Subscribe
India has warmed significantly over the past several decades—but not as much as the rest of the Northern Hemisphere. Satellite data from the US National Aeronautics and Space Administration (NASA), released on January 10, shows that since the 1950s, the country has warmed about 0.6oC, which is only about half the warming seen in other regions of the hemisphere. At first glance, this might seem like a potential silver lining in the climate crisis. But scientists say it could be a warning sign rather than a cause for comfort.
One possible explanation for this unusual trend lies in the thick blanket of aerosols—fine particulate matter that remain suspended in the air—lingering over large parts of the country, especially the Indo-Gangetic plains.
Aerosols, largely released through fossil fuel combustion, crop burning and industrial activities, act like a massive sunshade. “They reflect sunlight back into space like tiny mirrors or absorb it in the upper atmosphere. Either way, they reduce the amount of solar radiation reaching the surface,” says Loretta J Mickley, senior research fellow and co-lead, Atmospheric Chemistry Modeling Group, Harvard University, US. The result is a net cooling effect at the ground level—even as greenhouse gases continue to accumulate.
However, Mickley says, little has been studied to understand what governs the temperature patterns in India and the role of aerosols in suppressing warming. “In the eastern US, we have historically dealt mostly with reflective sulphate aerosols,” says Mickley. “But India also has a significant presence of black carbon—emitted from industries and crop burning—which makes the aerosol mix much more absorbing,” she adds.
The complexity does not end there. Aerosols also interact with clouds in unpredictable ways. “Generally, aerosols can increase cloud cover, but under certain conditions, they can also reduce it,” says Mickley. This unpredictability was clearly illustrated in a 2021 study published in Environmental Research. The researchers analysed satellite data during Delhi’s first COVID-19 lockdown (March-May 2020) and found a 25 per cent reduction in particulate matter. To coincide with it, night-time land surface temperatures also dropped by 2-4°C. This defied expectations: with fewer aerosols in the air, more sunlight should have reached the ground, potentially raising temperatures. The study thus suggests that under the usual conditions, black carbon aerosols (primarily produced by combustion of fossil fuels such as from vehicles and biomass such as from wildfires) near the surface absorb heat and warm the surrounding air. But when these particles were removed during the lockdown, the ground cooled instead. “The study shows how little we understand India and what governs the temperature patterns there,” says Mickley.
Kaighin McColl, assistant professor of Earth and Planetary Sciences at Harvard University, US, says that other than aerosols, expanding irrigation or a combination of factors could be the reason India is warming at a slower rate than the rest of the Northern Hemisphere.
Interactions between the troposphere and stratosphere could well be another reason, says Roxy Mathew Koll, climate scientist at the Indian Institute of Tropical Meteorology, Pune, adding that conclusive evidence is still lacking.
What scientists do agree on is that aerosols are wreaking havoc on public health. “The most recent estimate is that air pollution causes 1.5 million excess deaths each year in India,” says David Shumway Jones, professor at Harvard University’s School of Medicine. Other assessments, such as the “State of Global Air 2024” report, place the toll even higher—at 2.1 million deaths annually. The damage extends beyond mortality. Air pollution is linked to a host of chronic health conditions— respiratory illnesses, cardiovascular diseases, strokes and complications related to pregnancy. “It diminishes quality of life and places immense pressure on our public health systems,” says Jones.
This brings India to a challenging paradox. Reducing air pollution—a clear necessity for public health—could inadvertently speed up climate warming. The health impacts of fine particulate matter (PM2.5) and sulphur dioxide (SO2), both largely released by burning fossil fuels, are currently far more severe than those from heat, says Bhargav Krishna, convener at the Delhi-based non-profit Sustainable Futures Collaborative. These pollutants can penetrate deep into the lungs and bloodstream, worsen asthma, heart disease and lung function. However, this balance could shift. As PM2.5 and SO2 levels decline, especially during the hotter months, increased solar radiation could accelerate the formation of ground-level ozone, he adds. Ground-level ozone, an air pollutant, is primarily formed when sunlight interacts with pollutants like nitrogen oxides and volatile organic compounds.
Krishna has co-authored a recent study that shows exposure to both heat and air pollution can have a synergistic effect—significantly increasing mortality risk. Add to this the cascading effects of rising heat and ozone—on agriculture, labour productivity, housing and the growing demand for cooling—and it becomes evident that siloed approaches would not suffice. Addressing this multi-faceted challenge requires viewing climate, energy, air quality and development policies through a unified lens. “Unless we start thinking about climate, energy and air quality in an integrated manner,” Krishna says, “progress in one area could lead to setbacks in another.”
This article was originally published in the May 1-15, 2025 print edition of Down To Earth