Sea-level rise and human migration

Abstract

Anthropogenic sea-level rise (SLR) is predicted to impact, and, in many cases, displace, a large proportion of the population via inundation and heightened SLR-related hazards. With the global coastal population projected to surpass one billion people this century, SLR might be among the most costly and permanent future consequences of climate change. In this Review, we synthesize the rapidly expanding knowledge of human mobility and migration responses to SLR, providing a coherent roadmap for future SLR research and associated policy. While it is often assumed that direct inundation forces a migration, we discuss how mobility responses are instead driven by a diversity of socioeconomic and demographic factors, which, in some cases, do not result in a migration response. We link SLR hazards with potential mechanisms of migration and the associated governmental or institutional policies that operate as obstacles or facilitators for that migration. Specific examples from the USA, Bangladesh and atoll island nations are used to contextualize these concepts. However, further research is needed on the fundamental mechanisms underlying SLR migration, tipping points, thresholds and feedbacks, risk perception and migration to fully understand migration responses to SLR.

Key points

  • A large proportion of the global population presently reside in coastal regions where sea-level rise (SLR) impacts are expected and, in many cases, may influence the migration of millions of people.

  • Migration from SLR is multifaceted, influenced by environmental hazards and political, demographic, economic and social factors embedded within policy incentives to encourage or obstruct migration — not just SLR itself.

  • Evidence suggests that there are strong economic, social and cultural reasons for households to resist migrating away from areas exposed to SLR until migration is the only remaining option.

  • Estimating the number of migrants is difficult because future exposure to SLR is dependent on choices about carbon emissions today, as well as the coastal-adaptation choices we make over time.

  • Policies addressing SLR migration via protection and accommodation are well developed but policies addressing relocation are still too abstract and lack guidance on ensuring equity.

  • Future research on thresholds related to SLR migration and the interplay between physical and social processes will be critical for informing climate-migration policies.

Introduction

Global sea levels have risen by approximately 0.2 m since 1900 (ref.1), with projections showing continued changes under anthropogenic warming. However, estimates of future global mean sea-level rise (SLR) vary widely; current projections for the year 2100, for example, range from a low of 0.4 m to a high of 2.5 m (refs2,3,4), depending on assumptions of future greenhouse gas emissions, thermal expansion, melt of glaciers and the Antarctic and Greenland ice sheets, and isostatic adjustment as ice sheets disappear5. These SLR projections are likely conservative, and continued improvements in ice-sheet modelling suggest high-end SLR predictions are increasingly likely6,7. With global coastal populations totalling more than 600 million (projected to surpass one billion people this century8), any level of SLR is expected to impact and potentially displace a large population9,10. As a result, SLR is anticipated to be one of the most expensive and irreversible future consequences of global climate change9,11,12,13, costing up to 4.5% of global gross domestic product14.

The implications of SLR on human migration first appeared in the scientific literature during the late 1970s16, when there was increased recognition that disintegration of the West Antarctic Ice Sheet could lead to major disruption in coastal cities16, resulting in migration. It is now understood that SLR influences human migration in multiple ways. The most apparent influence involves permanent, irreversible inundation of low-elevation areas, which, under SLR, renders land uninhabitable and unavailable for livelihoods10,12,17, necessitating relocation. However, various other hazards associated with SLR will also impact migration patterns and, in fact, will exert their influence considerably sooner than complete inundation. Hazards include saltwater intrusion into groundwater and agricultural soils18,19,20,21,22,23, coastal flooding24,25,26, shifts in sediment regimes27, coastal erosion28,29 and increased inland penetration of tropical storm surges30,31,32,33. These hazards could spur migration by permanently destroying irrigated coastal agriculture and fresh drinking water supplies34,35, disrupting vital human systems36,37,38, reducing property values39,40 and, ultimately, destroying property and infrastructure41,42. SLR also threatens coastal livelihoods such as tourism43, coastal aquiculture44, fisheries45 and silviculture46, indirectly pressuring migration through adverse impacts on job security.

