Collaborating Authors

Reframing strategic, managed retreat for transformative climate adaptation


Human societies will transform to address climate change and other stressors. How they choose to transform will depend on what societal values they prioritize. Managed retreat can play a powerful role in expanding the range of possible futures that transformation could achieve and in articulating the values that shape those futures. Consideration of retreat raises tensions about what losses are unacceptable and what aspects of societies are maintained, purposefully altered, or allowed to change unaided. Here we integrate research on retreat, transformational adaptation, climate damages and losses, and design and decision support to chart a roadmap for strategic, managed retreat. At its core, this roadmap requires a fundamental reconceptualization of what it means for retreat to be strategic and managed. The questions raised are relevant to adaptation science and societies far beyond the remit of retreat. Evolving social norms, technologies, and economies will create futures that fundamentally differ from our world today. Climate change constrains the range of possible futures and affects the level of transformation societies will experience.

Ocean recoveries for tomorrows Earth: Hitting a moving target


As the human population has grown, our demands on the ocean have increased rapidly. These demands have similarly increased the pressure we place on these systems, and we now cause considerable damage globally. If we want to maintain healthy ocean ecosystems into the future, we must learn to use ocean resources in a sustainable way and facilitate recovery in regions that have declined. Determining how to make these goals a reality, however, is no small challenge. Ingeman et al. review the challenge presented by attempting both to recover and to use ecosystems simultaneously and discuss several approaches for facilitating this essential dual goal. Ocean defaunation and loss of marine ecosystem services present an urgent need to recover degraded ocean ecosystems. Growing scientific awareness, strong regulations, and effective management have begun to fulfill the promise of recovery. Unfortunately, many efforts remain unsuccessful, in part because marine ecosystems and human societies are changing. Rapid shifts in environmental conditions are undermining previously effective recovery strategies. Moreover, divergent perceptions of recovery exist. Efforts toward reversing marine degradation must address the dynamic social-ecological landscape in which recoveries occur, or forever chase a moving target. Recovery efforts of tomorrow will require institutional and tactical flexibility to keep pace with a changing ocean, and an inclusive concept of recovery. Further, vital population-level efforts will be most successful when complemented by a broader ecosystem and social-ecological perspective. In this Review, we provide a synthesis of ocean-recovery goals as moving targets and highlight promising steps forward. While acknowledging the priority of basic conservation imperatives, successful recoveries can encompass a range of outcomes in the space between minimum ecological viability and maximum carrying capacity. Ongoing advances are improving our ability to predict the effects of environmental change on ocean productivity and to calibrate recovery targets to changing conditions. As a complement to predict-and-prescribe methods, research can also point the way toward robust approaches in the face of irreducible uncertainty.

Pathways to coastal retreat


There is an urgent need to take coastal retreat more seriously as an option for adapting to sea level rise (SLR) and as a strategy capable of providing positive outcomes, if planned ahead. Early signs of such thinking are emerging. We demonstrate how exploring pathways to managed retreat adds value in the context of irreversible long-term SLR. Retreat is typically framed and understood as a single action, largely used after events rather than preemptively, and considered as a last resort. However, implementing managed retreat constitutes a multidecadal sequence of actions (i.e., across pathways) including community engagement, vulnerability assessment, land use planning, active retreat, compensation, and repurposing. This Policy Forum advances practical knowledge on what pathways to coastal retreat may look like and how they can pave the way for flexible and positive transformational adaptation, if started now. SLR globally accelerated from 1.4 mm/year (1901–1990) to 3.6 mm/year (2006–2015) and will continue to do so during this century (10 to 20 mm/year in 2100) ([ 1 ][1]). Sea levels could rise between 0.43 and 0.84 m globally by 2100, relative to 1986–2005, as a median estimate under low and high emission scenarios, respectively. However, a rise of 2 m by 2100 cannot be ruled out ([ 1 ][1]). There is also a clear commitment to SLR centuries into the future due to inertia in both the climate and ocean systems; for every degree of warming, sea levels will eventually rise ∼2.3 m ([ 2 ][2]). Inexorable SLR makes some degree of relocation of coastal residents, buildings, infrastructure, and activities inevitable, even if global warming is mitigated to 1.5° or 2°C. The necessity of paying more serious attention to pathways to managed retreat is becoming urgent ([ 3 ][3]). To begin with, observed coastal flooding is already reaching unacceptable levels for communities and infrastructure in many low-lying coastal settlements around the world ([ 1 ][1]), and unless adaptation starts now, in a few generations, more regions (e.g., small islands, parts of the US coast, major deltas) will be at risk of coastal flooding ([ 1 ][1]). Additionally, retreat requires decadal lead time to plan and implement equitably ([ 3 ][3], [ 4 ][4]). Furthermore, many decisions taken today have a long legacy effect and create path dependencies, closing off some options in the future. For example, coastal defenses last for many decades and protected areas attract people and assets, which lead to expectations of further protection. On the other hand, creating space for wetlands to grow as sea levels rise provides a temporary buffer, keeping future options open for later development or a lower barrier to retreat. Ongoing and accelerating SLR, compounded with other climate-related changes (e.g., intensification of extreme events such as storms, heavy rainfall, and river flows) and increasing population at the coast, is already progressively shrinking the solution space of available adaptation options. Accommodation