1. Introduction

Cities are expanding at an unprecedented pace, driven by internal migration and global population shifts that are reshaping urban form and function [1]. In accommodating this growth, urban expansion has disrupted environmental equilibria, intensifying heat exposure, air pollution and resource stress. These physical changes do more than altering urban climates; they actively reshape social, economic and cultural relations. Rising temperatures and pollution intersect with housing, infrastructure and income, redistributing risks unevenly across urban populations [2]. As cities grow denser and warmer, environmental stressors amplify inequality, exposing the deep interdependence between urban climate dynamics and social equity.

There are now 1,000 overheated cities, and approximately 1.7 billion citizens living in temperature conditions far above the limits of thermal comfort [3]. The rate of urban overheating is continuously rising, with the number of overheating hours having increased by 300% since 1980 [3]. Particularly in Southeast Asia, the rate of urban overheating has dramatically increased, adding about 508 million human-days of exposure to heatwave conditions per year in India and China in the last decade alone [4]. It is projected that under a future overheating scenario of 1.5ºC, citizens living in low-income countries will experience 95 days per year above 35º C, against 23 days for the urban citizens of rich countries [5].

Remarkably, about 9% of the global population (~600 million people), lives out of the ‘human climate niche’ defined as the historically highly conserved distribution of relative human population density with respect to mean annual temperature [6]. It is estimated that under the current policies leading to a temperature increase close to 2.7°C by the end of the century, almost one-third (22–39%) of people will live outside the niche [6]. These projections show that climate change is not a straightforward process but a differentiated crisis, driven by deeper historical and structural inequalities.

The relationship between income and heat exposure reinforces the need to situate urban thermal inequity within the historical and ongoing asymmetries between the Global North and the Global South. Accelerated urbanisation, heightened exposure to extreme climatic phenomena and constrained adaptive capacity in many low- and middle-income countries are inseparable from colonial legacies, extractive development models and unequal terms of global economic exchange. While high-income countries have contributed disproportionately to cumulative greenhouse gas emissions, cities in the Global South now bear an increasing share of climate-amplified heat exposure under conditions of informal housing, infrastructure deficits and limited fiscal capacity. Urban overheating thus reflects a global misalignment between responsibility for climate change and vulnerability to its impacts, reinforcing long-standing patterns of environmental and social injustice [7,8].

Differential exposure to extreme urban heat is observed not only between populations of different countries, but even within the urban population of the same city. The major demographic determinants of heat exposure in a city scale seem to be income, race and age. In most US cities, people of colour are more exposed to heat than non-Hispanic whites [9], while lower income urban dwellers usually live in heavily populated areas, with less greenery and permeable surfaces, all parameters exacerbating the effects of Urban Heat Island [10] and establishing higher temperatures during heatwaves [11]. At the same time, about 1.12 billion citizens live in slums under precarious conditions, while by 2030, one in every four people will live in a slum [12]. Governments around the world are failing to recognize these patterns and to provide adaptation and mitigation solutions for the vulnerable urban populations making urban overheating a discernible marker of urban injustice. The growing divide in heat exposure across countries, cities, and even neighborhoods highlights a deeper systemic failure to link urban development with social justice and climate resilience, a challenge this perspective aims to address.

Urban overheating therefore cannot be understood solely as an environmental phenomenon but as a multidimensional system of inequity that shapes, and is shaped by, energy use, health outcomes, economic productivity and social vulnerability. In the sections that follow, we quantify these interlinked impacts with a focus on low‑income urban populations, drawing on emerging global evidence to illustrate how heat exposure amplifies disparities across energy systems, health risks, productivity losses and learning capacity. We then outline technological, financial and governance pathways capable of addressing these inequities, arguing for a thermal justice transition that embeds equity at the centre of urban climate adaptation.

2. Quantifying heat impacts on low-income populations

The consequences of urban overheating extend across many facets of human life. High temperatures increase energy consumption in buildings and cities, and consequently, peak electricity demand. In turn, increased demand leads electricity producers to build new power stations passing the cost down to the consumers. At the same time, high temperatures reduce the efficiency of thermal and nuclear power plants, elevate pollutant emissions and intensify the concentration of tropospheric ozone, all posing serious threat for human life. Urban overheating drives up heat-related deaths and health problems, exacerbates mental health issues, fuels crime, urban aggression, and suicides, diminishes human productivity, and undermines future human capital by impairing students’ learning capacity [13]. These are not isolated consequences but interconnected dimensions of what can be called a “thermal poverty trap,” where exposure, vulnerability, and limited adaptive capacity perpetuate disadvantage.