Since the first studies to quantify population displacement due to SLR15, our fundamental understanding of SLR and human migration has rapidly advanced with the development of basic theory on climate-change migration47,48,49,50, empirical case studies of historical analogues for future SLR51,52,53,54,55, integrated economic analysis and modelling of SLR retreat56,57,58, explicit models of SLR migration50,59,60,61, as well as contentious policy discussions on the need for coastal retreat62,63,64,65. In some cases, studies even question if SLR will spur widespread migration at all66,67, as demonstrated by island residents in the Philippines who would rather adapt in place to SLR-related hazards than follow a relocation programme to the mainland67.

Globally, however, SLR threatens millions of people68, and even with strong reductions in carbon emissions, we are committed to SLR that will impact coastal cities69. The cost to adapt to such hazards will be hundreds of billions of dollars per year9,14,70. Moreover, if SLR forces millions of people further inland, a potential domino effect could result, increasing migration to more distant destinations48 and significantly altering population distributions50,59,60,71. With such a large global population exposed to SLR, there are calls for governments and institutions to facilitate potential migration and protect vulnerable coastal populations72,73.

Given the societal significance surrounding the topic, this Review synthesizes knowledge of potential human mobility and migration responses to SLR, and, in the process, identifies gaps in the research to help further SLR research and migration-related policymaking. We first discuss predictions of SLR migration, before describing decision-making at the individual and household levels. We then consider the institutional or governmental obstacles and facilitators to SLR migration, illustrating differences across the regional contexts of the USA, Bangladesh and atoll island states. Finally, we provide future directions for SLR and human-migration research.

Defining at-risk populations

Under an assumption that exposure corresponds directly with displacement, numerous studies have sought to identify the numbers and locations of people exposed to SLR8,9,10,11,14,15,26,30,73,74,75,76,77,78. However, estimates of SLR displacement are highly divergent, ranging from 88 million14 to 1.4 billion, the variability driven by competing definitions of who is ‘at risk’11,12,30. The three most common ‘at-risk’ definitions are: populations living in the low-elevation coastal zone (LECZ)8,11,76,79,80,81,82, populations living in the 100-year floodplain9,30,73,74,75,83,84,85 and populations living in areas that would be inundated under selected SLR scenarios10,12,14,15,26,85,86,87,88,89,90. All three approaches have associated strengths, weaknesses and implications for understanding the links between SLR and migration.

Assessments using the LECZ approach — typically defined as any area under 10 m in elevation and sometimes within 100 km of a coast8,9,11,75,79 — employ the most generalized and broadest definition of exposure to SLR. Depending on assumptions of future population growth, global estimates of people residing in the LECZ by 2100 range from 634 million11 to 1.4 billion8. Such studies cast the widest net for identifying exposure to SLR and, thus, provide the largest estimates of who might need to relocate as a result. However, residency in the LECZ alone may not entail exposure to any given SLR hazard, let alone displacement due to SLR.

By considering the extent of extreme water levels expected under SLR, residency within the 100-year floodplain offers more precise estimates of population exposure9,30,73,74,75,83,84,85. In the floodplain, residents might experience various SLR-associated hazards that influence migration decisions, such as increased severe storm surges30,31,32,33,73,91, occasional or periodic flooding24,25,92, saltwater intrusion of surface water and soils and groundwater wells19,20,22,93,94,95, shifts in sediment regimes27,28,29,96 and coastal erosion. In comparison to the LECZ approach, the floodplain metric reduces the exposed population in 2100 by roughly two-thirds, from 1.4 billion to 444 million8. However, as with the LECZ, residence in the 100-year floodplain may not necessarily result in migration responses to SLR. Indeed, many low-lying areas in the 100-year floodplain, such as Asia’s densely populated ‘mega-deltas’, possess fertile soil and ample water, which is ideal for farming and fishing. Floodplains thus attract large numbers of migrants from other areas, notwithstanding the presence of coastal hazards97. Simple residency in the 100-year floodplain does not, therefore, result in migration; it is only when the costs of increasing exposure to SLR hazards exceed the benefits of coastal environments that migration may occur.

The most conservative definition of populations ‘at risk’ of relocation due to SLR involves demarcating those living below the future sea level and, thus, projected to be permanently inundated10,12,14. This approach directly links exposure to an SLR hazard that will likely spur human migration: permanent inundation. Unlike the 100-year floodplain, which holds millions of residents, virtually no one lives below sea level. However as much as 0.79% (95% credible interval: 0.22–1.60%) of the world’s population, approximately 88 million people, could be permanently inundated with a median rise of 0.79 m by 2100 (ref.14). While this approach more precisely identifies populations that will have little choice but to relocate under various SLR scenarios, it still only links exposure to a single SLR hazard (permanent inundation), setting aside exposure to salinity, routine flooding and extreme events.

