Widespread Increase in Atmospheric River Frequency and Impacts Over the 20th Century

American Geophysical Union (AGU) | December 11th, 2025

Atmospheric rivers (ARs) are long, narrow regions of atmospheric moisture transport that provide water resources to many regions, but can also cause hazards such as extre

Atmospheric rivers (ARs) are long, narrow regions of atmospheric moisture transport that provide water resources to many regions, but can also cause hazards such as extreme rainfall and moist heatwaves. Despite their importance, there is uncertainty about how the frequency and impacts of ARs may be changing with time. Our study extends the global record of AR activity back to 1940. We find that the frequency of ARs has increased broadly across the midlatitude regions, rather than shifting polewards as observed in recent decades. This increase in AR frequency leads to more rain and snow associated with these features in both the mid‐ and high‐latitudes. We also find that there is an increase in humid heatwaves associated with ARs over the same period

Wildfire and Drought: Impacts on wildfire planning, behavior, and effects

National Integrated Drought Information System | March 4th, 2016

The relationship between drought and wildfire can appear to be simple, but the drought and wildfire nexus encompasses a profusion of human and environmental impacts. Imag

The relationship between drought and wildfire can appear to be simple, but the drought and wildfire nexus encompasses a profusion of human and environmental impacts. Images of burning forests, neighborhoods, and “smoked out” communities are all now commonplace. The National Integrated Drought Information System, in partnership with NOAA’s Western Regional Climate Center (WRCC), the California Nevada Applications Program (CNAP, a NOAA Regional Integrated Science Applications team), the Desert Research Institute (DRI), and the National Drought Mitigation Center (NDMC), hosted the Integrating Drought Science and Information into Wildfire Management Workshop in Boise, Idaho on 21-22 October 2015.  The 22 participants in the workshop were invited as representatives of a broad spectrum of expertise including prescribed fire, wildland fire management, climatology, smoke and air quality, fuels management, fire behavior, drought, and policy. In this report, we present the opportunities and challenges the workshop participants identified and the impacts of drought on wildfire. The workshop also provided the opportunity to identify next steps in understanding and addressing drought impacts on wildfire.

Wildfire Impacts on Snowpack Phenology in a Changing Climate Within the Western U.S.

American Geophysical Union (AGU) | July 8th, 2022

Snowpack accumulation and melt are critical for water supply in the western United States. As the number and size of wildfires have increased, it is important to understa

Snowpack accumulation and melt are critical for water supply in the western United States. As the number and size of wildfires have increased, it is important to understand how these events impact the seasonal snowpack. Using data from the Snow Telemetery monitoring network, we quantify the impacts of wildfires on snowpack for various regions across the western U.S. and Alaska. The results indicate that decreases in snowpack tend to occur following fires. Also, the snowmelt period usually begins and finishes earlier in the spring due to the conditions in the burned forests. In regions of the western U.S. that are already experiencing earlier snowmelt due to climate changes, wildfires are further shifting the melt earlier, which could have substantial consequences for water supply and flood risks.

Wildfire Induces Changes in Receiving Waters: A Review With Considerations for Water Quality Management

American Geophysical Union (AGU) | September 15th, 2022

Wildfires have increased in frequency in many ecosystems, with implications for human health and the environment, including water quality. Increased fire frequency and ur

Wildfires have increased in frequency in many ecosystems, with implications for human health and the environment, including water quality. Increased fire frequency and urbanization also raise the prospect of fires burning into urban areas, mobilizing pollutants few have considered to date. As a result, water quality managers lack information to anticipate, respond to and potentially mitigate wildfire impacts. Here, we reviewed the scientific literature to assess wildfire effects on response endpoints of a conceptual model linking fire to water quality, quantifying response directionality, magnitude and duration. Physically, water yield, sediments, and temperature all increased post-fire. Chemically, nutrients, ions, organic chemicals, and metals increased in burned watersheds, sometimes by orders of magnitude over pre-fire or reference conditions. In select cases, post-fire concentrations exceeded aquatic life criteria or drinking water standards, at times even in the finished drinking water. Biological assemblages commonly declined after post-fire runoff events. The duration of effects was less than 5 yr for most endpoints (e.g., metals) on average following fire, although effects did extend 15 yr or more in some individual cases. We found only a few studies on pollutants mobilized from wildfire impacted urban areas with benzene contamination in drinking water and high metal concentrations in ash prominent exceptions. Overall, this review provides a resource for understanding wildfire impacts on water quality endpoints, with the goal of informing the response of managers and other decision makers to this growing problem.

