Drivers & Projections

Scientists measure changes in sea level at a variety of scales, from global to local levels. Global sea level is influenced by the expansion of ocean waters as they warm, the addition of freshwater to the ocean from melting land-based ice sheets and glaciers, and the extraction of groundwater. Most studies on sea level rise tend to rely on large-scale models that give global projections, such as those used in the Intergovernmental Panel on Climate Change (IPCC) reports, and then down-scale these models to regional or local levels. At these smaller scales, sea level is additionally affected by factors such as coastal ocean temperatures and circulation patterns, and tectonic activity (i.e., uplift or subsidence). Because sea level does not change uniformly around the globe, modifications for regional or local conditions can be used to refine future projections for adaptation plans. For an illustration of the climate-sensitive processes and components influencing sea level rise, click here.

• Global

  The most recent global-scale assessment of sea level rise is the 2013 IPCC 5th Assessment Report (AR5). Depending on the emissions scenario used, global average sea level projections indicate a rise by 10-39 in (26-98 cm) by the year 2100 relative to mean sea level from 1985-2005. These projections are approximately 50% higher than previously-reported global projections because only the most recent models account for increases in sea level due to the loss of land-based ice sheets, which are a major reservoir for the global water supply.

• National

  In May 2014, the United States released its third National Climate Assessment (NCA), which includes the current best-available science on climate change and sea level rise at the national scale. Sea level rise projections in the NCA were informed by a 2012 NOAA report, which provided a set of four global sea level rise scenarios ranging from 8 in-7 ft (0.2-2.0 m) by the year 2100, and reflected different amounts of future greenhouse gas emissions, ocean warming, and ice sheet loss. Current greenhouse gas emissions are tracking with the second-highest of these scenarios, indicating that future sea level rise will exceed historical trends.

• Regional & Local

  Tide gauges and satellite observations show that in the past century, mean sea level in California has risen 8 in (20 cm), keeping pace with global rise. However, in the past 15 years or so, mean sea level in California has remained relatively constant, and it has been suggested that this suppression may have been due to factors such as offshore winds and other oceanographic complexities associated with the Pacific Decadal Oscillation (PDO), a pattern of Pacific climate variability that alternates between multidecadal-long warm and cool ocean phases, and was in a cool phase until mid-2014. As the PDO shifts back to its warm phase, conditions are likely to allow for accelerated sea level rise along the West Coast.

  According to the 2012 National Research Council (NRC) report, which the Ocean Protection Council established as the best available science on sea level rise for California in March 2013, sea level is projected to increase by 17 to 66 in (42 to 167 cm) along much of the California coast by Year 2100. The NRC report provides a range of sea level rise projections that have been refined for the West Coast. These account for uncertainties in the data, and the report defines separate projections for the regions North and South of Cape Mendocino (see the following table). The break at Cape Mendocino is significant because this is where two tectonic regimes – the Cascadian Subduction Zone and the San Andreas Fault Zone – meet. Land North of Cape Mendocino is generally being uplifted, while land elevation South of Cape Mendocino is remaining relatively constant. This means that for the region North of Cape Mendocino, the consequences of sea level rise may be somewhat diminished; however, a large Cascadian earthquake could trigger a rapid subsidence of as much as 6 ft (2 m) in the region, meaning that relative sea level rise would correspondingly increase.

  Time Period*	North of Cape Mendocino	South of Cape Mendocino
  by 2030	-2 – 9 in (-4 – +23 cm)	2 – 12 in (4 – 30 cm)
  by 2050	-1 – 19 in (-3 – + 48 cm)	5 – 24 in (12 – 61 cm)
  by 2100	4 – 56 in (10 – 143 cm)	17 – 66 in (42 – 167 cm)

  

  * Year 2000 as a baseline. Adapted from: NRC 2012, Table 5.3

  The Humboldt Bay and Eel River Estuary represent important exceptions to the projections for North of Cape Mendocino because these areas are currently experiencing considerable subsidence. Thus, use of the North of Cape Mendocino projections may not be appropriate. For these areas, more detailed refinements of sea level rise projections are considered essential, though an alternative could be to simply use the projections for South of Cape Mendocino. For the rest of California, refinements to sea level rise projections may be considered but are not necessary; deriving these is a highly technical and data-intensive exercise, and local variability from the regional projections will often be minor (for more information about vertical land movement and relative sea level rise, see the Commission’s Sea Level Rise Policy Guidance Appendix B). However, it is important to recognize that while sea level rise projections are fairly similar throughout the state, the physical impacts may be quite different given the nature of local coastal conditions, and locally-specific analysis of impacts will be critical.