options (e.g., elevated buildings, early warning, and shelter) will not be enough to reduce coastal risks to acceptable levels under SLR-induced flooding and erosion. As sea levels rise, groundwater salinization will render water supplies unusable and limit food production to saline-tolerant crops. Nor will nature-based solutions, such as offshore reefs or wetland restoration, be likely to keep pace with combined climate change impacts ([ 1 ][1]) and human pressures that have reduced space and sediment supply to the coast. Such responses are therefore expected to be only temporary adaptations in many places ([ 5 ][5]). Hard protection, either through holding the line (protect) or advancing seaward (advance) using levees, barriers, or artificial islands, can be beneficial, for example, in resource-rich megacities but also has limitations, as sustained and rapid SLR would make it increasingly difficult to extend infrastructure within available time frames ([ 6 ][6]). Also, hard protection will not be an affordable long-term solution for every community, nor will it address the impacts of rising groundwater and river flows in every coast ([ 6 ][6]) or the existing and increasing residual risks (e.g., when levees fail). In low-lying coastal areas across different geomorphologies and levels of development, retreat offers an alternative option (see the first figure) that ultimately removes vulnerability and risk in situ. Retreat is not easy, for various reasons, including attachment to place, high costs, lack of risk awareness, impacts on inland settlements, and political resistance ([ 3 ][3]). For example, retreat means sunk costs of existing investments in public infrastructure and private property and does not address the risk to cultural assets that cannot be relocated. However, among the reasons that make managed retreat beneficial is that it enables long-term change at the coast to be anticipated and planned for in an orderly way, which can minimize both stress on people and agencies and inequitable outcomes. Exploring pathways can support staging retreat and help to break retreat into manageable steps over time, align it with maintenance or other social goals (e.g., economic development or environmental conservation), and implement retreat depending on how the future unfolds. This could help to overcome the societal resistance to retreat. Dynamic Adaptive Policy Pathways (DAPP) ([ 7 ][7]) planning is a practical approach developed to do exactly this and is increasingly used to support climate change adaptation decision-making. To date, DAPP planning has been used to address adaptation to SLR in several locations, including the Netherlands, the UK, the US, and New Zealand, where measures have included no-build zones and community and assets relocation ([ 5 ][5], [ 6 ][6], [ 8 ][8]). The long-term perspective puts retreat on the table next to protection and accommodation measures (see the first figure), avoiding increasing investments that eventually become higher sunk costs. A first step in pathways planning is to assess the hazard, vulnerabilities, and uncertainties and to identify adaptation options. An adaptation option may fail to achieve objectives and/or may reach a performance limit or threshold (also referred to as an adaptation tipping point) as conditions change (e.g., SLR); a new or additional measure is then needed. Similarly, opportunities may arise (e.g., when infrastructure needs replacing or when people cannot tolerate SLR impacts and the need for retreat becomes obvious). The first figure presents some thresholds and opportunities for adaptation to SLR that change the solution space. Next, by sequencing options, starting with low-regret and preparatory actions that can and/or need to be taken in the near term, pathways are designed while also testing options for their sensitivity to a range of SLR increments and to their path dependency. Pathways design is often done in a staged manner, with increasing depth of analysis. For part of the city of Miami, Florida, potential pathways were first developed using narratives, by asking stakeholders: What could be short-term, mid-term, and long-term adaptation options? What is the next option? Promising options and pathways were then further assessed using detailed models. In the Netherlands, a study assessed the solution space for multiple meters of SLR before exploring pathways. The study concluded that spatial planning that recognizes the consequences of long-term SLR is needed, because of the uncertain, potentially high SLR. Monitoring is typically used to evaluate success of implementation but is also needed for detecting early warning signals on approaching thresholds and windows of opportunities for preemptive actions (e.g., new insights on future risks or new social values). This helps to identify when a decision to shift to another action is necessary. For adaptation to SLR, signals can be derived from climate drivers (e.g., mass loss from Antarctica, local SLR), impact signposts (e.g., flooding or freshwater availability) based on observations, and scientific studies and assessment [e.g., the Intergovernmental Panel on Climate Change (IPCC)] and, maybe more critically, from social, economic, and cultural signposts (e.g., insurance withdrawal, increased costs, and others developed with communities). Monitoring levels of (in)tolerable risk, increasing exposure to damage through population changes, and infrastructure aging could warn about potential lock-in or lock-out situations. Potential signals need to be evaluated for timeliness and reliability, while considering the required lead time for planning and implementation of next actions. This is problematic in a context of increasing and accelerating coastal risks, where physical and societal thresholds occur close together, with limited time left for implementation, and where communities are dependent on critical infrastructure, the functioning of which is already threatened. For example, in Florida, several water infrastructure thresholds are close or have been reached, where nuisance flooding is observed and the septic systems are being compromised by rising groundwater tables. New infrastructure with pumps and drainage can only buy a limited amount of time ([ 8 ][8]). Beyond mapping the solution space that includes retreat, pathways thinking is also critical to supporting the design and implementation of the transition to retreat, as presented with the nested pathways in the second figure. Although the relevance, extent, rate, and modalities of managed retreat will vary depending on SLR