Wildfire response to changing daily temperature extremes in California’s Sierra Nevada

American Association for the Advancement of Science (AAAS) | November 17th, 2021

Burned area has increased across California, especially in the Sierra Nevada range. Recent fires there have had devasting social, economic, and ecosystem impacts. To unde

Burned area has increased across California, especially in the Sierra Nevada range. Recent fires there have had devasting social, economic, and ecosystem impacts. To understand the consequences of new extremes in fire weather, here we quantify the sensitivity of wildfire occurrence and burned area in the Sierra Nevada to daily meteorological variables during 2001–2020. We find that the likelihood of fire occurrence increases nonlinearly with daily temperature during summer, with a 1°C increase yielding a 19 to 22% increase in risk. Area burned has a similar, nonlinear sensitivity, with 1°C of warming yielding a 22 to 25% increase in risk. Solely considering changes in summer daily temperatures from climate model projections, we estimate that by the 2040s, fire number will increase by 51 ± 32%, and burned area will increase by 59 ± 33%. These trends highlight the threat posed to fire management by hotter and drier summers.

Wildfires, Floods, and Climate Variability

University of California, San Diego (UCSD) | May 10th, 2018

The first chapter surveys fire and fuels managers at local, regional, and national levels. Survey results in the form of fire managers’ decision calendars show how clim

The first chapter surveys fire and fuels managers at local, regional, and national levels. Survey results in the form of fire managers’ decision calendars show how climate information needs vary seasonally, over space, and through the organizational network. The study identifies opportunities to use climate information in fire management, including seasonal to inter-annual climate forecasts at all organizational levels, to improve the targeting of fuels treatments and prescribed burns, the positioning and movement of initial attack resources, and staffing and budgeting decisions. The second chapter analyzes National Flood Insurance Program (NFIP) data to quantify the economic impacts of flooding across the western United States from 1978 to 2007. The study compares National Flood Insurance Program data to National Weather Service measures of total damages, and presents a spatial and temporal analysis of daily claims and loss data over this period. The NFIP data reveals that a small number winter-season extreme hydrologic events, covering wide spatial areas, are responsible for a large proportion of total losses. In coastal southern California and across the southwest, El Niño conditions have had a strong effect in producing more frequent and higher magnitudes of insured losses while La Niña conditions significantly reduce both the frequency and magnitude losses. In the Pacific Northwest, the opposite pattern appears, though the effect is somewhat weaker, and less spatially coherent. The third chapter quantifies the economic impacts of flooding due to atmospheric river (AR) events in the western United States from 1978 to 2007, using NFIP claims and loss data. The study confirms that AR-related flood events cause significant economic damages and form the primary source of insurance claims and insured flood losses in the western coastal states. It provides spatial and temporal characterizations of damages as a function of integrated vapor transport (IVT) and antecedent hydrologic conditions. As the magnitude and frequency of wildfire and flood events change in response to anthropogenic climate change, and as economic and demographic contributions to vulnerability increase over time, public policy must adapt to respond. The results in these papers may be used to inform policies to mitigate losses and respond to future disaster scenarios, and may be of interest to policy makers and applied climate researchers alike.

Winter wet–dry weather patterns driving atmospheric rivers and Santa Ana winds provide evidence for increasing wildfire hazard in California

Climate Dynamics (Springer) | August 12th, 2022

Floods caused by atmospheric rivers and wildfires fanned by Santa Ana winds are common occurrences in California with devastating societal impacts. In this work

Floods caused by atmospheric rivers and wildfires fanned by Santa Ana winds are common occurrences in California with devastating societal impacts. In this work, we show that winter weather variability in California, including the occurrence of extreme and impactful events, is linked to four atmospheric circulation regimes over the North Pacific Ocean previously named and identified as the “NP4 modes”. These modes come in and out of phase with each other during the season, result- ing in distinct weather patterns that recur throughout the historical record. Some phase combinations favor atmospheric river landfalls and extreme daily or multi-day precipitation, while other phase combinations favor anomalously hot weather and drying Santa Ana wind conditions over Southern California. This historical perspective of atmospheric circulation and impacts over 70 years reveals that weather patterns are changing in a way that enhances wildfire hazard in California, while the frequency of weather patterns linked to historical floods is not diminishing. These changes highlight the rising hazards of cascading weather extremes in California’s present and future.

