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
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
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]) impacts2,31—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, 202133,34,35). 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
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
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).
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).
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 margins69,70 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 residents99,100,101—and lack of workforce will continue impacting culture, diversity, and economic productivity in coastal areas.99,102,103,104
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).
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
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
Blue carbon refers to carbon captured by marine and coastal ecosystems, such as mangroves, coastal wetlands, and seagrasses. Coastal ecosystems sequester carbon at a much faster rate than terrestrial ecosystems, and the carbon stored belowground can remain in place for decades to millennia if undisturbed by humans or extreme events.
Read MoreMangroves 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 US161,162 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 erosion164,165 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 documents149,167,168,169 are continually published, and implementation of NBS strategies is increasing. The ability to include adaptive elements in NBSs for future changing conditions144,170,171 makes them an important component of the adaptation landscape over the coming decades.
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 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).
This assessment builds upon and amplifies the Key Messages within the “Coastal Effects” chapter of the Fourth National Climate Assessment, as those Key Messages are still relevant, yet they have become even more urgent.188 The US Interagency Sea Level Rise Task Force’s (hereafter “Task Force”) report2 provides clear evidence that sea level rise (SLR) is already accelerating and that current SLR tends are tracking on the Intermediate-Low curve or higher along the Nation’s coasts.
Impacts associated SLR and extreme coastal storms are increasing per observations, coastal ecosystems and communities are facing increasing risks, and transformative adaptation grounded with nature-based solutions may provide our best hope to retain a sense of balance between the coasts and our coastal communities. The author team required a depth and breadth of expertise across the Atlantic, Gulf, and Pacific Coasts, as well as the coastlines of Hawaiʻi and the US-Affiliated Pacific Islands, the US Caribbean, and Alaska; the leading edge of SLR science; the physical processes that shape our coastlines; the systemic inequities that continue to put frontline communities at greatest risk; and the human actions that have altered the coasts and transformed the shoreline to suit societal desires.
Prospective authors were nominated by their respective agencies, universities, organizations, or peers. The chapter lead and federal coordinating lead authors discussed and vetted prospective authors with a goal of creating a cohesive author team committed to bringing their formidable experience and skillsets together to develop this chapter.
This chapter was developed through weekly teleconferences, email exchanges, technical discussions of the relevant evidence base, and expert deliberation by the authors. The author team, along with the US Global Change Research Program, held a public engagement workshop with participants from federal, state, and local agencies; consultants; and interested members of the public. The workshop used innovative approaches and breakout groups that explored what the participants loved most about the coast before diving into the key topics that framed this chapter and the development of the Key Messages.
The urgent need for adaptation, with an emphasis on nature-based solutions and planned relocation, was a clear driver for Key Messages 9.2 and 9.3. Additional literature is required that presents lessons learned and successful implementations, even if at a small scale, to achieve the scale of planned relocation that is expected to be required in the US over the next century.179 The authors extensively reviewed the literature on transformative adaptation and adaptation that centered equity, community values, and included strong community participation. The concept of transformative adaptation and planned relocation required dialogue across many chapters, with an emphasis on Chapter 20 (Social Systems and Justice) and Chapter 31 (Adaptation) to achieve consistency and portray a sense of urgency for the Nation along these paths.
Consensus on the Key Messages and supporting literature required multiple iterations, discussions with other chapters, and careful review and revisions in response to comments from the public and the National Academies of Sciences, Engineering, and Medicine.
The severity and risks of coastal hazards across the Nation are increasing , driven by accelerating sea level rise and changing storm patterns, resulting in increased flooding, erosion, and rising groundwater tables. Over the next 30 years (2020–2050), coastal sea levels along the contiguous US coasts are expected to rise about 11 inches (28 cm), or as much as the observed rise over the last 100 years . In response, coastal flooding will occur 5–10 times more often by 2050 than 2020 in most locations, with damaging flooding occurring as often as disruptive “high tide flooding” does now if action is not taken .