We illustrate the difference in estimates obtained by the three approaches using Bangladesh as an example. By the mid-21st century, Bangladesh is projected to have at least 110 million people living in the 10-m LECZ8, at least 12 million living in the 100-year floodplain8 and about 1 million people directly inundated by SLR10. The difference between each exposure estimate is about one order of magnitude. Thus, as few as 1 million people could be forced to migrate and as many as 110 million people could experience some SLR-related impact, depending on the selection of temporal horizons, vertical elevation thresholds and SLR forecasts.

In the absence of any adaptive measures, the estimate of populations directly inundated likely underestimates those who will migrate due to SLR impacts59, whereas the population estimates in the LECZ and 100-year floodplain likely overestimate potential future migrants. However, because exposure to hazard alone is not a valid indicator of migration potential, none of these estimates (alone or in combination with one another) reliably quantify the number of people likely to migrate due to SLR at global, regional or local scales49,98,99. Indeed, vulnerable communities have often shown an unwillingness or inability to migrate (Box 1), even under constant threat100, influenced by individual and household-level decisions.

Individual and household migration

Decisions by individuals and households to migrate are influenced by more than SLR risk. They instead fall on a multidimensional framework whereby individuals must also weigh up the costs and benefits of a myriad of economic, social, demographic, emotional and political factors97,101,102,103,104, as well as the onset and duration of the environmental hazard itself (Fig. 1). In the case of SLR, people migrate in response to policy incentives, employment opportunities, socioeconomics, and social and kin networks within perceptions of the risk before deciding to migrate. These factors might operate in concert or independently from each other. For instance, property damage from a storm surge alone might not be enough impetus for someone to migrate, but property damage in concert with a policy incentive such as home buyouts might be enough. These factors are then further mediated by individual/household preferences and institutional-level obstacles and facilitators. People make choices about when they move, their destination, who to move with and whether to return97,105,106 — all embedded within a multidimensional decision-making framework (Fig. 1). In different contexts and increasingly over time, SLR hazards, risk perception, adaptation policies and livelihood changes, in particular, will variably factor into migration decisions.

Fig. 1: Migration outcomes under conditions of SLR.
figure1

A schematic of the numerous factors influencing sea-level rise (SLR)-driven migration. Migration from SLR is multifaceted and is influenced by environmental hazards and political, demographic, economic and social factors embedded within policy incentives to encourage or obstruct migration — not just SLR itself. SLR can gradually pressure migration, such as inundation, or suddenly, such as tropical cyclones, and individuals might migrate in reaction to this change or in anticipation of this change. The decision to migrate is also made in conjunction with individual/household contexts where SLR migration might result from a loss of livelihood or due to institutional failure. Institutions also mediate this decision with obstacles and facilitators designed to either prevent migration by reducing SLR hazards or to accentuate migration through retreat. Adapted from ref.47, Springer Nature Limited.

The perception of climate risk forms a critical bridge between a change in SLR and a potential migration response107. Both contextual and cognitive factors influence a person’s risk perception, including proximity to a hazard or potential hazard, past experience with a hazard, existence of structural protections against hazards and the individual’s ideology, economic resources and demographic characteristics108,109. For SLR migration, the perception of flood risk is often paramount110,111,112. Past experience with severe flooding causes people to perceive future flooding events as riskier113,114,115, amplifying possible forward-looking migration responses98,116. Perceptions of the risks of SLR combine with many other factors to influence the decision to move into or away from coastal areas. Therefore, SLR may marginally increase existing migration out-flows and decrease migration in-flows to coastal areas76.

Environmental drivers of migration typically operate by negatively affecting natural-resource-based livelihoods, such as farming and fishing, that are sustained by combining different types of capital (for example, natural, social, financial, physical)117. A sustainable livelihood allows coastal residents to cope with and recover from stresses and shocks while remaining in place. Soil salinization that lowers agricultural yields is one such threat to the sustainable livelihoods of coastal residents59. Other livelihoods threatened by SLR include tourism43, aquiculture44, fisheries45 and silviculture46. When livelihoods deteriorate, people diversify their livelihood portfolio by sending household members to work elsewhere temporarily, with the goal of remitting earnings118,119,120. To date, there is little evidence51 of environmental deterioration leading to complete settlement abandonment121.

