Fifth National Climate Assessment
9. Coastal Effects

Accelerating Sea Level Rise

Global mean sea level is rising at an accelerated rate, with the average rate of about 0.05 ± 0.01 inches per year (1.2 ± 0.2 mm per year) over the pre-satellite era (1901–1990)¹⁶ nearly tripling to 0.13 ± 0.02 inches per year (3.4 ± 0.4 mm per year) during the 30-year satellite era (1993–2022)¹⁷ due to thermal expansion from warming waters and the growing contribution from melting glaciers and ice sheets.¹⁸ Global SLR rates further accelerated to 0.17 inches per year (4.4 mm per year) over the last decade (2013–2022), although this acceleration may include components of natural variability due to the short time period.¹⁹

To help communities plan for an uncertain future, the US Interagency Sea Level Rise Task Force established five future SLR scenarios that span the range of plausible SLR amounts by 2100 using the latest scientific consensus from the Intergovernmental Panel on Climate Change (IPCC) and other scientific bodies.² The five SLR scenarios represent the range on a global scale, with projected SLR amounts in 2100 and scenarios defined as follows:

• Low, 1 foot (0.3 m) rise in global mean sea level relative to year 2000 baseline

• Intermediate-Low, 1.6 feet (0.5 m)

• Intermediate, 3.3 feet (1.0 m)

• Intermediate-High, 4.9 feet (1.5 m); and

• High, 6.6 feet (2.0 m) (Figure 9.1)

The SLR scenarios are downscaled to local and regional levels, considering future changes in land elevation, ocean heating and circulation, and Earth’s gravitation and rotation from melting of land-based ice. They are constructed directly from the IPCC Sixth Assessment Report (AR6) emissions- and temperature-based projections (App. 3.3)²⁰ but use consistent framing (e.g., Sweet et al. 2017²¹) to support risk reduction planning.

Sea levels are rising along contiguous US coastlines faster than the global average, with about 11 inches (28 cm; likely range of 10–12 inches [25–30 cm]) occurring over the last 100 years (1920–2020) and with about half of this rise (5–6 inches [13–15 cm]) occurring in the last 30 years (Figure 9.1).² SLR rates vary across different regions. In the last 30 years, the greatest rise is observed along the US western Gulf Coast (about 9 inches [23 cm]), largely due to high rates of land subsidence²² from subsurface groundwater and fossil fuel withdrawal.²³ About 6 inches (15 cm) of rise is observed along the northeast and southeast Atlantic and eastern Gulf Coasts. Lower rates of rise are observed along the Hawaiian and US Caribbean island coastlines (4 inches [10 cm]) and the northwest (2 inches [5 cm]) and southwest (3 inches [8 cm]) Pacific coastlines.²

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Sea level is projected to continue to increase this century by amounts related to future global warming levels.

Figure 9.1. This figure shows accelerating sea level rise (SLR) trends and SLR scenarios along the contiguous US coastline. It also shows the relationship between projected SLR under different global surface temperature increases in 2100 (KM 2.2). The left panel shows observed increasing average sea levels during 1920–2020 (solid black line), an extrapolation out to 2050 based on observed sea levels over 1970–2020 (dashed black line), a range of scenarios describing plausible sea level rise out to 2150 (multicolored lines), and an overlapping stacked bar showing a range of projected changes in 2100 SLR under different levels of global surface temperature increase, based on the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. The right panel shows expanded versions of the projections shown in the stacked bar in the left panel. Black lines indicate the median value, the bars show the extent of the likely range (17th–83rd percentile) of SLR by 2100, and the associated warming levels are indicated above each bar. The “High warming, low confidence” case (yellow bar) refers to the potential range of rising seas under higher temperatures with rapid ice melt. The lack of overlap in 2100 between the High sea level scenario and the “High warming, low confidence” case in 2100 is not an indication of overestimation but rather a result of how the low-confidence processes are analyzed. Adapted from Sweet et al. 2022.²

SLR rate suppression and acceleration along the northwest and southwest Pacific coastlines is in part due to oceanographic forcings associated with the El Niño–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO).²⁴ Along the Pacific Coast, ENSO and PDO will continue to drive decadal variability in SLR, with rates that are above or below the global average.²⁴ The current rate remains higher than the global average.²⁴ Characterizing past (and future) rise for Alaska and the US-Affiliated Pacific Islands is complicated due to tectonic effects that cause both uplift and subsidence. Year-to-year changes associated with natural variability can also change the rates over different analysis periods, such as the 8–12-inch (20–30 cm) variability in sea levels that can occur along the Pacific Coast during different phases of the ENSO.^25,26,27

