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Global evidence of rapid urban growth in flood zones since 1985

Nature volume 622, pages 87–92 (2023)Cite this article

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Abstract

Disaster losses are increasing and evidence is mounting that climate change is driving up the probability of extreme natural shocks1,2,3. Yet it has also proved politically expedient to invoke climate change as an exogenous force that supposedly places disasters beyond the influence of local and national authorities4,5. However, locally determined patterns of urbanization and spatial development are key factors to the exposure and vulnerability of people to climatic shocks6. Using high-resolution annual data, this study shows that, since 1985, human settlements around the world—from villages to megacities—have expanded continuously and rapidly into present-day flood zones. In many regions, growth in the most hazardous flood zones is outpacing growth in non-exposed zones by a large margin, particularly in East Asia, where high-hazard settlements have expanded 60% faster than flood-safe settlements. These results provide systematic evidence of a divergence in the exposure of countries to flood hazards. Instead of adapting their exposure, many countries continue to actively amplify their exposure to increasingly frequent climatic shocks.

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Fig. 1: Riverside-settlement expansion.
Fig. 2: Global settlement growth is fastest where hazards are highest.
Fig. 3: Settlement growth by hazard level.
Fig. 4: Flood-safe versus flood-exposed growth: relative difference between settlement growth in safe and high-hazard areas from 1985 to 2015.
Fig. 5: Top 20 countries in terms of settlement area by flood-hazard level.

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Data availability

The WSF-Evo dataset is publicly available for download from https://geoservice.dlr.de/web/maps/eoc:wsfevolution. We use proprietary global fluvial and pluvial flood-hazard data with the permission of Fathom Global, who provide the data for academic purposes and can be contacted at https://www.fathom.global/contact-us/. We use coastal flood maps developed by Vousdoukas et al.40, which are publicly available for download from https://doi.org/10.5281/zenodo.8057902. Country-level summary results and subnationally and annually disaggregated results are provided in the Supplementary Information to this study.

Code availability

The source code for this study is available at https://doi.org/10.5281/zenodo.7987230.

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Acknowledgements

This study has benefited from feedback and input by L. Bernard, R. Damania, V. Deparday, K. Garrett, C. Gevaert, N. Holm-Nielsen, B. Jongman, N. Lozano Gracia, S. Ramesh, C. Riom and L. Southwood. It was supported by the Global Facility for Disaster Reduction and Recovery (GFDRR).

Author information

Authors and Affiliations

  1. The World Bank, Washington, DC, USA

    Jun Rentschler, Paolo Avner, Rui Su & Stéphane Hallegatte

  2. German Aerospace Center (DLR), Munich, Germany

    Mattia Marconcini

  3. MindEarth, Biel, Switzerland

    Mattia Marconcini & Emanuele Strano

  4. Department of Marine Sciences, University of the Aegean, Mytilene, Greece

    Michalis Vousdoukas

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  1. Jun Rentschler
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Contributions

J.R. and P.A. led the conception, study design, analysis and drafting, with input from S.H., R.S. and M.M. M.M. and E.S. developed the WSF-Evo dataset, designed and implemented the computational process and performed sensitivity tests. M.V. developed the coastal flood-hazard data and performed sensitivity tests. All authors critically revised the manuscript and gave final approval for publication.

Corresponding author

Correspondence to Jun Rentschler.

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The authors declare no competing interests.

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Nature thanks Thomas Corringham, Long Yang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Flood-exposed settlement extents as a share of overall settlements over time.

a, By region, settlement extent, normalized. b, By income group, settlement extent, normalized. Hazard classes are defined on the basis of the estimated inundation depth experienced during a 1-in-100-year flood: none (flood depths of 0 cm), low (up to 15 cm), moderate (between 15 and 50 cm), high (between 50 and 150 cm) and very high (over 150 cm).

Extended Data Fig. 2 Year-on-year settlement growth (%).

a, By region. b, By income groupings. Hazard classes are defined on the basis of the estimated inundation depth experienced during a 1-in-100-year flood: none (flood depths of 0 cm), low (up to 15 cm), moderate (between 15 and 50 cm), high (between 50 and 150 cm) and very high (over 150 cm).

Extended Data Fig. 3 Proportional representation of settlement extent.

a, Global settlement (1.3 million km2 total in 2015). b, Settlement extent in high-hazard flood areas (144,600 km2 total in 2015). c, New settlement extent in high-hazard flood areas (76,400 km2 total during 1985–2015). These figures offer proportional representations of settlement areas in different countries. In each subfigure, the sum of all tiles corresponds to the total settlement extent (in km2) specified in parentheses.

Extended Data Fig. 4 Sensitivity and robustness of estimates.

a, Flood-exposed settlement area for different inundation depth thresholds. b, IDC score. c, Vietnam: settlement expansion by hazard zones and flood return periods (1985–2015). China: settlement expansion by hazard zones and flood return periods (1985–2015).

Extended Data Table 1 Changing flood exposure in the world’s built-up areas
Full size table
Extended Data Table 2 Flood-hazard categorization applied in this study
Full size table

Supplementary information

Supplementary Information

This Supplementary Information file contains the following three sections: relative accuracy of the WSF-Evo dataset (Kappa coefficients); settlement exposure growth at different flood return periods; country-level summary statistics. It includes two figures: Supplementary Fig. 1: Kappa coefficients for urban settlement datasets; Supplementary Fig. 2: settlement expansion by hazard zones and flood return periods (1985–2015) in Vietnam and China.

Reporting Summary

Supplementary Data

Subnational (admin-1) estimates.

Peer Review File

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Rentschler, J., Avner, P., Marconcini, M. et al. Global evidence of rapid urban growth in flood zones since 1985. Nature 622, 87–92 (2023). https://doi.org/10.1038/s41586-023-06468-9

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