Climate Change Effects on California Precipitation and Soil Moisture
Projecting 21st century changes in California precipitation and soil moisture is accomplished through examining global climate model simulations alongside dynamical and statistical downscaling techniques. Over the Los Angeles region, a novel hybrid statistical-dynamical downscaling procedure is developed to project mid- and end-of-21st century changes to local precipitation according to 36 global climate models under an aggressive warming scenario. For both time slices, large uncertainty exists on the sign of mean precipitation change, though the spread of these changes is only around 0.2 to 0.4 times as large as natural interannual variability in the historical period. Uncertainty in these precipitation projections can be attributed to varying model placements of the jet stream and storm tracks over the eastern Pacific Ocean. While mean precipitation changes over the Los Angeles region are projected to be small compared to natural variability levels, global climate models project significant changes to interannual extreme precipitation events over central and northern California. The strongest and most statistically significant changes are found by the second half of the 21st century, 2060--2100. During this time period, dry extremes become 1.5 to 2 times more common than historical levels, while wet extremes generally triple in their historical frequency. The mechanism behind this signal of increased extremes is rooted in large increases to interannual precipitation variability seen in a clear majority of global climate models. Finally, the hybrid statistical-dynamical downscaling technique is used to provide for a high-resolution multi-model projection of soil moisture changes over the California Sierra Nevada. Large spatial heterogeneity is found across the region, as the central and northern Sierra Nevada experience drier end-of-century soils and the highly-elevated southeastern peaks experience wetter soils. These signals reflect the complex effects of a much warmer future climate acting on snow-dominated landscapes. Across the entire Sierra Nevada, multiple lines of evidence indicate that the region will experience a net decline in soil moisture on the order of 2-3% per year by the end of the 21st century.