The Fifth National Climate Assessment assesses the many ways climate change affects people, nature, and infrastructure across the Nation and around the world. The chapters are divided up by sectors and topics, such as water (Ch. 4), food (Ch. 11), economics (Ch. 19), and social justice (Ch. 20). They are also organized by region, from the Northeast (Ch. 21) to Hawaiʻi and the US-Affiliated Pacific Islands (Ch. 30).
Mach, K.J., R. Vallario, J.R. Arnold, C. Brelsford, K.V. Calvin, A.N. Flores, J. Gao, K. Jagannathan, D. Judi, C.E. Martín, F.C. Moore, R. Moss, E. Nance, B. Rashleigh, P.M. Reed, L. Shi, and L.L. Turek-Hankins, 2023: Ch. 18. Sector interactions, multiple stressors, and complex systems. In: Fifth National Climate Assessment. Crimmins, A.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, B.C. Stewart, and T.K. Maycock, Eds. U.S. Global Change Research Program, Washington, DC, USA. https://doi.org/10.7930/NCA5.2023.CH18
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But in reality, the impacts and risks of climate change unfold across interacting sectors and regions. For example, a forest fire in one region (Ch. 7) can affect air quality (Ch. 14) and human health (Ch. 15) in other regions as well (Chs. 21–30), depending on where winds blow. Further, climate-related hazards interact with multiple stressors that might seem like they have nothing to do with climate change, such as the COVID-19 pandemic, economic recessions, or social inequities. For instance, different households with different levels of wealth can have very different capacities to evacuate in advance of a hurricane or recover if their homes are damaged (Chs. 9, 20, 22, 23).
As a result, if the perspective of only a single sector, topic, or region were considered, many climate impacts might be missed or overlooked. The consequences of climate change would be unexpected and surprising. And at the same time, the prospects for climate responses, whether through adaptation or mitigation, also fundamentally depend on these same interactions across sectors and regions. For example, using water (Ch. 4) for hydropower (Ch. 5) can impact fish in rivers (Ch. 8), as well as water supply for agriculture in rural communities (Ch. 11) and residential use in big cities (Ch. 12). Without considering such types of interactions, climate responses will be less effective, and there could be missed opportunities.
This chapter is about these deep connections inherent to climate impacts, risks, and responses. The chapter considers these interactions and interdependencies across sectors and regions as complex systems that can lead to cascading impacts and sudden failures, as well as sometimes surprising potential for reducing our emissions of heat-trapping gases and preparing for climate risks that can’t be avoided.
The chapter is organized as follows. First, it introduces what we know about complex systems and explains how complex systems—involving interactions across sectors and regions—can lead to climate impacts that happen faster than expected or can limit future options (KM 18.1). Second, the chapter assesses how complex, interacting climate impacts and responses can be most stressful for overburdened communities (KM 18.2). Third, the chapter evaluates how collaborative approaches to generating knowledge about complex systems can lead to better climate responses (KM 18.3). Finally, the chapter considers the degree to which current governance approaches are adequately prepared to handle the complexity of climate change (KM 18.4).
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Human–natural systems are dynamic and complex. Interconnected networks of people, infrastructure, commodities, goods, and services influence changing climate risks and are increasingly vulnerable to their impacts (high confidence). The vulnerabilities in these networks, and their effects on human–natural systems, strongly depend on human responses and other compounding stressors (high confidence). Decision-makers seeking to reduce climate change risks have to navigate diverse and sometimes competing objectives and perspectives across many actors, institutions, and geographic scales while reconciling deep uncertainties and limits to predictability (high confidence).
In a changing climate, interconnections among human and natural systems give rise to both successes and failures. For example, power systems can fail if exposed to extreme wind or extreme heat (KM 5.2). When they fail, transportation, water and wastewater treatment, telecommunications, health services, and many other economic activities are also disrupted (KM 19.3).1,2 The interactions among people, systems (e.g., networks, nature), and sectors thereby transmit opportunities and risks from one to another (e.g., KM 27.4), such as from the power system to the transportation system. These interactions lead to new risks and can both increase and limit existing threats.3,4,5,6 Figure 18.1 shows six interconnected and overlapping features of human–natural systems; each of these features contributes to the complexity of the systems and influences the effectiveness of climate mitigation and adaptation actions. Box 18.1 illustrates these types of interactions through a specific example, data centers.
