Engineering and land use intensified climate-driven Yangtze River flood hazards since the Little Ice Age.

Understanding the interplay between climate variability and human activities is essential for assessing long-term river flood hazards. Yet, the relative contributions of natural and anthropogenic drivers to flood magnitude remain poorly constrained due to limited multicentury records and insufficient integration of physical mechanisms with land-use histories. Here, we present the reconstruction of a 500-y history of Yangtze River flood variability using lake sediment archives, historical documents, and high-resolution climate reanalysis data. Our results reveal that flood frequency peaked during the Little Ice Age (~1500-1850 CE), driven by a weakened Western Pacific Subtropical High (WPSH), a southward-shifted Intertropical Convergence Zone (ITCZ), and enhanced southwesterly monsoonal moisture flux into the basin. These conditions were associated with El Niño-like tropical sea surface temperature (SST) anomalies and extratropical Rossby wave activity, highlighting the role of tropical-extratropical coupling in shaping flood-prone circulation regimes. Despite increased atmospheric temperature since 1850 CE, flood frequency declined, coinciding with a strengthened WPSH, a northward-shifted ITCZ, and the dominance of La Niña-like or neutral SST patterns. A nonstationary flood frequency model reveals that embankments and lake reclamation amplified the climate-driven 100-y flood magnitude by around 18% and 8%, respectively. These findings demonstrate that human interventions have intensified, rather than mitigated, the hydrological consequences of climatic forcing. Consistent with the recognition that flood risk is inherently nonstationary, our results provide empirical, multicentennial constraints on how atmospheric dynamics and landscape modification interact to shape flood hazards, highlighting the need for process-informed flood risk assessment in large river basins.