Looking toward the future, an 11-inch (28 cm) average rise along the contiguous US coastline is expected by 2050 (relative to 2020, with a likely range of 9–13 inches [23–33 cm]; see Table A1.2 in Sweet et al. 2022² for 2000 to 2020 offsets) based on an observation-based trajectory of SLR (Figure 9.1). An 11-inch (28 cm) rise by 2050 matches the observed average SLR along the contiguous US coastline over the last 100 years (1920–2020), representing ongoing SLR acceleration that falls between the Intermediate-Low and Intermediate sea level scenarios.² By 2050, SLR amounts will continue to vary geographically, with regional differences like those observed in the recent historic record (e.g., 1990–2020). For example, under the Intermediate sea level scenario, which closely aligns with most regional SLR trajectories,² SLR is expected to be higher along the Atlantic versus the Pacific Coast and greatest along the western Gulf Coast (Figure 9.2).

Beyond 2050, future global emissions and resultant ocean and atmospheric warming and ice sheet responses will determine future SLR. As of 2021, global temperatures have increased by 2° –2.2°F (1.1°–1.2°C) beyond preindustrial levels (KM 2.1) and are headed for a warming level of about 5.4°F (3°C) by 2100 under the current trajectory,²⁸ which is consistent with the IPCC AR6 intermediate and high scenarios (SSP2-4.5 and SSP3-7.0). With such warming, it is likely that the Intermediate-Low sea level scenario with 2+ feet (0.6+ m) of SLR relative to 2020 levels will be exceeded by 2100, and 3.6+ feet (1.1+ m) will be exceeded by 2150 (Figure 9.1).²

Failing to curb future emissions increases the probability of SLR equivalent to the Intermediate sea level scenario or perhaps even higher, such as the Intermediate-High and High sea level scenarios associated with the IPCC very high scenario (SSP5-8.5) that includes the addition of rapid ice sheet melt or disintegration during this century.²⁰ The probability of this low-likelihood outcome increases with higher global warming levels.²⁹ Under the Intermediate to High sea level scenarios, an average SLR of about 3.6–6.9 feet (1.1–2.1 m) along contiguous US coastlines by 2100 and 6.9–12.5 feet (2.1–3.8 m) by 2150 relative to 2020 would occur (Figure 9.1; App. 3.3).² Under the Intermediate-High and High sea level scenarios, contributions from the Antarctic ice sheet dominate and reduce overall SLR differences across US regions (KMs 2.1, 2.3).² Beyond 2150, global (and US) SLR will continue for millennia due to the long-term effects of warming this century. About 7–33 feet (2–10 meters) of global SLR over the next 2,000 years is likely if temperatures warm by 3.6° to 5.4°F (2° to 3°C) above preindustrial levels by 2100, similar to conditions about 125,000 years ago.^20,30

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By 2050 and 2100 under the Intermediate sea level scenario, sea level rise is projected to be higher along the Atlantic versus the Pacific Coast and greatest along the western Gulf Coast.

Figure 9.2. The figure shows relative sea level rise along the US coastlines under the Intermediate sea level scenario of the US Interagency Sea Level Rise Task Force² for 2050 (left) and 2100 (right). Relative sea level rise for the contiguous US is shown on the top, and for Alaska, Hawai‘i (left insets), and Puerto Rico (right insets) on the bottom. The black dots along the coastline indicate tide-gauge locations used to characterize past SLR. Characterizing past (and future) SLR for Alaska and the US-Affiliated Pacific Islands is complicated due to tectonic effects that cause both uplift and subsidence. Figure credit: NOAA National Ocean Service.