and local context, three generic steps can be highlighted across coastal settlements: preparation, active retreat, and cleanup ([ 5 ][5]). Enabling decision-makers to progressively prepare includes engagement to gain community understanding of the risks and to understand social values and vulnerabilities; planning to identify options, exploring pathways, and establishing monitoring plans to detect signals of opportunities (e.g., early moves, end of lifetime of infrastructure); funding for property acquisition and infrastructure provision in alternative areas; and adjustment of land use plans and regulations. These preparatory actions support active retreat, which comprises the acquisition of property, buyout, and removal of structures or relocation of houses, people, and economic activities. The last step, cleanup, comprises land rehabilitation and repurposing (e.g., for coastal amenity and recreational uses that can relocate readily) until that land is permanently flooded by the sea. Because implementing managed retreat can take decades, it needs to be considered well ahead of any climate-induced societal and physical thresholds ([ 9 ][9]). The time needed depends on each society's willingness and ability to anticipate the climate risks and to act on them before observed impacts. Time is also needed to plan and engage with those affected about the urgency to start the retreat process now, so that individuals can make relocation decisions as opportunities arise. For example, in the Netherlands and New Zealand, retreat to enable river floodplain restoration was signaled well ahead of project implementation in anticipation of the effects of climate change ([ 5 ][5], [ 10 ][10]), which gave time (25 and 10 years, respectively) for eventual removal of houses and purchase of at-risk properties on a voluntary basis. This contrasts with instances where retreat has been triggered after damaging climate events (e.g., after hurricanes Sandy in New York and Katrina in New Orleans ([ 4 ][4], [ 10 ][10]); where protection proved ineffective and retreat was forced, creating additional community stress and costs [e.g., after a storm and mudslide in New Zealand ([ 11 ][11])]; or where forced retreat to a flood-safe area was unsustainable because work was unavailable in the new location [e.g., in the Philippines ([ 12 ][12])]. In the Carteret Islands, Papua New Guinea, resettlement of island populations created negative outcomes owing to a lack of economic opportunities in the relocation areas, land tenure conflicts with established populations, and disruptions to local communities that were not planned for ([ 13 ][13]). These examples illustrate the social consequences of retreat if it does not take a planned and staged pathways approach. ![Figure][14] Indicative adaptation pathways of retreat Retreat is presented as a nested pathway within a broader pathways map, including advance, protect, and accommodate. Retreat comprises three stages: preparation, active retreat, and cleanup. Engagement and monitoring support planning and implementation (gray lines). After designing a plan, land use regulations and temporary measures can be implemented, followed by buyout. Enabling investments and regulations are precursor actions. GRAPHIC: KELLIE HOLOSKI/ SCIENCE BASED ON M. HAASNOOT ET AL. To determine when to start active retreat, one can assess under what conditions retreat is required because of limitations of other strategies, indicating the latest moment at which active retreat should be realized. Another way is to assess the conditions under which retreat becomes more beneficial than other strategies accounting for flood risk, alignment with social goals, and costs. For example, Kool et al. ([ 14 ][15]) worked backward from an infrastructure threshold for SLR of 30 cm, at which point a gravity-based storm-water and wastewater system would need to be replaced by a pumped system. Before that point, the costs for a new system, its lifetime, and the opportunity costs to the community would need to be assessed against the costs and benefits of a retreat option that helps remove the ongoing impacts from SLR. Using pathways for adjacent locations, they identified opportunities for drainage system redesign to buy time for engagement with the community before eventual retreat. Such a strategy consisting of progressive steps can result in a beneficial transition that is supported by the community. An increasing number of studies ([ 3 ][3], [ 5 ][5], [ 10 ][10], [ 15 ][16]) provide lessons for developing robust pathways to coastal retreat: (i) engaging early with affected communities to build understanding of their risk tolerance, vulnerabilities, and values; (ii) enhancing the policy and public understanding of higher risk levels than in the past; (iii) early design of and contributions to design of funding mechanisms and regulations that can enable implementation of retreat; (iv) avoiding developments in places recognized as risky and where existing urbanization trends can be reversed through no-build zones and prohibited land uses; (v) considering locations for new developments or designing them to be movable; and (vi) considering whether buying time through temporary accommodation, protection, or nature-based measures will trigger greater risk exposure and therefore worsen the problem over time, or whether these approaches facilitate a transition to retreat. Inexorable SLR that will continue for centuries means that for many low-lying coastal areas worldwide, retreat is an inevitable adaptation action. If planned now and integrated with social, economic, and cultural goals, the anticipatory and dynamic pathways to retreat can be a positive approach to reduce coastal risks and minimize regret of investments and social inequities. To allow retreat to be considered a serious option and implemented where appropriate, there are a number of necessary enablers that require further attention by the research and policy communities. These include: (i) improved understanding of how SLR is a changing risk over time that requires a shift from static to dynamic pathways decision-making and how this affects communities differently now than in the past; (ii) improved understanding of what managed retreat comprises and how it can be staged over time through monitoring and sharing experiences; (iii) development of policies and regulations that are grounded in anticipatory planning supported by sustainable funding arrangements; (iv) further development of analytical methods relevant to changing risk, such as for mapping the shrinking solution space and identifying