Increasing precipitation volatility in twenty-first-century California

Study key findings & significance

• Abrupt transitions in California from a parched winter to a soggy one, as observed in the mid-2010s, will become more common if greenhouse gases continue to increase.
• California’s wet season is likely to contract, and the shorter wet seasons will become more variable, with an increase in both extra-dry and extra-wet winters.
• The study projects a 25% to 100% increase in extreme dry-to-wet precipitation events, despite only modest changes in mean precipitation
• One of the most worrisome findings is an enhanced risk of extremely wet winters. The modeling indicates that 40-day-long rainfall stretches on par with those observed during the “Great Flood” winter of 1861-62 will be more than three times more likely by the 2070s-2090s than they were prior to human-produced greenhouse gases. 
• Such hydrological cycle intensification would seriously challenge California’s existing water storage, conveyance and flood control infrastructure

Author quotes

California’s human and natural systems can usually withstand the kind of variations we’ve seen in the past. But as the frequency and intensity of these swings increases, it could push some species to the edge. For example, the warming climate is already stressing our forests faster than they can adapt—we’re seeing a dramatic example of this right now with widespread Sierra Nevada tree mortality. And warming temperatures are making it difficult to manage for salmon and other fishes that rely on cold water.

Daniel Swain, lead author and climate scientist at UCLA

Abstract

Mediterranean climate regimes are particularly susceptible to rapid shifts between drought and flood—of which, California’s rapid transition from record multi-year dryness between 2012 and 2016 to extreme wetness during the 2016–2017 winter provides a dramatic example. Projected future changes in such dry-to-wet events, however, remain inadequately quantified, which we investigate here using the Community Earth System Model Large Ensemble of climate model simulations. Anthropogenic forcing is found to yield large twenty-first-century increases in the frequency of wet extremes, including a more than threefold increase in sub-seasonal events comparable to California’s ‘Great Flood of 1862’. Smaller but statistically robust increases in dry extremes are also apparent. As a consequence, a 25% to 100% increase in extreme dry-to-wet precipitation events is projected, despite only modest changes in mean precipitation. Such hydrological cycle intensification would seriously challenge California’s existing water storage, conveyance and flood control infrastructure.