Attributing Global Changes in Flood Seasonality to Anthropogenic Climate Change and Land-Surface Human Activities

Flood seasonality strongly influences flood risk and its management worldwide. Anthropogenic climate change and land-surface human activities are widely recognized as the two major drivers of its evolution, yet their relative contributions remain poorly quantified. In this study, the factorial experiment design of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) was used to disentangle the individual and combined effects of these two drivers on global flood seasonality. Using observation-driven ISIMIP2a simulations, the ability of the ISIMIP framework to reproduce two key seasonality metrics—mean flood timing and flood timing concentration—was first evaluated. Historical and future changes in flood seasonality under RCP6.0 and RCP8.5 were then assessed at multiple spatial scales, including global grid cells, 32 major river basins, and Köppen–Geiger climate zones. Quantitative results show that north of 40° N, under the influence of land-surface human activities, the proportion of grid cells exhibiting earlier mean flood timing under RCP6.0 increases from 54.43% (climate-only) to 65.76%, with the largest change observed in the cold climate zone (median: −0.0708). In contrast, the tropical zone shows a delayed trend (median: +0.0051). Spatially, a pronounced polarization pattern emerges: high-latitude cold regions experience widespread earlier and more dispersed flood timing, while low-latitude tropical regions exhibit later and more concentrated flood timing. At the basin scale, under RCP8.5, the Pearl River Basin shows the strongest delay (ΔF = +0.1356), whereas the St. Lawrence River Basin shows the strongest advance (ΔF = −0.07445). Land-surface human activities amplify or even reverse the direction of changes in mean flood timing. When human activities are included, 31 out of 32 major basins exhibit earlier flood timing, with seven basins reversing from delay to advance. The anthropogenic climate change signal emerges over most of the globe during the first half of the 21st century, and the timing of emergence is closely linked to the warming threshold. This study reveals the modulating role of land-surface human activities in the evolution of flood seasonality and quantifies the emergence time and warming conditions associated with the anthropogenic signal, providing a scientific basis for basin-scale flood-risk management and climate adaptation strategies.