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Summary S ea level rose during the 20th century, and obser- rise in the global oceans and along the coasts of Cali- vations and projections suggest that it will rise fornia, Oregon, and Washington for 2030, 2050, and at a higher rate during the 21st century. Rising 2100. The charge to the committee is given in Box S.1. seas increase the risk of coastal flooding, storm surge The most comprehensive estimates of global sea- inundation, coastal erosion and shoreline retreat, and level rise are made by the Intergovernmental Panel on wetland loss. The cities and infrastructure that line Climate Change (IPCC), which assesses the state of many coasts are already vulnerable to damage from knowledge on climate change every 5 to 6 years. The storms, which is likely to increase as sea level continues last IPCC assessment, published in 2007, evaluated to rise and inundate areas further inland. research results published until mid-2006. This report Global mean sea level is rising primarily because summarizes the IPCC (2007) findings on global sea- global temperatures are rising, causing ocean water to level change and updates them with more recent results. expand and land ice to melt. However, sea-level rise is In contrast, no comprehensive assessments of the rate not uniform; it varies from place to place. Sea-level rise of sea-level rise off the coasts of California, Oregon, along the coasts of California, Oregon, and Washington and Washington have been carried out. Consequently, (referred to hereafter as the U.S. west coast) depends this report summarizes published research results on on the global mean sea-level rise and also on regional the processes that contribute to sea-level change in the factors, such as ocean and atmospheric circulation pat- region and also presents the committee's analysis of terns in the northern Pacific Ocean, the gravitational relevant data and model results. Projections of global and deformational effects of land ice mass changes, and and local sea-level rise for 2030, 2050, and 2100 are tectonics along the coast. The comparative importance based on model results and data extrapolations, as of these factors determines whether local sea level is described below. higher or lower than the global mean, and how fast it is changing. Such information has enormous implica- GLOBAL SEA-LEVEL RISE tions for coastal planning. California Executive Order S-13-08 directed state Following a few thousand years of relative stabil- agencies to plan for sea-level rise and coastal impacts, ity, global sea level has been rising since the late 19th and it also requested the National Research Council or early 20th century, when global temperatures began (NRC) to establish a committee to assess sea-level rise to increase. The IPCC (2007) estimated that global to inform these state efforts. The states of Washington sea level rose an average of 1.7 ± 0.5 mm per year and Oregon, the U.S. Army Corps of Engineers, the over the 20th century, based on tide gage measure- National Oceanic and Atmospheric Administration, ments from around the world. Rates for 19932003 and the U.S. Geological Survey subsequently joined were 3.1 ± 0.7 mm per year, based on precise satellite California in sponsoring this study to evaluate sea-level altimetry measurements and confirmed by tide gage 1
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2 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON BOX S.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 asso ciated 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 Niño-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. records. More recent tide gage and altimetry data con- quantified but were thought to account for less than firm that the higher rate of sea-level rise is continuing. 10 percent of the observed rise. More recent data have However, because of natural climate variability, which changed these estimates. After the IPCC (2007) report affects sea level on decadal and longer timescales, more was published, a bias was discovered in some ocean data are needed to determine whether the higher rates temperature measurements, which gave systematically since the 1990s mark an acceleration in the long-term warmer temperatures than the true values. Data sets sea-level trend. corrected for this bias yield significantly lower rates of thermal expansion for the 19932003 period than were Components of Global Sea-Level Rise found by the IPCC (2007). New research results also indicate that the relative A warming climate causes global sea level to rise contribution of land ice to global sea-level rise is in- by (1) warming the oceans, which causes sea water to creasing. Since 2006, the ice loss rate from the Green- expand, increasing ocean volume, and (2) melting land land Ice Sheet has increased, and, according to most ice, which transfers water to the ocean. Human ac- analyses, the contribution of Antarctic ice to sea-level tivities that transfer water between the land and ocean change has shifted from negative (lowering sea level by also affect global sea-level change. In particular, water accumulating ice) to positive (raising sea level). Ice loss withdrawn from aquifers eventually reaches the ocean, rates from glaciers and ice caps have