  Although the NRC report only provides estimated sea level rise ranges through the year 2100, sea level will continue to rise, possibly at an accelerating rate, beyond the end of the century. Appendix B of the Commission's Sea Level Rise Policy Guidance describes the process for modifying projections to account for specific time periods.

  The Coastal Commission will be using and recommends that local governments and permit applicants use best available science, currently identified as the projections provided in the NRC 2012 report, in all relevant local coastal planning and coastal development permitting decisions.

Scenario-Based Analysis

Sea level rise is a complex response to Earth’s shifting climate. Because climate change is largely dependent on greenhouse gases, from both natural and human activities, it is challenging to accurately predict changes, including the timing and magnitude of sea level rise. To account for this, sea level rise projections are typically expressed as ranges with values bounded by expectations based on multiple scenarios of future conditions.

• Sources of Uncertainty

  At the global scale, uncertainty about the nature of sea level rise is primarily driven by future greenhouse gas emissions, sulfate aerosol concentrations, and rates of land ice loss. At the regional and local scales, variations in the influence of thermal expansion and future vertical land motion further contribute to the challenge of predicting future sea level rise.

• Analytical Approaches

  There are many ways to project future changes in global sea level, including using extrapolations from historical trends and observations, estimations from physical models, and combinations of observations and modeling, known as semi-empirical methods. For a description of these various techniques, please see Appendix A in the Commission's Sea Level Rise Policy Guidance.

  In general, the Commission recommends using the best available science (currently the 2012 National Research Council's report) and scenario-based analysis to accommodate the uncertainty in sea level projections. Scenario-based analysis (or planning) refers to the idea of developing multiple scenarios from which to analyze vulnerabilities, generate new ideas and adaptation options, and/or test strategies. In the context of sea level rise, it involves selecting several possible amounts of sea level rise as a starting point to evaluate impacts to coastal resources and the potential risks to development over time. Such an approach is useful because it reveals a full range of potential consequences that can be reasonably expected and helps to identify tipping points indicating if, or when, sea level rise will become a serious issue in a particular location. Using multiple sea level rise scenarios can help planners anticipate the types of hazards that need to be prepared for, including those to coastal resources and human health and safety.

  In practice, the process for choosing sea level rise projections and performing scenario-based analysis will be slightly different for Local Coastal Program (LCP) planning and Coastal Development Permit (CDP) applications due to differences in planning goals and the levels of technical detail necessary. For an LCP, the general goal is to assess the potential impacts from sea level rise over the entire planning area and a range of time horizons so that both short and long-term adaptation strategies can be identified and implemented. For a CDP application, the goal is narrower: to understand how sea level rise will impact a specific site and a specific project over its expected lifetime so that the proposed development will be safe from hazards and avoid impacting coastal resources. For more detail on choosing sea level rise scenarios in the context of LCP planning or CDP applications, see Chapter 5 and Chapter 6 in the Commission’s Sea Level Rise Policy Guidance, respectively.

Physical Impacts

As sea level rises, it will adversely affect California’s coastal communities and resources. The main physical effects of sea level rise include increased flooding, inundation, wave impacts, coastal erosion, changes in sediment dynamics, and saltwater intrusion to groundwater supplies. These impacts are interrelated and often occur together.

• Flooding & Inundation

  Low-lying coastal areas may experience more frequent flooding (temporary wetting) or inundation (permanent wetting), and the inland extents of 100-year floods may increase. As riverine and coastal waters come together at river mouths, coastal lagoons, and estuaries, higher water levels at the coast may cause water to back up and increase upstream flooding. Drainage systems that discharge close to sea level could have similar problems, and inland areas may become flooded if outfall pipes back up with saltwater. In addition, other climate change impacts such as increases in the amount of precipitation falling as rain rather than snow will add to river flooding in some areas.

• Waves

  As seas rise, waves will force water further and further inland, particularly during coastal storms. They will also trigger consequences such as high amounts of erosion and damage to development. Erosion rates along coastal cliffs, beaches, and dunes are expected to increase with rising sea level and are likely to further increase if waves become larger or more frequent.

