Climate Change, Floods, and Human Health

Flooding is by far the most common climate hazard and wreaks devastating consequences (see Section S1 in the Supplementary Appendix, available with the full text of this article at NEJM.org).1 From 2000 to 2019, more than 1.65 billion people were affected by floods, with approximately 104,614 lives lost.1 A changing climate alters precipitation patterns, evapotranspiration, soil moisture, and the global cryosphere, leading to changes in the frequency, magnitude, and duration of floods.2 Most countries had a higher average number of population-weighted flood days during the period of 2001–2021 than during 1985–2000 (Figure 1, and see the interactive graphic, available with the full text of this article at NEJM.org). This increase is particularly evident in Australia, Russia, North America, central and southeastern Europe, West and East Africa, and South and Southeast Asia.

Projections based on a scenario of high greenhouse-gas emissions indicate that flood frequency will increase in 42% of worldwide land-grid cells during 2071–2100 (from 1971–2000 levels), including Southeast Asia, peninsular India, eastern Africa, and the northern half of the Andes (in South America).3 Correspondingly, annual worldwide flood exposure is projected to increase by a factor of approximately 14 from the period of 1971–2000 to 2071–2100.3 By 2100, an annual 20-year flood risk (i.e., a 5% chance of flooding each year) will be faced on land that is currently home to 59.4 million people (under a scenario of low greenhouse-gas emissions), 72.8 million (with intermediate levels of greenhouse-gas emissions), and 97.6 million (with high greenhouse-gas emissions).4 The United Nations International Children’s Emergency Fund (UNICEF) estimates that riverine floods could displace approximately 96 million children worldwide over the next 30 years.5

Flood exposure affects health both directly and indirectly, across short- and long-term time frames (Figure 2). In this article, we summarize current knowledge — and gaps in our knowledge — about the health risks of floods and solutions to reduce those health risks.