Regions at risk: end-of-century impacts of extreme climate change on global population

The human population is projected to grow to between 8.9 billion and 12.4 billion by 2100, with 85% expected to live in cities. Over 80% of global GDP is generated in cities, demonstrating the importance of urban environments to human flourishing. As urban infrastructure systems are vulnerable to climate change it is vital that the full range of potential impacts on urban areas are understood.

High-impact, low-likelihood (HILL) extreme climate change (ECC) events do not receive as much attention as scenarios considered most likely in the existing literature, despite the severe threat these tail risks pose to many aspects of human society. This includes impacts on urban infrastructure, supply chains, population displacement and migration, food, water and energy security and associated consequences such as conflict. There remains significant uncertainty regarding the likelihood of ECC due to, for example, potential climate tipping elements that may cascade, trigger feedbacks and lead to runaway climate change, even if global warming remains under Paris Agreement thresholds, which is growing more unlikely. The potentially existential impacts of such scenarios necessitates that we exercise the precautionary principle and explore the boundaries of plausibility.

In this study we present synthesised evidence from extreme climate change scenarios, assessing the potential worst-case impacts on urban systems with a focus on heterogeneous regional impacts and the potential need for significant pre-emptive adaptation efforts. Global regions are ranked according to their potential vulnerability to ECC impacts with the aim of identifying urban areas where critical infrastructure is at risk of failure and what effects are likely to be experienced. We highlight gaps in current understanding and the need to focus research in this area, including case studies of infrastructure systems in cities identified as vulnerable, with the aim of generating evidence for use in policy development.