declined over the raising global sea level, whereas water stored behind same period, but not enough to offset the increases in dams effectively lowers global sea level. ice sheet melt. As a result of higher observed ice loss The IPCC (2007) estimated that ice melt from rates and a lower (corrected) contribution from thermal glaciers, ice caps, and ice sheets contributed about expansion, land ice is currently the largest contributor 40 percent of the observed sea-level rise for 19612003 to global sea-level rise. In the most recent published and that thermal expansion of ocean water contributed estimate, land ice accounted for about 65 percent of one-quarter of the observed rate for 19612003 and the total sea-level rise from 1993 to 2008. one-half for 19932003. Contributions from ground- The contributions of groundwater withdrawal and water extraction and reservoir storage were poorly reservoir storage to sea-level change remain poorly
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SUMMARY 3 constrained, largely due to sparse data and inadequate Factors That Affect Land Elevation in California, models. Each process likely has a significant but oppo- Oregon, and Washington site effect on sea-level change, on the order of 0.5 mm per year. Although modern melting of land ice has a sig- nificant effect on sea-surface heights in the northeast Pacific Ocean, the melting and eventual disappearance SEA-LEVEL RISE OFF CALIFORNIA, of North American ice sheets that began more than OREGON, AND WASHINGTON 20,000 years ago has a significant effect on land levels The sea level at any particular place along the coast in California, Oregon, and Washington. The massive is commonly measured using tide gages, which record loss of ice from the ancient ice sheets continues to the height of the sea surface with respect to the land cause uplift of about 1 mm per year in northernmost surface, both of which may change over time. Relative Washington, which had been covered by an ice sheet, sea level will rise if ocean levels rise and/or land levels and subsidence of about 12 mm per year in areas at fall. Records from 12 west coast tide gages indicate local the ice margin and beyond, which includes the rest of variability in sea-level change along the coast, although Washington, Oregon, and California. most of the gages north of Cape Mendocino, C alifornia, Tectonics causes substantial regional uplift along show that relative sea level has been falling over the past much of the Washington, Oregon, and northernmost 610 decades, and most of the gages south of Cape California coast, where ocean plates are descending Mendocino show that relative sea level has been rising. below North America at the Cascadia Subduction Zone. South of Cape Mendocino, California, the Pacific and North American plates are sliding past one Factors That Affect Northeast Pacific Ocean Levels another along the San Andreas Fault Zone, creating Along the west coast of the United States, climate relatively little vertical land motion along the coast. patterns such as the El Niño-Southern Oscillation and, Local tectonics, as well as compaction of sediments, to a lesser extent, the Pacific Decadal Oscillation, af- pumping of water or hydrocarbons from subsurface fect winds and ocean circulation, raising local sea level reservoirs, and fluid recharge can produce locally high during warm phases (e.g., El Niño) and lowering sea rates of land subsidence or uplift. Water or hydrocarbon level during cool phases (e.g., La Niña). Large El Niño extraction, which can lower surface elevations up to events can raise coastal sea levels by 10 to 30 cm for tens of centimeters per year if fluids are not returned to several winter months. the subsurface, is most important in California. The large mass of glaciers and ice sheets exerts The total vertical land motion from all of these a gravitational pull that draws ocean water closer. geological processes and human activities can be es- As the ice melts, the gravitational pull decreases, ice timated from Global Positioning System (GPS) mea- melt enters the ocean, and the land and ocean basins surements, which show that much of the coast is rising both deform as a result of this loss of land ice mass. about 1.53.0 mm per year north of Cape Mendocino. These gravitational and deformational effects produce The coast south of Cape Mendocino is sinking at an a spatial pattern of regional sea-level change called a average rate of about 1 mm per year, although GPS- sea-level fingerprint. Melting from Alaska and, to a measured rates vary widely (-3.70.6 mm per year). lesser extent, Greenland, causes relative sea level to fall at decreasing rates from northern Washington to PROJECTIONS OF SEA-LEVEL RISE FOR southern California, whereas melting from Antarctica 2030, 2050, AND 2100 causes relative sea level to rise along all three states. The net effect is a reduction in the contribution of the Global Projections three ice sources to relative sea-level rise by 42 percent along the north coast (Neah Bay), 24 percent along the Projections of global sea-level rise are generally central coast (Eureka), and 14 percent along the south made using models of the ocean-atmosphere-climate coast (Santa Barbara) for 19922008. system, extrapolations, or semi-empirical methods.
