Spatial distribution, determinants, and implementation barriers of green-blue-grey infrastructure-based urban heat island mitigation strategies in Budapest, Hungary

Urban nature-based infrastructure is increasingly recognised as a powerful tool to tackle climate change-induced challenges. However, knowledge gaps remain regarding the use of hybrid infrastructure in heat mitigation, including context-specific factors that impact efficacy. This is particularly true in Budapest, Hungary, where between 1902 and 2019, a 4°C difference in urban heat levels was observed. This study aims to systematically evaluate where the urban heat island effect is most significant, what factors influence the urban heat island effect, what barriers exist to installing and scaling mitigating infrastructure, and what governmental levels and organs, civil society actors, and private actors are involved in urban heat mitigation efforts. Using an online survey (n = 216), key informant interviews (n = 13) and transect walks, findings show residents perceive the most important factors green surface size (98.7 %), water bodies (87.7 %), structural materials (92.9 %), construction materials (70.8 %), building types, density and surface albedo/colour (68.8 %), and domestic cooling (60.4 %). The heat-affected areas of Budapest include the dense historic core, outer industrial belt, railway stations, and Liszt Ferenc Airport. Key barriers relate to limited space, political will, funding, as well as unfavourable legal frameworks and bureaucratic environments. Moreover, a lack of vertical and horizontal coordination results in a disconnect between stakeholders’ agendas and understanding of the issue. Opportunities lie in expanding green spaces, improving rainwater management and retrofitting building structures through improved insulation. Further priorities include reducing impermeable surfaces and increasing the proportion of cool roofs and other hybrid infrastructure. In parallel, advancing knowledge of public acceptability of government policies and of societal responses to these policies is needed. This requires robust, transparent information dissemination and consultation mechanisms to foster legitimacy and trust. Finally, ensuring accountability across all levels of governance, with clearly delineated responsibilities to monitor progress and adaptive management, is essential for responding effectively to evolving social-ecological conditions.