Social vulnerability amplifies global inequality in urban heat risk

Assessing urban heat risk is essential for developing heat adaptation and mitigation strategies. A key challenge in global-scale risk assessments is the common assumption of spatially homogeneous social vulnerability, often due to data constraints. Yet, the extent to which this assumption underestimates global climate inequality remains unclear. Here, we conduct a comparative heat risk assessment across 1,029 global cities during 2000–2019 by introducing a multidimensional social vulnerability index. Results reveal increasing urban heat risk worldwide, with the upward trend in the Global South nearly five-fold that in the Global North. We demonstrate that the traditional homogeneous assumption leads to a nearly 65% underestimation of North-South inequality in long-term heat risk. Furthermore, intraregional inequality measured by the Gini index is larger within the Global North and is underestimated by nearly 15% under the homogeneous assumption. Our study quantifies systematic bias in existing global heat risk assessments, providing a foundation for advancing climate justice in a rapidly warming world.