1

Introduction

Sea-level change is one of the most visible consequences of changes in the Earth’s climate. A warming climate causes global sea level to rise principally by (1) warming the oceans, which causes sea water to expand, increasing ocean volume, and (2) melting land ice, which transfers water to the ocean. Tide gage and satellite observations show that global sea level has risen an average of about 1.7 mm yr-1 over the 20th century (Bindoff et al., 2007), which is a significant increase over rates of sea-level rise during the past few millennia (Shennan and Horton, 2002; Gehrels et al., 2004). Projections suggest that sea level will continue to rise in the future (Figure 1.1). However, the rate at which sea level is changing varies from place to place and with time. Along the west coast of the United States, sea level is influenced by changes in global mean sea level as well as by regional changes in ocean circulation and climate patterns such as El Niño; gravitational and deformational effects of ice age and modern ice mass changes; and uplift or subsidence along the coast. The relative importance of these factors in any given area determines whether the local sea level will rise or fall and how fast it will change.

Sea-level change has enormous implications for coastal planning, land use, and development along the 2,600 km shoreline of California, Oregon, and Washington (referred to hereafter as the U.S. west coast). Rising sea level increases the risk of flooding, inundation, coastal erosion, wetland loss, and saltwater intrusion into freshwater aquifers in many coastal communities (e.g., Heberger et al., 2009, 2011). Valuable infrastructure, development, and wetlands line much of the coast. For example, significant development along the edge of central and southern San Francisco Bay—including two international airports, the ports of San Francisco and Oakland, a naval air station, freeways, housing developments, and sports stadiums—has been built on fill that raised the land level only a few feet above the highest tides. The San Francisco International Airport will begin to flood with as little as 40 cm of sea-level rise (Figure 1.2), a value that could be reached in several decades (Figure 1.1).

Coastal infrastructure and ecosystems are already vulnerable to high waves during ocean storms (e.g., Figure 1.3), especially when storms coincide with high tides and/or El Niño events. For example, a strong El Niño, combined with a series of large storms at times of high astronomical tides, caused more than $200 million dollars in damage (in 2010 dollars) to the California coast during the winter of 1982–1983 (Griggs et al., 2005). Higher sea levels and heavy rainfall caused flooding in low-lying areas and increased the level of wave action on beaches and bluffs (Storlazzi and Griggs, 2000). More than 3,000 homes and businesses were damaged, 33 oceanfront homes were completely destroyed, and roads, parks, and other infrastructure was heavily damaged. The damage will likely increase as sea level continues to rise and more of the shoreline is inundated.

In November 2008, Governor Arnold Schwarzenegger issued Executive Order S-13-08 directing California state agencies to plan for sea-level rise and coastal impacts.1 Included in the executive

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1 See <http://gov.ca.gov/executive-order/11036/>.