if and when retreat will be needed; (v) further assessment of the effectiveness of the range of adaptation responses under alternative futures and how retreat can be integrated with wider societal goals; and (vi) enhancement of the role of political leadership in building community trust in preparation for managed retreat, and embedding commitment devices to maintain the long-term dynamic approaches for reducing SLR risks. Notably, the development and the implementation of any retreat pathway fundamentally depends on the past trajectory of coastal risks; the present situation (governance, coastal strategy, observed impacts, individual and institutional values and attitudes toward climate-related risks); the envisioned future; and when and under what conditions adaptation opportunities and limits appear. Whatever the context considered, it is increasingly evident that the shrinking solution space for adaptation in low-lying coastal areas calls for long-term dynamic pathways planning now. 1. [↵][17]1. H.-O. Pörtner et al. 1. M. Oppenheimer et al ., “Sea level rise and implications for low-lying islands, coasts and communities” in IPCC Special Report on the Ocean and Cryosphere in a Changing Climate, H.-O. Pörtner et al., Eds. (IPCC, 2019). 2. [↵][18]1. A. Levermann et al ., Proc. Natl. Acad. Sci. U.S.A. 110, 13745 (2013). [OpenUrl][19][Abstract/FREE Full Text][20] 3. [↵][21]1. A. R. Siders, 2. M. Hino, 3. 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Decolonize climate adaptation research


Climate-forced population displacement is among the greatest human rights issues of our time, presenting unprecedented challenges to communities and the governments responsible for protecting them. Sea level rise, heat, drought, and wildfires will cause people to move, losing homes and places they love, often with no ability to return. Indigenous Peoples have done the least to cause this crisis and face the loss of lands and connections to ancestral, cultural, and spiritual heritage. To ensure that their right to self-determination is protected and the horrific legacy of government-forced relocations is not repeated, communities must lead and define research on climate-forced displacement and managed retreat that involves them and the lands upon which they dwell and subsist. A focus on human rights, and decolonization of research to change institutional structures of knowledge production, can help communities define their future in a climate-altered world. The government responsibility to protect people may require relocation against peoples' will. Determining which communities are most likely to encounter displacement requires sophisticated assessment of the vulnerability of a community's ecosystem, but also its social, economic, and political structures. Human rights principles, which include rights to food, to safe and sanitary housing, and to water, must be embedded in any relocation governance framework. The right to self-determination ensures that communities make the decision of whether, when, and how relocation will occur and that cultural and spiritual heritage is protected if relocation is the best strategy. Human rights principles also ensure that racial and economic inequities, legacies of colonization and slavery, are addressed when responding to climate-forced displacement. Scholars continue colonization when Indigenous Tribes are not represented in, or consulted for permission to do, research on their communities and lands. Decolonization is the restoration of cultural practices, spirituality, and values that were taken away or abandoned through colonization and that are important for survival, well-being, and subsistence lifestyles. Decolonization advances and empowers Indigenous Peoples and stops perpetuating their subjugation and exploitation. Indigenous-led research can help determine whether inclusion of human rights protections averts or minimizes severe consequences associated with government-mandated relocation. For example, in a letter to the US National Science Foundation expressing concerns with its Navigating the New Arctic program, four Alaska Native organizations explained the danger and damage to their communities when outside academics define food security, resilience, and adaptation, highlighting the importance of Indigenous scholarship and voices in research.[*][1] Self-determination and decolonization mean that communities control the narrative about how the climate crisis affects them. Colonization continues when non-Indigenous scholars write narratives about “vanishing cultures.” The Alaska Native Science Commission and Inuit Circumpolar Council provide a promising model, having protocols that ensure Indigenous communities lead research efforts, defining the questions and methodologies. Non-Indigenous scholars need to build relationships and trust with Tribes before submitting funding applications to understand how skills offered by academic researchers can benefit and complement skills and expertise of Indigenous knowledge holders. Community-based environmental monitoring, and coproduction of knowledge, are important decolonizing tools that can facilitate empowerment and capacity building. Community-based monitoring is important to understand local ecosystem change, which is critical to implementing community-based adaptation strategies; global, regional, and national climate change assessments generally aggregate information above the level of resolution required for effective community policy. We reflect here on our experience in the North American Arctic and Subarctic, but such issues arise in communities around the globe. Countries such as Kiribati and Maldives face inundation from sea level rise, possibly leaving residents stateless. Sea level rise and extreme weather threaten lives and livelihoods in coastal communities in Egypt, Panama, and elsewhere. Research must support and build the capacity of Indigenous Tribes and local communities so that they have tools to respond dynamically to support adaptation that protects their human rights. [1]: #fn-1

Assessing human habitability and migration