The following subsections provide indicative quantitative data on the various impacts of urban overheating on energy production and use, social disparities, human health, performance and Gross Domestic Product (GDP).

3. Technological and financial pathways to equitable urban heat mitigation

Methods to reduce urban overheating have been widely studied and implemented across numerous cities. These approaches range from improving energy efficiency and thermal comfort in both existing and new buildings, thereby reducing reliance on air conditioning, to strategic urban planning that integrates greenery and heat sinks to dissipate excess heat. Over the past two decades, intensive and largely successful scientific research has also focused on the development of advanced materials specifically designed to mitigate urban overheating, expanding the portfolio of tools available for climate-resilient urban adaptation. Photonic super-cool materials, for example, have achieved unprecedented thermal performance, reducing surface temperatures by up to 8°C below ambient temperature [62,63]. The application of super-cool materials in Riyadh, Saudi Arabia, is expected to reduce ambient peak urban temperatures by up to 4.5°C, alongside adapted green technologies [64].

Beyond their impact on the urban microclimate, the application of modern thermal mitigation technologies to low-income housing can substantially reduce indoor heat exposure. Empirical studies show that such interventions can lower peak indoor temperatures by up to 10 °C, offering meaningful thermal relief without reliance on air conditioning and protecting the health of vulnerable populations significantly [65]. In South Africa, for example, the application of cool coatings to informal dwellings constructed from timber frames and galvanised iron sheets reduced the number of days with indoor temperatures above 40 °C from 76 to just 6 per year [66]. In India, large scale deployment of cool roofs in informal settlements has been shown to prevent up to 317,000 premature deaths, reduce cooling related emissions by 68 million tons of CO₂ equivalent by 2030, and generate an average of 186 additional labour hours per household annually through reduced heat stress and improved sleep quality [67]. These benefits translate into approximately USD 22 billion in productivity gains between now and 2030. While these findings demonstrate the profound social value of thermal mitigation, their widespread adoption depends critically on affordability, policy support, and institutional capacity, especially in low-income and informally governed urban areas. The scale of investment required to decarbonise the built environment and mitigate urban overheating creates both a challenge and an opportunity. Current estimates indicate that approximately USD 1.6 trillion per year is required for global decarbonisation until 2050, with an additional USD 400 billion annually needed specifically for urban thermal mitigation [68]. At present, the economic cost of urban overheating is estimated at USD 700 billion per year and is expected to exceed USD 2.5 trillion by 2050 [69]. These losses are disproportionately concentrated in tropical middle- and low-income countries, where annual GDP losses may reach 3.2–6.4%, compared to near negligible impacts in high income economies [69].

Consequently, pathways to equitable heat mitigation must be understood within deeply unequal political and economic relations between high- and low-income areas, especially between the Global North and the Global South. Historical patterns of extraction, emissions-intensive development, and capital accumulation in high-income countries have disproportionately contributed to the climate conditions that now intensify heat exposure in low- and middle- income cities, while simultaneously limiting their adaptive capacity. Critical urban research has shown that climate adaptation initiatives can unintentionally reinforce inequality when driven primarily by market incentives, property value enhancement, or technology-led showcase projects implemented without strong equity safeguards [70,71]. Cities in the Global South are often positioned as recipients of adaptation technologies and financing models developed in the Global North, lacking the institutional capacity, fiscal autonomy, and political leverage required to ensure socially just outcomes.

Without explicit equity safeguards, heat mitigation financing mechanisms risk privileging investment returns over social protection and concentration of benefits in already advantaged countries or neighbourhoods, mirroring failures observed in carbon markets and other forms of climate greenwashing. Any “market for urban heat mitigation” must therefore operate within strong public governance frameworks, ensuring that financial instruments such as green bonds or resilience credits prioritise low-income and heat-vulnerable communities, complement public investment in social housing and urban infrastructure, and support redistributive climate policies rather than replace them [70,72].

