When do city networks cover regions prone to hot summer extremes? The ICLEI network case

Cities are increasingly at the forefront of climate change impacts, particularly as extreme heat intensifies and spreads across the globe. At the same time, transnational city networks such as ICLEI have emerged as key actors in urban climate governance, yet it remains unclear how environmental risk, economic capacity, and historical connectivity shape participation in these networks. We start from an in-depth investigation of the development of extreme hot summers in different regions of the world, the geographic spread of the world’s population, the localization of populated and urban regions, and the membership of city governments in ICLEI. Utilizing observations of extreme hot summers from 1990-2020, this study provides a large-scale, long-term retrospective on city engagement in transnational climate governance and contributes to discussions on how climate extremes shape the global development of urban climate networks. Using ERA5 reanalysis data on hot summer extremes alongside contextualizing social data on the global population density, ICLEI member city locations, and World Bank GDP data, we analyze spatial and temporal patterns of network developments.

We find that early ICLEI membership was concentrated in economically resourced and historically connected cities in Europe and North America, while later expansion increasingly reached cities in regions experiencing high absolute and intensifying hot summer extremes, including parts of West and Southern Africa. Our results further show that regional clustering and local diffusion play a central role in network expansion, with membership often spreading from early adopters to neighboring cities. Overall, the findings highlight how transnational urban climate governance emerges at the intersection of climate exposure, economic resources, and existing relationships.

The contribution bridges geoscience and social sciences by mapping geospatial and temporal climate data and data on the ICLEI network, contextualized with economic data. Importantly, our approach transcends outsourcing climate observation and reanalysis by engaging in deep interdisciplinary collaboration to gauge how changes in the network are aligned with climate extremes. It aims to take up geoscientific contributions into the theoretic thought of social scientific thought, providing a basis for an assessment that recognizes the natural environment as a factor in the social, economic, and political developments – such as the management of sustainability-oriented networks.