Read about Confidence and Likelihood
Multiple lines of evidence, including satellite and tide-gauge observations and model simulations, show that substantial SLR has occurred to date, globally and for the US, as documented and synthesized in the Working Group I contribution to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (AR6) and the Task Force report on SLR.2,189 Observations show that SLR is accelerating at global, national, regional, and local levels, and AR6 projections and the sea level scenarios from the Task Force report suggest that these trends are expected to continue over the next several decades and through the end of this century and beyond (see https://sealevel.nasa.gov/data_tools/18).2,189 Beyond 2150, SLR is expected to continue for the next several thousand years due to the long-term effects of emissions and warming over this past century, irrespective of future emissions occurring after 2100. These lines of evidence are synthesized, and a large body of relevant literature is documented (e.g., Dangendorf et al. 2019; Frederikse et al. 2020; Fox-Kemper et al. 2021; Hamlington et al. 2021; Edwards et al. 202120,190,191,192,193), in the Task Force report on SLR2 and IPCC AR6.189
An additional body of literature and references therein links this increase in average sea level to a broad range of risks and adverse impacts in the coastal zone. Extreme water levels will continue to rise with SLR, causing deeper, more frequent, more severe, and more widespread flooding (e.g., Sweet et al. 2021, 2022; Taherkhani et al. 2020; Thompson et al. 2021; Vitousek et al. 20172,35,36,41,194). Observational and model simulation evidence also indicates that many types of extreme events are increasing in intensity, frequency, and geographic extent as a result of human-caused climate change and that hurricanes, in particular, are intensifying and causing heavier rainfall and higher storm surges, all of which compounds these flood risks (see evidence base underlying Key Messages 2.2, 3.6, and USGCRP 2017195). Extreme water levels and flooding lead, in turn, to additional coastal zone impacts (e.g., erosion, damage to property and infrastructure, ecosystem impacts). Compound flooding associated with other coastal storm types (e.g., atmospheric rivers, extratropical cyclones) is also projected to increase with a warming climate.196,197,198
A further body of literature documents how population growth, migration, and development trends in the coastal zone have exacerbated societal risks and exposure of populations and the built environment to increasing SLR- and flooding-related hazards.199,200,201
For near-term impacts (to 2050), uncertainties and research gaps include the impact of natural climate variability on the observation-based trajectories, coastal adaptation, and policy actions to reduce future hazards and improved incorporation of interacting and compounding drivers into projections of coastal water levels and overall coastal flood hazards, such as winds, surge, waves, rising water tables, and extreme rainfall. In addition, more detailed understanding of, and data on, compound flood hazards is a key area of research needed to better understand and communicate flood risks and inform adaptation efforts.
For longer-term impacts (after 2050), major uncertainties and research gaps include improved modeling and observational capabilities to assess long-term global average SLR trajectories as a function of uncertainties in both emissions pathways and the sensitivity of ice sheet dynamical processes to a given level of warming, particularly the “low-confidence” ice sheet processes, as per IPCC AR6.20 Projections that include these ice sheet processes, particularly under higher-emissions futures, result in substantially higher global average SLR values by the end of this century and beyond. Pathways to such futures include outcomes such as earlier-than-projected ice shelf disintegration in Antarctica; abrupt, widespread onset of marine ice sheet instability and/or marine ice cliff instability in Antarctica; and faster-than-projected changes in surface-mass balance on Greenland, potentially associated with changes in atmospheric circulation, cloud processes, or albedo changes.2 Monitoring the sources of ongoing SLR and the processes driving changes in sea level is critical for assessing scenario divergence and tracking the trajectory of observed SLR, particularly during the period when future emissions pathways might increase the risk of triggering these low-confidence processes.
Based on a spatially weighted average of about 100 NOAA tide gauges and following methodologies in Sweet et al. (2022),2 there is high confidence that sea levels along the contiguous US have risen about 11 inches (likely range between 10 and 12 inches) on average over the 1920–2020 period, with about 5–6 of those inches occurring since 1990, indicating that sea level rise is accelerating. There is also high confidence that the probability of minor, moderate, and major coastal flooding increased by about 2–3 times between 1990 and 2020 (as defined by contemporary NOAA weather-related impact thresholds calibrated to historic NOAA tide-gauge water level heights). Thus, it is very likely that the severity and risks of hazards are increasing.