Habitability loss is increasingly recognized as an important dimension of climate risk assessment and one with complex linkages to migration. Most habitability assessments, like climate risk assessments more generally, are based on “top-down” approaches that apply quantitative models using uniform methodologies and generalizable assumptions at global and regional scales, privileging physical sciences over social science–informed understandings of local vulnerability and adaptive capacity. Many assessments have focused on a single climate hazard threshold (such as permanent inundation or the 1-in-100-year flood), and a subset have implied that outmigration may be one of the few viable adaptation responses ([ 1 ][1]). There is a risk that such climate determinism minimizes the potential for human agency to find creative, locally appropriate solutions. Although top-down modeling can serve a useful purpose in identifying potential future “hot spots” for habitability decline and potential outmigration, only by integrating “bottom-up” insights related to place-based physical systems and social contexts, including potential adaptive responses, will we arrive at a more nuanced understanding. This integrated framework would encourage development of policies that identify the most feasible and actionable local adaptation options across diverse geographies and groups, rather than options that are deterministic and one-size-fits-all and encourage binary “migrate or not” decisions. We propose a set of recommendations centered around building the research and assessment knowledge base most needed to inform policy responses around habitability loss and migration. We define habitability as the environmental conditions in a particular setting that support healthy human life, productive livelihoods, and sustainable intergenerational development. Climate change may undermine one or more of the following associated, interacting, dimensions of habitability: basic human survival ([ 2 ][2]), livelihood security ([ 3 ][3]), and societies' capacity to manage environmental risks ([ 4 ][4]). Rapid rates of climate change and departures from historical variability ranges can increase risks, especially when coupled with nonclimate stressors. In such instances, threats to habitability may be evident in changing flows of human migration, whether forced or voluntary ([ 5 ][5]). Most habitability assessments have relied on outputs from top-down models. This approach is conducive to system-level prediction, producing quantitative outputs that are globally comparable, such as single physical hazard thresholds that are either assumed or empirically based. Much recent work reflects a blend of long-term, high-resolution historical climate data where available, combined with projections across a large suite of global climate models driven by multiple representative concentration pathways (RCPs) representing trajectories of greenhouse gas concentrations. Another critical element is inclusion of extreme events, often expressed as a frequency of occurrence or a magnitude associated with a given recurrence period. In turn, top-down demographic and economic models, which form the basis for the shared socioeconomic pathways (SSPs) projecting global socioeconomic trajectories, provide a picture of future population and development that can also inform projections of people and assets at risk. Climate projections can also drive sectoral impact assessments—for example, empirically by extending historical statistical relationships between climate variability and the affected sector. More commonly, projections from standardized climate simulations drive sectoral impact models that dynamically simulate key features, such as crop growth. Top-down migration models use relative changes in sectoral impacts across regions along with other information as a means of projecting future population flows. Thus, these models project responses to habitability changes in regions where varying conditions may lead to outmigration, inmigration, or both. The standardized nature of top-down methods facilitates comparisons—for example, of regions most at risk of crossing habitability thresholds associated with a climate hazard, and when. The top-down perspective can also reveal large-scale trends and interconnected features of global systems. However, there are several limitations. First, local and regional geophysical and sector-specific factors can drive hazards and risks at scales missed by global analyses. Second, less-modeled, place-specific characteristics of populations, such as health and socioeconomic status, shape both exposure and vulnerability. Third, adaptation choices and activities are embedded in historical context and culturally specific individual and community values and objectives that cannot easily be incorporated in models. Fourth, high-impact outcomes—associated, for example, with compound extreme events and abrupt changes in climate, ecological, and social systems—may be underestimated because of top-down model limitations such as the inability to credibly resolve evolving correlation structures across variables, space, and time, and key system sensitivities and feedbacks within and across systems ([ 6 ][6]). For example, climate phenomena teleconnected across great distances may lead to “breadbasket” failures in key food-producing regions and price shocks that can seriously reduce food security among vulnerable populations far away from the regions experiencing the climate stress. Fortunately, top-down approaches are increasingly being paired with bottom-up approaches that offer a specificity that can help address these challenges. Bottom-up conceptual and/or computational modeling of complex adaptive systems can be designed to simulate the local experience of losing habitability over time. In the breadbasket case above, models of local responses can be paired with global models of international food trade that set boundary conditions. For example, agent-based models (ABMs) set up simulations with agents empirically calibrated to behaviorally respond to changing environmental conditions: the loss of assets and livelihood opportunities, threats to life, and changing structure of social networks. Modeling can be trained on local data to understand and predict important feedbacks at higher spatial and temporal resolution than is possible with global models. ABMs can be calibrated to examine a range of individual-actor preferences and test the effect of local decision-making to plausibly depict tradeoffs among adaptation options, including migration ([ 7 ][7]). As another bottom-up example, qualitative information can be coproduced with diverse stakeholders, including subject matter experts, to explore high-impact scenarios and local solutions that will be missed by top-down approaches. Of course, bottom-up approaches have their limitations as well. For example, their specificity makes it difficult to compare across geographies and groups, and individual methodological decisions can appear arbitrary. Furthermore, bottom-up computational models