Increases in flood magnitudes in California under warming climates

•Climate model projections agree: flooding to increase in California Sierras.•More than one factor was at play to increase the flood.•Changes in flood magnitudes may pose challenges in California.

SummaryDownscaled and hydrologically modeled projections from an ensemble of 16 Global Climate Models suggest that flooding may become more intense on the western slopes of the Sierra Nevada mountains, the primary source for California's managed water system. By the end of the 21st century, all 16 climate projections for the high greenhouse-gas emission SRES A2 scenario yield larger floods with return periods ranging 2-50 years for both the Northern Sierra Nevada and Southern Sierra Nevada, regardless of the direction of change in mean precipitation. By end of century, discharges from the Northern Sierra Nevada with 50-year return periods increase by 30-90% depending on climate model, compared to historical values. Corresponding flood flows from the Southern Sierra increase by 50-100%. The increases in simulated 50 year flood flows are larger (at 95% confidence level) than would be expected due to natural variability by as early as 2035 for the SRES A2 scenario.