Due to sea level rise (SLR), coastal storms and high tides have amplified coastal flooding and erosion impacts, and this trend will continue into the future, with some regions more vulnerable than others (Ch. 2: Climate, KM 9).^{6,7,8,9,36,37,38} High tide flooding is already forcing some East Coast cities to install costly pump stations to frequently clear floodwaters from the streets (such as Miami Beach, as shown in Figure 8.3) (see also Ch. 19: Southeast, KM 2) and to mobilize emergency responders to routinely close flooded streets. Along with increases in tidally driven flooding, storm surges are higher due to SLR.^36,39,40 Warmer air temperatures have increased the probability of heavy precipitation events,^41,42,43 permafrost thawing, and earlier season sea ice loss, leading to increased erosion over significant miles of coastline (Ch. 26: Alaska, KM 2). The severity of compound events—the coupling of surge, discharge from rivers, and heavy precipitation—has increased in many coastal cities (Ch. 19: Southeast, KM 2; Ch. 3: Water, KM 2).^18,19 In addition, modeling suggests that tropical cyclone intensity will increase,^40,44,45 which would lead to greater damage upon landfall. Collectively, these factors already threaten coastal economies, public safety, and well-being, and continued growth and development along the coast increase the risk to more people and infrastructure.

Even under a very low scenario (RCP2.6) (see the Scenario Products section of App. 3 for more on scenarios), projections indicate that the frequency, depth, and extent of both high tide and more severe, damaging coastal flooding will increase rapidly in the coming decades.^{7,8,36,46,47,48} With rapid ice loss from Greenland and Antarctica under the higher scenario (RCP8.5), an Extreme scenario of global sea level rising upwards of 8 feet by 2100 is a possibility.^{36,37,49,50,51,52} Under this rise, the average daily high tide would exceed the current 100-year (1% annual chance) coastal water level event in most U.S. coastal locations.^8,39,53 Because these low-probability, high-consequence risks cannot be ruled out, a robust risk management approach to future planning would involve their consideration.

Coastal property owners are likely to bear costs from SLR and storm surge, including those associated with property abandonment; residual storm damages; protective adaptation measures, such as property elevation; beach nourishment; and shoreline armoring.³⁵ The potential for future losses is great, with continued and often expensive development at the coasts increasing exposure (Ch. 5: Land Changes, KM 2).^54,55 Shoreline counties hold 49.4 million housing units, while homes and businesses worth at least $1.4 trillion sit within about 1/8th mile of the coast.⁵⁶ Flooding from rising sea levels and storms is likely to destroy, or make unsuitable for use, billions of dollars of property by the middle of this century, with the Atlantic and Gulf coasts facing greater-than-average risk compared to other regions of the country.^57,58,59 Recent economic analysis finds that under a higher scenario (RCP8.5), it is likely (a 66% probability, which corresponds to the Intermediate-Low to Intermediate sea level rise scenarios) that between $66 billion and $106 billion worth of real estate will be below sea level by 2050; and $238 billion to $507 billion, by 2100.⁶⁰

These market impacts have the potential to influence property developers, lenders, servicers, mortgage insurers, and the mortgage-backed securities industry.^58,61 Coastal property and infrastructure losses cascade into threats to personal wealth and could affect the economic stability of local governments, businesses, and the broader economy.⁶² Some coastal property owners are dependent on recouping losses from private or public insurance policies, and there are few private flood insurance policies currently available.^63,64 Mortgage holders located within the federally designated Special Flood Hazard Area defined by the Federal Emergency Management Agency are required to purchase flood insurance, which is almost always obtained through the National Flood Insurance Program (NFIP). Losses generated by the NFIP create substantial financial exposure for the Federal Government and U.S. taxpayers.^65,66 There are already indications in places like Atlantic City, New Jersey, and Norfolk, Virginia,^58,67 that homes subject to recurring flooding may become unsellable. The impacts of Hurricanes Harvey, Irma, and Maria in 2017 will only exacerbate the NFIP losses. (For more information on the 2017 Atlantic hurricane season, see Ch. 2: Climate, Box 2.5.) Additionally, diminished real estate values are likely to result in lower tax revenues and reduced community services (Ch. 28: Adaptation, KM 5).^68,69

