National heat exposure in India surged by 157% between 1983 and 2016, driven
primarily by population growth while being heavily amplified by urban warming
and high humidity.
What to know about Dual drivers of urban warming and population growth
Urban heatwaves have emerged as one of the deadliest natural hazards in contemporary India, driven by the dual forces of anthropogenic climate change and rapid urbanization. While the broader meteorological trends of heatwaves have been extensively studied, a critical gap has existed in understanding how city-level urban populations are exposed to extreme heat over time.
New research from the Indian Institute of Tropical Meteorology, Ministry of Earth Sciences evaluates urban population exposure to extreme heat across more than 3,000 Indian urban settlements from 1983 to 2016. By deploying high-resolution datasets and specialized wet-bulb globe temperature metrics, the study demonstrates that India’s total urban heat exposure more than doubled over this 34-year period, rising dramatically from 23 billion person-days in 1983 to 59 billion person-days in 2016.
The new study tracked both thermal hazards and demographic shifts through the Wet Bulb Globe Temperature metric, which integrates ambient temperature, humidity, solar radiation, and wind speed. High relative humidity is especially critical in tropical countries like India because it impairs the human body’s natural ability to dissipate heat through sweat, escalating health risks and reducing labor productivity.
To capture moist-heat stress accurately, the study utilizes the Global High-Resolution Daily Extreme Urban Heat Exposure dataset, mapping exceedance frequencies, which measures the the rate or number of times that a process crosses a specific critical threshold, against thresholds categorized into moderate, very high, and extreme risk levels. Furthermore, the researchers decomposed total exposure changes into two independent components: the urban warming effect, which captures climate-induced warming combined with the urban heat island effect while holding population constant at baseline 1983 levels, and the population-growth effect, which isolates demographic expansion.
Key context and latest developments
The findings reveal that at the national scale, population growth accounted for 64% of the total exposure increase, while urban warming contributed the remaining 36%. However, these drivers exhibit a spatial heterogeneity across the country, meaning there are significant variations between the different regions. Megacities such as Delhi, Mumbai, Chennai, and Ahmedabad are predominantly population-driven, where rapid urban expansion and dense informal settlements concentrate vulnerable populations in high-risk zones.
Conversely, southern coastal cities, particularly along the Kerala coast, including Thiruvananthapuram, Kollam, and Kozhikode, exhibit exposure profiles that are overwhelmingly dominated by climate-induced urban warming. In these areas high humidity and urban morphology exacerbate heat stress despite slower population growth. Other major centers like Kolkata and Hyderabad display balanced contributions, facing compounded risks from simultaneous climate and demographic pressures.
Earlier global assessments usually provided broad, macro-level evaluations of population exposure to extreme heat, but offered limited granularity for regional dynamics within climatically diverse and rapidly urbanizing countries like India. By applying a robust decomposition framework to thousands of individual Indian settlements, this research bridges the gap between macro-scale climate projections and localized urban vulnerabilities. It provides the first systematic ranking of India’s major million-plus cities based on heat exposure trends, allowing urban planners to differentiate between warming-dominated and population-driven contexts.
Despite its comprehensive scope, the analysis relies on a historical dataset spanning 1983 to 2016, and does not capture the most recent post-2016 shifts in extreme heat events. Furthermore, uncertainties within underlying climate datasets remain uncharacterized, and area-averaged wet-bulb globe temperature values can obscure significant intra-urban variations driven by micro-scale land-use heterogeneity and building morphology.
By identifying specific cities experiencing rapid increases in exposure and pinpointing their dominant drivers, the study lays the groundwork for tailored adaptation strategies. Rapidly growing cities can utilize these findings to integrate heat-risk considerations into urban planning through green and blue infrastructure, cool roofs, and building code enhancements. Meanwhile, warming-driven coastal cities can prioritize public health interventions, heat-health early warning systems, and labor protection measures. Addressing these complex environmental and demographic challenges fosters the development of sustainable, climate-resilient cities capable of protecting vulnerable populations from escalating heat hazards.