such as ABMs are still limited by a lack of empirical data with which to calibrate model parameters. Here, we walk through the habitability challenges of two climate hazard examples, demonstrating the strengths and limitations of top-down approaches and how bottom-up perspectives lead to different policy-relevant insights. ### Sea level rise and extreme sea level events Recent years have seen growing complexity and nuance in assessments. Global assessments have supplemented climate model outputs by considering a broad range of sea level change components and including, for example, expert elicitation as a means of estimating low-probability, high-consequence outcomes ([ 8 ][8]). High-spatial-resolution digital elevation models and consideration of changes in the frequency and intensity of societally relevant metrics such as recurrence intervals and extreme values of coastal high water have been integrated into global products. Using many of the above advances, Kulp and Strauss estimated that the number of people exposed annually to coastal flooding under constant population could increase from 250 million people today to, by 2100, 310 million to 420 million under an intermediate scenario to 380 million to 630 million under a high-end scenario ([ 1 ][1]). Other studies have included changes in storms, hyper-local positive correlations between population density and subsidence, population projections consistent with SSP-RCP combinations, and assets at risk. Additional refinements have focused on specific coastal locations, adding critical context at the expense of global information. For example, Storlazzi et al. framed their assessment of tipping-point risks to atolls around two metrics—annual overwash events that threaten infrastructure, and salinization of groundwater—that are specifically relevant for atolls given their small size, uniformly low elevation, and relative isolation and found that habitability is threatened in most atoll islands by the middle of the 21st century, far sooner than permanent-inundation–based studies would suggest ([ 9 ][9]). Some local studies have included dynamic interaction between coastal waters and adjacent landforms. Other local and regional studies have considered social dimensions of human vulnerability, as well as in situ adaptation, using empirically calibrated agent-based livelihood decision models that span multiple climate, RCP, and SSP scenarios ([ 7 ][7]). The three dimensions of habitability demonstrate why no single coastal flood metric threshold can be determined in a top-down way. For the direct survivability dimension, key factors include future flood control, feasibility of evacuation, and the stochasticity of individual storms. For livelihood, saline intrusion, for example, could benefit some sectors such as specialized aquaculture, even as it harms most sectors and people. And for the societal resilience dimension, large-scale factors such as levels of inequity, strength of governance and social networks, and quality of infrastructure will be critical. As sea levels rise and coastal flooding becomes more common, social, economic, and political factors in some locations will conspire to induce sudden loss of habitability far sooner than physical hazard–based thresholds such as permanent inundation would suggest, as risk perception and long-term economic viability shift. For example, increases in insurance premiums could negatively affect asset values and tax revenues, leading to deteriorating infrastructure and services. The timing of such threshold-crossing cannot be predicted on the basis of top-down models alone. In some instances, shocks can lead to rapid learning, adjustment, and in situ adaptation, at least temporarily. ![Figure][10] Frequent exceedance by 2100 of historically rare climate thresholds Under the high-emissions scenario RCP8.5, at most coastal locations extreme sea level events historically defined as 1-in-100-year events are projected to range in frequency from once per year to more than 10 times per year due to the effects of sea level rise alone. Only point locations where historical event data are available are shown. Projected number of days per year by 2100 exceeding a 33°C wet bulb globe temperature (WBGT) in a high-emissions scenario are also depicted. Under standard assumptions of wind and solar radiation, a WBGT of 33°C corresponds to a wet bulb temperature of roughly 31.5°C. [Sea level data are from figure 4.12 in ([ 8 ][8]); WBGT data are from fig ure 3 in ([ 12 ][11]).] GRAPHIC: N. DESAI/ SCIENCE BASED ON HORTON ETAL. ### Extreme heat Most assessments of future heat hazards have considered temperature only, although recent efforts are increasingly adopting a compound events framework—for example, considering how co-occurring extremes of high temperature and high humidity can modulate threats to habitability. Humid heat is particularly harmful to human health and the ability to engage in outdoor activities. Sherwood and Huber described a wet bulb temperature of 35°C as a threshold above which humans could not survive beyond approximately 6 hours owing to physiological and thermodynamic limits on the ability to cool through perspiration ([ 2 ][2]). Model-based studies have projected that this threshold could be crossed in the Persian Gulf and South Asia during the second half of the 21st Century under a high-emissions scenario ([ 10 ][12]). However, a finer-scale study found that this threshold has already been briefly crossed multiple times in populous cities. Although an absolute habitability threshold exists for the survivability dimension of extreme humid heat, some people will lose their ability to thermoregulate at much lower wet bulb temperatures. Mortality rates of the elderly, those with chronic health conditions, and those involved in strenuous activity rise dramatically well below the 35°C wet bulb threshold. In terms of the livelihood dimension, at ∼3.5°C of global warming above preindustrial levels, de Lima et al. project that in Sub-Saharan Africa and Southeast Asia increases in humid heat may decrease agricultural labor productivity by 30 to 50%, leading to larger agricultural sector impacts than are associated with direct temperature and CO2 effects on crops ([ 11 ][13]). However, air conditioning and other adaptations will enable—indeed, have enabled—some people to continue to live in places that exceed the 35°C threshold. Such an outcome increases inequity because those with no option but to work outdoors, or no access to affordable air conditioning, would be forced to migrate. And even for those with air conditioning, the