Increases in Flooding Frequency Will Continue

SLR will continue to cause permanent inundation for formerly dry lands and an escalation in the severity (depth, geographic extent, and frequency) of coastal flooding, ranging from powerful storm events to more frequent high tide flooding (HTF). As of 2020, the highest annual frequencies of coastal flooding—defined in a nationally consistent manner as minor (disruptive HTF, about 1.75–2 feet [0.5–0.6 m] above average high tide), moderate (damaging HTF, about 2.75–3 feet [0.8–0.9 m]), and major (destructive HTF, about 4 feet [1.2 m]) impacts^2,31—are along the northeast Atlantic and western Gulf coastlines (Figure 9.3), due in part to greater exposure to strong storms and wide, shallow continental shelves allowing for higher storm surges.³²

Annual frequencies of both minor and moderate coastal flooding increased by a factor of 2–3 along most Atlantic and Gulf coastlines between 1990 and 2020 (Figure 9.3). Minor HTF events, which are the most common impact of SLR, occur several times a year with accelerating frequencies (e.g., Sweet et al. 2019, 2020, 2021^33,34,35). A typical HTF event lasts about two days and several high tides.² Along the coastlines of Hawai‘i, the US-Affiliated Pacific Islands, and US Caribbean islands, as well as some US Pacific coastlines, SLR is a growing problem. Flood impacts are occurring with much smaller flood heights than those shown in Figure 9.3, including in some cases where water levels are elevated only slightly above high tide.

By 2050 under the Intermediate sea level scenario (Figure 9.1), minor, moderate, and major coastal flood frequencies will all increase by a factor of about 5–10 in many regions relative to 2020 in the absence of adaptation (Figure 9.3). In effect, a flood regime shift would occur; for example, the frequencies of moderate flooding are projected to occur as often as minor, disruptive HTF occurs now (circa 2020). By 2100 under the Intermediate sea level scenario, major flooding would occur almost daily along US coastlines.² These increases in flood frequency could be further amplified with higher amounts of SLR, worsening storm conditions, natural climatic variability (e.g., ENSO), or other reasons such as long-term tidal cycles and land subsidence or uplift.^31,36

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Minor, moderate, and major coastal flood frequencies will increase by a factor of about 5–10 in many regions of the US relative to 2020 in the absence of adaptation.

Figure 9.3. (top) Descriptions of three coastal flood types—minor (disruptive), moderate (damaging), and major (destructive)—provided by NOAA National Weather Service, reflect today’s vulnerabilities within coastal communities. (bottom) These graphs show annual average frequencies of minor, moderate, and major flooding by region (multicolored bars). The flood frequencies are based on a set of 187 NOAA tide gauges, with 14 in Hawai‘i and the US-Affiliated Pacific Islands, 4 in Puerto Rico, and 4 in the US Virgin Islands (collectively shown as Islands). Note that this figure uses the US regions defined in Sweet et al. (2022),² which differ from the NCA5 regions: NE = northeast Atlantic, SE = southeast Atlantic, NW = northwest Pacific, SW = southwest Pacific, E. Gulf = eastern Gulf, W. Gulf = western Gulf. Observations are shown for 1990 and 2020, and projections are shown for 2050 under the Intermediate sea level scenario for all US coastal regions. The amount of SLR for each region during 1990–2020 and 2020–2050 under the Intermediate scenario is provided below the graph. The amount of SLR for Alaska is not shown because Alaska’s SLR varies along the shoreline due to both tectonic uplift and subsidence; the SLR values for Alaska are shown on Figure 9.2. The annual average frequencies of minor, moderate, and major flooding are projected to increase more in the next 30 years (2020–2050) than they did in the past 30 years (1990–2020) regardless of any future worsening of storm events. In some Atlantic and Gulf Coast regions exposed to hurricanes, more severe and catastrophic coastal flood levels are possible and will become more likely as sea levels rise. Figure credit: NOAA National Ocean Service. Photo credits: (left) City of Norfolk Staff Photographer Andrew Cooper; (center and right) Jeff Orrock, NOAA.

Waves, Storminess, and Landscape Variability Amplify Flood Risk

Climate-driven changes to coastal water levels, including waves, storm surge, river flows, and landscape changes, are important considerations when planning for future flood risk.^37,38,39 Wave-driven water levels, for example, comprise 25%–90% of extreme coastal water levels along exposed US coastlines.^2,40,41 Across most US coasts, many extreme events are increasing in intensity, frequency, and geographic extent (KM 2.2) because of human-caused climate change (KM 3.1). For example, hurricanes are intensifying more rapidly and decaying more slowly, leading to stronger storms extending farther inland with heavier rainfall and higher storm surges, resulting in less time for communities to prepare (KM 2.2). Climate change is also increasing coastal hazards through changes in the frequency, magnitude, and impacts of compound events (Figure 9.3; Focus on Compound Events).^42,43 In the coastal zone, compound flood events are commonly due to the joint occurrence of heavy precipitation, high river flows, elevated groundwater levels, soil saturation, and elevated ocean water levels.^38,44,45