• Coastal Erosion

  Large sections of the California coast consist of oceanfront bluffs that are often highly susceptible to erosion. As sea level rises, the amount of time that these bluffs are exposed to wave attack will increase, further aggravating erosion. This in turn could trigger landslides and the loss of structural and geologic stability of bluffs. This would impact development such as homes, infrastructure, the California Coastal Trail, Highway 1, and other roads and public utilities. Over the long term, rising sea levels can also contribute to landward migration of beaches due to the combined effects of inundation and shoreline erosion. The use of hard armoring structures (e.g., seawalls) to protect onshore development from erosion impedes the migration of beaches and will accelerate the loss of the remaining beach.

• Sediment Supply & Movement

  Sediment is important to coastal systems for many reasons, including the formation of beaches and mudflats, and as the substrate for wetlands. As water levels increase and precipitation patterns change, the dynamics of erosion and deposition processes will alter sediment availability. Losses of sediment could intensify beach erosion and potentially increase the need for beach nourishment projects. At the same time the effectiveness and long-term viability of nourishment projects may be reduced if the new sand is quickly washed away. Sediment supply in wetland areas is also important for long-term marsh accretion and survival; however, the rate of sea level rise may outpace the rates at which wetlands naturally trap sediment and grow vertically.

• Saltwater Intrusion

  Research indicates that rising sea level is likely to push saltwater through the ground, introducing it to and degrading fresh groundwater resources in some areas; however, the degree of impact will vary greatly depending on local hydrogeological conditions. Generally, the most vulnerable areas will be where aquifers are unconfined along low-lying coasts, or aquifers that have already experienced overdraft and saline intrusion. In California, this is a problem in a number of areas including, but not limited to, the coastal agricultural lands in the Pajaro and Salinas Valleys, the Oxnard Plain, and the heavily urbanized coastal plains of Los Angeles and Orange Counties. Additional research is needed to understand the site-specific consequences of sea level rise and saltwater intrusion to coastal aquifers.

Storms, Extreme Events, & Abrupt Change

Much of the California coast is currently vulnerable to flooding and wave damage during large storm events, and even more of the coast is vulnerable to storm impacts when they occur during times of heightened water levels, such as high tides, El Niño events, a warm phase of the Pacific Decadal Oscillation (PDO), or any combination of these factors. Sea level rise will further increase the amount of coastal area vulnerable to storm impacts.

Climate change could also contribute to increased storm frequency or intensity (i.e., storminess). Extreme events stand to interact with sea level rise in ways that further intensify its effects. The extremes associated with high-intensity events may be particularly devastating since they have the potential to cause broad-scale damage, as seen from events such as Hurricanes Katrina and Rita, Superstorm Sandy, and the Tohoku tsunami. Abrupt change in sea level is another potential consequence of climate change.

• Extreme Events & Storms

  There are several ways to describe extreme events, and most definitions tend to frame them in terms of consequences or past observations. By their very nature, extreme events are those beyond the normal occurrences considered in most shoreline studies. For example, for storm waves and flood conditions, an extreme event will normally be interpreted as anything worse than the 100-year event.

  Extreme events are of particular concern in the examination of coastal vulnerability and damage because they tend to cause the greatest community upheaval and can result in irreversible changes to the coastal landscape. In the El Niño winter of 1982-1983, for example, a series of storms, several of which coincided with high tides, caused more than $200 million in damage (in 2010 dollars) to coastal California (OPC 2013). The National Research Council (NRC) notes that "waves riding on these higher water levels will cause increased coastal damage and erosion — more than that expected by sea level rise alone" (2012, p 107). In its report, the NRC goes on to observe that if the frequency or intensity of storms changes, then so will the frequency and intensity of extreme sea level events. Although evidence that storminess will change in the North Pacific Ocean is currently inconclusive, even if it does not change, sea level rise will exacerbate storm surge and high waves, magnifying their impact on the coastline. For this reason, it is important to include these factors in the analysis of sea level rise hazards.

• Abrupt Change

  Sea level rise projections tend to indicate gradual changes of sea level with time. However, abrupt change is possible and there are several mechanisms that could drive such change at global or regional scales.

  • The first is a rapid collapse of land-based ice sheets, which would lead to a sudden acceleration of sea level rise; as the likelihood of such a collapse is uncertain (but believed to probably be low), this remains a topic in need of future research.
  • Another possible driver at local and regional scales is rapid change in land elevation during an earthquake. The National Research Council's 2012 report finds that a large earthquake in the Cascadia Subduction Zone could cause land in northern California, Oregon, and Washington to suddenly subside, causing a sudden rise in relative sea level by 3-6.5 ft (1-2 m) and adding to the increases in sea level already being forced by climate change.
  • A third potential mechanism for abrupt sea level rise relates to oceanographic regime shifts. From 1983 to 2011, sea level rise in California appeared to be suppressed relative to global observations, and it has been suggested that this inconsistency was due to persistent alongshore winds and extended offshore upwelling, which replaced warm surface water with cooler, denser deep ocean water. However, such conditions cannot continue indefinitely and as the PDO, wind, and other conditions shift, California’s sea level will continue rising, likely at an accelerated rate (see Bromirski et al. [2011, 2012]).

