State of California Sea Level Rise Guidance

California Natural Resources Agency (CNRA) | March 14th, 2018

The climate across California is changing, and the effects, such as rising average temperatures, shrinking mountain snowpack, more intense storms, and higher sea

The climate across California is changing, and the effects, such as rising average temperatures, shrinking mountain snowpack, more intense storms, and higher sea levels are expected to continue and worsen in the coming decades. Sea-level rise is caused by the thermal expansion of warming ocean water and melting of land ice as the Earth warms. It is one of the most obvious manifestations of the trend of climate change and is an immediate and real threat to lives, livelihoods, transportation, economies, and the environment in California. In April 2017, catalyzed by direction from Governor Brown and the need to ensure that best available science was informing sea-level rise planning decisions in California, a Working Group of the California Ocean Protection Council’s Science Advisory Team (OPC-SAT) released a report, entitled “Rising Seas in California: An Update on Sea-Level Rise.” The Rising Seas Report was prepared and peer-reviewed by some of the nation’s foremost experts in coastal processes, climate and sea-level rise science, observational and modeling science, the science of extremes, and decision-making under uncertainty. The report synthesized the current state of sea-level rise science, including advances in modeling and improved understanding of the processes that could drive extreme global sea-level rise as a result of ice loss from the Greenland and Antarctic ice sheets. The report found that: Scientific understanding of sea-level rise is advancing at a rapid pace. The direction of sea-level change is clear; sea levels are rising. The rate of ice loss from the Greenland and Antarctic ice sheets is increasing, and California is particularly vulnerable to sea-level rise caused by ice loss from West Antarctica. New scientific evidence has highlighted the potential for extreme sea-level rise. Probabilities of specific sea-level increases can inform decisions. Current policy decisions are shaping our coastal future. Waiting for scientific certainty is neither a safe nor prudent option. The increased understanding of sea-level rise projections and polar ice sheet loss warranted an update to the State’s sea-level rise guidance document to ensure decisions were based on the best available science. Additionally, an increased policy focus requiring state and local governments to incorporate climate change into decision making merited an update to address the needs of both state and local audiences. This updated document, the “State of California Sea-Level Rise Guidance” (Guidance), provides a bold, science-based methodology for state and local governments to analyze and assess the risks associated with sea-level rise, and to incorporate sea-level rise into their planning, permitting, and investment decisions. This Guidance provides: 1. A synthesis of the best available science on sea- level rise projections and rates for California; 2. A step-by-step approach for state agencies and local governments to evaluate those projections and related hazard information in decision making; and 3. Preferred coastal adaptation approaches.

State of California Sea-Level Rise Guidance—2018 Update

Ocean Protection Council (OPC) | March 4th, 2018

This updated document, the “State of California Sea-Level Rise Guidance” (Guidance), provides a bold, science-based methodology for state and local governments to ana

This updated document, the “State of California Sea-Level Rise Guidance” (Guidance), provides a bold, science-based methodology for state and local governments to analyze and assess the risks associated with sea-level rise, and to incorporate sea-level rise into their planning, permitting, and investment decisions. This Guidance provides:  A synthesis of the best available science on sea-level rise projections and rates for California;  A step-by-step approach for state agencies and local governments to evaluate those projections and related hazard information in decision making; and  Preferred coastal adaptation approaches. 

Summarizing Relationships among Landfalling Atmospheric Rivers, Integrated Water Vapor Transport, and California Watershed Precipitation 1982–2019

American Meteorological Society (AMS) | September 16th, 2022

Atmospheric rivers (ARs) are defined as corridors of enhanced integrated water vapor transport (IVT) and produce large fractions of annual precipitation in regions with c

Atmospheric rivers (ARs) are defined as corridors of enhanced integrated water vapor transport (IVT) and produce large fractions of annual precipitation in regions with complex terrain along the western coastlines of midlatitude continents (e.g., 30%–50% along the U.S. West Coast in California). This study investigates this relationship among landfalling ARs, IVT, and watershed mean areal precipitation (MAP) for a 38-yr period over California. On average, the daily average IVT magnitude at different coastal locations explains ∼34% of the variance in annual watershed MAP across 140 Hydrologic Unit Code 8 (HUC-8) watersheds with large spatial variability across California. Further investigation of the IVT magnitude and direction at coastal locations illustrated that accounting for water vapor transport direction increases the explained variance in annual MAP to an average of 45%, with highest values (∼65%) occurring in watersheds over Northern and coastal California. Similar investigation of the lower-tropospheric water vapor flux vector at 850 and 925 hPa revealed further increases in the explained variance in annual MAP to an average of >50%. The results of this study 1) emphasize the importance of both IVT direction and water vapor flux altitude to watershed MAP, 2) align well with previous studies for select locations that highlight the importance of upslope (i.e., lower tropospheric) water vapor flux during landfalling ARs and precipitation, and 3) motivate the development of AR-related and watershed-centric forecast tools that incorporate IVT direction and water vapor flux altitude parameters in addition to IVT magnitude.