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On the coast, natural landscapes are intertwined with the cultures, economies, and built infrastructure of humans (Figure 9.4). Coastal landscapes (e.g., beaches, dunes, barrier systems, coastal wetlands, and cliffs) evolve across a range of timescales (from minutes to millennia) in response to physical forcing (e.g., tides, waves, storms, climate variability), as well as biological (e.g., vegetation type and density, ecosystem characteristics) and geological (e.g., sediment flows, tectonics, substrate composition) controls.⁴⁶ Climate change is exacerbating coastal hazards, with rising seas and more intense storms leading to increases in both flood risks and shoreline change and erosion (KM 9.1).^47,48,49,50

Coastal communities face heatwaves, heavy rainfall, landslides, compound flooding, and other climate hazards that are not unique to coastal environments.¹⁴ The health, function, and productivity of coastal ecosystems are also being degraded by stressors from human actions (e.g., development, dredging, wetland infill, sediment diversions). Combined, these threats jeopardize attachment to place,⁵¹ economies, and safety (Figure 9.5).^52,53 Understanding the interactions and interconnections between hazards (KM 9.1), communities, and coastal ecosystems is necessary for taking informed action to mitigate and adapt to climate change (KM 9.3).

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Coastal landscapes and man-made interventions provide economic, cultural, and community protection from existing climate hazards under existing conditions.

Figure 9.4. This hypothetical coastal community shows some of the natural and built environments found in our Nation’s actual coastal communities. This community has several types of open coast shorelines, including 1) cliffs, 2) low-lying beaches, and 3) a barrier island. Behind the barrier island is 4) a tidal estuary fringed with marshes. There are two residential neighborhoods, 5) a commercial hub, and 6) an industrial zone that is water dependent. The riprap at the cliff base and beach groins (narrow perpendicular structures extending from the beach into the ocean) provide protection from coastal erosion due to waves. Subsequent figures in this chapter will illustrate the possible impacts of climate change on this community (Figure 9.5) and adaptation strategies to increase its resilience (Figure 9.6). Adapted from Dupigny-Giroux et al. 2018.⁵⁴

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Coastal communities are expected to flood due to rising sea levels and rising groundwater levels.

Figure 9.5. Future climate impacts to our coastal landscapes and communities will be variable. 1) Increased sea level and wave energy will result in shoreline erosion and the collapse of coastal cliffs, which can alter iconic landscapes and damage places of value, including historical or cultural sites. 2) Flooding and wave hazards, including 3) flooding from higher groundwater tables, are anticipated, threatening homes and businesses as well as infrastructure and utilities. 4) Some ecosystems may be able to adapt or migrate to keep pace with future sea levels, and others may become inundated and converted to open water. Adapted from Dupigny-Giroux et al. 2018.⁵⁴

Impacts on Communities and People

Our Nation’s coasts underpin substantial sectors of our economy, serving as the entry and exit for goods and services (Focus on Risks to Supply Chains), generating revenue through recreation and tourism, and supporting thriving and diverse fisheries and other water-based industries. Coastal counties contribute $11 trillion annually (in 2022 dollars) in goods and services and employ 58.3 million people.⁸ Increasing impacts to coastal systems due to exacerbating hazards will ripple across the US.

As extreme storms intensify and/or the impacts are exacerbated by SLR (KMs 2.2, 3.5), damages are increasing by the billions, with significant damage centered where tropical cyclones (e.g., hurricanes) make landfall⁵⁵ and where extratropical cyclones are the more common driver of coastal hazards.^48,56 Extreme storms and more frequent high tide flooding (HTF) bring cascading impacts, including loss of energy accessibility and continuity (KM 5.2); loss of ecosystem services (KMs 8.1, 8.3); impacts to agriculture from flooding and saltwater intrusion into groundwater (KM 30.1); flooding, erosion, and landslide disruptions to transportation (KM 13.1), utilities, infrastructure, emergency services, and teleconnections (KM 12.2); and population migration and displacement (KM 20.3).

