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Is your city prepared for flooding caused by extreme rainfall under climate change? In many regions, the uncertainty surrounding this threat is a major cause for concern and an obstacle to adaptation. However, according to researchers from Japan, their new statistical method increases the accuracy of flood risk projections across about 70% of Earth's landmass.

Earth's climate is notoriously complex to model, with considerable uncertainty stemming from the internal variability of this chaotic system. Flood risks are particularly important to predict accurately, because human populations often gather around waterways. A key approach to improve future flood risk projection accuracy is to increase the sample size of the climate scenarios used for analysis, but the number of large-ensemble experiments available for these future projections remains limited.

In a new study in Scientific Reports, researchers from the Institute of Industrial Science, The University of Tokyo, developed a method to expand the statistical sample size of the available ensemble data by merging parts of future climate scenarios with the same level of warming but different socioeconomic pathways. These pathways account for various socioeconomic factors including economic growth, urbanization, and technological development.

"Previously, it was believed that flood risk changes would vary under different future socioeconomic scenarios," explains study lead author Yuki Kimura. "However, under the same level of global warming, the geographic distribution of flood risks is actually broadly similar across socioeconomic pathways."

This approach, using large-scale simulations with a global flood model and climate projection data, allowed the research team to more accurately evaluate future flood risks across much of Earth's land surface. These findings offer policymakers powerful new insights for adaptation and preparedness strategies, by separating out the unpredictable effects of real socioeconomic factors.

Specifically, the area around the Mississippi River, U.S., and a region extending from China to Southeast Asia were identified as areas that particularly benefited from the increased accuracy of flood risk prediction achieved in this study.

According to study senior author Dai Yamazaki, "Using this method, we can now report flood risk information that is both reliable and practical, regardless of the socioeconomic scenario, based on specific warming levels such as 2°C or 3°C." Basing projections on these warming levels, rather than time-based predictions, aligns well with the climate policy targets set by the Paris Agreement, further highlighting the practical value of this approach.

Considering warming levels independently from time may not be appropriate for all climate parameters—for example, particularly rapid warming could affect ecosystems in ways that differ markedly from more gradual scenarios at the same warming level. However, given its practical benefits for predicting flood risks, this method will likely become widely used in future to provide accurate, actionable information to prepare for climate change effects.