Globally, this study shows that overall settlements expanded by 85 percent between 1985 and 2015 on average, while those exposed to the highest flood hazard level increased by 122 percent. These differences suggest the influence of a host of underlying causes for the increasing flood exposure of urbanization, including geographic and climatic features, socioeconomic trends, planning practices, and institutions.

These findings carry concrete implications for urban planners and policy makers. In areas, where flood exposure is already high, investments in disaster preparedness and protection are crucial to mitigate losses; for instance, through early warning systems, evacuation protocols, insurance, social protection, and retrofitting of infrastructure. In areas, where flood exposure is still low but increasing rapidly, revision of land use and urbanization plans will be essential, along with reviewing and reforming risk-informed building codes and infrastructure masterplans. While land scarcity and geographic constraints may mean that settling in flood zones cannot always be avoided, this necessitates careful planning of flood protection systems and disaster preparedness measures to enable resilient socio-economic development.

In an age of increasing disaster losses due to flood events, effective flood risk mitigation efforts must be based on a robust understanding of the different contributing factors to flood risk. These can include intensifying hazards (for instance, due to climate change), increasing exposure to hazards (due to expanding at-risk settlements), and rising vulnerability to shocks (for instance, due to inadequate building standards or social protection systems). This study shows with high geographic granularity that rising exposure is key to understanding the global rise in flood disaster losses.

# **References**

> 1 <mark>Otto, F. The art of attribution.</mark> _<mark>Nature Clim Change</mark>_ <mark>6, 342–343 (2016).</mark>

> 2 Schiermeier, Q. Droughts, heatwaves and floods: How to tell when climate change is to blame. _<mark>Nature</mark>_ <mark>560, 20-22 (2018)</mark>

> 3 <mark>Otto, F., van Oldenborgh, G., Eden, J.</mark> _<mark>et al.</mark>_ <mark>The attribution question.</mark> _<mark>Nature Clim Change</mark>_ <mark>6, 813–816 (2016).</mark>

> 4 <mark>Philip, S., Kew, S., van Oldenborgh, G. J., Otto, F., Vautard, R., van der Wiel, K., King, A., Lott, F., Arrighi, J., Singh, R., and van Aalst, M.: A protocol for probabilistic extreme event attribution analyses, Adv. Stat. Clim. Meteorol. Oceanogr., 6, 177–203 (2020)</mark>

> 5 <mark>Raju, E., Boyd, E. & Otto, F. Stop blaming the climate for disasters.</mark> _<mark>Commun Earth Environ</mark>_ <mark>3, 1 (2022).</mark>

> 6 <mark>Lahsen, M., & Ribot, J. (2022). Politics of attributing extreme events and disasters to climate change.</mark> _<mark>Wiley Interdisciplinary Reviews: Climate Change</mark>_ <mark>, 13( 1), e750.</mark>

7 Hallegatte, S., Vogt-Schilb, A., Bangalore, M., Rozenberg, J. Unbreakable : Building the Resilience of the Poor in the Face of Natural Disasters. Washington, DC: World Bank (2017) 8 Henderson, J.V., Cities and development. Journal of Regional Science, 50: 515-540. (2010)

> 9 Rosenthal, S. S. & Strang, W.C. Evidence on the nature and sources of agglomeration economies. _Handbook of Regional and Urban Economics_ , **4** , 2119–71, (Elsevier, 2004).

> 10 Duranton, G. & Puga, D. Micro-foundations of urban agglomeration economies. _Handbook of Regional and Urban Economics_ **4** , 2063–2117 (2004).

11 DLR(2021) https://geoservice.dlr.de/web/maps/eoc:wsfevolution

> 12 Hallegatte, S. _An Exploration of the Link between Development, Economic Growth, and Natural Risk_ (World Bank, Washington DC, 2012).

> 13 Hallegatte, S. _How Economic Growth and Rational Decisions Can Make Disaster Losses Grow Faster Than Wealth_ (World Bank, Washington DC, 2011).

> 14 Kuhnreuter, H. Mitigating disaster losses through insurance. _J. Risk Uncertain._ **12** , 171–181 (1996).

20