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1 Introduction S ea-level change is one of the most visible con- of the coast. For example, significant development sequences of changes in the Earth's climate. A along the edge of central and southern San Francisco warming climate causes global sea level to rise Bay--including two international airports, the ports of principally by (1) warming the oceans, which causes San Francisco and Oakland, a naval air station, free- sea water to expand, increasing ocean volume, and ways, housing developments, and sports stadiums--has (2) melting land ice, which transfers water to the ocean. been built on fill that raised the land level only a few Tide gage and satellite observations show that global feet above the highest tides. The San Francisco Inter- sea level has risen an average of about 1.7 mm yr-1 national Airport will begin to flood with as little as over the 20th century (Bindoff et al., 2007), which is 40 cm of sea-level rise (Figure 1.2), a value that could a significant increase over rates of sea-level rise during be reached in several decades (Figure 1.1). the past few millennia (Shennan and Horton, 2002; Coastal infrastructure and ecosystems are already Gehrels et al., 2004). Projections suggest that sea level vulnerable to high waves during ocean storms (e.g., will continue to rise in the future (Figure 1.1). How- Figure 1.3), especially when storms coincide with high ever, the rate at which sea level is changing varies from tides and/or El Nio events. For example, a strong El place to place and with time. Along the west coast of Nio, combined with a series of large storms at times of the United States, sea level is influenced by changes in high astronomical tides, caused more than $200 million global mean sea level as well as by regional changes in dollars in damage (in 2010 dollars) to the California ocean circulation and climate patterns such as El Nio; coast during the winter of 19821983 (Griggs et al., gravitational and deformational effects of ice age and 2005). Higher sea levels and heavy rainfall caused modern ice mass changes; and uplift or subsidence flooding in low-lying areas and increased the level along the coast. The relative importance of these factors of wave action on beaches and bluffs (Storlazzi and in any given area determines whether the local sea level Griggs, 2000). More than 3,000 homes and businesses will rise or fall and how fast it will change. were damaged, 33 oceanfront homes were completely Sea-level change has enormous implications for destroyed, and roads, parks, and other infrastructure coastal planning, land use, and development along was heavily damaged. The damage will likely increase the 2,600 km shoreline of California, Oregon, and as sea level continues to rise and more of the shoreline Washington (referred to hereafter as the U.S. west is inundated. coast). Rising sea level increases the risk of flooding, In November 2008, Governor Arnold inundation, coastal erosion, wetland loss, and saltwater Schwarzenegger issued Executive Order S-13-08 di- intrusion into freshwater aquifers in many coastal com- recting California state agencies to plan for sea-level munities (e.g., Heberger et al., 2009, 2011). Valuable rise and coastal impacts.1 Included in the executive infrastructure, development, and wetlands line much 1 See . 9

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10 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON FIGURE 1.1 Estimated, observed, and projected global sea-level rise from 1800 to 2100. The pre-1900 record is based on geological evidence, and the observed record is from tide gages (red line) and satellite altimetry (blue line). Example projections of sea-level rise to 2100 are from IPCC (2007) global climate models (pink shaded area) and semi-empirical methods (gray shaded area; Rahmstorf, 2007). SOURCES: Adapted from Shum et al. (2008), Willis et al. (2010), and Shum and Kuo (2011). order was a request that the National Research Council fornia, Oregon, and Washington for 2030, 2050, and (NRC) establish a committee to assess sea-level rise in 2100 (see Box 1.1 for the committee charge). The years California to inform state planning and development for the assessment represent planning horizons: 2030 efforts. Prior to release of the NRC report, the state is a typical planning horizon for many local managers; agencies were instructed to incorporate sea-level-rise 2050 is the latest date for which conventional popula- projections into their planning process. The range of tion projections are available; and 2100 is the limit projections adopted by California as interim values are beyond which uncertainties become too high for plan- 1321 cm for 2030, 2643 cm for 2050, and 78176 cm ning.2 The report primarily focuses on how much sea for 2100 (CO-CAT, 2010). level is likely to rise globally (Task 1) and along the west Following the California executive order, the states coast of the United States (Task 2). Processes that have of Oregon and Washington, the U.S. Army Corps of only transient effects on sea level (e.g., tides, tsunamis) Engineers, the National Oceanic and Atmospheric were considered only if the nature of the process affects Administration, and the U.S. Geological Survey joined trends in sea level (e.g., changes in frequency of inten- California in sponsoring this NRC study. These agen- sity of storms [Task 2a]). Coastal impacts or measures cies need sea-level information for a variety of purposes, to lessen them were considered only in the context of including assessing coastal hazard vulnerability, risks, summarizing what is known about how coastal habi- and impacts; informing adaptation strategies; and im- tats and natural and restored environments respond to proving coastal hazard forecasts and decision support and protect against future sea-level rise and storms tools. (Tasks 2b and 2c). This report provides an assessment of current 2 Jeanine Jones, California Department of Water Resources, knowledge about changes in sea level expected in Cali- personal communication, December 3, 2008.

