If India chokes less, it will fry more

Over the past 40 years South Asia has warmed far more slowly than elsewhere in the world—temperatures in the region have risen by around 0.09°C per decade, compared with 0.30°C over the rest of the world’s land mass and 0.23°C for places at a similar latitude. Scientists are not yet sure how to explain the South Asian “warming hole”, but there is strong evidence that both smog and changes in irrigation have played important roles. If so, then when pollution wanes, and the expansion of irrigation slows, that will change. Dirty air kills millions of people in the region each year; cleaning it up is imperative. But that clean-up may well cause a dangerous surge in warming.

South Asia is already a hot place. The time between April and June, before the monsoon, is typically the worst. In these months, temperatures on the Indo-Gangetic plain—the area that spreads from eastern Pakistan, across North India and into Bangladesh—can regularly exceed 40℃. More than 40% of people experiencing very strong or extreme heat stress each year—a measure that includes the degree to which humidity makes high temperatures more dangerous—live in South Asia. High levels of heat stress can be fatal, especially for the elderly or those with health problems.

Such dangerously hot days are becoming more frequent. Already this year the mercury has hit 44°C in some parts of India. But though temperatures are rising, South Asia has avoided the full brunt of global warming. One explanation for that is the region’s high levels of pollution. The long-lived greenhouse gases responsible for global warming are well distributed around the world; other types of pollution are more local and regional. And much of this localised pollution cools the surface rather than warming it.

The Indo-Gangetic plain is among the world’s most polluted areas. Heavy industry, traffic emissions, agricultural-waste burning and the use of solid fuels for cooking all contribute to high aerosol levels. Several studies have suggested that higher levels of these pollutants over the past two decades may have somewhat counteracted the effect of rising temperatures in the region. But the fact that sooty particles absorb sunlight rather than reflect it, cooling the surface but warming the atmosphere, complicates matters. One recent study found that in the spring of 2020, when lockdowns caused a drastic reduction in pollutants in many Indian cities, temperatures did not spike as some scientists expected but rather were unusually cool. The paper only examined temperatures over a few months, and so changes could be due to chance fluctuations, but “it’s still a little puzzling,” says Loretta Mickley, a climate scientist at Harvard University.

The expansion of irrigation is another explanation for South Asia’s slower-than-elsewhere warming. Water absorbs heat as it evaporates, cooling the air around it. In India, the area of irrigated land has doubled since 1980. Scientists think that the cooling effect of expanding irrigation in the region may have masked the impact of global warming. One study, published in Nature Communications in 2020, estimated that, without irrigation, South Asia could have between two and eight times more days of extreme heat than it does now.

There is still no scientific consensus on how much these factors have slowed warming in South Asia—scientists need to better understand the intricacies of how pollution and irrigation influence temperature, rainfall and monsoon timing in the region and how all these factors interact. But experts agree on what lies ahead. At a recent meeting between Indian and American scientists in Delhi, David Battisti, an atmospheric scientist at the University of Washington, said that over the next 20 years India “is pretty much assured” to warm at twice the rate of the past 20. Daniel Schrag, of Harvard, says temperatures in India will rise faster than in the rest of the world.

In the coming decades neither pollution nor irrigation will continue to grow in the way they have over the past 40 years. Both conceal additional warming only as long as their levels are increasing. If pollution and irrigation remain stable, South Asia will feel the full effects of any new global warming. If they decrease, the region will feel the effect of the rising temperatures from the past few decades that these factors were masking.

And decrease they will. “The truth is, India can’t continue to irrigate,” says Dr Schrag. Groundwater levels in the region are becoming depleted. “And they also can’t continue to have the level of air pollution they have.” Tackling pollution has been a flagship programme for the Indian government. By 2024 the country aimed to reduce particulate matter by 20-30% relative to 2017, by 2026 the administration aimed for 40% . So far, the project’s main achievement has been building aerosol-monitoring stations in places that did not previously have them. Many cities missed the target for 2024, although a study published earlier this year found that there has been some improvement in pollution levels over the past decade.

Other countries have shown it can be done. In the 1960s Japan was one of the most polluted places in the world. Throughout the following two decades the country enacted new laws to try to reduce pollution and introduced environmental taxes and charges for waste. By the 1980s aerosol levels had fallen dramatically. More recently, China has slashed its aerosol emissions.

Currently, air pollution is a far greater blight to South Asia than extreme heat is. According to the Global Burden of Disease study, in 2021 alone aerosol pollution killed between 2m and 3m people in the region, while extreme heat led to 100,000-600,000 deaths. “It’s crazy, the numbers for air-pollution-related mortality,” said Bhargav Krishna of the Sustainable Futures Collaborative, a research institute in Delhi. “It’s simply because of the scale of exposure and the year-round exposure that people are having to deal with.”

Even a modest increase in temperature in South Asia could be devastating. Already, there are days when the combined effect of heat and humidity push parts of the region close to the limits of human habitability (roughly equivalent to temperatures of 45°C with relative humidity of 50%). These conditions are expected to become far more common. According to Dr Battisti, by 2047, there could be a four-fold increase in the number of days that the average Indian will experience heat stress—assuming pollution and irrigation levels stay stable. With both set to drop, the number will be greater still.

Sweltering heat is unavoidable for many people in South Asia. According to an analysis by the Hindustan Times in 2022 around half of Indians work outdoors and only 10% of households in the country have air-conditioning. Many Indian cities have adopted heatwave action-plans, laying out how they will provide cooling and water to people in extreme heat. But few are taking steps to make cities more resilient or mitigate their exposure to heat. Without drastic changes the coming temperature surge will be deadly. ■