In addition to private property risks, coastal infrastructure, such as roads, bridges, tunnels, and pipelines, provides important lifelines between coastal and inland communities, meaning that damage to this infrastructure results in cascading costs and national impacts (Ch. 12: Transportation, KM 1 and 2).⁷⁰ Oil and gas from critical energy infrastructure along the coast is distributed to the entire nation.^71,72 Similarly, the entire country depends on coastal seaports for access to goods and services, as they handle 99% of overseas trade (Ch. 12: Transportation, KM 1). Incorporating adaptation into infrastructure upgrades will be expensive. For instance, the estimated cost to elevate and retrofit the major commercial ports of California (such as San Diego, Los Angeles/Long Beach, San Francisco) to adapt to 6 feet of SLR is $9–$12 billion.⁷³ Investing in these interconnected lifelines would support community stability and the Nation’s economy (Ch. 3: Water, KM 2; Ch. 11: Urban, KM 3; Ch. 17: Complex Systems, KM 1 and 3).⁷⁰

Coastal ecosystems such as estuaries, deltas, marshes, mangroves, seagrasses, beaches, and reefs provide valuable benefits to the economy and society.³⁵ They support fisheries, reduce shoreline erosion from waves, improve water quality, and create valuable recreation opportunities.⁷⁴ Between 2004 and 2009, it was estimated that U.S. coastal wetland environments have been lost at an average rate of about 80,160 acres per year, with 71% of coastal wetland loss occurring in the Gulf of Mexico.⁷⁵ At this rate, by 2100 the United States will have lost an additional 16% of coastal wetlands.⁷⁵ Sea level rise in the Atlantic is contributing to the declining health and integrity of Atlantic marshes. Marsh degradation is expected to occur faster in the Atlantic than in the Pacific due to the higher SLR expected along the U.S. Atlantic coast.^76,77

Coastal wetlands generate climate mitigation benefits by serving as natural sinks for atmospheric carbon dioxide.^78,79,80 As these ecosystems are degraded or lost, their carbon uptake potential will be diminished and their stored carbon potentially released. In addition, wetlands are a first line of natural defense against erosion, waves, flooding, and storm surge.⁸¹

Natural and nature-based infrastructure provides alternatives to traditional hard structure approaches such as seawalls, levees, and dikes and can improve the resilience of coastal communities and the integrity of coastal ecosystems.^81,82,83 This approach includes a range of efforts, such as the protection or restoration of natural habitats to mitigate waves and erosion (Figure 8.4) (see also Ch 19: Southeast, KM 3)^{84,85,86,87,88,89} and hybrid approaches that combine built and natural features, such as some living shorelines options.^83,90 These types of approaches are being considered in the Superstorm Sandy Rebuild by Design challenge, the Changing Course competition focused on the Lower Mississippi River delta, and in experimental studies and the development of guidance conducted within estuaries.⁹¹ Studies suggest that healthy coastal ecosystems provide important cost savings in terms of flood damages avoided,^92,93,94 but more research would be useful to increase the level of confidence.

Flooding and erosion impact many populations along the coast. However, for socially and economically marginalized and low-income groups, climate change and current and future SLR could exacerbate many long-standing inequities that precede any climate-related impacts (Figure 8.5) (see also Ch. 11: Urban, KM 1; Ch. 18: Northeast, KM 3).^95,96 Underrepresented and underserved communities facing additional threats from climate change span a variety of regions and contexts, ranging from the elderly in Florida⁹⁷ to rural and subsistence-based fishing communities in Alaska (Ch. 26: Alaska, KM 4).⁹⁸ The 2017 hurricane season provided grim imagery of the impacts to these socially and economically vulnerable coastal residents, and the long-term impacts on these communities are as yet unclear (Figure 8.6) (see also Ch. 2: Climate, Box 2.5). Given limited resources, the core of this challenge rests on questions about who is most vulnerable to the impacts, who should pay for losses incurred, who should pay for protecting coastal communities in the future, and how governments and communities set protocols and policies for keeping people safe. These types of questions bring to light the divergent views of various stakeholders regarding the role of individuals, businesses, and governments in assuming the risks and benefits of living and working near the coast (Ch. 14: Human Health, KM 2 and 3).⁹⁹