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4 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON Ocean-atmosphere models are based on knowledge which affects the steric component; and any future of the physical processes that contribute to sea-level changes in the rate of ice flow, which affects the total rise, and they predict the response of those processes ice contribution. These uncertainties, and hence the to different scenarios of future greenhouse gas emis- ranges, grow with the length of the projection period. sions. These models provide a reasonable estimate of The committee's global projections for 2030 and the water density (steric) component of sea-level rise 2050 are similar to the Vermeer and Rahmstorf (2009) (primarily thermal expansion), but they underestimate projections for the same periods, but they have a wider the land ice contribution because they do not fully ac- range. For 2100, when IPCC (2007) projections are count for rapid changes in the behavior of ice sheets and also available, the committee's projection is substan- glaciers as melting occurs (ice dynamics). The IPCC tially higher than IPCC's projection (1859 cm with (2007) projections were made using this method, and an additional 17 cm if rapid dynamical changes in ice they are likely too low, even with an added ice dynamics flow are included), mainly because of a faster growing component. Estimates of the total land ice contribution cryosphere component, and lower than Vermeer and can be made by extrapolating observations of recent ice Rahmstorf 's projection (78175 cm). loss rates from glaciers, ice caps, and ice sheets into the future. Extrapolations of future ice melt are most reli- Projections for California, Oregon, and able for time frames in which the dynamics controlling Washington behavior are stable, in this case, up to several decades. Semi-empirical methods, exemplified by Vermeer and Sea-level rise off the west coast of the United Rahmstorf (2009), avoid the difficulty of estimating States is influenced by a variety of local factors; there- the individual contributions to sea-level rise by simply fore, sea-level projections for California, Oregon, postulating that sea level rises faster as the Earth gets and Washington differ from global projections. The warmer. This approach reproduces the sea-level rise ob- factors that affect local sea-level projections include served in the past, but reaching the highest projections steric variations; wind-driven differences in ocean would require acceleration of glaciological processes to heights; gravitational and deformational effects (sea- levels not previously observed or understood as realistic. level finger prints) of melting of ice from Alaska, Given the strengths and weaknesses of the different Greenland, and Antarctica; and vertical land motions projection methods, as well as the resource constraints along the coast. The local steric and wind-driven com- of an NRC study, the committee chose a combination ponents were estimated by extracting northeast Pacific of approaches for its projections. The committee pro- data from the same ocean models used for the global jected the steric component of sea-level rise using out- projections. The cryosphere component was adjusted put from global ocean models under an IPCC (2007) for gravitational and deformational effects and then mid-range greenhouse gas emission scenario. The land extrapolated forward. Finally, vertical land motion was ice component was extrapolated using the best avail- projected using continuous GPS measurements for two able compilations of ice mass accumulation and loss tectonically distinct areas: Cascadia, where the coastline (mass balance), which extend from 1960 to 2005 for is generally rising, and the San Andreas region, where glaciers and ice caps, and from 1992 to 2010 for the the coastline is generally subsiding. Greenland and Antarctic ice sheets. The contributions The projections for California, Oregon, and were then summed. The committee did not project the Washington are illustrated in Figure S.1. The steep land hydrology contribution because available estimates change in projected sea-level rise at Cape Mendocino suggested that the sum of groundwater extraction and reflects the transition from land subsidence in California, reservoir storage is near zero, within large uncertainties. which effectively increases sea-level rise, to land uplift Based on these calculations, the committee esti- in Oregon and Washington, which effectively decreases mates that global sea level will rise 823 cm by 2030 sea-level rise. The slight slope in the projection curves relative to 2000, 1848 cm by 2050, and 50140 cm by from north to south reflects the sea-level fingerprints, 2100. The ranges reflect uncertainties related to the fit which lower relative sea level, especially along the Wash- of the data; the level of future greenhouse emissions, ington coast. For the California coast south of Cape
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SUMMARY 5 -124° -120° -116° 2030 2050 48° 2100 48° Seattle 48° Cascadia Subduction Zone Newport 44° 44° 40° 40° MTJ Sa San nA Francisco nd re as 36° Fa 36° ul t Los Angeles 32° 32° 0 50 100 150 cumulative SLR (cm) FIGURE S.1 Projected sea-level rise off California, Oregon, and Washington for 2030 (blue), 2050 (green), and 2100 (pink), relative to 2000, as a function of latitude. Solid lines are the projections, and shaded areas are the ranges. Ranges overlap, as indicated by the brown shading (low end of 2100 range and high end of 2050 range) and blue-green shading (low end of 2050 range and high end of 2030 range). MTJ = Mendocino Triple Junction, where the San Andreas Fault meets the Cascadia Subduction Zone.