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DISCLAIMER: The inundation data used in these maps do not account for shoreline protection or wave activity. These maps INTRODUCTION are for informational purposes only. Users agree to hold harmless and blameless the State of California and its representatives 11 and its agents for any liability associated with the use of the maps. The maps and data shall not be used to assess actual coastal hazards,insurance requirements, or property values or be used in lieu of Flood Insurance Rate Maps issued by the Federal Emergency Management Agency (FEMA). South San Francisco San Francisco Bay Millbrae NORTH 0 1.5 3 MILES Sea level rise data provided by: FIGURE 1.2 Expected inundation of low-lying areas, including the San Francisco International Airport (center), in the San Francisco Bay Area with a Knowles, SOURCE: 40 cmN. rise in 2008. sea S.W. Siegel, level and(light blue P. A. M. shading). Bachand, 2002. SOURCE: Bay Conservation and Development Commission, . COMMITTEE APPROACH climate change, including sea-level change. For Task 1, this report summarizes the latest IPCC (2007) findings Assessments are intended to yield a judgment on on global sea-level rise and its major components, then a topic, based on review and synthesis of scientific updates them with more recent results. knowledge. Beginning in 1989, the primary assess- For Task 2, the committee drew on published re- ments of global sea-level change have been carried out search on sea-level change along the west coast of the by thousands of scientists working under the auspices United States and also carried out its own analyses. of the Intergovernmental Panel on Climate Change Prior assessments of the rate of local sea-level rise (IPCC). IPCC assessments, which are made every 5 or have been made for Washington (Mote et al., 2008) 6 years, evaluate observations, models, and analyses of

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12 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON FIGURE 1.3 High surf during a high tide of nearly 2.7 m removed the front lawn of the Pacific Sands Resort at Neskowin, Oregon, on January 9, 2008. SOURCE: Courtesy of Armand Thibault. BOX 1.1 Committee Charge The committee will provide an evaluation of sea-level rise for California, Oregon, and Washington for the years 2030, 2050, and 2100. The evalu- ation will cover both global and local sea-level rise. In particular, the committee will 1. Evaluate each of the major contributors to global sea-level rise (e.g., ocean thermal expansion, melting of glaciers and ice sheets); combine the contributions to provide values or a range of values of global sea-level rise for the years 2030, 2050, and 2100; and evaluate the uncertainties as- sociated with these values for each timeframe. 2. Characterize and, where possible, provide specific values for the regional and local contributions to sea-level rise (e.g., atmospheric changes influencing ocean winds, ENSO [El Nio-Southern Oscillation] effects on ocean surface height, coastal upwelling and currents, storminess, coastal land motion caused by tectonics, sediment loading, or aquifer withdrawal) for the years 2030, 2050 and 2100. Different types of coastal settings will be examined, taking into account factors such as landform (e.g., estuaries, wetlands, beaches, lagoons, cliffs), geologic substrate (e.g., unconsolidated sediments, bedrock), and rates of geologic deformation. For inputs that can be quantified, the study will also provide related uncertainties. The study will also summarize what is known about a. climate-induced increases in storm frequency and magnitude and related changes to regional and local sea-level rise estimations (e.g., more frequent and severe storm surges); b. the response of coastal habitats and geomorphic environments (including restored environments) to future sea-level rise and storminess along the west coast; c. the role of coastal habitats, natural environments, and restored tidal wetlands and beaches in providing protection from future inundation and waves.

