太热，太潮湿：为什么印度和巴基斯坦持续的热浪如此危险

India and Pakistan are no strangers to heat. This time of year is the worst, as heat peaks before the monsoon brings cooler conditions from June.

But this year’s heat is something else. Intense, sustained heat began in mid-April. Daily maximum temperatures have topped 46°C in many locations, with some areas running around 5–8°C above seasonal norms.

The unrelenting heat has driven record demand for electricity in India as people turn on air conditioners – and worsened drought conditions affecting more than a million square kilometres across both countries.

When extreme heat combines with humidity, it can be lethal. Human bodies cannot cool themselves easily in these conditions. The heatwave has claimed at least 37 lives in India and 10 in Pakistan. These figures are likely to be a major underestimate, as heat-related deaths are systemically undercounted in India.

Why is it so hot?

It’s usually a hot wait for the monsoon. But several factors can line up to make a bad season much worse.

One reason it’s been so bad this year is due to persistent high-pressure weather systems. When these systems sit in place, they make heatwaves more likely by suppressing cloud formation and reducing the chance of cooling rain. This year, strong high-pressure systems have lingered over parts of India and Pakistan, trapping hot air near the surface and allowing temperatures to build over many days.

With less rain, there’s more heat at ground level and soils dry out. Drier soils make things worse, because less heat is used up evaporating moisture in the soil and more goes into heating the land. High pressure systems can often hang around for many days, allowing extreme heat to build up.

It’s often worst in cities, as concrete and asphalt absorb heat during the day and release it slowly overnight. This means cities stay hotter overnight, boosting health risks for people without access to cooling.

Behind these immediate reasons is the big one: climate change. As the world gets steadily hotter, heatwaves get worse and worse. Estimates from World Weather Attribution suggest the first big heatwave from 15–29 April 2026 was made about three times more likely and about 1°C hotter due to climate change.

At current global levels of global warming (~1.4°C) , this means the subcontinent faces similar events about once every five years. At present, we’re tracking towards 2.6°C of warming by 2100. At that level of heat, heatwaves like this would hit every 2-3 years and be 2.2°C hotter.

Humidity makes heat much more lethal

The number on a thermometer is only part of the danger.

Many parts of India and Pakistan are intensely humid. When sustained extreme heat arrives, humidity acts to intensify the threat to health. Humidity levels are worsening in parts of the region.

That’s because it’s harder to cool down naturally in humid conditions. Human bodies use sweating as the main method of cooling. When these beads of warm water evaporate off the skin, heat is carried away.

Humid air makes sweating a much less effective method of cooling. When the air already holds a lot of moisture, it takes longer for sweat to evaporate. The body can keep getting hotter even as it sweats.

This is why scientists are increasingly concerned about lethal humidity – when heat and humidity combine to rapidly sicken or kill.

Dying like this is deeply unpleasant. It begins with the core body temperature rising. People sweat more to try to shed the heat, but sweating doesn’t work well. If there’s no reprieve, the body temperature can keeps rising past 40°C and heatstroke can set in, damaging the brain and other vital organs. This can be fatal without rapid cooling and urgent care.

To gauge the combined danger of heat and humidity, scientists use measures such as the wet-bulb temperature. This reflects how much cooling is possible through sweating.

It used to be thought the limit for human survival was a wet-bulb temperature of 35°C. But new research shows heat and humidity can be lethal across a range of temperature and humidity combinations. For example, for older people who are outdoors, 35°C and 90% humidity is as deadly as 45°C and 30% humidity. These levels have already been reached during heatwaves in Southern Asia in recent years. For instance, even healthy 18-35 year olds are at risk of dying with humidity of 40% and temperatures of 45°C.

It’s likely some areas of the subcontinent have hit those limits at times during this intense period of heat. But we can’t say for sure, as most weather bulletins give air temperatures rather than wet-bulb temperature.

A threat faced unequally

The risks of heat and humidity are not faced equally. Wealthier people can turn on the air conditioner and avoid going outdoors.

But poorer people in informal settlements can’t escape the heat. Neither can construction workers, farmers, delivery riders and others doing physically demanding work outdoors.

There’s another risk too. The body needs cooler temperatures overnight to recover from intense heat. When the heat continues overnight, there’s no relief.

While cities are hotter than the surrounding areas, rural communities still face threats from heat and humidity. That’s because more work tends to be outdoors, healthcare is often far away and cooling is limited.

When could relief come?

When the monsoon arrives, it usually brings cooler conditions. Cloud cover and widespread rainfall help lower daytime temperatures, though humidity often stays high. The monsoon usually arrives in early June in southern India and covers the whole country by mid-July. In Pakistan, the monsoon typically arrives later, usually beginning in early July. The monsoon often lasts till September.

Relief can’t come too soon for the region.

Unfortunately, it won’t be the last threat. But as climate change ramps up, extreme heat and humidity will hit these nations more often – and more severely.

Oluwafemi E. Adeyeri receives funding from the Australian Research Council.

Sarah Perkins-Kirkpatrick receives funding from the Australian Research Council and is President of the Australian Meteorological and Oceanographic Society.