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6 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON Mendocino, the committee projects that sea level will 2100, however, projections made using process-based rise 430 cm by 2030 relative to 2000, 1261 cm by 2050, numerical models, extrapolations, and semi-empirical and 42167 cm by 2100. For the Washington, Oregon, methods all have large uncertainties. The actual sea- and California coasts north of Cape Mendocino, sea level rise will almost surely fall somewhere within the level is projected to change between -4 cm (sea-level fall) wide uncertainty bounds, although the exact value can- and +23 cm by 2030, -3 cm and +48 cm by 2050, and not be specified with high confidence. 10143 cm by 2100. Major sources of uncertainty in the regional projections are related to assumptions about fu- SEA-LEVEL RISE AND STORMINESS ture ice losses and a constant rate of vertical land motion over the projection period. Uncertainties are larger for Most of the damage along the California, Oregon, the regional projections than for the global projections and Washington coasts is caused by storms--particularly because more components are considered and because the confluence of large waves, storm surges, and high uncertainties in the steric and ocean dynamic compo- astronomical tides during a strong El Niño. The water nents are larger at a regional scale than at a global scale. levels reached during these large, short-term events The combination of land uplift and gravitational have exceeded mean sea levels projected for 2100, so and deformational effects reduces the threat of future understanding their additive effects is crucial for coastal sea-level rise for Washington and Oregon. However, planning. the land is rising along the Washington and Oregon coasts likely because interseismic strain is building in Changes in Storm Frequency and Magnitude the Cascadia Subduction Zone. A great earthquake (magnitude larger than 8), which has occurred in the Climate change has been postulated to induce area every few hundred to 1,000 years, would cause changes in storm frequency, magnitude, and direction. some coastal areas to immediately subside and rela- To date, there is no consensus among climate model tive sea level to suddenly rise. If this occurs, relative simulations about whether the number and sever- sea level could rise an additional meter or more over ity of storms will change in the northeast Pacific. A projected levels. number of climate models predict a northward shift The committee's projections for the California in the North Pacific storm track over the course of the coast are slightly higher than its global projections, pri- 21st century, which could lessen the impact of winter marily because much of the coastline is subsiding. The storms in southern California and possibly increase California projections are somewhat lower but have their impact in Oregon and Washington. However, wider ranges than the Vermeer and Rahmstorf (2009) these changes may not emerge for a few decades, and global projections, which are being used by California most observational records are not yet long enough to on an interim basis for coastal planning. determine conclusively whether storm tracks are mov- The projections of future sea-level rise have large ing north. uncertainties resulting from an incomplete understand- Several observational studies have reported that ing of the global climate system, the inability of global the largest waves have been getting higher and that climate models to accurately represent all important winds have been getting stronger in the northeastern components of the climate system at global or regional Pacific over the past few decades. Interpretation of scales, a shortage of data at the temporal and spatial these trends is controversial because wave and wind scales necessary to constrain the models, and the need to records are short, extending back only about 35 years. make assumptions about future conditions (e.g., green- At least part of the observed increase likely reflects house gas emissions, large volcanic eruptions) that drive natural climate variability of the Pacific atmosphere- the climate system. As the projection period lengthens, ocean system, particularly the occurrence of large El uncertainty in the projections grows. At short timescales Niños and interdecadal fluctuations. If some or all of (2030 and perhaps 2050), when the models more closely the increase represents a long-term trend, the frequency represent the future climate system, confidence in the and magnitude of extremely high coastal wave events global and regional projections is relatively high. By will likely increase.
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SUMMARY 7 Even if storminess does not increase in the future, ther increase if waves become higher. Although seawalls sea-level rise will magnify the adverse impact of storm and revetments can make the shoreline more resistant surges and high waves on the coast. For example, a to wave attack, they prevent beaches from migrat ing model using the committee's sea-level projections pre- landward and will eventually be overwhelmed by sea- dicts that the incidence of extreme high water events level rise. (1.4 m above historical mean sea level) in the San Marshes and mudflats protect inland areas by stor- Francisco Bay area will increase substantially with sea- ing flood waters and damping wave height and energy. level rise, from less than 10 hours per decade today to To continue providing these services as sea level rises, a few hundred hours per decade by 2050 and to several marshes must be able to maintain their elevation rela- thousand hours per decade by 2100. tive to sea level and to move inland in places where they are subject to erosion at the seaward edge. Building Coastal Responses to Sea-Level Rise and elevation requires a sufficient supply of sediment and Storminess accumulation of organic material. Most studies of west coast marshes have focused on the supply of sediment. The natural shoreline can provide partial protec- The frequent storms and associated floods in central tion for coastal development against sea-level rise and and southern California potentially provide enough storms. Coastal cliffs, beaches, and dunes take the sediment for marshes to keep pace with the sea-level brunt of storm waves and are therefore eroding over rise projected for 2030 and 2050 by the committee. In the long term. The net result of storms and sea-level Oregon and Washington, rivers also potentially carry rise is coastline retreat, with rates ranging from a few enough sediment for marshes to maintain elevation, de- centimeters per year for cliffs made of resistant bedrock spite upstream dams, especially because the projections to several meters per year for beaches and dunes, which of sea-level rise are lower. For 2100, marshes will need consist primarily of unconsolidated sand. These rates room to migrate, a high sediment supply, and uplift or will increase with rising sea level and are likely to fur- low subsidence to survive the projected sea-level rise.
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