Precipitation shifts over western North America as a result of declining Arctic sea ice cover: The coupled system response

American Meteorological Society (AMS) | July 21st, 2009

Changes in Arctic sea ice cover have the potential to impact midlatitude climate. A previous sensitivity study utilizing the National Center for Atmospheric Resea

Changes in Arctic sea ice cover have the potential to impact midlatitude climate. A previous sensitivity study utilizing the National Center for Atmospheric Research’s (NCAR) atmospheric general circulation model [AGCM; Community Climate Model, version 3 (CCM3)] to explore climate sensitivity to declining Arctic sea ice cover suggested that, as Arctic sea ice cover is reduced, precipitation patterns over western North America will shift toward dryer conditions in southwestern North America and wetter conditions in northwestern North America. Here, three complementary lines of research validate and explore the robustness of this possible climate change impact: 1) repetition of the previous sensitivity study (specified constant Arctic sea ice cover and atmospheric CO2) with an updated version of the NCAR AGCM [third Community Atmosphere Model (CAM3)], 2) investigation of the climate response to dynamically reduced Arctic sea ice cover (driven by a quadrupling of atmospheric CO2) in the coupled NCAR Community Climate System Model.

Precipitation variability increases in a warmer climate

Nature Portfolio (Springer Nature) | December 21st, 2017

Understanding changes in precipitation variability is essential for a complete explanation of the hydrologic cycle’s response to warming and its impacts. Whil

Understanding changes in precipitation variability is essential for a complete explanation of the hydrologic cycle’s response to warming and its impacts. While changes in mean and extreme precipitation have been studied intensively, precipitation variability has received less attention, despite its theoretical and practical importance. Here, we show that precipitation variability in most climatemodels increases over a majority of global land area in response to warming (66% of land has a robust increase in variability of seasonal-mean precipitation). Comparing recent decades to RCP8.5 projections for the end of the 21st century, we find that in the global, multi-model mean, precipitation variability increases 3–4% K−1 globally, 4–5% K−1 over land and 2–4% K−1 over ocean, and is remarkably robust on a range of timescales from daily to decadal. Precipitation variability increases by at least as much as mean precipitation and less than moisture and extreme precipitation for most models, regions, and timescales. We interpret this as being related to an increase in moisture which is partially mitigated by weakening circulation. We show that changes in observed daily variability in station data are consistent with increased variability.

Predicting Streamflow Elasticity Based on Percolation Theory and Ecological Optimality

American Geophysical Union (AGU) | July 20th, 2023

Priorities for California’s Water -- Stewarding the Wet Years

Public Policy Institute of California (PPIC) | November 2nd, 2023

The 2023 water year provided a case study of California’s increasingly volatile climate, with large swings between wet and dry conditions both within and between years.

The 2023 water year provided a case study of California’s increasingly volatile climate, with large swings between wet and dry conditions both within and between years. It highlighted our need to better navigate multiple imperatives: storing more water during intense wet periods to build water supply reliability, protecting communities and infrastructure against the threat of increasingly damaging floods, and taking better advantage of wet years to improve the health of freshwater ecosystems and native fish and wildlife.

Probabilistic estimates of future changes in California temperature and precipitation using statistical and dynamical downscaling

Climate Dynamics (Springer) | March 30th, 2012

Sixteen global general circulation models were used to develop probabilistic projections of temperature (T) and precipitation (P) changes over California by the 2060s.

Sixteen global general circulation models were used to develop probabilistic projections of temperature (T) and precipitation (P) changes over California by the 2060s. The global models were downscaled with two statistical techniques and three nested dynamical regional climate models, although not all global models were downscaled with all techniques. Both monthly and daily timescale changes in T and P are addressed, the latter being important for a range of applications in energy use, water management, and agriculture. The T changes tend to agree more across downscaling techniques than the P changes. Year-to-year natural internal climate variability is roughly of similar magnitude to the projected T changes. In the monthly average, July temperatures shift enough that that the hottest July found in any simulation over the historical period becomes a modestly cool July in the future period. Januarys as cold as any found in the historical period are still found in the 2060s, but the median and maximum monthly average temperatures increase notably. Annual and seasonal P changes are small compared to interannual or intermodel variability. However, the annual change is composed of seasonally varying changes that are themselves much larger, but tend to cancel in the annual mean. Winters show modestly wetter conditions in the North of the state, while spring and autumn show less precipitation. The dynamical downscaling techniques project increasing precipitation in the Southeastern part of the state, which is influenced by the North American monsoon, a feature that is not captured by the statistical downscaling.

Projected Changes in California’s Precipitation Intensity-Duration-Frequency Curves

California Public Utilities Commission (CPUC) | August 15th, 2018

Traditionally, infrastructure design and rainfall-triggered landslide models rely on the notion of stationarity, which assumes that the statistics of hydroclimati

Traditionally, infrastructure design and rainfall-triggered landslide models rely on the notion of stationarity, which assumes that the statistics of hydroclimatic extremes (e.g., rainfall, streamflow, etc.) do not change significantly over time. However, during the last century, we have observed a warming climate with more intense precipitation extremes in some regions, likely due to increases in the water holding capacity of the atmosphere. Consequently, infrastructure and natural slopes will likely face more severe climatic conditions, with potential human and socioeconomic consequences. Here, we outline a framework for quantifying climate change impacts on natural and man-made infrastructure using bias-corrected multi-model simulations of historical and projected precipitation extremes. The approach evaluates changes in rainfall intensity-duration- frequency (IDF) curves and their uncertainty bounds using a non-stationary model based on Bayesian inference. We show that highly populated areas across California may experience extreme precipitation that is more intense and twice as frequent, relative to historical records, despite the expectation of unchanged annual mean precipitation. Since IDF curves are widely used for infrastructure design and risk assessment, the proposed framework offers an avenue for assessing infrastructure resilience and landslide hazard in a warming climate.