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INTRODUCTION 13 and California (e.g., Cayan et al., 2009), and numerous Uncertainty in projecting climate-related sea-level studies have been published on individual contributors changes arises from three sources: internal variability of to sea-level change along the U.S. west coast. The the climate system, which fluctuates on interannual to committee also analyzed tide gage records and Global multidecadal and longer timescales and on regional to Positioning System data from California, Oregon, and global spatial scales; model uncertainty; and scenario Washington for their local (around the station) and uncertainty (Hawkins and Sutton, 2009). The first is regional (along the coast of one or more states) trends, particularly important for projections based on extrapo- and extracted regional information from satellite lation of observations because observational records altimetry data and glacial isostatic adjustment models. tend to be short relative to the timescale of variability in The most challenging aspect of the committee the climate system. Models have uncertainties because charge was the projections of sea level for 2030, 2050, they are mathematical approximations that depart in and 2100. The numerical global climate models devel- important ways from the actual system. Uncertainty oped for the IPCC Fourth Assessment Report3 project in models used to describe key elements of sea-level global sea-level rise to 2100. However, they do not change results from uncertainties in model parameters account for rapid changes in the behavior of ice sheets (e.g., initial conditions, boundary conditions) as well as and glaciers as melting occurs (ice dynamics) and thus structural uncertainties from incomplete understanding likely underestimate future sea-level rise. The new suite of some climate processes or an inability to resolve the of climate models for the Fifth Assessment Report was processes with available computing resources (Knutti et not available at the time of writing this report. Conse- al., 2010). Finally, future emissions of greenhouse gasses quently, the committee projected global sea-level rise and other factors that drive changes in the climate sys- (Task 1) using model results from the IPCC Fourth tem depend on a collection of human decisions at local, Assessment Report, together with a forward extrapola- regional, national, and international levels, as well as po- tion of land ice that attempts to capture an ice dynamics tential but unknown technological developments. The component. The committee also considered results IPCC deals with this uncertainty by providing a range from semi-empirical projections, which are based on of possible futures (scenarios) based on assumptions the observed correlation between global temperature about trends in concentrations of greenhouses gases and and sea-level change (e.g., Vermeer and Rahmstorf, other influences on the climate (e.g., Moss et al., 2010). 2009). For the projections of sea-level rise along the This report uses both model and extrapolation U.S. west coast (Task 2), the committee derived local approaches to make projections. Each approach has values using regional ocean information extracted from different uncertainties (e.g., extrapolations take no global models, GPS data from along the coast, and ice account of emission scenarios), which were combined loss rates of large or nearby glaciers. into a single uncertainty range for the projections. Al- though isolating the various sources of uncertainty may Uncertainty have been useful for some applications (e.g., evaluating costs and risks of various mitigation strategies), it was In the IPCC Fourth Assessment Report, the major not required in the committee charge and would have components of global sea-level rise were estimated at required a different analysis approach. the 90 percent confidence level (Bindoff et al., 2007). That is, values given as x e mean that there is a 90 per- OVERVIEW OF SEA-LEVEL CHANGE cent chance that the true value is in the range x - e to x + e. This report follows the IPCC convention unless Sea level is neither constant nor uniform every- specified otherwise. where, but changes continually as a result of interact- ing processes that operate on timescales ranging from hours (e.g., tides) to millions of years (e.g., tectonics). 3 More than 20 such models from around the world were Processes that affect ocean mass, the volume of ocean analyzed and compared through the World Climate Research water, or sea-floor topography cause sea level to change Program's Coupled Model Intercomparison Project (CMIP3). See on global scales. On local and regional scales, sea level is .

