Introduction

Accurately assessing urban heat risk is vital for developing effective adaptation and mitigation strategies. This research addresses a critical gap in global-scale risk assessments: the frequent assumption of spatially homogeneous social vulnerability. By conducting a comparative heat risk assessment across 1,029 global cities from 2000 to 2019, the study introduces a multidimensional social vulnerability index. The results demonstrate that the traditional homogeneous assumption leads to a systematic underestimation of climate inequality, particularly between the Global North and Global South.

Results

Worldwide increasing urban heat risk

Global urban residents experienced extreme heat risk (Risk) with an average intensity of 2.11 million person-events during the study period. The most acute risk hotspots are concentrated in major urban agglomerations across South Asia, Southeast Asia, and Central and West Africa. In these regions, cities such as Delhi, Dhaka, Jakarta, and Bangkok face a dual crisis of surging heatwave frequency combined with dense, highly vulnerable populations. Widespread increasing trends in Risk are evident across more than 84.4% of global cities, with 38.8% of cities exhibiting a statistically significant increasing trend. This trend affects nearly 398 million urban residents, predominantly in Asia (57.5%), Europe (16.1%), and Africa (14.3%).

North-South inequality in urban heat risk

The study finds that residents in the Global South experience Risk that is 203.0% higher than those in the Global North. On average, residents in the Global South recorded 3.13 million person-events compared to 1.03 million in the Global North. Crucially, the traditional assessment method, which assumes homogeneous social vulnerability, only identifies a 138.0% disparity. This indicates that the homogeneous assumption leads to a 65.0% underestimation of North-South inequality. When disaggregated by income levels, the disparity becomes more pronounced; the increasing trend of Risk in the Global South is nearly five times that of the Global North. The increasing trend in low-income countries (LICs) reached 3.76 × 10⁵ person-events per year, whereas high-income countries (HICs) remained moderate at 0.24 × 10⁵.

Intraregional inequality in heat risk

Intraregional inequality, measured by the Gini index, is larger within the Global North (0.47) than within the Global South (0.41). The study reveals that 69.5% of cities in the Global North exhibit ‘High-High’ or ‘Low-Low’ spatial association patterns between exposure and social vulnerability, compared to only 25.7% in the Global South. This spatial clustering represents a localized ‘Matthew Effect’, where regions with high exposure simultaneously suffer from high social vulnerability. Over the past two decades, Gini indices for Risk have gradually declined in the Global North while increasing in the Global South, suggesting that the distribution of heat risk within the Global South is becoming increasingly uneven.

Discussion

Accounting for the spatial heterogeneity of social vulnerability reveals that conventional assessments severely underestimate global heat risk inequality. The research suggests a shift in focus from merely identifying ‘who is exposed’ to ‘who is truly vulnerable’. Reducing social vulnerability is essential for mitigating climate impacts, especially in developing countries. While economic growth offers potential benefits, these can be offset by rapid population ageing. The study underscores the urgent need to integrate demographic-targeted vulnerability reduction into broader climate adaptation strategies, supporting the pursuit of climate justice and the Sustainable Development Goals (SDGs).

Methods

The researchers selected 1,029 cities with urban areas larger than 100 square kilometres, including 493 in the Global North and 536 in the Global South. These cities represent over 55% of the global urban population. Heatwaves were defined as periods of three or more consecutive days where both daily maximum and minimum temperatures exceeded the 90th percentile from a reference period (1970–1999). The multidimensional social vulnerability index integrated eight indicators across economic, environmental, and demographic dimensions, including air-conditioning penetration, GDP per capita, urban greenspace, and the proportion of susceptible groups such as children, elderly adults, and women.