Those who are most likely to move away from SLR hazards are those who can best absorb the emotional and financial costs and extract benefits associated with migrating: healthy, skilled, working-age adults, who can increase lifetime potential earnings by moving to higher-wage labour119,122,123,124,125,126. For example, in atoll island nations heavily threatened by SLR, evidence already suggests that SLR hazards translate into reduced housing values40 and migration of young, working-age people for economic opportunities127,128,129,130.

Tropical cyclones provide important analogues131 for SLR and human migration, as increased tropical cyclone intensity is associated with SLR. Generally, cyclones and associated flooding produce temporary, short-term mobility, and not permanent out-migration. For example, a study using millions of mobile network subscribers quantified mobility before, during and after Cyclone Mahasen, which struck Bangladesh in May 2013. They found evidence of slight anomalies in temporary mobility around the storm, but virtually no permanent migration132. In New Orleans, Louisiana, widespread destruction of housing from Hurricane Katrina produced near complete evacuation of the city and increased residents’ mobility over the next several years, but many residents returned after homes and neighbourhoods had been rebuilt133,134. Both examples point to a reluctance to permanently relocate to new destinations (Box 1). Cyclones have yet to produce long-term changes in coastal populations41,135, but higher-intensity cyclones could compromise economic growth in many regions of the world136,137 and, in the long term, affect the viability of coastal communities that are unable to adapt69.

While SLR may displace coastal populations in the future, urbanization and coastal amenities support large coastal populations, and continue to drive pro-coastal migration. For centuries, people have settled in river deltas and coasts for their natural resources and amenities, including fresh water, ecosystem services, transportation and recreational opportunities. These environmental resources and amenities, as well as the disamenities associated with SLR and cyclones, are capitalized in housing prices and wages138,139,140. The processes influencing migration are difficult to incorporate into demographic projections. Instead, those seeking to quantify how SLR will affect future populations identify geographic ‘hotspots’71,141. The extent of coastal urbanization provides clear motivation for institutions in these ‘hotspots’ to prevent SLR-related migration or, in the case where prevention is unfeasible, facilitate migration to safer locales.

Institutional influences

Migration is often described as being one of a wide range of potential household-level adaptation choices to reduce exposure47,142,143. Yet, in the case of SLR-related hazards, there are relatively few long-term adaptation choices available to households apart from migration144. Thus, household-level responses to SLR and other hazards are typically contingent upon or ‘downstream’ from government and institutional responses145,146,147.

Institutional adaptive responses to SLR operate as either obstacles or facilitators to migration (Fig. 1) and fall under three broad categories: protection, accommodation and retreat (Fig. 2). A combination of environmental and socioeconomic conditions influences which response (or mixture of responses) governments employ to cope with, and adapt to, SLR. Protection and accommodation are policy actions designed to prevent migration by either reducing SLR hazards (through protection) or increasing capacity to cope with the hazard (via accommodation). Retreat, by contrast, directly facilitates migration.

Fig. 2: Responses to SLR hazards.
figure2

A schematic illustration of the potential responses to sea-level rise (SLR). a | Protection, which refers to armouring designed to prevent the hazards. b | Accommodation, which refers to adaptation measures designed to facilitate living with the hazards. c | Migration or retreat, which refers to the relocation of individuals or communities away from the hazard.

Much of the adaptation literature focuses on protection measures designed to hold back the sea, prevent the negative impacts of SLR and, thus, reduce the need for migration9,30,73,74,148. These solutions include hard armouring like seawalls, groins and other infrastructure that maintain and expand the current shoreline and, in some cases, provide protection against storm surges. Soft-armouring methods, such as beach nourishment149,150,151 or ‘living shorelines’152,153 further replenish lost sediment and encourage more natural defences to SLR. The costs associated with protecting the world’s coastal populations via protection are astounding73,85 and are projected to reach nearly $100 billion by the end of the century70. Given both the costs and scale, it is unlikely that governments will armour every coastline in the world144,154.