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14 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON also affected by vertical land motions and local climate that period, ice sheets covered much of North America, and oceanographic changes. The primary factors that northern Europe, and parts of Asia, and sea levels were contribute to global and local sea-level change are illus 125135 m lower than present (Peltier and Fairbanks, trated in Figure 1.4 and discussed below. 2006; Clark et al., 2009). The onset of deglaciation more than 20,000 years ago (Peltier and Fairbanks, Global Sea-Level Change 2006) caused sea level to rise at an average rate of about 10 mm yr-1 (Alley et al., 2005). Empirical and glacial Global sea level has varied significantly through- isostatic modeling studies suggest that the rate of ice out Earth's history. Sediment and ice-core records of melt dropped significantly 7,000 years ago (Gehrels, these changes provide a pre-anthropogenic context 2010), then declined steadily to a value of zero change for understanding the nature and causes of current around 2,000 years ago (Fleming et al., 1998; Peltier, and future changes. Over the past 2.5 million years, 2002b; Peltier et al., 2002). Geological data from salt large continental ice sheets grew during long inter- marshes show a clear acceleration from relatively low vals of cold global temperatures (glacial periods or ice rates of sea-level change during the past two millennia ages) and retreated during intervals of warm global (order 0.25 mm yr-1; Figure 1.5) to modern rates (order temperatures (interglacial periods). Traces of paleo- 2 mm yr-1) sometime between 1840 and 1920 (Kemp shorelines, found along many of the world's coastlines, et al., 2011). provide robust evidence that global mean sea level was Since the industrial era began, changes in global at least 6 m higher during the last interglacial period sea level have been driven in part by the accumulation (~116,000130,000 years ago) than at present (Kopp et of greenhouse gases in the atmosphere, which trap al., 2009). During the Last Glacial Maximum (~26,000 heat and raise global temperatures. The primary pro- years ago), approximately 40 106 km3 of sea water was cesses responsible for modern sea-level rise are thermal transferred to the continents and stored as ice. During expansion of ocean water and melting from glaciers, Oceanatmosphere interaction Terrestrial water Ice storage melting Gravitational attraction of ice Groundwater withdrawal Ocean circulation Density changes Glacial isostatic Uplift and adjustment subsidence FIGURE 1.4 Processes that influence sea level on global to local scales. SOURCE: Modified from Milne et al. (2009).

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INTRODUCTION 15 Sea-Level Estimates Proxy reconstructions 0.2 Observations (tide gauges) Sea Level (m) Model 0.0 -0.2 rate of sea-level 0 mm/yr +0.6 mm/yr -0.1 mm/yr +2.1 rise (mm/yr) 0 500 1000 1500 2000 Year (AD) FIGURE 1.5 Sea-level estimates for the past 2000 years, adjusted for glacial isostatic effects, from proxy (geological) evidence (blue), tide gage observations (green), and modified semi-empirical model hindcasts (red). Dotted red line shows where the model hindcast deviates from the proxy record. The lower panel shows rates of sea-level change in mm yr-1 based on the proxy reconstruc- tions. SOURCE: Data from Jevrejeva et al. (2008), Vermeer and Rahmstorf (2009), and Kemp et al. (2011). ice caps, and the Greenland and Antarctic ice sheets circulation are driven primarily by changes in winds (Figure 1.6). Changes in the amount of water stored in and ocean surface density associated with the El Nio- land reservoirs have a smaller effect on global sea level. Southern Oscillation (ENSO), which has a period of 2 In general, groundwater extraction transfers water to to 7 years, and the Pacific Decadal Oscillation, which the ocean and causes sea level to rise, and filling of land has a typical period of several decades. During a strong reservoirs causes sea level to fall. El Nio, a pulse of warm water in the eastern equatorial Pacific moves northward, forming a bulge in sea level Local and Regional Sea-Level Change Along the along the California, Oregon, and Washington coasts. U.S. West Coast The low atmospheric pressures and west-southwest winds induced by an El Nio further elevate sea levels, Relative (or local) sea level is the mean level of the which can reach 30 cm above normal levels for several sea with respect to the land, both of which change with months (Komar et al., 2011). Sea level is lower along time, as summarized below. the U.S. west coast during cooler La Nia conditions. Large storms raise coastal sea level for the dura- Changes in Ocean Levels tion of the storm, usually several hours. The path and propagation speed of storms dictate wind direction and Sea level in the Pacific Ocean is affected by ocean changes in barometric pressure, which in turn influence circulation, short-term climate variations, storms, and wind waves and high water. The strongest winds and gravitational and deformational effects of land ice hence the biggest waves along the west coast of the changes. Changes in ocean circulation affect regional United States are typically generated during winter sea level on seasonal to decadal and longer timescales storms. Large waves along the California coast are by redistributing ocean mass and altering seawater tem- also generated by tropical storms that reach the eastern perature and salinity patterns. These changes in ocean Pacific in summer and early fall.

