The city that traps its own heat

Anyone who has spent an April afternoon in Ho Chi Minh City knows that the heat there does not simply come from the sky, nor does it fade when the sun goes down. It radiates from pavements, bounces off glass towers and pours out of the backs of air conditioners, so that the streets are still warm to the touch long after dark. It is not all in the mind, either. Scientists call this the urban heat island effect, and it helps to explain why the city centre can be several degrees hotter than the rice fields and mangroves on its outskirts.

The causes are well understood. Concrete and asphalt absorb sunlight during the day and release it slowly at night, whereas soil and vegetation stay cooler because they hold moisture. Over the past three decades, Ho Chi Minh City has grown at a remarkable pace, and much of that growth has come at the expense of green space and open water. Canals have been filled in to make way for roads, and gardens have given way to shophouses and apartment blocks. On top of this, millions of motorbikes and air conditioners pump waste heat directly into the streets, thereby making a hot city hotter still.

The consequences are far from trivial. Extreme heat disrupts sleep, reduces the productivity of outdoor workers and puts the elderly and young children at serious risk. Delivery riders, street vendors and construction workers bear the brunt of it, since they have little choice but to stay outside during the hottest hours. Given that these are often the people with the lowest incomes, the heat island effect is not only an environmental problem but also a question of fairness.

Granted, the city cannot simply undo decades of construction, and land in the centre is far too expensive to turn into parks on a large scale. Nonetheless, a great deal can be done at street level. Planting shade trees, painting roofs in light colours and protecting the canals that remain are relatively cheap measures, yet studies from other tropical cities suggest that they can lower local temperatures by a degree or two. That may not sound like much, and sceptics are quick to dismiss such efforts as a drop in the ocean. On a day when the thermometer is already approaching forty degrees, however, a degree or two can make a real difference.

Equally important is the way new districts are designed. Developers have often favoured sealed glass towers that depend entirely on air conditioning, partly because such buildings look modern and sell well. By contrast, traditional Vietnamese houses, with their high ceilings, deep verandas and open inner courtyards, were built to let air circulate. Architects who borrow from that tradition are not being nostalgic; rather, they are rediscovering solutions that worked long before electricity was cheap.

Ho Chi Minh City will keep growing, and that is no bad thing in itself. But the heat island is a reminder that a city is not simply built; it is shaped by countless small decisions about what to pave, what to plant and what to protect. The real question is whether the city can continue to grow without slowly cooking the people who live in it.