Adaptation responses include accommodation of higher water levels, adjusting usage in and of the coastal zone145 to reduce negative SLR-related hazards that drive migration. Accommodation strategies include elevating homes and structures55,155,156, flood proofing157, managing land use, deploying flood warnings and pumps158, changing groundwater-extraction techniques145 or elevating roads. Many port cities’ coastal management plans already contain provisions to accommodate higher water levels155, such as coastal buffer zones in Ghana159 or multi-tiered terraces in Mokpo, Korea160. Much like costs for protection, those associated with accommodation can be high157, but remain lower than extensive protection. Thus, accommodation seems to be the most feasible adaptation measure154, as demonstrated by its already widespread adoption across the world55,144,161,162.

Retreat includes interventions that aim to facilitate migration out of SLR hazard areas or relocate residents and settlements to safer locations145. Although less desirable than protection and accommodation55,67,163, relocation is already seen as inevitable for a number of ‘hotspot’ communities62,164 now or in the future, when protection and accommodation become too costly or ineffective74. The Carteret Islands in Papua New Guinea, Vunidogoloa in Fiji and Kivalina in Alaska, USA, are some communities either undergoing relocation or have already relocated due to SLR-associated hazards62,164. We identify two types of retreat: planned or managed retreat and unplanned retreat or migration.

Managed retreat

Managed-retreat interventions might include a purposeful and coordinated process of relocation away from the path of eroding coastlines and coastal hazards62. Based on past analogues, managed retreat in response to SLR will likely be limited to small populations living in highly exposed areas49. Barring a catastrophic storm63, it is unlikely for many large cities due to the scale and value of infrastructure, or sunk costs. Instead, these cities are likely to commit to protection in the nearterm165.

Resettlement of a whole community requires centralized planning, where relocation includes considerations for infrastructure and service provisions, such as new roads, schools, markets, clinics and houses62,164,166,167,168. For many countries and areas where most property is privately owned, agreements to resettle are difficult to achieve and the cost is considerable62,167,169,170. Governments also lack coherent and coordinated regulatory approaches to address who exactly is vulnerable, what parameters determine habitability, where communities will relocate and when relocation will occur171. The success of a centrally planned managed retreat also depends on the availability of safer inland areas to host migrants. In many countries, private land owners can wield property rights and ‘institutional muscle’ to legally exclude others in a semi-permanent barrier to migrant entry172. In contrast, countries with capacity for strong centralized planning, like China, are more able to implement retreat strategies requiring involuntary relocation and large-scale mobilization of resources172. Likewise, retreat strategies may be more feasible in countries with large communally owned lands, like the Pacific Island countries of Fiji and Samoa49. For example, Fiji successfully relocated 26 households from the village of Vunidogoloa in 2012 (ref.173), whereas resettlement has been delayed in highly threatened Alaskan villages62.

Unplanned retreat

Many countries lack comprehensive federal approaches for planned relocation174 but have a range of legal mechanisms to support individual and household-level retreat at multiple governmental scales (national, state and local). These policy responses lead to ‘unmanaged’ or ‘unplanned’ retreat175. These include ‘downzoning’ flood-prone areas, creating setbacks or buffers, securing easements from developers and protective zoning176,177,178. Market-based interventions, such as small-scale home buyouts63,112,179,180 may be popular, if not expensive, in higher-income countries with strong private property rights. For example, at least 40,000 voluntary buyouts have occurred in the USA since 1989 (ref.181).

Both managed and unmanaged retreat are generally contentious, with deep and persistent equity concerns. Factors of age, class, race, property ownership and historical structural/institutional disadvantage influence the experience of displacement and retreat180. Relocation can be perceived as ‘thinly veiled forms of social engineering’169 and those who are relocated might suggest that a government is ‘picking sides’169, deeming some ‘victims’ and unworthy of protection65,169. Relocations that follow ‘principles of equitable adaptation’182,183 and retreat can increase the success of a retreat programme, build social capital, deepen civic engagement and networks, and, ultimately, build resilience184,185.