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16 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON FIGURE 1.6 IPCC (2007) estimates of the primary contributions to global mean sea-level change for 1961 to 2003 (blue) and for 1993 to 2003 (brown), compared to the observed rate of global sea-level rise from tide gages and satellite altimetry. The bars represent the 90 percent error range. The relative contributions of these components has changed in recent years, as discussed in this report. SOURCE: Figure 5.21 from Bindoff et al. (2007). Finally, the large mass of glaciers and ice sheets last ice age, tectonics, compaction of sediments, and exerts an additional gravitational pull that draws ocean the removal or addition of fluids from underground water closer. As the ice melts, the gravitational pull de- reservoirs. During the last glacial maximum, the weight creases, ice melt is transferred to the ocean, and the land of the ice depressed the land under the ice mass. As and ocean basins deform in response to the loss of land the ice melted, the land beneath rose at rates up to ice mass. These gravitational and deformational effects 50100 mm yr-1 (e.g., Shaw et al., 2002), and the ocean create regional patterns of sea-level change. Modern floor subsided as ice melt was added to the ocean basins, melting of ice masses that are nearby (Alaska glaciers) exerting a considerable load (on the order of 100 t m-2 or large (Greenland and Antarctic ice sheets) has the for a sea-level rise of 100 m; Figure 1.7). These isostatic largest effect on sea level in the northeast Pacific Ocean, adjustments produced a characteristic pattern of sea- reducing the land ice contribution to local sea-level rise level change, with land uplift and relative sea-level fall on the order of tens of percent. The influx of fresh melt near the major ice centers, and relative sea-level rise water to the ocean also decreases seawater salinity and everywhere else. Box 1.2 illustrates the effect of glacial thus density near shore, which further contributes to isostatic adjustment on relative sea level along the west regional sea-level variations. coast of the United States over the past 18,000 years. The west coast of the United States is tectoni- Changes in Land Levels cally active, straddling three plate boundaries: the North American and Pacific plates, which slide past Regional and local land motion along the U.S. one another along the San Andreas Fault Zone in west coast is caused by the ongoing response of the California, and the Juan de Fuca plate, which subducts solid earth to a massive loss of ice at the end of the under the North American plate along the Cascadia

