When the sun goes down, heat vulnerability remains: evidence from the I4C Barcelona demonstrator
A new study, led by the Barcelona Supercomputing Center (BSC) and carried out by the Earth System Services group at BSC-CNS, in collaboration with the Universitat Autònoma de Barcelona, highlights how night-time heat is becoming an increasingly serious and unequal climate challenge for urban populations.
Why cities remain hot at night: implications for health, comfort and social inequality
Using Barcelona as a case study, the research shows that urban heat-island effects are often most pronounced after sunset. Buildings, roads, and other impermeable surfaces gradually release the heat absorbed during the day, significantly limiting night-time cooling. In densely built neighbourhoods with limited green space and restricted access to climate shelters, residents can therefore remain exposed to elevated temperatures for prolonged periods.
The study further highlights how higher night-time temperatures interact with other urban stressors, such as traffic-related noise, exacerbating discomfort, health risks and social inequalities. For vulnerable groups, including older people and individuals with chronic health conditions, insufficient nocturnal cooling reduces opportunities for physiological recovery and increases cumulative heat stress during heatwaves.
The role of the I4C Barcelona demonstrator city
The I4C Barcelona demonstrator city plays a key role in advancing research on the evolution of elevated night-time temperatures and their implications for urban resilience. This work, led by the Barcelona Supercomputing Center, builds on climate information derived from the latest Euro-CORDEX simulations at a 0.11-degree resolution, generated through the downscaling of CMIP5 global climate model projections analysed within the I4C project. These projections provide detailed insights into the evolution of tropical and torrid nights and their projected trends through to the end of the 21st century.
Within the broader I4C framework, which operates across four European demonstrator cities, Barcelona focuses specifically on extreme heat, with particular attention to night-time conditions and their interaction with public health, social vulnerability, and urban morphology. In this context, the Barcelona demonstrator contributes to the development of an advanced climate service designed to support urban planning, housing retrofitting, and public health decision-making. The service delivers tailored climate projections, alongside clearly defined thresholds and formats, to provide actionable references for local authorities and health agencies when designing and triggering city-level interventions.
By integrating heat-related indicators, such as heat stress indices, the frequency of tropical nights, and drought metrics, with socio-demographic data and information on infrastructure quality, the Barcelona demonstrator enables a detailed assessment of the unequal distribution of extreme heat impacts across the urban population. This integrated approach supports the identification of priority areas for intervention, informs the design of public spaces capable of functioning as climate shelters, and provides an evidence base for public policies on housing retrofitting and urban regeneration.
Against this background, the study calls on both researchers and practitioners to adopt a night-time perspective that brings together climate science, social equity, and urban policy, thereby strengthening the capacity of cities to protect vulnerable population groups in a warming climate.
Rethinking urban heat adaptation
The findings underline the need to rethink how cities address heat risk. By focusing not only on daytime extremes but also on night-time conditions, urban planners, public authorities, and climate services can develop more effective and fair adaptation strategies. Integrating night-time heat indicators into risk assessments, early warning systems, and urban planning will be essential to protect those most exposed.
Next steps: from research to dialogue with cities
Building on this work, the I4C team is preparing a workshop with the Network of Cities and Towns for Sustainability in the Province of Barcelona, to be held on 27 January. Heat, health and, among other topics, night-time heat will be central to the discussions, helping to connect scientific evidence with the practical needs of local authorities.