Regional contexts

Neumann et al.8 provide a comprehensive global analysis of the countries with the largest populations in the LECZ and in the 100-year floodplain (Fig. 3a). Countries with more than 50 million people in the LECZ are China (244 million), India (216 million), Bangladesh (109 million), Indonesia (93 million), Vietnam (80 million), Egypt (63 million) and Nigeria (57 million). Countries with more than 10 million people in the 100-year floodplain are China (103 million), India (63 million), Vietnam (50 million), Egypt (20 million), Indonesia (14 million) and Bangladesh (12 million). These countries represent the anticipated ‘hotspots’ of SLR migration, though neither the LECZ nor the 100-year floodplain metrics guarantee SLR-driven migration.

Fig. 3: At-risk populations in the LECZ.
figure3

a | Population projections in the low-elevation coastal zone (LECZ) for 2060 (ref.11); countries with at least 5 million people in the 100-year floodplain but lacking considerable sea-level rise (SLR) and migration research are highlighted with a yellow border. b | Projected populations at risk from SLR in the USA under an SLR scenario of 1.8 m by 2100 (ref.10). c | Projected migrants in coastal Bangladesh due to SLR-induced salinization59. dh | Populations in atoll island nations in 2060 (ref.11). Grey shading indicates countries/counties where data are unavailable or no coastal region is present. Data for part a from ref.8. Data for part b from ref.10. Data for part c from ref.59.

Some of these countries have few, if any, comprehensive studies on SLR and migration, lacking consideration of the potential destinations of migrations or when migration may occur. In some cases, there is more research on the migration of natural systems in response to SLR than of people186,187. Countries with at least 5 million people in the 100-year floodplain but lacking considerable SLR and migration research include China, India, Indonesia, Vietnam, Egypt, Nigeria, Thailand, the Philippines, Japan, Pakistan, Myanmar and Iraq8. The behavioural dynamics of SLR migrants needs greater attention, with a focus on the potential destinations of these migrants50,59,60,71.

Governmental resources and adaptive capacity will combine with local geography to affect how SLR influences migration. Here, we discuss three regional contexts — the USA, Bangladesh and atoll island nations — to highlight similarities and differences in migration signals across contexts (Fig. 3).

USA

Nearly 40% of the US population presently lives in coastal communities188 that are also predicted to see continued growth and development in the future. As a result, SLR (and its corresponding hazards) are projected to threaten between 3 million and 43 million people by 2100, with as many as 13 million that could face permanent inundation and displacement without protective measures8,10,12,76 (Fig. 3b). Half of those exposed to SLR reside in Florida and nearly a quarter in Miami, Florida alone10. Managed retreats in Alaska, Louisiana, New York and Texas offer a potential glimpse of broader-scale retreat in the USA63,65,180, including migration into less vulnerable coastal areas as a form of ‘climate gentrification’39,40, sometimes stemming from retreat itself189.

Numerous historical analyses in the US context51,52,54,55,190,191 find that SLR can overwhelm resilient coastal residents with strong emotional ties to place55, leading to abandonment51. In particular, the 1918 abandonment of Holland Island in the Chesapeake Bay due to SLR, triggered by population levels falling below a level to support community services, is a powerful analogue for potential future abandonment51. Moreover, when people do migrate in response to SLR hazards in the USA, they more often migrate to nearby urban job-growth centres190, rather than making small, incremental migrations52. Accordingly, future migration modelling suggests coastal adjacent, major inland cities, such as Austin, Texas, Orlando, Florida, or Atlanta, Georgia, might become major migration destinations for those migrating in response to SLR inundation50,76. For those migrating in response to a short-term risk, however, evidence suggests that people tend to migrate back to their home community once the risk has receded; for example, with Hurricane Katrina in New Orleans and Hurricane Maria in Puerto Rico42,133.

A wide range of protection and accommodation measures are routinely studied, discussed and deployed in anticipation of SLR in the USA155,192,193,194. Major US cities such as New York and Miami are actively working towards protection and accommodation. How many people these measures will protect from migrating is unknown. Many highly visible and contentious managed retreats are also presently underway in mainly indigenous communities across the USA65,167,170,195, such as the Isle De Jean Charles relocation in Louisiana or the Kivalina relocation in Alaska, where disagreements between tribal members and government agencies have slowed relocation efforts. No national government agency has the financial resources to coordinate or facilitate widespread adaptation and relocation policies196, leading to ad hoc policy deployment. With relocation cost estimates ranging between $200,000 and $1 million per capita62,167,196, widespread managed retreat seems unlikely in the US context, but a number of policy levers may be used to reduce incentives to live on vulnerable coastlines.