Summary of Climate Change Impacts to Water and Sanitation for Frontline Communities in the United States: Water, Sanitation, and Climate Change in the United States Part 1

Pacific Institute | December 6th, 2023

Climate change is among the most urgent, wide-ranging crises we face. It poses significant challenges and is already resulting in severe impacts to drinking water and san

Climate change is among the most urgent, wide-ranging crises we face. It poses significant challenges and is already resulting in severe impacts to drinking water and sanitation access—both for those currently living without access and for communities where forces like drought, flooding, sea-level rise, wildfires, and intensifying storms threaten to degrade or strip basic services from the people. Access to sufficient, safe, acceptable, physically accessible, and affordable water and sanitation is essential for human health and wellbeing, as well as economic prosperity—and climate change will inevitably make ensuring all have access more challenging. Without water and sanitation access, people do not have water to drink, cannot clean, shower, or wash their hands, and cannot safely dispose of human waste. They may have to travel long distances to purchase and haul water back to their homes. Without access to water, they also have less ability to cool their homes, protect themselves from wildfires, and work or go to school. An economic assessment by DigDeep found that for every household in the United States without access to water and sanitation, the domestic economy loses $15,800 annually in extra health care costs, time and money spent on bottled water, lost educational or work opportunities, and premature death (DigDeep 2022). (Click here for Part 2)

Sustainable floodplains through large-scale reconnection to rivers

American Association for the Advancement of Science (AAAS) | December 11th, 2009

Flooding is the most damaging natural disaster worldwide, and the flood-vulnerable population is expected to grow in coming decades. Flood risks will likely increase beca

Flooding is the most damaging natural disaster worldwide, and the flood-vulnerable population is expected to grow in coming decades. Flood risks will likely increase because of both climate change and shifting land uses, such as filling of wetlands and expansion of impervious surfaces, that lead to more rapid precipitation runoff into rivers. In the United States, annual river flood losses continue to rise, punctuated by major events in the Midwest (1993, $30 billion in total costs; 2008, $15 billion) and California's Central Valley (1995 and 1997; $4 billion each event). Meanwhile, pressure to develop new housing in floodprone areas near rivers (floodplains) continues , even as levee-system maintenance is chronically underfunded.

Synoptic conditions associated with cool season post-fire debris flows in the Transverse Ranges of southern California

Natural Hazards (Springer) | April 22nd, 2017

The Transverse Ranges of southern California often experience fire followed by flood. This sequence sometimes causes post-fire debris flows (PFDFs) that threaten life a

The Transverse Ranges of southern California often experience fire followed by flood. This sequence sometimes causes post-fire debris flows (PFDFs) that threaten life and property situated on alluvial fans. The combination of steep topography, highly erodible rock and soil, and wildfire, coupled with intense rainfall, can initiate PFDFs even in cases of relatively small storm rainfall totals. This study identifies common atmospheric conditions during which damaging PFDFs occur in the Transverse Ranges during the cool season, defined here as November–March. A compilation of 93 PFDF events during 1980–2014 triggered by 19 precipitation events is compared against previous studies of the events, reanalysis, precipitation, and radar data to estimate PFDF trigger times. Each event was analyzed to determine common atmospheric features and their range of values present at and preceding the trigger time. Results show atmospheric rivers are a dominant feature, observed in 13 of the 19 events. Other common features include low-level winds orthogonal to the Transverse Ranges and other conditions favorable for orographic forcing, a strong upper level jet south of the region, and moist-neutral static stability. Several events included closed low-pressure systems or narrow cold frontal rain bands. These findings can help forecasters identify more precisely the synoptic-scale atmospheric conditions required to produce PFDF-triggering rainfall and thus reduce uncertainty when issuing warnings.

