# 1. Introduction

Disaster losses are rising, and evidence is mounting that climate change is driving up the probability of extreme natural shocks<sup>1,2,3,4</sup> . Yet, it has also proven politically expedient to invoke climate change as an exogenous force that supposedly places disasters beyond the influence of local planners and authorities<sup>5,6</sup> . However, locally determined patterns of urbanization and spatial development are key factors to people’s exposure and vulnerability to climatic shocks<sup>7</sup> . In this study we show that since 1985 human settlements around the world – from rural villages to large cities – have expanded continuously and rapidly into present flood zones. Globally, settlement growth in high-risk zones is outpacing growth in safe zones by a large margin. This provides systematic evidence that rather than adapting to increasingly frequent climatic shocks, countries around the world are actively increasing their exposure to flood hazards.

Around the world, urban populations are growing rapidly, as people from agricultural peripheral regions move into cities searching for economic opportunities. As a consequence, cities continue to grow, in terms of population and spatial footprint. Urbanization and economic development have traditionally gone hand in hand as cities enable agglomeration economies<sup>8</sup> ; for instance, they enable matchmaking between employers and job seekers, sellers and buyers, and capital and projects; support expensive productivityenhancing infrastructure such as public transport systems; and favor the circulation of ideas<sup>9,10</sup> .

However, on the downside, rapid urban growth can cause congestion effects; for instance, by increasing exposure to natural hazards and pressure on public services and infrastructure. This is particularly relevant in developing countries, which often lack the capacity for comprehensive urban planning and implementation, and the financial resources to invest in transport and protection infrastructure. These challenges are often compounded by fast demographic growth and internal migration, which can mean that risk-informed urban and infrastructure planning cannot keep pace. Moreover, as safe spaces are increasingly occupied, the resulting land scarcity can drive new developments disproportionately into previously avoided areas such as riverbeds, flood plains, or wetlands (figure 1).

This study offers a systematic and global analysis of spatial urbanization patterns and their evolving exposure to flood hazards. It leverages the 2019 high resolution global flood hazard layers by Fathom and the World Settlement Footprint Evolution (WSF-Evo) data set, developed by the German Aerospace Center (DLR) in 2021, which offers high-precision measurements of yearly settlement extents from 1985 to 2015 at 30 by 30 meters spatial resolution and with global coverage (figure 2)<sup>11</sup> . These data allow us to track the speed and shape of urban expansion in large cities through to small rural settlements and estimate flood exposure with high precision, and therefore understand what share of countries’ built-up land is exposed to high flood hazards and how this evolves over time. We show that settlements across the world grew by 85 percent between 1985 and 2015, from 693,000 to over 1.28 million square kilometers. In the same period, settlements exposed to the highest flood hazard level increased by 122 percent – thus highlighting that in many places, risky growth is outpacing safe growth.

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