Art × Climate

Xavier Cortada
Elevation Drive: 7 Feet Above Sea Level
(2018, water-based paint on asphalt)

Artist’s statement: This street intersection mural in Pinecrest, Florida depicts the location’s elevation above sea level. Painted intersections, coupled with Underwater Markers (elevation yard signs) that residents place in their front yards, make the issue of climate change impossible to ignore. Mapping the topography of their community, neighbors reveal an alarming reality: declining property values, increased flood insurance costs, failing septic tanks, compromised infrastructure, climate gentrification, and collapsing ecosystems. The socially engaged art work is aimed at revealing the vulnerability of coastal communities to rising seas, sparking climate conversations, and catalyzing civic engagement.

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Artworks and artists’ statements are not official Assessment products.

Coastal hazard assessments that consider SLR, storm surge, waves, rainfall, and coastal change (e.g., beach and dune change, cliff change) can better depict potential future coastal response and societal impacts.^37,57,58 Compared to assessing only SLR-driven flooding, including these processes greatly expands the floodplain region in the northern Gulf of Mexico⁵⁸ and triples the estimated number of people on the Pacific Coast exposed to flooding.³⁷

During an extreme event, more ocean water can wash over barrier islands and flow into bays via inlets, enhancing flood risk and amplifying storm surge within inland coastal bays by more than 20%.^59,60,61 Continued population growth and urbanization will expose an ever-increasing number of people to coastal flood risks.^3,62,63,64

Although extreme storm events make newspaper headlines, SLR brings chronic challenges that could be equally or more damaging over the long term.² Coastal groundwater investigations in Pacific Island settings,⁶⁵ low-lying atolls,⁶⁶ karst aquifers,⁶⁷ barrier island systems,⁶⁸ and active tectonic margins^69,70 have demonstrated that climate-driven groundwater rise will impact coastal communities and ecosystems due to saltwater intrusion into groundwater sources, more saturated soils, and ponding at the surface comparable in magnitude to SLR-driven overland flooding. Seawater intrusion into coastal aquifers can increase salinity beyond potable levels, endangering access to fresh water for millions of people.⁷¹

The combination of rising groundwater and HTF in coastal communities will continue to impact stormwater and wastewater infrastructure, including septic systems, and increase the occurrence of urban flooding.^{72,73,74,75,76} This could cause public health concerns, such as pollutant discharges into the environment⁷⁷ and the spread of environmental infectious diseases (KM 15.1). Additionally, contaminated sites, such as Superfund sites, face increasing exposure to rising groundwater and flood damages, which could lead to future public health and environmental concerns if buried contaminants are mobilized and enter groundwater or river systems (KM 28.2). HTF and rising groundwater will also increase occurrences of roadway flooding, potentially impeding traffic, delaying emergency response efforts, flooding properties, and negatively impacting real estate values and commerce.^{78,79,80,81,82} In agricultural areas, rising groundwater and saltwater intrusion in irrigation systems are reducing crop productivity, resulting in barren farmlands in the absence of salt-tolerant crops.^83,84

The impacts of worsening coastal hazards are not equally distributed across US communities (KM 20.1; Box 20.1).^85,86,87 Disparities in wealth, economic and educational opportunities, infrastructure quality and quantity, and investment in flood risk-reduction measures all contribute to variable physical and socioeconomic impacts on coastal residents.^88,89,90 Many Tribal and Indigenous communities face severe impacts from extreme storms, erosion, permafrost thaw, and SLR, with limited resources to support adaptation (KMs 16.1, 29.4, 29.7; Ch. 30). Historic redlining policies forced communities of color into the least valuable, often low-lying lands that have increased flood risks, higher exposure to toxic substances, and more climate change–exacerbated hazards than non-redlined neighborhoods.^91,92,93 Communities that are economically disadvantaged have a higher statistical risk of flood exposure than wealthier communities.^86,87,88 This inequity is further increased because the impact of coastal flooding on individuals and communities is not only based on flood damages but also the ability to pay for the costs of recovery.⁸⁵ Decades of limited community inclusion in decision-making and disinvestment in critical infrastructure and community services have generated greater risk to physical and socioeconomic impacts of coastal hazards.⁹⁴

In addition to direct impacts from acute events, chronic impacts are also experienced unequally among coastal residents. Changes in ecosystem services such as fisheries habitats will impact Indigenous practices in which culture and biodiversity are inextricably linked. In the Hawaiian Islands, loko iʻa (Hawaiian fishponds) are low-intensity forms of aquaculture that traditionally provided food security, contributing to coastal community resilience (KMs 30.1, 30.5).^95,96 These systems are threatened by SLR, with consequences on local livelihoods and cultural practices. Other communities, such as subsistence fishers and fisheries-based rural villages, will similarly suffer as negative impacts on coastal fisheries habitat threaten their way of life (KMs 10.1, 10.2).

