The phrase “extreme heat” used to belong to weather bulletins and record books. In Asia’s rising heat belt, it is becoming a description of normal life. Stretch a band from Karachi along the Arabian Sea, across the Indo-Gangetic plain to Dhaka, then arch it through the Bay of Bengal and up toward Manila, Ho Chi Minh City and Bangkok, and you trace a corridor where population density, humidity and climate physics are colliding. It is here, in the sweltering streets of megacities built for a cooler era, that the limits of human tolerance are being tested first.

From “Hot” to Physically Dangerous: Enter the Wet-Bulb World

The technical phrase that now preoccupies climate scientists is not simply temperature, but “wet-bulb temperature” — a measure that combines heat and humidity to capture how easily the human body can cool itself through sweat. At a sustained wet-bulb of around 35°C, even a healthy person resting in the shade with unlimited water cannot shed heat fast enough; survival becomes a matter of hours. South Asia and parts of Southeast Asia are now seeing short episodes that brush up against this threshold. Projections for the 2030s suggest that such conditions will become more frequent, and that they will spread across wider regions of Pakistan, India, Bangladesh and the maritime tropics.

These numbers are not academic abstractions. They define who can work outdoors, when children can walk to school, whether cramped dwellings turn lethal at night, and how labour, leisure and sleep are carved up in a megacity day. A heatwave in Paris is uncomfortable; a heatwave in Karachi, when night-time temperatures refuse to fall and humidity stays high, can be mortal.

Karachi, Dhaka, Manila: Three Cities, One Zone of Strain

No two megacities are identical, yet Karachi, Dhaka and Manila share a common predicament. They are coastal, low-lying, thick with concrete, and expanding faster than their infrastructure can adapt. Each is already grappling with flooding and storms. Now they must live with new patterns of heat that undermine the very routines on which their economies are built.

In each city, the heat crisis intersects with existing fault-lines: inequality, governance gaps, and aging or inadequate infrastructure. Climate does not arrive on a blank slate; it lands on top of political and social structures that decide whose neighbourhoods cool down, whose do not, and who pays for a world where the baseline has shifted.

The Economy of Heat: When Labour Hours Melt

For planners, one of the most worrying metrics is not the thermometer but the calendar of safe working hours. Agricultural economists estimate that in parts of South and Southeast Asia, outdoor labour capacity during hot months has already fallen significantly over the past three decades. Construction workers, rickshaw pullers, street vendors, dock labourers and waste pickers are the invisible cooling system of urban life — they move goods, repair roads, build housing, keep the city breathing. They are also the most exposed to ambient conditions.

In Karachi’s industrial zones, factories increasingly adjust shifts around peak heat, but many cannot afford to shut doors altogether when the mercury climbs. In Dhaka, garment workers who spend their days inside poorly ventilated buildings may avoid direct sun but not heat, especially when power outages kill fans and air-conditioning. In Manila, jeepney drivers and market vendors face the double cost of lost hours and rising expenditure on water and basic cooling. As heat intensifies, the informal and semi-formal economy bleeds productivity quietly, in millions of shortened workdays and compromised bodies.

Power Grids at the Breaking Point

The obvious adaptation to more heat is more cooling: fans, air-conditioners, cold storage, shaded public spaces. But cooling draws on electricity, and electricity grids in many Asian cities are already strained by rapid growth, under-investment and volatile fuel prices. A hot day in Manila or Karachi pushes air-conditioning use to new peaks, putting transformers, distribution lines and power plants under stress. When the system fails, the very technology meant to protect people becomes an amplifier of risk.

Dhaka’s grid, fed by a mix of domestic gas, imported fuels and rising renewables, must balance demand surges with the financial realities of high global energy prices and limited fiscal space. Rolling blackouts during heatwaves turn apartments into ovens. For wealthier households, backup generators and inverters offer relief; for the majority, especially in informal settlements, grid failure is simply an additional layer of vulnerability they cannot hedge away.

Housing: Concrete, Corrugated Metal and the Physics of Trapped Heat

The thermal behaviour of a city is written in its materials. In many neighbourhoods of Karachi, Dhaka and Manila, homes are built from concrete, brick and corrugated metal with minimal insulation or cross-ventilation. These surfaces absorb heat during the day and release it slowly after sunset, ensuring that night-time temperatures inside remain several degrees higher than outside air. In crowded lanes where buildings press against each other and air barely moves, the “urban heat island” effect is not a metaphor; it is a bodily experience of suffocation.

Retrofitting such housing stock is a monumental task. Cool roofs, reflective paints, added shading, redesigned windows — all are technically straightforward but depend on cash, legal security and landlords willing to invest. Informal homes, lacking clear titles and built incrementally, sit in a grey zone where neither the state nor the market sees an obvious role. Without targeted programmes, the people who need thermal improvements most are those least likely to receive them.