There is high confidence and it is likely that sea levels will rise about 11 inches (likely range of about 9–13 inches) between 2020 and 2050 based on both extrapolating rates and accelerations estimated from historical tide-gauge observations and model projections, with both approaches producing projections within similar ranges. In response to 11 inches of SLR by 2050, there is high confidence and it is very likely that the probability of minor, moderate, and major coastal flooding will occur 5–10 times more often by 2050 in many regions without additional flood risk-reduction measures, as compared to contemporary standards.
Climate change–driven sea level rise, among other factors, is affecting the resilience of coastal ecosystems and communities . The impacts of climate change and human modifications to coastal landscapes, such as seawalls, levees, and urban development, are both limiting the capacity of coastal ecosystems to adapt naturally and are compounding the loss of coastal ecosystem services . Proactive strategies are necessary to avoid degraded quality of life in the coastal zone, as the combination of reduced ecosystem services and damage to the built environment from exacerbated coastal hazards increasingly burdens communities, industries, and cultures .
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A growing body of literature captures the limited ability of coastal ecosystems to adapt to climate-driven changes, particularly due to human modification. Multiple lines of evidence show that physical changes in the coastal zone in response to climate change are occurring, including upland conversion and marsh expansion,121,122,202 expansion of mangrove systems,123 and marsh and beach loss due to erosion and barriers that limit inland migration of these ecosystems.47,50,110,119,138,203 The consequent loss of ecosystem services, such as storm protection, wetland carbon sequestration, sensitive habitat, and industry, including agriculture, tourism, recreation, and fishing, has been well documented (e.g., Siverd et al. 2020; Weiskopf et al. 2020204,205), and the amplification of these losses via human modifications of the coast are well supported in the peer-reviewed literature.105,113,206
With ecosystem loss, exacerbated coastal hazards, and growing coastal populations, increasing damages and costs have been observed (e.g., Bouwer 2019; Hino et al. 2019; Smiley et al. 2022; Al-Attabi et al. 202379,207,208,209), and ongoing health and safety concerns due to increasing flood frequencies, contaminated water supplies, degraded water quality, exposure to toxic substances, and strains on mental health due to the ongoing threat of disasters have been documented (e.g., Coutu 2018; Makwana 2019; Erickson et al. 2019; Gobler 2020; Raker 2022210,211,212,213,214). Additionally, many studies have shown that 1) overburdened, under-resourced, economically disadvantaged, or otherwise vulnerable populations (e.g., children, people with disabilities) face a greater burden from disasters (e.g., Conzelmann et al. 2022; Raker 2022; Smiley et al. 2022209,214,215) and are limited in their ability to recover from these impacts, and 2) existing inequities continue to be magnified (e.g., Erman et al. 2020; Griego et al. 2020; Sou et al. 2021; Dundon and Camp 2021; Bento and Elliott 2022216,217,218,219,220). Municipal coastal officials, elected officials, and staff continually document increasing challenges within their communities through participation in professional organizations (e.g., National League of Cities, Association of State Floodplain Managers, regional communities of practice). Specific challenges include the combination of increasing development and land-use pressures and exacerbating coastal hazards that put more homes, businesses, and individuals at risk. Numerous municipalities are installing backflow preventers, documenting high tide flooding, and attempting to manage magnified impacts from rainfall occurring concurrently with high tide flooding and other coastal hazards (e.g., EcoSystems 2014; WSAV 2018; Coutu 2021221,222,223).
Future coastal landscape change is difficult to model and predict broadly in the spatially detailed form required by decision-makers, due to the multitude and complexity of the processes and feedbacks acting within and across different coastal ecosystems.137,224,225
National-scale efforts are emerging to assess the risk of losing vital coastal wetland habitats112,226 and to monitor the daily to annual status of sandy beaches using satellite imagery.227,228,229 However, monitoring alone cannot save these at-risk ecosystems; improved understanding and ability to model the thresholds and/or tipping points associated with ecosystem loss versus survival are needed broadly to support proactive planning for management of coastal resources and communities.58,137 Information is needed about when and where saltwater intrusion may occur and its impacts.69,230,231
Anticipating and accounting for future human modifications that may reshape the coast and/or affect ecosystem behaviors are areas of considerable uncertainty.232 Multidisciplinary scenario development can help to explore the physical changes that may occur, how humans may choose to respond to these changes, and the resources that may be available to support these modifications, such as emplacement or removal of gray and green infrastructure, planned relocation, or trade-offs.233,234 A better understanding of when and in what form humans may take future action can in turn help inform understanding of the landscape response, which can better frame the immediate and longer-term risks to coastal populations in the future.232,235
Based on observations and predictive modeling the authors have high confidence that the long-term sustainability of our coastal ecosystems and human systems is very likely being affected by climate changes, particularly due to land loss. Observations and modeling have also given us high confidence that human measures that have historically been used to limit coastal change and have predominantly relied on hard, fixed infrastructure solutions to protect development are very likely to make coastal areas less resilient to future change amplified by climate drivers. With this reduction in resilience, numerous studies have shown there is high confidence that coastal ecosystems are very likely to be limited in their ability to provide the services on which humans depend. There is high confidence based on the array of literature and studies available that the loss of these services are very likely to require proactive strategies to address significant and cascading impacts on cities, communities, and ways of life in the coastal zone.
