Climate change and California drought in the 21st century

National Academy of Sciences (NAS) | March 31st, 2015

Climate science has advanced over decades from an initial focus on the development and use of numerical models of Earth’s climate and compilation of rich netw

Climate science has advanced over decades from an initial focus on the development and use of numerical models of Earth’s climate and compilation of rich networks of obser- vational data, to now being in a position to“detect” and “attribute” specific impacts and events to anthropogenic climate change. Recent analyses have thus established the“fingerprint” of anthropogenic climate change in an increasingly diverse array of meteorological and hydrological phenomena around the world, from heat waves to coastal damages during extreme tides and storms, flooding from more intense precipitation events, and severe drought. In a new study published in PNAS, Diffenbaugh et al. now add weight to the accumulating evidence that anthropogenic climatic changes are already influencing the frequency, magnitude, and duration of drought in California. The authors show that the increasing co-occurrence of dry years with warm years raises the risk of drought despite limited evidence of a trend in precipitation itself, highlighting the critical role of elevated temperatures in altering water availability and increasing overall drought intensity and impact.

Climate Change and Future Water Availability in the United States, Chapter E of U.S. Geological Survey Integrated Water Availability Assessment—2010–20

U.S. Geological Survey (USGS) | January 15th, 2025

The following are the key points of this chapter:  The steady rise in global temperature as a result of human activity is causing changes in Earth’s water cycle.

The following are the key points of this chapter:  The steady rise in global temperature as a result of human activity is causing changes in Earth’s water cycle. The amount of water stored within and moving between vapor, liquid and frozen components of the water cycle is shifting, with significant consequences for water availability. Climatic drivers underlie regional or local changes in water supply and their socioeconomic impacts. Threats to the quantity and quality of water available for humans and ecosystems in North America include increases in drought, aridification, and fire weather; heavy precipitation and flooding; cryosphere decline; rising surface-water temperatures; and saltwater intrusion in coastal areas. An understanding of how changes in the major climatic drivers impact the water cycle and the accurate simulation of impacts in hydrological models are crucial to predicting future water availability and developing adaptation strategies to ensure human and ecosystem water supplies. Companion chapters: U.S. Geological Survey Integrated Water Availability Assessment—2010–20, Chapter A Water Supply in the Conterminous United States, Alaska, Hawaii, and Puerto Rico, Water Years 2010–20, Chapter B Status of Water-Quality Conditions in the United States, 2010–20, Chapter C  Water Use Across the Conterminous United States, Water Years 2010–20, Chapter D  Integrated Water Availability in the Conterminous United States, 2010–20, Chapter F 

Climate change and the aridification of North America

National Academy of Sciences (NAS) | May 19th, 2020

Discussions of droughts and their impacts often center on the lack of precipitation, just as assessments of hydrologic impacts under a changing climate most ofte

Discussions of droughts and their impacts often center on the lack of precipitation, just as assessments of hydrologic impacts under a changing climate most often focus on how average precipitation in a given locale is likely to change in the future. Within climate science, however, focus has begun to include the growing role warming temperatures are playing as a potent driver of greater aridity: hotter climate extremes; drier soil conditions; more severe drought; and the impacts of hydrologic stress on rivers, forests, agriculture, and other systems. This shift in the hydrologic paradigm is most clear in the American Southwest, where declining flows in the region’s two most important rivers, the Colorado and Rio Grande, have been attributed in part to increasing temperatures caused by human activities, most notably the burning of fossil fuels. Warmer summers are also likely to reduce flows in the Columbia River, as well as in rivers along the Sierra Nevada in California. Now, an important study [Increased drought severity tracks warming in the United States’ largest river basin, J.T. Martin et al] documents how warming is also causing flow declines in the northern Rocky Mountains and in the largest river basin in the United States, the Missouri. This work further highlights the mechanisms behind the temperature-driven river flow declines and places more focus on how anthropogenic climate warming is progressively increasing the risk of hot drought and more arid conditions across an expanding swath of the United States.

Climate Change and Water Supply Security: Reconfiguring Groundwater Management to Reduce Drought Vulnerability

California Energy Commission (CEC) | July 31st, 2012

Periodic droughts, projected to become more frequent and severe with climate change, present a significant planning challenge for California’s water agencies. This r

Periodic droughts, projected to become more frequent and severe with climate change, present a significant planning challenge for California’s water agencies. This research examined approaches to reducing drought vulnerability, focusing on five water agencies on California’s north and central coast that rely on local and regional sources of water. Curtailing water use is the principal response to drought. In contrast, this project highlights an important but underutilized proactive adaptation to improve water supply security during drought: the development of locally based groundwater drought reserves. While this approach represents an obvious solution in principle, it is uncommon to find it in practice, and this research provides insight into (1) motivating factors, (2) legal barriers and opportunities, (3) tools, and (4) policy options to support increased drought resilience and the development of drought reserves.