Workshop report—Earthquakes and High Water as Levee Hazards in the Delta

Delta Stewardship Council (Delta Council) | September 30th, 2016

Earthquakes and high water as hazards to Delta levees were reviewed in a seven-hour workshop organized by the Delta Independent Science Board and held at the campus of th

Earthquakes and high water as hazards to Delta levees were reviewed in a seven-hour workshop organized by the Delta Independent Science Board and held at the campus of the University of California, Davis. Earthquake hazards in the Delta were described in terms of ground motions from Bay Area earthquakes, infrequent earthquake recurrence on faults beneath the Delta, and levee fills prone to earthquake-induced liquefaction. Large uncertainties attend all these seismic elements of levee hazard. Those uncertainties, according to presentations in the workshop, include whether the Delta ground motions previously computed for Bay Area earthquakes were too large.  Hazards from high water were deemed greatest from the confluence of high river discharge, wind-driven surge and waves, and high tides. Major risk assessments have used available data on these hazards without mandates to advance the science. Research needs and opportunities identified in the workshop include expanded observations of Delta ground motions, improved estimates of geologically recent displacement on faults beneath the Delta, further identification of liquefiable materials and mechanisms beneath levees, continued airborne measurements of land-level change, updated mapping of the contracting area of remaining peat, and fuller documentation of past levee failures. Recurring assessments of earthquake hazards and climate change provide precedents for periodic reappraisal of Delta levee risk. The workshop brought together different parts of the diverse community of Delta levee specialists. Positive responses to the workshop suggest that it served levee specialists and outsiders alike.

Workshop report—Earthquakes and High Water as Levee Hazards in the Sacramento-San Joaquin Delta

Delta Stewardship Council (Delta Council) | September 30th, 2016

Yolo Bypass Salmonid Habitat Restoration and Fish Passage Final Environmental Impact Statement / Environmental Impact Report (Chapters 1-8)

U.S. Bureau of Reclamation (USBR) | May 1st, 2019

The Yolo Bypass Salmonid Habitat Restoration and Fish Passage Project (Project) has been developed to improve fish passage and increase floodplain fisheries rearing habit

The Yolo Bypass Salmonid Habitat Restoration and Fish Passage Project (Project) has been developed to improve fish passage and increase floodplain fisheries rearing habitat in the Yolo Bypass and the lower Sacramento River basin. The United States Department of the Interior, Bureau of Reclamation (Reclamation), as the Federal lead agency under the National Environmental Policy Act (NEPA), and the California Department of Water Resources (DWR), as the State of California (State) lead agency under the California Environmental Quality Act (CEQA), have prepared this joint Environmental Impact Statement/Environmental Impact Report (EIS/EIR) to assess impacts of the Project. The Project actions would implement Reasonable and Prudent Alternative (RPA) action I.6.1 and, in part, RPA action I.7, as described in the 2009 National Oceanic and Atmospheric Administration National Marine Fisheries Service (NMFS) Biological Opinion and Conference Opinion on the Long-Term Operations of the Central Valley Project and State Water Project and the 2012 Yolo Bypass Salmonid Habitat Restoration and Fish Passage Implementation Plan (Reclamation and DWR 2012).  Authority for combined Federal and State documents is provided in Title 40, Code of Federal Regulations (CFR), Sections 1502.25, 1506.2, and 1506.4 (Council on Environmental Quality’s Regulations for Implementing NEPA [CEQ Regulations]) and California Code of Regulations (CCR) Title 14, Division 6, Chapter 3 (State CEQA Guidelines), Section 15222 (Preparation of Joint Documents). This document was prepared consistent with United States Department of the Interior regulations specified in 43 CFR, Part 46 (United States Department of the Interior Implementation of NEPA, Final Rule).  This EIS/EIR evaluates reasonably foreseeable potential direct, indirect, and cumulative impacts on the environment that could result from implementing the Project alternatives. In addition, this EIS/EIR includes feasible mitigation measures to avoid, minimize, rectify, reduce, or compensate for adverse impacts.