Environmental Justice

Environmental justice demands that all people, regardless of their race, ethnicity, or level of income, are able to enjoy the benefits of the environment and environmental protection programs. The California Coastal Act speaks to this concept, recognizing that the protection of the coast is of vital interest to all people, of paramount concern to present and future residents of the state and nation, and that careful planning and development is essential to the economic and social well-being of the people. Safeguarding California, the State's plan for climate adaptation, also highlights the importance of environmental justice as a cross-sector theme for resilience planning efforts. With respect to rising seas, two particular aspects of environmental justice should be considered across the state: public access and the capacity of communities to adapt.

Planners and decision makers should consider environmental justice concerns in the analysis of alternative project designs and adaptation measures, and make certain that low-income and underserved communities are involved in decision-making and planning efforts. This will better ensure that adaptation efforts benefit all Californians, fairly, and that vulnerability to sea level rise does not disproportionally increase for any particular group or demographic, and does not have any unintended consequences that lead to social or environmental injustices.

• Public Access

  Many people and communities use and enjoy public beaches, generally at low or no cost. Public access – to beaches, recreational amenities, and even surf spots – is one of the coastal resources most at risk from accelerating sea level rise. Where development prevents beaches and other coastal ecosystems from migrating inland in response to sea level rise, these public resources will shrink and change in nature as water steadily encroaches from the sea. Shoreline armoring used to protect development, whether public or private, potentially impacts widely available public beach resources by inhibiting the natural processes that allow a shoreline to migrate inland and continue to exist. Loss of public access is significant and especially important to those with fewer economic resources.

• Adaptive Capacity

  Another aspect of environmental justice that needs to be considered in conjunction with sea level rise is the disproportionate impacts imposed on vulnerable communities that have relatively limited capacity to adapt. Due to current development patterns along the coast, sea level rise hazards may affect various sections of the population differently, as could the implementation and effectiveness of various adaptation measures. A 2009 study by Heberger et al. found that the number of people living along the open coast in areas exposed to flooding from a 100-year flood would increase to 210,000 with a 4.6 ft (1.4 m) increase in sea level; approximately 27% or 56,000 of these are lower income people (those earning less than $30,000 annually); 45,000 are renters; and 4,700 are linguistically-isolated and less likely to understand flood warnings. The study also identified Los Angeles, San Diego, Ventura, Humboldt, and San Luis Obispo counties as those where the greatest increases in the number of people vulnerable to flooding will occur. Sea level rise will likely result in the loss of key infrastructure, the intrusion of saltwater into freshwater sources, and the creation of additional coastal hazards. Hazards in vulnerable areas will have disproportionate impacts on communities with the least capacity to adapt, which could deepen and expand existing environmental injustice if adaptation responses are not managed appropriately.

Coastal Act Resources

The California Coastal Act protects a multitude of coastal resources, many of which are threatened by sea level rise. In this context, hazard avoidance is also a key consideration for coastal development, including public infrastructure and ports. The consequences of rising seas will be widespread and as communities develop means to address the issue, it will be important to integrate future projections of sea levels into existing hazard analyses, siting, design and construction processes, ecosystem management, and community planning activities. The following are samples of considerations for coastal resources and sea level rise.

• Coastal Development

  The likelihood of property damage from flooding, inundation, or extreme waves will increase with sea level rise, as will the number of people living in areas exposed to significant flooding. Changes in erosion and sediment dynamics will influence the spatial extent of bluffs and shorelines, and could threaten coastal structures and recreation areas.

  A rise in water levels, extreme wave conditions, and the loss of buffering wetlands also increase the susceptibility of levee systems to damage and overtopping. Public infrastructure such as low-lying roads and railways, wastewater treatment facilities, energy facilities, stormwater infrastructure, and utility infrastructure are at risk of damage and impaired function due to erosion, flooding, and inundation. Flooding and inundation also threaten port infrastructure such as piers and marina facilities, and may impose challenges with vessel access to port facilities where bridge clearance is reduced. Industrial development such as refineries and petrochemical facilities are facing the impacts of sea level rise as well – inundation of contaminated lands near industrial development can potentially lead to problems with water quality and polluted runoff, the availability of areas suitable for siting or expansion of industrial development will probably become increasingly limited, and damage to industrial facilities exposed to saltwater is likely to increase.

