A temperature of 43.5°C in Delhi this week is not merely a statistic. It is a physical reality that feels closer to 47°C for the city’s 30 million inhabitants, according to British climate scientists who have analysed the impact of the urban heat island effect. The phenomenon, driven by concrete, asphalt and a dearth of vegetation, is adding a dangerous multiplier to an already warming planet.
Dr. Aris Thorne of the University of Cambridge’s Centre for Climate Science explains that the urban heat island effect results from the replacement of natural surfaces with materials that absorb and re-radiate solar energy. “A city like Delhi is essentially a heat battery. During the day, buildings and roads soak up shortwave radiation. At night, they release it as longwave radiation, preventing the temperature from dropping. This can increase ambient temperatures by 3°C to 6°C compared to surrounding rural areas, and more in densely built-up districts.”
On a day when the baseline temperature reaches 43.5°C, that extra 3°C to 6°C pushes the effective heat load on the human body beyond survivable limits for prolonged exposure. The wet-bulb globe temperature, a measure that accounts for humidity, wind and radiant heat, likely exceeded 35°C in parts of Delhi. That is the threshold at which the human body can no longer cool itself through sweating.
This is not a future projection. This is the present reality for hundreds of millions of people living in rapidly urbanising regions of South Asia. The Intergovernmental Panel on Climate Change’s sixth assessment report noted with high confidence that the frequency and intensity of extreme heat events have increased globally, and that urbanisation exacerbates this trend. Delhi, with its population density and infrastructure, is a case study.
The immediate cause of this week’s heat wave is a persistent high-pressure system over northern India, drawing hot air from the Thar Desert. But the background warming from climate change has shifted the probability of such events. A 2022 attribution study by World Weather Attribution found that the deadly heat wave in India and Pakistan that year was made 30 times more likely by climate change. Similar attribution for this event would likely show a comparable increase.
What can be done? The answer lies in re-engineering the urban fabric. Green roofs, reflective “cool” pavements, expanded tree canopy and water features can reduce local temperatures significantly. A study in Ahmedabad showed that a combination of white roofs and tree planting could lower neighbourhood temperatures by up to 2°C. But implementation lags behind the need. Policy inertia, cost and competing priorities slow adoption.
The deeper issue is that adaptation must be matched by mitigation. No amount of urban greening will keep Delhi habitable if global emissions continue on their current trajectory. Every fraction of a degree of warming amplifies the urban heat island effect, making adaptation harder and more expensive.
For the residents of Delhi, the immediate advice is to stay indoors during peak heat, hydrate and check on the elderly. But for policymakers in Delhi, London and every capital in between, the message is clear: the urban heat island effect is a force multiplier for climate change. We are building our own ovens, and the thermostat is rising.
As Dr. Thorne put it: “We know how to fix this. The physics is not in dispute. What is in dispute is whether we have the collective will to act before the heat becomes truly unbearable.” The temperature in Delhi today is 43.5°C. The real question is what it will be tomorrow.