Public Space and the Politics of Shade

In wealthy districts, heat can be mitigated by parks, tree-lined avenues and air-conditioned malls. In the working-class fabric of these megacities, public space is often sparse, paved and unshaded. Street trees are cut to make way for road widening; small parks are converted to parking lots or encroached by construction. The design of the city quietly decides who has access to shade.

Experiments in some Asian cities — heat-resilient urban planning in Ahmedabad, for example, or small-scale cooling centres in Philippine barangays — hint at what a different politics of shade could look like: bus stops designed as shelters rather than bare poles, schoolyards with canopies, markets rebuilt to allow air circulation, riverside or coastal promenades restored as cooling corridors instead of fenced-off dumping grounds. But these remain islands in a much larger sea of asphalt and bare concrete.

Migration: When Heat Meets the Urban “Last Stop”

Climate migration in Asia is often imagined as a rural phenomenon: farmers leaving dry fields or flooded deltas to seek work elsewhere. In practice, many of those journeys end in already overheated cities. Dhaka’s slums are filled with families from riverine districts hit by recurrent flooding and erosion. Karachi attracts migrants from Pakistan’s interior provinces and from across the border. Metro Manila draws people from typhoon-prone islands and impoverished regions of Luzon and Mindanao.

The paradox is stark. People move partly to escape environmental stress, yet they often arrive in neighbourhoods whose exposure to heat, flooding and pollution is as bad or worse than what they left. The city offers income, schools and access to services, but it also concentrates risk. As the heat belt intensifies, urban migration will both relieve and compound climate pressure, depending on whether infrastructure and planning can keep pace.

Health Systems: The Slow Burn

Hospitals in these megacities are built to handle infectious disease outbreaks, chronic conditions and the normal flow of accidents and emergencies. Heat adds a new layer of demand. During heatwaves, admissions for dehydration, kidney stress, heart problems and complications of diabetes and hypertension all climb. Mortality spikes are often undercounted because deaths are attributed to underlying conditions rather than to heat as a trigger.

For under-resourced health systems, heat is a multiplier. Staff must work in their own stressful conditions; equipment runs hotter; electricity failures disrupt care; and the poorest patients, who live in the hottest areas, reach care late if at all. Early warning systems and public advisories help, but they can only do so much when people must choose between staying home in stifling rooms or going out to earn daily wages.

What Serious Adaptation Would Look Like

Adaptation to Asia’s new heat belt is often framed as a list of options: early warning systems, green roofs, urban forests, revised building codes. The harder question is whether these can be delivered at the scale and speed that Karachi, Dhaka, Manila and their peers require. Serious adaptation would mean redesigning core parts of the urban metabolism, not just adding cosmetic greenery or issuing more warnings on radio.

It would involve rethinking work hours and labour protections, so that outdoor workers are not forced into midday shifts when conditions are physically dangerous. It would require integrating heat into the logic of housing policy, treating shade and ventilation as basic infrastructure, not luxuries. It would demand that power-sector planning explicitly account for rising cooling demand and the cost of failures during heatwaves. And it would push municipal budgets to prioritise the unglamorous work of fixing drains, planting trees and enforcing setbacks that keep some space for air and water to move.

The Political Question: Who Gets to Stay Cool?

In the end, the map of heat vulnerability will not be decided only by climate models. It will be decided by politics. In all three cities, and in many others across the belt, the people with the least formal power live in the hottest places, work the hottest jobs and have the fewest escape routes. They also have the weakest voice in how adaptation funds are allocated, where infrastructure is built and which neighbourhoods receive upgrades first.

That imbalance turns climate adaptation into a question of justice. Who pays for the extra electricity when tariffs rise? Who funds better housing for those who cannot pay market rents? Who decides whether a cooling centre is opened in a slum or in a downtown commercial district? The answers will determine whether the new heat belt becomes a story of managed risk or of quietly tolerated mass discomfort — and preventable loss of life.

Asia’s Heat Belt in the 2030s: From Shock to Background Condition

By the mid-2030s, if current projections hold, many of the extremes that now make headlines will have faded into the background of urban life. Heatwaves will still be named and tracked, but the underlying average will be higher. Young people growing up in Karachi, Dhaka and Manila will have no memory of cooler baselines; for them, extreme heat will not be a deviation but the starting point.

What can still change is the institutional response. A decade is long enough to redesign building codes, to plan and begin to implement new power infrastructure, to adjust labour laws, to shift public investment toward shade, ventilation and drainage, and to fold heat risk into every major decision about where and how cities grow. It is not time enough to avoid the new climate zone. It is time enough to decide whether living in it is merely punishing — or impossibly cruel.