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INTRODUCTION 17 along the San Andreas Fault Zone have less impact on sea level because the primary motions are horizontal and much of the fault is further inland. Land subsidence resulting from sediment compac- tion and fluid (water, petroleum) withdrawal may cause relative sea level to rise. Compaction is particularly important in deltas and other coastal wetlands, where sediments have high water contents. Withdrawal of groundwater and petroleum increases the effective stresses in the surrounding sediments, resulting in consolidation and subsidence, which may be partially reversed by returning fluids to the subsurface. GEOGRAPHIC VARIATION ALONG THE U.S. WEST COAST How much coastal inundation can be expected with sea-level rise depends on the local geomorphology, which varies significantly along the west coast of the United States. The geomorphologic features along the coast are primarily the result of a collision between the North American and Pacific plates that began more than 100 million years ago and created steep coastal mountains, uplifted marine terraces, and sea cliffs. Over time, coastal lowlands developed, dominated by long sandy beaches, estuaries, and other wetlands. Most of the California coastline (72 percent or about 1,265 km) is characterized by steep, actively eroding sea cliffs, including about 1,040 km of relatively low-relief FIGURE 1.7 Response of the solid earth (brown) to the growth cliffs and bluffs, typically eroded into uplifted marine and melting of an ice sheet (blue) at increasing distances from terraces (Figure 1.9), and 225 km of high-relief cliffs the ice (A, B, and C). The addition of an ice sheet causes the and coastal mountains (Figure 1.10). The remain- land below it to subside and pushes (red arrow) a peripheral bulge outward. With deglaciation, the subsurface material flows ing 28 percent of the coastline is relatively flat and back toward the area formerly covered by ice until equilibrium comprises wide beaches, sand dunes, bays, estuaries, is again reached. SOURCE: Modified from Kemp et al. (2011). lagoons, and wetlands. The coast of Oregon is dominated by resistant volcanic headlands separated by areas of lower relief. Subduction Zone offshore Washington, Oregon, and The latter are characterized by uplifted marine ter- northernmost California (Figure 1.8). In subduction races, valleys where rivers emerge at the shoreline, and zones, strain builds within the fault zone, causing the associated estuaries, sand spits, beaches, and dunes. land to rise slowly before subsiding abruptly during a The most extensive sand spits occur along the north- great (magnitude greater than 8) earthquake. The last ern Oregon coastline. The longest continuous beach great earthquake in the region occurred in 1700, caus- extends about 96 km, from Coos Bay to Heceta Head, ing a sudden rise in relative sea level of up to 2 m due near Florence. The largest coastal dune complex in the to subsidence (Atwater et al., 2005). Since that event, United States backs this region (Figure 1.11). Many much of the coastline of northern California, Oregon, of the estuarine wetlands have been diked, primarily and Washington has been slowly rising. Land motions to provide pasturelands.

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18 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON BOX 1.2 Changes in Relative Sea Level Along the U.S. West Coast Since the Last Glacial Maximum During the last ice age, northern Washington was covered by the Laurentide Ice Sheet. When the ice sheet retreated, coastal areas that had been depressed under the weight of the ice sheet were flooded. Relative sea level peaked in that area about 17,000 years ago, reaching values of about 90 m above present in Anacotes (#1 in the Figure) and about 40 m above present in Seattle (#2). Subsequent glacio-isostatic uplift caused relative sea level to fall to its lowest levels about 12,000 years ago (about -40 m in Anacortes and -55 m in Seattle). Relative sea level then rose as ice meltwater was transferred to the oceans and the Laurentide Ice Sheet peripheral bulge began to collapse, causing coastal subsidence. Glacio-isostatic contributions were much lower in southern Washington, Oregon, and northern California (#3#9 in the Figure) than for northern Washington, but they were still a dominant influence on sea level. In this area, rates of relative sea-level rise slowed as the effects of glacio-isostatic subsidence decreased. In Eureka, California (#7), for example, relative sea level rose at an average rate of about 7.5 mm yr-1 between 10,000 and 6,000 years before present, then rose at a decreasing rate. 1. Anacortes, WA 220 90 -40 2. Seattle, WA 40 -10 -60 3. Long Beach, WA 10 -45 -100 4. Pacific City, OR 10 -20 Relative Sea Level (meters) -50 Index Point 5. Waldport, OR 10 Terrestrial Limiting -20 Marine Limiting -50 ICE-5G VM5a 6. Coos Bay, OR 10 ICE-6G VM5a -20 -50 7. Eureka, CA 10 -20 -50 8. Sacramento/San Joaquin Valley, CA 10 -20 -50 9. South San Francisco Bay, CA 10 -20 -50 18 12 6 0 Age (ka) FIGURE Reconstruction of changes in relative sea level over the past 18,000 years for nine locations in Washington, Oregon, and California. Green crosses (index points) represent former sea levels inferred from dated organic sediment in salt and fresh water marshes. Limiting data are from marine shells (blue crosses) and terrestrial peat (orange crosses) that must have been laid down below and above mean sea level, respectively. Red and black lines are model predictions (Peltier and Drummond, 2008; Argus and Peltier, 2010; Peltier, 2010). SOURCE: Data provided by Richard Peltier, University of Toronto.