  The Pacific Institute estimated that the replacement value of property at risk from 4.6 ft (1.4 m) of sea level rise for the California coast, excluding San Francisco Bay, is approximately $36.5 billion (in Year 2000 dollars). Note that this value does not take into account the financial impacts on business operations.

  Sea level rise may also lead to an increase in demand for construction of shoreline protection. However, shoreline protection devices alter natural shorelines and generally have negative impacts on other coastal resources including beaches, nearshore marine habitats, and the scenic and visual qualities of coastal areas.

• Coastal Habitats

  Coastal habitats – including bluffs and cliffs, rocky intertidal areas, beaches, dunes, wetlands, estuaries, lagoons and tidal marshes, tidal flats, eelgrass beds, and tidally-influenced streams and rivers – are likely to be affected by sea level rise. Inundation and increased erosion can potentially convert habitats from one type to another and generally reduce the amount of nearshore habitat, such as sandy beaches and rocky intertidal areas. In the absence of seawalls or other barriers, some beaches may be able to migrate inland over the long-term, but the dune habitats behind them that serve as buffers for interior lands during storms are likely to retreat at increasingly rapid rates. Wetland habitats, including California's few remaining saltwater marshes, will also be impacted by sea level rise as intertidal zones shift inland. The Pacific Institute estimated that California's 550 square miles (885 km) of critical coastal wetland habitat, including those in San Francisco Bay, would be converted to open water with a 4.6 ft (1.4 m) rise of sea level if they are unable to accrete upward or to migrate inland due to natural or anthropogenic barriers.

  Importantly, there are many endemic and endangered species in California that are dependent on these coastal environments. For example, grunion need a sandy beach environment in order to reproduce and survive, the California clapper rail is dependent on marshes and wetlands, and the black abalone requires rocky intertidal habitat. Nesting habitat, nursery areas, and haul-out sites important for birds, fish, marine mammals and other animals could also disappear as sea levels rise.

• Cultural Resources

  Archaeological and paleontological resources are potentially threatened by inundation, flooding, and increases in erosion due to sea level rise. This includes areas of traditional cultural significance to California's Native American tribes, including villages, religious and ceremonial locations, middens, burial sites, and other areas. For example, the Santa Barbara Channel area has thousands of archaeological sites dating over 13,000 years that are at risk.

• Biological Productivity

  By changing the composition and distribution of coastal habitats, sea level rise may also impact the biological productivity of coastal waters. Such a change in habitat is likely to alter species composition, and could potentially result in cascading effects through coastal ecosystems. For example, the loss of intertidal habitats to inundation, especially when combined with other climate impacts, may affect seabird populations that use these areas for nesting and feeding.

• Coastal Agriculture

  Sea level rise could affect low-lying agricultural areas by flooding or inundating land, which can potentially limit productivity and cause major impacts on local businesses, national food supplies, and the State's economy. Additionally, the intrusion of saltwater into groundwater supplies may reduce the amount of freshwater available for agricultural uses.

• Public Access & Recreation

  Coastal access and recreational opportunities are threatened by rising seas as permanent inundation, episodic flooding, or the erosion of beaches, recreational areas, or trails occurs. In areas where beaches cannot migrate inland due to development or more resistant landforms, beaches will narrow or disappear completely. Access to, and the quality of, water-oriented activities may also be affected. For instance, increased water levels and altered sediment patterns driven by sea level rise could lead to a change in surfing conditions, or affect the safety of harbors and marinas.

• Water Quality

  Coastal water quality is threatened by rising seas in a number of ways including the mobilization of previously "dry" pollutants, intrusion of saltwater to groundwater aquifers, and the loss of coastal habitats that provide natural water filtration services. The inundation of toxic soils and increases in nonpoint source pollution from flooding potentially expose humans and coastal ecosystems to hazardous materials as well as pathogens. Sea level rise could also impact critical infrastructure, such as wastewater facilities near the coast, damaging equipment and biological treatment systems, blocking discharges, and increasing the risk of releasing untreated wastewater into the surrounding environment. Saltwater intrusion associated with increased sea level potentially renders existing wells unusable and thereby reduces the total groundwater supply available to communities in coastal areas.