Accelerating sea level rise and climate change will transform the coastal landscape, requiring a new paradigm for how we live with, or adapt to, these changes . Although incremental in nature, nature-based solutions and planned relocation strategies may help communities adapt to increasing coastal hazards if they are community-led and equity-centered . Maintaining cultural and economic connections within coastal communities will require equitable transformative adaptation that addresses systemic interconnections between ecosystems, communities, and governance .
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Because coastal hazards will continue to worsen and the impacts to the natural and built environment will increase, coastal communities will have to adapt (or continue to adapt) to climate change. Business-as-usual strategies are not expected to be sufficient in the future because they do not address the root causes of vulnerability in coastal communities9,14,15 nor acknowledge that sea levels will continue to rise beyond typical infrastructure planning time horizons.141,236
Consensus is growing to support nature-based solutions (NBSs) and strategies such as planned relocation as essential components of climate adaptation.149,153,169,237 There is a growing body of literature that demonstrates that NBSs can successfully provide flood risk protection.115,160,164,165,237 Multiple studies based on laboratory experiments have demonstrated the capacity of NBSs to attenuate wave energy, currents, and storm surges under a range of controlled conditions.238,239 Additional studies based on field measurements during extreme coastal events have validated these findings within a range of geographical settings and environmental and extreme weather conditions.154,156,240 Numerical modeling studies have expanded these findings to low-frequency events and a broader range of extreme conditions.114,161,162 There is growing evidence on the functionality and performance of NBSs for flood risk reduction. This body of literature supports an increasing number of guidelines and practical guidance for NBS planning, design, and implementation.149,169 State agencies are beginning to require prioritizing NBSs for coastal adaptation, where possible, in lieu of hardened infrastructure.
Planned relocation continues to be a topic of contentious debate in coastal communities, but there is growing evidence that demonstrates openness by communities to include these strategies in long-term planning discussions.241 This is particularly true if the definition of planned relocation is broadened to include different land-use policy levers that are common planning tools, such as setbacks or easements,242 as well as discussions and planning that are led by the community.243
Transformative adaptation to SLR is possible, in part, due to the array of efforts used to provide meaningful and understandable information to coastal stakeholders. In the coastal zone, stakeholders span a wide array of sectors, grappling with different priorities, timelines, and urgencies that often lead to differing needs. For example, ecologists designing a wetland restoration require probabilistic estimates of near-term SLR, while planners of critical infrastructure need to understand the full suite of possible risks across both the near term and long term to make wise decisions and investments for the communities they serve. The current state of the science and the corresponding guidance on how to make decisions in the face of the knowns and unknowns around rising seas (e.g., The Application Guide for the 2022 SLR Technical Report) are essential indicators that transformative adaptation, inclusive of NBSs and migration, is achievable.1
Although it is generally understood that riverine flood risk-reduction projects, such as increasing levee heights, could exacerbate flood risks in downstream communities, the potential for a similar deflection of flood risks from one community in coastal environments is less understood. In San Francisco Bay, a modeling study showed that the addition of a levee or seawall to protect one community could increase flood risks elsewhere on the estuarine shoreline.6 This concept may be important to consider more broadly along the coast, as it intersects with equity considerations if communities with fewer resources to adapt are confronted with increased risks diverted from communities with greater resources.