Adaptation strategies, including the decision to retreat from, accommodate, or protect against a particular impact, are dependent on several factors. Economically, a property owner’s access to capital or insurance to fund these strategies contributes to adaptation choices, making poverty a driver of vulnerability in the face of climate-based impacts.¹⁰⁰ Some property owners can afford to modify their homes to withstand current and projected flooding and erosion impacts. Others who cannot afford to do so are becoming financially tied to houses that are at greater risk of annual flooding.⁶⁷ Additionally, communities are composed of renters and other individuals who do not own property, making it more difficult for them to contribute their voices to conversations about preserving neighborhoods. Culturally, coastal communities have ties to their specific land and to each other, as is the case from the bayous of Louisiana, to the beaches of New Jersey, to the sea islands of South Carolina and Georgia. These ties can impede people’s ability and willingness to move away from impacted areas. For Indigenous villages to most effectively respond to critical climate impacts, decision-makers should consider identifying a suitable place to relocate that does not infringe on the needs and territories of other populations, is large enough for the entirety of the village, and is suitable for building and accessing infrastructure (Ch. 15: Tribes, KM 3).¹⁰¹

Climate change impacts are expected to drive human migration from coastal locations, but exactly how remains uncertain.^102,103,104 As demonstrated by the migration of affected individuals in the wake of Hurricane Katrina, impacts from storms can disperse refugees from coastal areas to all 50 states, with economic and social costs felt across the country.¹⁰⁵ Sea level rise might reshape the U.S. population distribution, with 13.1 million people potentially at risk of needing to migrate due to a SLR of 6 feet (about 2 feet less than the Extreme scenario) by the year 2100.¹⁰² The Biloxi-Chitimacha-Choctaw tribe on Isle de Jean Charles in Louisiana was awarded $48 million from the U.S. Department of Housing and Urban Development to implement a resettlement plan.^106,107 The tribe is one of the few communities to qualify for federal funding to move en masse. (Ch. 15: Tribes, KM 3; Ch. 19: Southeast, KM 1).

Coastal Adaptation

Coasts will confront a more diverse and, to a great extent, unique range of climate stressors and impacts compared with the rest of the country. Rising sea levels will force many more coastal communities to grapple with chronic high tide flooding, higher storm surges, and associated emergency response costs over the next few decades.^6,7,36,75 The growing concentration of people and economic activity in coastal areas will introduce a greater degree of risk, including impacts that will ripple far beyond coastal communities themselves.^70,108 Understanding these realities, coastal cities such as Boston, New York City, Miami, San Francisco, New Orleans, and Los Angeles are beginning to make investments to adapt to SLR (see the Case Study: “Key Messages in Action”) (see also Ch. 19: Southeast, KM 1). From these efforts, and others like them, examples of successful adaptation planning are being collected to provide guidance to other communities facing similar challenges (Figure 8.2) (see also Ch. 28: Adaptation).^109,110,111

However, while many current plans call for risk identification, monitoring, research, and additional planning, there is still little focus on the major investments or immediate implementation actions and cost-dependent tradeoffs required to successfully adapt.¹¹⁰ The financial resources currently being devoted to adapt to or mitigate coastal climate change impacts are insufficient to meet the projected challenges ahead.^112,113,114 Additionally, with the limited and often expensive adaptation opportunities currently under consideration, including elevating properties or constructing seawalls, climate-driven impacts may lead to a great deal of unplanned and undesired community change that is likely to disproportionately impact communities that are already marginalized. Resilience planning that considers cultural heritage and incorporates community-driven values, experiences, concerns, needs, and traditional knowledge promotes social inclusivity and equity in adaptation decisions (Ch. 15: Tribes, KM 3).^115,116