The steady rise in flood insurance prices reduces home affordability in coastal regions, with many heirs and low-income and moderate-income property owners unable to afford flood insurance (KMs 16.1, 21.5).^97,98 Aside from home affordability, cascading effects such as climate gentrification—when affluent residents move into low-income areas less exposed to climate hazards, displacing the previous residents^99,100,101—and lack of workforce will continue impacting culture, diversity, and economic productivity in coastal areas.^{99,102,103,104}

Natural Resilience of the Coast Is Changing

For centuries, humans have been reshaping the coast to meet societal needs through urban development, sediment retention and diversion, and coastal defense structures.^105,106,107 These interventions have driven many coastal systems dangerously close to irreversible and profound change (KM 8.1).^108,109 Ecosystem losses due to erosion, more frequent flooding, and coastal squeeze (where human development or natural elevation change limits or prevents inland migration of coastal habitats) will increasingly limit the capacity of coastal landscapes to adapt naturally and diminish their ability to provide valuable ecosystem services (Figure 9.5; KM 8.1).^{47,110,111,112,113}

Mangroves and salt marshes, collectively referred to as tidal wetlands, provide culturally and economically essential fisheries habitat and absorb and store floodwaters (Focus on Blue Carbon).^114,115 SLR and increasing coastal hazards (KM 9.1), as well as eutrophication, sediment availability, poor drainage, and coastal squeeze can all drive tidal wetland loss.^116,117 Some tidal wetlands may survive in place due to accretion, while others may migrate upland and convert other ecosystems (e.g., upland habitat, agriculture, and forests) into tidal wetlands.^118,119

Throughout the US, a net loss of tidal wetlands is expected, but the rate and extent to which the loss occurs will vary significantly by geography and climate change scenario.¹²⁰ For example, in Chesapeake Bay,¹²¹ Florida,¹²² and New Jersey,¹¹⁷ a net loss of tidal wetlands is expected. Along the Gulf Coast, mangroves are overtaking salt marshes, reflecting a shift in vegetation dynamics and habitat.¹²³ Coastal development and steep topography limit inland migration along the Pacific Coast, and tidal wetland conversion to open water and net tidal wetland loss due to SLR appear inevitable.¹²⁴

Barrier islands and reef systems act as a first line of flood defense, absorbing wave impacts as large storms make landfall, thereby reducing flood risk for coastal and inland communities.^{125,126,127,128,129} Barrier island and mainland beach systems may migrate landward naturally to keep pace with SLR, or they may be outpaced and narrow and/or flatten depending on their elevation, how frequently storm waves wash over them, sediment supply, and the persistence of vegetation, all of which can be affected by human modifications.^{61,130,131,132,133,134,135}

Long-term observations, projections of coastal change and erosion, and improved understanding of complex coastal feedback processes help define the conditions and tipping points that may limit natural adaptation (KM 8.1).^136,137 Climate adaptation that restores natural processes and works with coastal ecosystems and landscapes may reduce flood risks while providing multiple co-benefits, including carbon sequestration (KM 8.1; Focus on Blue Carbon). For example, acquired or restored open-space areas (e.g., undeveloped, agricultural, or park lands) along the coast can provide accommodation space for inland wetland and coastal habitat migration as seas rise.^138,139

Allowing coastal ecosystems to evolve naturally may negatively impact some communities and wildlife species, such as the reshaping of barrier islands in response to extreme events that can increase inland storm surge (KM 9.1); however, these natural changes may have beneficial impacts for other species and communities through habitat creation and water quality improvements.^50,134 All changes across the landscape have implications for changes to biodiversity via species declines, species range and phenological shifts, disease, and impacts from invasive species (KM 8.2), affecting seagrasses, corals, mangroves, fisheries, shorebirds, and marine mammals (KMs 21.2, 22.1, 23.2, 26.3, 27.3, 28.2, 30.4).