Climate change decisive for Asia’s snow meltwater supply

Nature Portfolio (Springer Nature) | June 24th, 2021

Streamflow in high-mountain Asia is influenced by meltwater from snow and glaciers, and determining impacts of climate change on the region’s cryosphere is essential to

Streamflow in high-mountain Asia is influenced by meltwater from snow and glaciers, and determining impacts of climate change on the region’s cryosphere is essential to understand future water supply. Past and future changes in seasonal snow are of particular interest, as specifics at the scale of the full region are largely unknown. Here we combine models with observations to show that regional snowmelt is a more important contributor to streamflow than glacier melt, that snowmelt magnitude and timing changed considerably during 1979–2019 and that future snow meltwater supply may decrease drastically. The expected changes are strongly dependent on the degree of climate change, however, and large variations exist among river basins. The projected response of snowmelt to climate change indicates that to sustain the important seasonal buffering role of the snowpacks in high-mountain Asia, it is imperative to limit future climate change.

Climate change scenarios forecast increased drought exposure for terrestrial vertebrates in the contiguous United States

Nature Portfolio (Springer Nature) | November 15th, 2024

Climate warming enhancement of catastrophic southern California debris flows

Nature Portfolio (Springer Nature) | June 29th, 2020

The sequence of wildfires followed by debris flows, frequently affects southern California, reflecting its drought-heavy precipitation climate bipolarity. Organic debris

The sequence of wildfires followed by debris flows, frequently affects southern California, reflecting its drought-heavy precipitation climate bipolarity. Organic debris from incomplete burning is lighter than inorganic matter, and partially inviscid. Hence lower rainfall totals can trigger downslope motion than typically required by the underlying clasts of loose inorganic granular material. After advection downslope, the pebble-laden organic debris has a higher capacity for rilling; a positive feedback process. A mechanism is proposed whereby boulders are ‘rafted’ by organic debris. This coordinated movement of boulders greatly enhances the debris flow erosion capacity. This climate change sensitive debris flow enhancing mechanism, through organic–inorganic granular material interaction, is supported by observations and the numerical simulations. Using a model explicitly parameterizing erosion processes, including runoff entrainment, rilling incision, and bank collapse, the lifecycle of the Montecito debris flow of January 9, 2018 is simulated, providing quantitative estimates of mass conveyed and debris sorting at the terminus. Peak rafting speeds are ~ 12.9 m/s at ~ 300 m asl. Total boulder (effective diameter > 25 cm) volume involved for the Ysidro Creek area alone is ~ 5 × 104 m3, scattered between the region 50–260 m asl. Debris flows are highly repeatable and locations prone to debris flows are identified and their likelihood of realization estimated.

Climate-driven disturbances amplify forest drought sensitivity

Nature Portfolio (Springer Nature) | June 7th, 2024

Climate, Drought, and Sea Level Rise Scenarios for the Fourth California Climate Assessment

California Energy Commission (CEC) | August 27th, 2018

Daily temperature and precipitation over California at a resolution of 1/16° (about 6 km, or 3.7 miles) were generated to support climate change impact studies for the

Daily temperature and precipitation over California at a resolution of 1/16° (about 6 km, or 3.7 miles) were generated to support climate change impact studies for the energy system and other sectors featured in the California’s Fourth Climate Change Assessment. The data, derived from 32 coarse-resolution (~ 100 km) global climate models (GCMs), were bias corrected and downscaled using the Localized Constructed Analogues (LOCA) statistical method. The data cover 1950-2005 for the historical period and 2006-2100 for two future climate projections using medium and high greenhouse gas and aerosol emissions scenarios. Statewide, temperature is projected to increase 2-4 °C (medium emissions scenario) to 4-7 °C (high emissions scenario) by the end of this century. Precipitation shows fewer wet days, wetter winters, drier springs and autumns, and an increase in dry years as well as maximum precipitation in a single day. Ten GCMs that closely simulate California’s climate are identified for studies where all 32 GCMs cannot be used. Additional variables were downscaled for these 10, including wind speed, specific and relative humidity, and surface solar radiation. Four models that span the temperature and precipitation changes from the 10 are identified for studies that cannot accommodate the 10. Wind speed shows small decreases, while relative humidity changes are more complicated, with coastal increases but decreases inland. Surface solar radiation shows small Southern California increases in spring. The downscaled fields were applied to the Variable Infiltration Capacity (VIC) land surface model to develop snow cover, soil moisture, runoff, water loss from plants, surface heat fluxes, and other parameters. Moisture deficit is projected to increase over much of the state, particularly Northern California and the Sierra Nevada, while top level soil moisture is projected to decrease, particularly in Southern California. Most streamflows shift to earlier in the year, with the bigger shifts experienced in basins which currently have substantial contributions from snowmelt. Two versions of a 20-year dry spell were identified from one of the GCM simulations to investigate future drought: the original episode from 2051-2070, and one shifted earlier in the century with temperatures consistent with 2023-2042. For both, we provide downscaled temperatures and precipitation along with VIC hydrological output. Sea level rise (SLR) projections for California were generated using a probabilistic approach employing estimates of the components that contribute to global and regional SLR, including new science on the possibility of increased contribution from Antarctica. Hourly projections of sea level at selected coastal locations were generated out to 2100 that include tides, regional and local weather influences, and short period Pacific climate fluctuations along with the aforementioned sea level rise scenarios. The climate scenario and SLR data are available online from cal-adapt.org.

Climatic Fluctuations, Drought, and Flow in the Colorado River Basin

U.S. Geological Survey (USGS) | August 16th, 2004