Taming wildfires in the context of climate change: The case of the United States

Organisation for Economic Co-operation and Development (OECD) | January 22nd, 2024

Shifts in climate cycles (e.g., Pacific Decadal Oscillation, El Niño-Southern Oscillation), combined with ongoing changes in drought, temperature and snowmelt, together,

Shifts in climate cycles (e.g., Pacific Decadal Oscillation, El Niño-Southern Oscillation), combined with ongoing changes in drought, temperature and snowmelt, together, have contributed to the overall warmer, drier conditions that fuel extreme wildfires (Environmental Protection Agency, 2022[4]; Kitzberger et al., 2007[10]; Westerling, 2016[11]). The period with the largest increase in area burned coincides with many of the warmest years on record nationwide (Environmental Protection Agency, 2022[4]). Such changes are especially visible in the western United States, where the peak extent of area burned now occurs about one month earlier than just two decades ago. Other factors including longer dry seasons and drier soils and vegetation are increasingly creating the conditions for catastrophic wildfires (Environmental Protection Agency, 2022[4]; US Global Change Research Program, 2018[9]). Wildfires are also further spreading into areas historically unaccustomed to fire, including high elevation areas in the west of the country (Figure 1.2) (OECD, 2023[2]; Alizadeh et al., 2021[12]), which may be less prepared for wildfires and potentially harmed by its impacts.

The challenge of closing the climate adaptation gap for water supply utilities

Nature Portfolio (Springer Nature) | June 29th, 2024

The challenge of unprecedented floods and droughts in risk management

Nature Portfolio (Springer Nature) | August 3rd, 2022

Risk management has reduced vulnerability to floods and droughts globally, yet their impacts are still increasing. An improved understanding of the causes of changing imp

Risk management has reduced vulnerability to floods and droughts globally, yet their impacts are still increasing. An improved understanding of the causes of changing impacts is therefore needed, but has been hampered by a lack of empirical data. On the basis of a global dataset of 45 pairs of events that occurred within the same area, we show that risk management generally reduces the impacts of floods and droughts but faces difficulties in reducing the impacts of unprecedented events of a magnitude not previously experienced. If the second event was much more hazardous than the first, its impact was almost always higher. This is because management was not designed to deal with such extreme events: for example, they exceeded the design levels of levees and reservoirs. In two success stories, the impact of the second, more hazardous, event was lower, as a result of improved risk management governance and high investment in integrated management. The observed difficulty of managing unprecedented events is alarming, given that more extreme hydrological events are projected owing to climate change.

The Changing Nature of Atmospheric Rivers

American Meteorological Society (AMS) | March 15th, 2025

Atmospheric rivers (ARs) are expected to strengthen in a warming climate, largely due to the thermodynamic (moistening) effect. Here, we show that this trend is already e

Atmospheric rivers (ARs) are expected to strengthen in a warming climate, largely due to the thermodynamic (moistening) effect. Here, we show that this trend is already evident in historical reanalysis data using the AR Tracking Method Intercomparison Project (ARTMIP) tier 2 AR detection tools (ARDTs) and variants of our own global AR detection applied to ERA5, MERRA-2, and JRA-55 reanalysis data. Over the 1980–2019 (ARTMIP) and 1980–2023 (variants) periods, total AR area increased by 6%–9%. AR integrated water vapor (IWV) increased by 1.5%–2.5% while integrated vapor transport (IVT) increased by less than 1% and 850-hPa wind speed (|V850|) and vertically integrated moisture flux convergence (VIMFC) both decreased. IWV increases were the most robust overall. All trend magnitudes were sensitive to subsetting, with fixed-frequency subsets consisting of the most intense AR grid points showing larger increases of 3%–4% IVT, 4%–6% IWV, and 6%–10% VIMFC, this last opposing a decreasing AR-mean VIMFC trend likely associated with large area increases. For individual ARs, maximum IVT and IWV increased at ;3–63 and ;1.5–23 the rate of AR-mean values, respectively. Regional changes were often even larger, particularly for extreme events, though most geospatial trends had low detectability under a false discovery rate control framework. Ultimately, despite considerable ARDT and methodological diversity, we found robust consensus moistening and expansion of ARs between 1980 and 2023. However, further research is required to determine the extent to which these trends are affected by reanalysis observational assimilation changes. For example, previous studies indicate that certain reanalyses misrepresent 1980–94 atmospheric moistening and so may underestimate historical AR expansion and intensification rates.