CITIES are not meant to hold heat the way a kiln does, yet many now behave as though they were built for that purpose. The phenomenon commonly described as the urban heat island is neither abstract nor distant; it is felt in the heavy stillness of late afternoons, in the absence of relief after sunset and in the growing strain on bodies and systems that were never designed for such persistent warmth. At its simplest, a heat island emerges when built environments register higher temperatures than their rural surroundings, or when certain parts of a city grow markedly hotter than others. The cause lies in transformation. Where forests, wetlands and open soil once absorbed and moderated solar radiation, there now stand roads, rooftops and concrete expanses that capture heat through the day and release it slowly into the night. This altered landscape produces pockets of elevated temperature that can exist across entire cities or within neighbourhoods, depending on how structures, greenery and water bodies are distributed.
The mechanics are straightforward, though their consequences are not. Urban materials such as brick, asphalt and concrete store solar energy efficiently, while the geometry of dense construction restricts airflow and traps warmth between buildings. These ‘urban canyons’ reduce the natural cooling that wind might otherwise provide. Human activity compounds the problem. Vehicles, air-conditioning systems, generators, and industrial operations release additional heat into already burdened environments. Weather patterns and geography further intensify the effect: still air and clear skies maximise heat absorption, while topographical barriers can block dispersing winds. Research has long indicated that as cities expand, becoming denser and more spatially complex, these heat-retaining characteristics will only strengthen. Global warming sets the wider context, but local conditions — population density, land cover, industrial activity and the balance between terrestrial and aquatic ecosystems — determine how acutely any given city will feel the rise.
Dhaka stands as a clear and troubling example. It is no longer simply a dense city; it is an ecosystem that has been so extensively modified that its capacity to regulate temperature has been deeply compromised. Over the years, wetlands have been filled, vegetation cleared and natural surfaces replaced with impermeable layers of concrete and asphalt. What remains of green and blue spaces is fragmented, limiting evapotranspiration, one of the most effective natural cooling processes. High-rise buildings crowd together, obstructing airflow and reinforcing the canyon effect, while dark roofs and paved roads absorb heat throughout the day and release it well into the night. The result is a city where nightfall offers little respite.
This artificial heat load is intensified by the rhythms of urban life. Traffic congestion generates constant thermal output; air conditioners and generators discharge heat even as they attempt to cool interiors; industrial activity adds to the burden. At the same time, the degradation of water bodies and the loss of their connectivity diminish their role as heat sinks. Poor waste management and the persistence of open burning further contribute to local warming and air pollution. In such conditions, temperature is not merely a measure of weather but an index of systemic strain. Dhaka’s heat is cumulative, layered through years of unplanned expansion and ecological neglect.
The consequences extend well beyond discomfort. Human health is the most immediate concern. Elevated daytime temperatures combined with limited night-time cooling increase the risk of heat stress, dehydration, and cardiovascular complications. Those most exposed — outdoor workers, children, the elderly and low-income communities — bear a disproportionate share of this burden. The city’s healthcare system reflects this pressure, with rising admissions during heatwaves offering a visible indicator of environmental decline. Heat also shapes the urban economy. As temperatures rise, labour productivity declines, particularly in sectors reliant on physical work, while household expenditure increases due to the need for cooling.
Energy demand follows a predictable but troubling trajectory. Hotter conditions drive greater reliance on air conditioning and mechanical cooling, pushing electricity consumption to peak levels during already strained periods. This demand often necessitates increased reliance on fossil fuel-based power generation, which in turn raises greenhouse gas emissions and exacerbates air pollution. The cycle is self-reinforcing: higher temperatures demand more energy, and greater energy use contributes to further warming. Air quality deteriorates as heat accelerates chemical reactions in the atmosphere, leading to the formation of ground-level ozone and fine particulate matter. These pollutants carry their own health risks, compounding the effects of heat exposure.
Environmental systems are equally affected. Heated surfaces transfer warmth to stormwater runoff, raising the temperature of water bodies and disrupting aquatic ecosystems. Changes in temperature alter oxygen levels, metabolic rates, and reproductive cycles, often with fatal consequences for sensitive species. On land, increased heat accelerates soil moisture loss, raising evapotranspiration rates and leading to water stress. Vegetation suffers, biodiversity declines and the ecological balance that once moderated urban climates continues to erode. In this sense, the heat island is not a singular issue but a convergence of multiple environmental failures, each reinforcing the other.
The trajectory is not irreversible, but it demands deliberate intervention. Urban greening offers one of the most immediate and effective responses. Expanding tree cover, restoring parks, and introducing rooftop and vertical gardens can reduce surface temperatures and improve air quality. Equally important is the protection and rehabilitation of wetlands, canals, and floodplains, ensuring that water bodies can function as natural cooling systems. Urban planning must move beyond density alone and consider airflow, spacing and orientation, creating ventilation corridors that allow heat to dissipate rather than accumulate. Materials matter as well; reflective roofing and lighter-coloured pavements can significantly reduce heat absorption.
Transport and energy systems require parallel attention. Reducing congestion, promoting public transport, and limiting emissions from vehicles and industry can lower both heat output and pollution levels. At the building level, passive cooling strategies — designs that maximise shade, ventilation and insulation — can reduce dependence on energy-intensive air conditioning. Waste management reforms, including the elimination of open burning, would address a persistent and avoidable source of urban heat and pollution.
None of these measures is novel, and that is precisely the point. The knowledge exists, as does the evidence of what works. What has been lacking is consistent application, guided by an understanding that urban development cannot be separated from ecological function. Dhaka’s heat is not an accident of geography; it is the outcome of decisions made over time. Reversing its course will require decisions of equal scale and intent, grounded in the recognition that a city’s viability depends not only on its capacity to grow, but on its ability to remain habitable.
Md Sohrab Ali is former additional director general of department of environment.
