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.

Using a combination of dynamical and statistical downscaling techniques, we projected mid-21stcentury warming in the Los Angeles region at 2-km (1.2-mile) resolution. To account for uncertainty associated with the trajectory of future greenhouse gas emissions and other factors affecting the planet's energy balance, we examined projections for both "business-as-usual" (RCP8.5) and "mitigation" (RCP2.6) emissions scenarios of the Fifth Coupled Model Intercomparison Project. To account for the considerable uncertainty associated with choice of global climate model, we downscaled results for all available global models. For the business-as-usual scenario, we find that by the mid-21stcentury, the most likely warming is roughly 4.6°F averaged over the region's land areas, with a 95% confidence that the warming lies between 1.7 and 7.5°F. The high resolution of the projections reveals a pronounced spatial pattern in the warming: High elevations and inland areas separated from the coast by at least one mountain complex warm 20 to 50% more than the areas near the coast or within the Los Angeles basin. This warming pattern is especially apparent in summertime. The summertime warming contrast between the inland and coastal zones has a large effect on the most likely expected number of extremely hot days per year. Coastal locations and areas within the Los Angeles basin see roughly two to three times the number of extremely hot days, while high elevations and inland areas typically experience approximately three to five times the number of extremely hot days. Under the mitigation emissions scenario, the most likely warming and increase in heat extremes are somewhat smaller. However, the majority of the warming seen in the business-as-usual scenario still occurs at all locations in the most likely case under the mitigation scenario, and heat extremes still increase significantly. Therefore adaptation to a changing climate over the next few decades is likely to be inevitable in the Los Angeles region.