Projected Changes in Precipitation, Temperature, and Drought across California’s Hydrologic Regions

Climate (MDPI) | April 23rd, 2018

This study investigates potential changes in future precipitation, temperature, and drought across 10 hydrologic regions in California. The latest climate model p

This study investigates potential changes in future precipitation, temperature, and drought across 10 hydrologic regions in California. The latest climate model projections on these variables through 2099 representing the current state of the climate science are applied for this purpose. Changes are explored in terms of differences from a historical baseline as well as the changing rate (trend slope). Results indicate that warming is expected throughout the 21st Century across all regions in all temperature projections. RCP 4.5 and RCP 8.5 warm about the same amount by mid-21st Century, but thereafter the higher rate of warming produces greater increases in temperature under RCP 8.5. There is no such consensus in precipitation, with projections ranging from -25 percent to +50 percent different from the historical baseline. There is no statistically significant increasing or decreasing trend in historical precipitation and in a majority of the projections of precipitation. On average, projected precipitation changes are small compared to the natural variability observed in historical precipitation. Compared to wet regions, dry regions are projected to have higher increases in temperature and more severe droughts. Droughts are increasingly extreme in the late 21st Century, especially in RCP 8.5 simulations. The study also shows that the coolest North Lahontan region tends to have the highest increases in both minimum and maximum temperature. The region is also projected to experience increases in wet season precipitation. For the driest region, the Colorado River region, all projections consistently show higher increasing trends in temperature and drought risk compared to their historical counterparts. Overall, these findings are meaningful from both scientific and practical perspectives. From a scientific perspective, these findings provide useful information that can be utilized to improve the current flood and water supply forecasting models or develop new predictive models. From a practical perspective, these findings can help decision-makers in making better-informed adaptive strategies for different regions to address adverse impacts posed by those potential changes.

Projecting groundwater storage changes in California’s Central Valley

Nature Portfolio (Springer Nature) | August 27th, 2018

Accurate and detailed knowledge of California’s groundwater is of paramount importance for statewide water resources planning and management, and to sustain a

Accurate and detailed knowledge of California’s groundwater is of paramount importance for statewide water resources planning and management, and to sustain a multi-billion-dollar agriculture industry during prolonged droughts. In this study, we use water supply and demand information from California’s Department of Water Resources to develop an aggregate groundwater storage model for California’s Central Valley. The model is evaluated against 34 years of historic estimates of changes in groundwater storage derived from the United States Geological Survey’s Central Valley Hydrologic Model (USGS CVHM) and NASA’s Gravity Recovery and Climate Experiment (NASA GRACE) satellites. The calibrated model is then applied to predict future changes in groundwater storage for the years 2015–2050 under various precipitation scenarios from downscaled climate projections. We also discuss and project potential management strategies across different annual supply and demand variables and how they affect changes in groundwater storage. All simulations support the need for collective statewide management intervention to prevent continued depletion of groundwater availability.

Projections of declining surface-water availability for the southwestern United States

Nature Portfolio (Springer Nature) | December 23rd, 2012

Global warming driven by rising greenhouse-gas concentrations is expected to cause wet regions of the tropics and mid to high latitudes to get wetter and subtropi

Global warming driven by rising greenhouse-gas concentrations is expected to cause wet regions of the tropics and mid to high latitudes to get wetter and subtropical dry regions to get drier and expand polewards. Over southwest North America, models project a steady drop in precipitation minus evapotranspiration, P−E, the net flux of water at the land surface, leading to, for example, a decline in Colorado River flow. This would cause widespread and important social and ecological consequences. Here, using new simulations from the Coupled Model Intercomparison Project Five, to be assessed in Intergovernmental Panel on Climate Change Assessment Report Five, we extend previous work by examining changes in P, E, runoff and soil moisture by season and for three different water resource regions. Focusing on the near future, 2021–2040, the new simulations project declines in surface-water availability across the southwest that translate into reduced soil moisture and runoff in California and Nevada, the Colorado River headwaters and Texas.

Prolonged California aridity linked to climate warming and Pacific sea surface temperature

Nature Portfolio (Springer Nature) | September 15th, 2016

California has experienced a dry 21st century capped by severe drought from 2012 through 2015 prompting questions about hydroclimatic sensitivity to anthropogen

California has experienced a dry 21st century capped by severe drought from 2012 through 2015 prompting questions about hydroclimatic sensitivity to anthropogenic climate change and implications for the future. We address these questions using a Holocene lake sediment record of hydrologic change from the Sierra Nevada Mountains coupled with marine sediment records from the Pacific. These data provide evidence of a persistent relationship between past climate warming, Pacific sea surface temperature (SST) shifts and centennial to millennial episodes of California aridity. The link is most evident during the thermal-maximum of the mid-Holocene (~8 to 3 ka; ka = 1,000 calendar years before present) and during the Medieval Climate Anomaly (MCA) (~1 ka to 0.7 ka). In both cases, climate warming corresponded with cooling of the eastern tropical Pacific despite differences in the factors producing increased radiative forcing. The magnitude of prolonged eastern Pacific cooling was modest, similar to observed La Niña excursions of 1o to 2 °C. Given differences with current radiative forcing it remains uncertain if the Pacific will react in a similar manner in the 21st century, but should it follow apparent past behavior more intense and prolonged aridity in California would result.