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Despite projected climate change impacts, coastal communities remain valued places for living and working. Relentless growth in, and enthusiasm for, the coast creates a tension between the need to adapt to climate change and our existing relationships with the coast.^14,140 Although adaptation is occurring in some locations, small-scale and incremental adaptations are not sufficient for the pace and scale of changes that are already occurring (KMs 9.2, 31.1).^13,14,15 Accelerating SLR and increasing coastal hazards (KM 9.1) are affecting larger geographic areas along the coast, expanding the scale and complexity of the adaptation responses and the number and diversity of stakeholders at risk.¹³

Adaptation that includes a broad suite of strategies that address the root causes of coastal vulnerability, consider the needs of diverse stakeholders, center equity (KM 31.2), and reframe societal values and assumptions can lead to transformative and systemic change that can allow coastal communities to thrive and maintain a relationship with the coast (KMs 22.1, 31.3).¹⁴¹ Example strategies can include updated land-use policies,^142,143 community infrastructure investments, nature-based solutions (NBSs), and planned relocation.^14,144 Individually, these strategies are incremental steps, but when combined in a manner that considers long-term community goals and inclusive and sustained engagement with frontline communities, they can lead to equitable transformative adaptation (Figures 9.6, 22.6, 31.3; Box 9.1; KMs 31.2, 31.3).^14,145

Transformative adaptation requires fundamental shifts in systems, values, and practices to equitably address the risks of climate change (KM 31.3), including integration of local perspectives, which leads to more equitable distribution of resources.⁹ Community-led adaptation actions and NBSs can also enhance a sense of place by recreating lost relationships with the coast or fostering new ones between people and the environment.¹⁴

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Timely implementation of adaptation strategies, including planned relocation, can reduce the impacts of climate change on coastal communities.

Figure 9.6. Many strategies can reduce climate-driven coastal hazards. 1) Critical infrastructure, housing, and businesses can relocate out of harm’s way. Retreated lands create space for parks and recreational areas, nature-based solutions (NBSs) for flood risk-reduction, or migration space for coastal ecosystems, while also accommodating rising waters. 2) Relocated communities may move into established communities, or 3) they may create new residential centers. Intentional and equitable stakeholder engagement helps ensure that historic inequities are not perpetuated during relocations. 4) NBSs, such as restoring wetlands, can slow and store rising waters. 5) Housing and structures can be relocated away from rising groundwater tables. 6) Combinations of green and gray infrastructure (hybrid strategies) may be required; for example, a living seawall provides shoreline protection and beneficial habitat for marine organisms. 7) Cultural assets that define a community’s character, such as this lighthouse, can be elevated or moved. Adapted from Dupigny-Giroux et al. 2018.⁵⁴

Art × Climate

Michael O. Snyder
The Coming Coast
(2021, photographic print)

Artist’s statement: “The Coming Coast” documents a journey along the 11,000-mile coastline of the Chesapeake Bay as it may yet be in the year 2100. Along the way, I use blue tape to mark the path of the coming coast as it snakes through neighborhoods that are sometimes miles away from the water. I also meet a diverse group of people who are working to adapt to the rising tides. I aim to understand how individuals from different backgrounds and perspectives can, quite literally, find common ground and work together to protect a shared resource.

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Artworks and artists’ statements are not official Assessment products.

Nature-Based Solutions in Coastal Communities

NBSs integrate natural processes with traditional engineering approaches to reduce flood risk while also preserving or enhancing the ecological value of natural landscapes (e.g., maintaining essential habitat for protected species) and providing potential societal, economic, and other co-benefits (Focus on Blue Carbon).^149,150 NBSs can include ecosystem conservation and restoration or recreation of natural processes that reduce flood risks, hybrid solutions (e.g., living shorelines), and the greening of traditional infrastructure (e.g., ecological riprap).^151,152 Although NBSs are effective in reducing temporary flooding resulting from storms, they may provide only modest benefits in preventing permanent inundation from SLR.^149,153 However, when NBSs are paired with planned relocation, protection from flooding and SLR is provided by moving a community out of harm’s way while also reestablishing the natural flood risk-reduction benefits of coastal ecosystems.⁵²

