was earlier expected to emerge around July. During a La Niña event, trade winds are even stronger than usual, pushing more warm water toward Asia. The stronger winds and more dynamic atmospheric circulation help disperse pollutants in northern India.
Beig says pollution over north India should not be seen as being caused only by local emissions or regional factors. It is influenced by global weather systems too. Tackling it requires an airshed approach. Beig also cautions against overemphasis on arresting dust or 10-micron particulate matter (PM10), when PM 2.5 is a greater health hazard.
(Top photo of a burnt field in Ludhiana by Vivian Fernandes)