Despite the growing number of studies investigating and validating the performance of NBSs for flood risk reduction, research gaps remain with respect to uncertainty in flood risk-reduction benefits under a range of future environmental conditions and hazards, given the intrinsic dynamic nature of NBS systems. Specifically, what strategies work in active coastal zones with high wave energy? Furthermore, research is lacking with respect to NBS strategies in extreme environments, especially the Arctic, where traditional vegetation-centric approaches are not practical. This is especially relevant in western Alaska, where communities are experiencing increasing flood hazards and traditional flood protection is extremely costly. While there has been progress on developing standards and guidelines for using NBSs to reduce flood risk,149,167,168,169 there remains a need for professional engineering organizations and nongovernmental organizations to expand the existing documentation.
Uncertainties and research gaps on planned relocation tend to focus on the process and willingness of communities to relocate: What are the tipping points that encourage a community to adopt community-wide planned relocation? Where should people move to, and are receiving communities prepared to take in increased populations? How does relocation get paid for? How can the psychological barriers to planned relocation be overcome?
There is a lack of literature exploring the governance structures, laws, and policies necessary to support transformative adaptation, including planned relocation. Spanning the gap between adaptation planning and successfully implementing adaptation solutions on the ground requires overcoming governance challenges.244 Although the number of legal analyses relevant to adaptation is growing, these analyses are still limited in their practical application and scope.245,246 Research has demonstrated the value of multidirectional policies, laws, and efforts at stimulating climate planning and adaptation, particularly the benefit of top-down laws directing the need to plan and implement adaptation without being overly prescriptive;53 however, this work is in its early stages. Comprehensive analyses that explore how current policies impede or foster transformative climate adaptation would help to synthesize and identify where improvements could be made within governance structures to support successful adaptation.
There is limited research on the economic and social drivers of, and impediments to, transformative adaptation in coastal communities. Research is also lacking on the social psychology prerequisites needed for successful transformative adaptation. How will a coastal community know when residents are ready to start down the path of transformative adaptation? How can psychological readiness be fostered, including effective communication of future conditions that may compel this sort of action?
Beyond NBSs and planned relocation, many incremental adaptations are well understood in terms of implementation and impact alone. For example, many communities have experience with raising roads, resizing drainage culverts, and building shoreline stabilization structures. However, research that results in effective guidance is lacking for larger measures, such as infrastructure abandonment or relocation and the aggregation of smaller measures. While there is an emerging body of research on this topic for individual actions,174,247,248 analysis at the community level across a range of adaptation actions and timelines is lacking.
The body of research supporting transformative adaptation is growing; however, much of this research is not tailored for coastal communities. There is a gap in research on the preconditions for, and drivers of, success in coastal communities. Furthermore, there is also a gap in research that assesses the effectiveness of mitigation and adaptation approaches under low-likelihood, high-impact scenarios, given that these approaches necessarily change under extreme SLR scenarios.
Increasing coastal hazards, changing weather patterns, and extreme storms are causing widespread and rapid changes along our Nation’s coasts (high confidence). At present, adaptation efforts are most often incremental in nature and sector-specific (e.g., focused on adapting a wastewater treatment plant or stretch of roadway) as opposed to community-wide in scale (high confidence). As SLR accelerates, exposing greater populations and geographies to coastal hazards, this adaptation approach will become ineffective.
Adaptation responses that move beyond traditional solutions may include nature-based solutions and planned relocation. There is medium confidence that nature-based solutions and planned relocation strategies, when they are community-led and equity-centered, can provide an equitable response to coastal hazards and climate change impacts. There is medium confidence that transformative adaptation that centers the community within the planning, design, and implementation of multi-benefit solutions can better maintain the social, cultural, and economic connections communities require to thrive.
The statements in this Key Message are supported both in the literature cited and in the author team’s understanding of the state of the adaptation practice across a wide variety of coastal communities, both in geography and in social systems. This community-focused lens supplements the published literature and allows the authors to reflect the most current consensus on this topic.
Virtually Certain | Very Likely | Likely | As Likely as Not | Unlikely | Very Unikely | Exceptionally Unlikely |
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99%–100% | 90%–100% | 66%–100% | 33%–66% | 0%–33% | 0%–10% | 0%–1% |
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