Mangroves and other coastal wetlands reduce wave energy,^154,155 decrease coastal erosion,^156,157 and provide flood attenuation.^114,158,159 Wetlands helped communities avoid $795.2 million (in 2022 dollars) in direct flood damages during Hurricane Sandy.¹²⁷ Beaches and dunes reduce storm surges and absorb wave energy.¹⁶⁰ Coral reefs damp wave energy and provide flood protection for adjacent communities, with an estimated flood risk-reduction benefit of over $2.2 billion annually (in 2022 dollars) in the contiguous US^161,162 and more than $1.1 billion (in 2022 dollars) annually in Hawaiʻi, Guam, American Sāmoa, the Commonwealth of the Northern Mariana Islands, Florida, Puerto Rico, and the US Virgin Islands.¹⁶³

Hybrid solutions can reduce shoreline erosion^164,165 and enhance the engineering design life and flood risk-reduction performance of traditional infrastructure.¹⁶⁶ Flood risk-reduction benefits of hybrid solutions have been demonstrated across varying hydrodynamic conditions.¹⁶⁵ NBS guidance documents^{149,167,168,169} are continually published, and implementation of NBS strategies is increasing. The ability to include adaptive elements in NBSs for future changing conditions^144,170,171 makes them an important component of the adaptation landscape over the coming decades.

Planned Relocation Strategies in Coastal Communities

Planned relocation is the process of moving individual properties, infrastructure, or whole communities preemptively away from, or in response to, the impacts of natural hazards.¹⁷² Historically, most communities have remained in place post-disaster by adapting or rebuilding using engineered solutions (KM 20.3).¹⁷³ However, as climate change impacts increase, adapting and rebuilding in place will become more challenging (KM 31.1). With accelerating SLR, particularly under low-likelihood, high-impact SLR scenarios (Figure 9.1), planned relocation may become more cost effective than adapting in place, with a lower long-term risk of loss of life and property if engineered solutions fail.¹⁷⁴

In the US, planned relocation generally occurs reactively (i.e., post-disaster) rather than proactively (i.e., relocating at-risk communities before a disaster). For example, targeted buyouts of assets most at risk of future repetitive damage occurred after Hurricane Sandy in Staten Island, New York.¹⁷⁵ Residents and communities have also relocated after natural disasters, such as Isle de Jean Charles, Louisiana;¹⁷⁶ Kivalina, Alaska;¹⁷⁷ and the Quinault Indian Nation (Box 20.1). As planned relocation expands, there is an urgent need to assess lessons learned from past relocations and lean into transformative adaptation that improves community well-being and addresses social, ecological, and intergenerational justice.^178,179

Proactive planned relocation may become the most viable response for many future coastal communities as SLR continues and coastal lands become submerged.^38,180 However, discussions of planned relocation remain challenging and controversial.^12,174 Impediments include resistance to change;¹⁸¹ disagreements about when communities and infrastructure may be irrevocably lost and, thus, the appropriate timing for relocation; lack of community-led decision-making; cost effectiveness compared to defending in place;^174,182 disruptions to community cohesion and social capital;^183,184 and identification of suitable replacement locations.¹⁸⁵

Transformative Adaptation Opportunities in Coastal Communities

Transformative adaptation that is proactive and intentional and involves fundamental shifts in systems, values, and practices (KM 31.3) provides opportunities to meet the challenges of shifting and receding shorelines. Transformative adaptation along the coast considers aspects such as funding and economic security, alignment of governmental entities, attachment to place and livelihoods, and technical expertise.^52,53

Intentional and equitable transformative adaptation is an opportunity to redress root causes of inequities and disparate impacts of climate change in coastal communities.^14,15 Achieving this would require sustained funding dedicated to proactive planning, design, and execution.¹⁸⁶ This would prevent reactive strategies that have historically exacerbated inequities and focused resources on wealthy, typically White communities (KM 20.3)¹⁸⁷—a particular challenge in areas where there are large disparities in wealth and where lower-income homeowners and renters lack affordable flood insurance.¹⁰³

Current adaptation strategies that are increasingly being implemented on the coast could shift toward transformative adaptation if local communities are centered and inequities are transparently addressed (KMs 31.2, 31.3; Figure 31.2). This will require an incremental shift in practice, resulting in additional shifts in systems (e.g., permitting), values (e.g., recognizing and addressing past injustices), and risk tolerance (e.g., increasing comfort in natural shoreline protection over more traditional hardened structures).

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