Bangladesh

Owing to a long-standing concern for future SLR impacts, Bangladesh has been historically regarded as a major SLR hotspot197,198 and is the third most at-risk country to SLR, with 2 million to 110 million people at risk to SLR and its associated hazards8,60 (Fig. 3c).

Episodic tidal inundation and storm surge are major SLR hazards, amplified by riverine flooding associated with cyclones and seasonal monsoons132. These hazards threaten subsistence farmers and fishers living in low-lying delta, interrupting access to fresh water, driving soil salinization and eroding human settlements and arable farmland32,199,200,201. As 30% of the total cultivatable land of the country lies along the coast, the impacts of salinization on agriculture could undermine food security far beyond the coast202 and are estimated to displace more than 200,000 people annually59. Permanent inundation could ultimately displace upwards of 2.1 million people primarily towards Dhaka60. SLR-related migration in Bangladesh intentionally increases household resilience203 by migrating short distances204 towards pre-existing migration destinations132, spurred by both SLR and socioeconomic vulnerabilities205. This migration is likely to be internal, rather than international206,207.

As in the USA, many climate migrants in Bangladesh gravitate to wage opportunities in urban economic centres132. However, unlike the USA, many of these migrant destinations include cities under similar risk of future SLR. SLR-induced migration may, therefore, contribute to the further expansion of the nation’s informal settlements208. The permanence of these migration patterns has been relatively unexplored due to the paucity of migration data. However, recent work leverages data from millions of mobile network subscribers132 and existing longitudinal data in one survey site209 to monitor migration responses to other SLR hazards, such as cyclone incidence and torrential flooding. Although flooding, in these contexts, clearly disrupts livelihoods, these studies contribute to a growing consensus that the observed migration patterns around extreme events are relatively short-lived132,209,210,211 (Box 1). At present, coastal adaptation in Bangladesh almost exclusively consists of accommodation212. Without a concerted effort to facilitate retreat from coastlines98, SLR impacts may intensify the need to migrate, even while reducing people’s ability to absorb the losses, transition to urban-wage labour and relocate to urban slums.

Atoll island nations

Unlike the USA and Bangladesh with tens of millions of people threatened by SLR, the sparsely populated atoll island nations contain comparatively far fewer people at risk to SLR. For example, SLR threatens 6,000 people in Nauru, 9,000 in Guam, 25,000 in the Northern Mariana Islands, 31,000 in Vanuatu, 91,000 in the Marshall Islands, 133,000 in Fiji, 190,000 in Kiribati and 234,000 in the Solomon Islands8 (Fig. 3dh).

The SLR forecast is so severe for the most low-lying nations that the possibility of deterritorialization has captured much of the discourse on SLR and human migration for many atoll island nations. The United Nations High Commissioner for Refugees noted 10 years ago that early action was needed to prevent statelessness in the low-lying atoll nation states of the Maldives, Tuvalu, Kiribati and the Marshall Islands213. Loss of an entire territory or the exile of an entire population is unprecedented213, introducing unparalleled scenarios of state dissolution and possible statelessness — even if it is unclear that a state would cease to exist if submerged214. This anticipated deterritorialization or substantial territorial loss encompasses legal concerns regarding statehood, national identity, refugee status, state responsibility and access to resources, among other things215 (Box 2). While inundation is a significant concern, atoll nations are likely to face uninhabitability before complete submersion due to lack of fresh water and increased soil salinization215.

There is growing consensus that migration should be planned and coordinated171,182, facilitating movement and admission to other countries for displaced persons214 but strict migration-eligibility criteria and the lack of financial assistance restricts access to neighbouring countries216. The Maldives, Micronesia, the Marshall Island, Kiribati and Fiji have included migration in their national adaptation policies173,217. Kiribati’s noted ‘Migration with Dignity’ approach seeks to ensure the best outcome for cross-border migration I-Kiribati people who flee the impacts of climate change218. However, the policy only paves the way for those already willing and ready to migrate, possibly excluding those with limited literacy skills or those who rely on agriculture and place-based livelihoods218,219.

