In coastal areas, projected increases in the amplitude of the sea level seasonal cycle will lead to the intensification of coastal flood hazards. Conversely, projected decreases in the amplitude may act to suppress flood hazards in some regions, partially offsetting the impact of sea level rise. Simulations of the sea level seasonal cycle by the Coupled Model Intercomparison Project Phase 6 (CMIP6) models suggest changes up to ±30% are projected under a high emission scenario. We demonstrate that a currently unaccounted increase of 5–10 cm in the sea level seasonal cycle will increase the frequency of extremes from once in 20 years to once in 10 years. However, the sea level seasonal cycle amplitudes in CMIP6 models are underestimated compared to seasonal variability observed by satellite altimetry for almost all coastlines, suggesting that future changes might be underestimated, leading to a large impact on extreme sea levels in coastal areas. Including the changes in the future sea level seasonal cycle will advance future projections of sea level extremes and contribute to the crucial decisions about adaptation in coastal areas.

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