Technological and financial solutions must also be accompanied by broader social and institutional transformations. Thermal justice cannot be achieved without addressing structural drivers of vulnerability, including informal housing, insecurity of tenure, racialised segregation, limited political representation, and unequal access to public space. Community level interventions such as climate shelters or climate oases, i.e., public, accessible, and thermally safe spaces designed to protect residents during extreme heat events, have emerged as important complementary strategies, particularly in cities where housing conditions cannot be rapidly upgraded. These interventions foreground human wellbeing over market value and emphasise care, accessibility, and social inclusion as core adaptation principles [73].

A just thermal transition therefore requires not only technology transfer, but context-specific co-design, local capacity building, and the redistribution of adaptation finance toward highly vulnerable urban communities. Financing interventions could be underpinned by established instruments, including green bonds and broader green banking mechanisms, to channel capital toward measurable thermal risk reduction. Although the scale of required investment is substantial, it is not prohibitive. The necessary capital clearly exists. A modest annual tax increase of just 1% on the wealthiest 1% of the global population could generate approximately 7.4 trillion US dollars, sufficient to finance large-scale, equitable urban heat mitigation and adaptation efforts worldwide.

According to new analysis by the Fight Inequality Alliance, Institute for Policy Studies, Oxfam and the Patriotic Millionaires, an annual wealth tax of up to 5% on the world’s multi-millionaires and billionaires could raise $1.7 trillion a year, enough to lift 2 billion people out of poverty, fully fund the shortfalls on existing humanitarian appeals, deliver a 10-year plan to end hunger, support poorer countries being ravaged by climate impacts, and deliver universal healthcare and social protection for everyone living in low- and lower middle-income countries [74]. Allocating even a modest share of global wealth to thermal mitigation and climate adaptation is a non-negotiable obligation in a warming world, capable of dismantling structural inequities while repositioning climate policy as a central mechanism of redistribution, protection and collective resilience.

4. Conclusions

Urban overheating has emerged as one of the most consequential challenges of the twenty-first century, driven by the convergence of global climate change, local urbanisation processes and entrenched socio-economic inequalities. As we argue in this Perspective, these forces do not operate independently but form a reinforcing feedback system in which heat exposure deepens vulnerability and vulnerability in turn amplifies exposure. This dynamic helps explain why, despite advances in climate science and mitigation technologies, thermal disparities within cities persist and, in many cases, intensify. Evidence across energy systems, health outcomes, productivity and spatial exposure demonstrates that overheating functions as a socio-economic amplifier, disproportionately burdening low-income populations who face higher relative costs, weaker infrastructure and fewer adaptive options.

We argue that breaking this feedback loop requires a thermal justice transition, defined as a systemic reorientation of urban climate adaptation that places equity at its core. Such a transition entails the strategic deployment of technological innovations, including super-cool materials and urban greening, within the most vulnerable communities, the development of metrics capable of measuring and tracking intra-urban thermal disparities, the establishment of thermal safety nets to guarantee minimum levels of thermal protection, and the mobilisation of innovative financial instruments such as urban heat bonds and resilience credits to direct capital toward populations most at risk. We further emphasise the need to recognise and support the Global South not only as a site of vulnerability, but as a source of locally appropriate, scalable, and context sensitive adaptation strategies capable of informing just urban heat transitions globally.

Ultimately, achieving thermal justice in cities requires more than the deployment of innovative technologies and the mobilisation of financial resources. While such instruments are essential, they must be accompanied by deeper social and institutional transformations, including equitable urban planning, inclusive governance, and the protection of vulnerable populations. In addition, meaningful progress depends on the accountability of high emitting industries and nations, predominantly in the Global North, whose historical contributions to climate change have disproportionately shaped present day heat risks elsewhere. Addressing urban overheating therefore demands a comprehensive approach that integrates technological innovation, financial redistribution, social change, and responsibility across scales. We reframe urban overheating not simply as a problem of temperature management but as a challenge of justice and governance that demands interdisciplinary collaboration among climate scientists, urban planners, economists, social scientists, policymakers and communities themselves. As cities continue to grow and warming intensifies over the coming decade, we warn that failure to confront urban thermal inequity risks locking billions of people into cycles of heat vulnerability and socio-economic exclusion. Conversely, embedding equity within thermal adaptation offers a powerful opportunity to protect health, enhance productivity, reduce poverty and build cities that are not only cooler but also fairer and more resilient. Urban overheating is no longer a marginal environmental concern. It represents a defining test of whether societies can govern climate change in ways that are just, inclusive and transformative, and how this challenge is met will shape urban livability for generations to come.