third dimension of habitability—society's capacity to manage environmental risks—will be tested in unforeseen ways because it will be critical that air conditioning not fail. Sea level rise and extreme humid heat are far from the only climate hazards that have been assessed in the literature for potential habitability thresholds. For example, changes in surface moisture fluxes as mean precipitation and temperature shift are projected to have large impacts on dryland agriculture, fire regimes in forests, and water availability downstream from snow and glacier reservoirs. These and other hazards and impacts may overlap and interact across scales to affect habitability in complex ways, such as by potentially increasing the risk of conflict. Areas where current-day rare extreme sea level and humid heat events will occur with high frequency by the end of the century under a high emissions scenario of sea level rise and warming are identified in the figure ([ 8 ][8], [ 12 ][11]). The two metrics, corresponding to the current 1-in-100-year extreme sea level event and a wet bulb globe temperature of 33°C, respectively, are emblematic of top-down approaches. They thus represent an important point of entry for engagement with the bottom-up insights described above, as a step toward more nuanced habitability and migration assessments. Migration may result from threats to survival, upended livelihoods, or the breakdown in the collective capacity to adapt ([ 5 ][5]). However, research on climate change and migration makes clear that an even broader set of factors undergird migration decision-making. A decision to move is ultimately a personal or household judgment on factors that include local habitability. Involuntary migration occurs when people lack agency about the key dimensions of mobility, including the timing, destination, or duration of mobility or whether to migrate at all. Where agency is extremely low, involuntary migration may take different forms, including temporary or permanent displacement and distress migration. Distress migration—mass migration or displacement related to rapid deterioration in local circumstances—is a humanitarian concern because of the need for emergency interventions to avoid poor outcomes. Distress migration has been a common phenomenon throughout history but has risen and fallen on the global policy agenda largely as a function of whether or not wealthy industrialized countries are destinations. Also of humanitarian concern is the phenomenon of involuntary immobility, in which people are unable to move without help—the population most likely to require assistance relocating under managed retreat programs. Avoiding distress migration and involuntary immobility in favor of safe and orderly migration, as advanced by the Global Compact on Migration, is now a global policy priority, and the Compact calls on governments to “strengthen joint analysis and sharing of information to better map, understand, predict, and address migration movements” as a result of climate change impacts—all of which are essential aspects of habitability assessment. Many assessments posit some form of forced migration as an inevitable outcome of declining habitability. Yet, environmental stress rarely directly results in migration but works through a complex array of economic, demographic, social, and political proximate determinants that both initiate and sustain or modify flows. In any given population exposed to climate risks, different segments of the population respond to hazards differently and at different points in time, and as such, migration evolves with habitability through time. Whereas some may be able to migrate from deteriorating conditions without assistance, others may become immobile owing to limited options and insufficient resources, suffering progressive impoverishment and vulnerability unless social protection or planned relocation efforts are implemented ([ 5 ][5]). In situ adaptation, facilitated migration, and improving reception of migrants in (largely urban) destination areas are often more appropriate policies in these regions. Managed retreat has been proposed as a strategy for regions with declining habitability, but as a largely technical package of responses that includes buyouts, incentives, and planned relocation, among others, it does not currently translate well to most developing-world circumstances. The relationship between habitability and migration may be counterintuitive, as illustrated by the lack of evidence for migration away from low-lying delta areas despite acute risks ([ 7 ][7]). Migration itself affects habitability for those who are unable or unwilling to leave increasingly vulnerable circumstances, either positively, such as through incoming remittances, or negatively, such as through outmigration of the working-age demographic stratum and subsequent changes in economic dynamism and livelihood options. Flows may begin owing to entrenched poverty and environmental risks and then be sustained as migrant social networks lower barriers for those who initially remained behind. Although migration offers possibilities for advancing human well-being, as multiple dimensions of habitability are compromised, resulting forced migration will negatively affect human well-being. Migrants risk new constraints in urban informal settlements, and displaced persons may become permanently disconnected from their original communities and livelihoods in resettlement communities or refugee camps ([ 13 ][14]). Although top-down assessments oversimplify likely migratory responses to habitability declines, this does not necessarily imply that migration flows are overestimated. Multiple factors are driving migration in developing regions to varying degrees, including poor governance, perceived lack of opportunities, conflict, individual extreme events, and in some cases, climate-catastrophic discourses that add to a sense of hopelessness ([ 14 ][15]). Deeper and more contextualized understandings of migration dynamics aid in policy design, but the threats that result from declining habitability in combination with other drivers are real and may lead to substantial displacement of populations across a range of spatial scales. Top-down, threshold-based habitability assessments can serve a critical role in helping to identify priority regions and groups for integrated bottom-up work while revealing interactions in global systems that cannot be gleaned from the bottom-up work alone. Integration not only leads to better predictions of when and where habitability may diminish but also can be used