Conclusion and perspectives

SLR-driven human migration has the potential to alter population distributions at all scales. The work discussed in this Review highlights how hazards associated with SLR might spur human migration and the obstacles and facilitators for this migration. Nonetheless, several significant gaps remain in modelling, measuring and policy development around the implications of SLR for human migration.

First, quantifying the locations and numbers of people ‘at risk’ to SLR cannot be equated with the numbers of migrants responding to SLR. SLR hazards are highly variable across space and time, and their significance for migration, especially towards the end of this century, will largely be driven by greenhouse gas emissions. There must be more careful consideration of what exactly constitutes exposure to SLR and the time frames associated with these exposures. SLR impacts are often discussed in the far future5,12, yet impacts such as reduced housing prices, gentrification and migration are documented today where contemporary SLR is already minimal39,40,59. Additionally, many people presently reside in highly exposed coastal communities and it is necessary to connect the actual hazards of SLR to human migration on timescales of human decision-making.

Second, as many SLR hazards are still yet to manifest, the empirical linkages between SLR hazards and human migration are still too tenuous. Some commentators continue to erroneously describe a predetermined relationship between the inundation of coastal communities and the resultant waves of migration6,172. Research has only begun to turn to the underlying mechanisms that might drive this migration39,40,59, but it is abundantly clear that more research is critically needed to understand the numbers of future migrants, the decades in which migration may occur and their potential destinations. Human behaviour is complex and scientists should focus on how SLR hazards might translate into migration signals. The work on soil salinization is a start59 but is limited to agriculturally dominant contexts. Critically, our understanding of thresholds and tipping points beyond which human migration becomes inevitable is severely limited. Further, this Review highlights the dearth of science on SLR and migration for numerous countries highly threatened by SLR, most notably China, India, Indonesia, Vietnam, Egypt and Nigeria.

Third, common non-migration household adaptations to coastal hazards (such as elevating houses and storing valuables above ground) are not sensible if schools, clinics, workplaces or neighbouring households do not take similar actions. Protection — the most effective adaptations to reduce exposure to SLR and coastal hazards — requires resources generally only available through coordinated policy interventions. Policies addressing relocation are still too abstract and lack guidance to ensure equity. Without more concrete policy guidance for relocation across borders and to facilitate integration into destination communities, migrants from atoll island nations may endure a climate-change-related human rights catastrophe and magnified suffering of the most vulnerable populations. Just as with research on the mechanisms associated with SLR migration, research on immobile or trapped populations (those who are unable or unwilling to migrate) is crucially needed. An important priority is to identify the policies that will best alleviate the suffering of those trapped in increasingly flood-prone areas.

Fourth, global population projections from the United Nations show widespread ageing in every country by the century’s end220. The well-documented relationship between age and migration propensity221 suggests that youthful populations are more likely to migrate than older populations. What are the implications of an ageing coastal zone if ‘migration as adaptation’ is the primary adaptation strategy in many developing countries and older people migrate less than younger people? Migrants continue to migrate to the economic engines in coastal cities and mega-deltas222 but will changing demographics alter this migration dynamic? Relatively little research explores the important implication of ageing on human mobility and migration in the coastal zone.

Rigorous scientific research on SLR and human migration will result from multidisciplinary data, methods and research teams involving oceanographers, anthropologists, geographers, economists, remote sensors, sociologists and geomorphologists, to name a few. Alarmist predictions of ‘climate refugees’ garner press headlines in the Global North and fuel anti-immigrant sentiments. However, the research reviewed here paints a much more complex picture that allows us to anticipate how SLR migration may unfold in different scenarios to develop informed policies that avert crises and promote more equitable and humane outcomes.

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Acknowledgements

This work was supported by the National Socio-Environmental Synthesis Center (SESYNC) under funding received from the National Science Foundation DBI-1639145 and based upon work supported by the National Science Foundation under grant number 1600131.

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M.E.H., E.F., M.B. and V.M. substantially contributed to the discussion of content and wrote and edited the paper. M.C., K.A., R.M. and D.W. substantially contributed to the discussion of content and edited the paper.

Correspondence to Mathew E. Hauer.

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Hauer, M.E., Fussell, E., Mueller, V. et al. Sea-level rise and human migration. Nat Rev Earth Environ 1, 28–39 (2020). https://doi.org/10.1038/s43017-019-0002-9

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