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INTRODUCTION 19 FIGURE 1.8 Major tectonic features along the western United States. Subduction of the oceanic Juan de Fuca and Gorda plates beneath the North American Plate occurs along the Cascadia Subduction Zone, which extends more than 1,000 km from Mendocino, California, to Vancouver Island. South of Cape Mendocino, the North American and Pacific plates slide past one another along the San Andreas Fault Zone. The land west of the San Andreas Fault, from San Diego to Cape Mendocino, is moving northwest relative to the rest of North America. SOURCE: U.S. Geological Survey, . The shoreline of southern Washington is domi- ORGANIZATION OF THE REPORT nated by depositional landforms. Beaches, mostly backed by dunes, some developed, extend northward This report evaluates changes in sea level in the about 100 km from the mouth of the Columbia River global oceans and along the coasts of California, to the mountainous Olympic Peninsula (Figure 1.12). Oregon, and Washington for 2030, 2050, and 2100. The Long Beach Peninsula near the Columbia River Chapter 2 describes methods for measuring sea level and Grays Harbor include some of the most extensive and presents recent estimates of global sea-level rise. wetlands in Washington, outside of Puget Sound. Chapter 3 updates the IPCC (2007) estimates of the Some of these wetlands are being restored (e.g., Fig- major components of global sea-level change--thermal ure 1.13). Small coastal developments are present on expansion of ocean water, melting of glaciers and ice portions of the peninsula and on the low-lying coastal sheets, and transfers of water between land reser- areas to the north. voirs and the oceans. Chapter 4 assesses the factors that influence sea-level change along the U.S. west

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20 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON FIGURE 1.9 Uplifted marine terraces, Santa Cruz County, California. SOURCE: Copyright 20022012 Kenneth & Gabrielle Adelman, California Coastal Records Project, . FIGURE 1.10 Steep rocky cliffs of the Marin Headlands north of San Francisco, California. SOURCE: Copyright 20022012 K enneth & Gabrielle Adelman, California Coastal Records P roject, .

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INTRODUCTION 21 FIGURE 1.11 Oregon Dunes National Recreation Area. The largest coastal dune field in the United States has developed along the central Oregon coast and extends inland up to 3 km. SOURCE: Gary Griggs, University of California, Santa Cruz. FIGURE 1.12 Long Beach Peninsula, Washington. Sandy beaches backed by dunes dominate the southern coast of Washington. SOURCE: Courtesy of Phoebe Zarnetske, Oregon State University.

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22 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON FIGURE 1.13 Tidal wetlands along the mouth of the Nisqually River, Washington, are being restored following removal of a dike built a century ago to drain the area for cattle ranching. SOURCE: Courtesy of Carl Safina; photo taken for the PBS television series Saving the Ocean. coast, including regional changes in ocean circulation, Hawaii, appears in Appendix D. Chapter 5 summarizes climate-induced changes in storms, gravitational and recent projections of global and regional sea-level rise deformational effects of land ice change, and vertical and presents the committee's projections for 2030, land motions. It also summarizes the results of the 2050, and 2100. The method used to project the cryo- committee's analysis of tide gage and GPS records spheric component of global sea-level rise is described from the California, Oregon, and Washington coasts, in Appendix E. Chapter 5 also describes what rare, which is discussed in detail in Appendix A. Sea-level extreme events, such as a great earthquake along the data from the northeast Pacific Ocean is presented in Cascadia Subduction Zone, might mean for local sea- Appendix B. Data and uncertainties associated with level rise. Chapter 6 summarizes the literature on natu- the analysis of gravitational and deformational effects ral shoreline responses to and protection from sea-level of land ice change are given in Appendix C. The tide change. Biographical sketches of committee members gage and vertical land motion analyses draw on leveling are given in Appendix F, and a list of acronyms and data, and a description of leveling data compiled and abbreviations appears in Appendix G. analyzed for California by James Foster, University of