to inform adaptation responses that themselves help preserve or restore habitability. Bottom-up assessments by definition provide finer, local resolution, and their richness of detail means that they require diverse participation and methods. To date, most locales have not been subject to such integrated habitability assessment. We thus encourage transdisciplinary, long-term coupled top-down and bottom-up habitability assessment [for example, ([ 15 ][16])] to complement and augment efforts such as the Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP), which has contributed so much to our understanding of potential future climate impacts on sectors such as agriculture, water, ecosystems, and health. Initial model intercomparison could focus on what regions and groups face diminishing habitability under different model configurations. Particularly where models agree on potential habitability hot spots, bottom-up modeling experiments could be conducted and compared on specified challenges to human survival, livelihoods, and capacity to manage risk, although standardization would be needed. The Intergovernmental Panel on Climate Change (IPCC) and national efforts can also help to develop this still inchoate middle space between top-down and bottom-up approaches to habitability and migration. Migration is emerging as a cross-cutting theme throughout the current IPCC assessment, and a special report on habitability and migration would both advance the knowledge base and showcase emerging methodologies. As one example, a climate change detection and attribution dimension would help inform dialogues about loss and damage under the Paris Agreement. Likewise, a discussion on migration across the Reasons for Concern commonly used in IPCC assessments ([ 5 ][5]) would allow us to distinguish how climate-induced migration, distress or otherwise, is distinct from other forms of migration. The complexity of the assessment challenge calls for a holistic, people-centric approach in which models, data aggregation, and ethnographic work are all advanced. Sectors such as engineering, hydrology, and reinsurance, that have historically been overreliant on physical models and hazard thresholds, operate at a scale that is ripe for habitability-relevant innovations at the interface between top down and bottom up. In this middle space, models could be used to examine policy scenarios instead of learning occurring exclusively from costly, time-consuming, real-world policy interventions that may put vulnerable people at risk. Greater communication among modelers will be key, and models must be validated with on-the-ground local research. To support migration and habitability modeling specifically, this would include data on when, where, and why people have moved or considered moving, how they define habitability, and the policy conditions that determine mobility outcomes ([ 14 ][15]). Furthermore, bottom-up research must account for the place-specific characteristics of populations—such as assets, livelihood opportunities, and social networks—that shape both exposure and adaptation. Investments in place-based social science thus help address data gaps, providing ground-truthing that will strengthen simulations of the outcomes of interventions. Investments in early-warning systems could help to anticipate where distress migration may happen, a key step in informing policy. The shortcomings of adaptation planning and policy at current risk levels in wealthy countries hint at the global challenges ahead in a changing climate. In the United States, for example, federal and local risk assessments—let alone policies—are not presently centrally coordinated or comparable. There is woefully insufficient funding available for bottom-up adaptation efforts from the better-financed federal level. Policies toward population mobility—whether planned, internal responses or immigration from other countries—vary from inconsistent over time to incoherent and sometimes inhumane. Coproduction of knowledge across diverse groups will be a precondition for any breakthroughs. In some instances, a starting point may be to bring preexisting top-down habitability and migration assessments to communities, provided that community feedback is collected and integrated iteratively and before key policy decisions are made. In other instances, stakeholder engagement may begin with fewer top-down, nonprobabilistic approaches that can be developed with communities, such as storylines and scenarios. Storylines and scenarios lend themselves to exploration of the uncertainties that most influence habitability locally (for example, the potential for changing correlation structures in models) and which adaptation strategies should be explored for which groups. Deeper stakeholder engagement, coupled with the other recommendations above, thus provides a foundation for colearning, iteration, and developing flexible approaches to the challenge of diminishing habitability. To the extent that top-down, threshold-based approaches are used to define habitability universally, there is a risk of assuming a high likelihood of uniform outmigration or concluding with blanket policy recommendations around managed retreat. Basing assessments on nuanced definitions of habitability and integrating top-down with bottom-up approaches could encourage a broader range of policies tailored to specific locations and groups, including regions that have been put forth as likely receiving areas. A focus on the dimensions of habitability presented here, and bottom-up approaches, will invariably alter top-down projections of migration. Under wetbulb temperatures exceeding 35°C, high levels of outmigration from the Persian Gulf may be avoided if air conditioning is widely available and alternative livelihood options develop for those who would otherwise work outdoors. However, there will be regions where social tipping points and a sense of prevailing pessimism about the future—for example, owing to evolving risk perception or disinvestment by the private or public sectors—could contribute to outmigration far sooner and more suddenly than top-down habitability threshold–based methods would suggest. Global, regional, and national migration policies themselves will also play an important role in facilitating or impeding migration. What is already clear is that climate change will result in shifting population distributions and that this process will overall be harmful to the most vulnerable, including those who may be “trapped” in deteriorating circumstances. 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