Climate change makes the existing interconnections between human and natural systems more important for two reasons. First, many human and natural systems have conditions in which a small shock or change can produce very large impacts, but these conditions are rarely known in advance.7 For example, the winter storm in Texas in February 2021 was a relatively brief 10-day shock of extreme cold weather, but it caused cascading failures across many sectors (e.g., energy, water, health), as well as fatalities and long-lived economic impacts for the region and for individuals.8 There is evidence that warming in the polar north can increase the potential for these types of winter storm shocks in the US.9 Second, climate change puts natural systems into fundamentally new conditions, so there is a greater chance of experiencing these large potentially negative impacts. Climate change–driven aridification of the American Southwest is an example of a fundamental change that has resulted in more persistent and severe heat as well as drought extremes. The consequences of this fundamental change are complex, increasing the potential for compounding and cascading impacts. These effects include simultaneous increases in the potential risks for human health, disruptions to critical transportation services, stress on power systems, and water scarcity conflicts across regions and sectors.10,11 The overall risk of bad outcomes increases, and the sources of risk are more complex. There has been substantial progress in the study of complex, adaptive human–natural systems.3,12,13,14,15,16,17,18 Innovations across many disciplines are helping to address interconnected risks.19,20,21,22,23,24,25,26 Further, climate mitigation and adaptation actions depend on one another and interact. They both require resilience to intensifying and increasingly complex mixtures of future influences, shocks, and hazards. Figure 18.1 shows how these future influences can be climate-related hazards, such as floods, droughts, or wildfires; environmental shifts, such as changes in forest composition; or social trends or disruptions, such as from changes in economies, political contexts, cultures, or disease.2,5,12,16,22,23,26,27,28,29 Mitigation and adaptation involve major changes to human–natural systems with global to local implications (KMs 31.1, 32.5). They can create fundamental shifts in supply chains, consumption patterns, technologies, and competition over constrained natural resources (also see Focus on Risks to Supply Chains).13,27,30,31,32,33,34 These changes are expected to continue to have disproportionate impacts on exposed populations and ecosystems (KMs 4.2, 5.2, 8.3, 9.3, 15.2).
There are therefore trade-offs, linked benefits, and dependencies among energy transitions, adaptation actions, and sustainability goals (KMs 17.4, 25.4, 31.3, 32.2).12,15,19,22,28,35,36,37,38,39,40 For example, an increased supply of clean energy helps power air-conditioning to keep homes cool under intensifying heat while also ensuring that this increased energy usage does not cause emissions of heat-trapping gases. This situation also has implications for social equity and justice, as poverty reduction and improved energy security are needed for all households to afford adequate air-conditioning. These interactions demonstrate the importance of accounting for diverse forms of knowledge, social and institutional power dynamics, and justice in navigating risks, challenges, and benefits of interventions and policies (Figure 18.1; KMs 16.3, 17.4, 20.1, 31.3).31,41,42,43 Recent advances offer promise for assessing risk–benefit trade-offs, interconnections, and sequences of action or inaction across scales.44,45,46,47 However, rapidly changing human systems, their multisectoral dynamics, and their interconnections with natural systems create deeply uncertain futures. Many futures are possible, and there is disagreement on how probable they are or how they may shape different human–natural systems’ responses. Therefore, significant challenges remain for measuring risk–benefit trade-offs when evaluating potential actions.48,49,50,51
Recognition of these deep uncertainties and the difficulties in predicting complex interconnections among human and natural systems over long time periods has led to an approach called exploratory systems modeling.52,53,54 This approach considers diverse scientific perspectives and uses scenarios to better understand a wide array of possible future outcomes. For example, scenarios can examine the consequences of extreme weather conditions that have never previously happened but may happen under a future climate. The approach aims to discover what future conditions, actions, and outcomes are the most consequential.45,47,55,56,57 Capital investments associated with new energy sources and climate adaptation are both expensive and long-lived. Exploratory modeling of benefits and impacts of these investments can help avoid the unintended amplification of risks and increase future resilience. Exploratory modeling can support adaptation planning by providing a wider array of futures, such as for coastal systems under sea level rise,58 and by more clearly identifying responses as they occur among interconnected, complex human and natural systems.
Compounding and cascading interactions among sectors, hazards, and geographies magnify the impact of climate change and societal responses for already-overburdened groups (high confidence). However, social vulnerability assessments tend to evaluate risks and impacts by sector, hazard, or jurisdiction, and most complex-systems models do not yet account for social and political dynamics (high confidence). Data about how complex systems affect frontline communities under climate change are severely lacking, especially for hard-to-reach populations, and this can lead to disproportionate risks and impacts for these groups (high confidence).
The complex systems described in KM 18.1 can create cascading and compounding climate impacts that particularly affect people and communities with little flexibility to absorb additional stress. This includes smaller and more rural communities, lower-income households, racialized minorities, people with health conditions and disabilities, pregnant people, caregivers, young children, and older adults (KMs 4.2, 15.2, 16.2).18,65,66,67,68,69,70,71 The combination of complex societal factors (Figure 18.1) with people’s multiple roles and identities shapes their lived experiences of multiple shocks and stressors (Figure 18.2).72,73,74 It is not an accident that access to land, housing, infrastructure, food, and water in the US is highly unequal.67,75 Within a context of institutionalized inequality and uneven access to safe housing and quality infrastructure, new and conflicting private- and public-sector responses to complex climatic and non-climate events often reinforce existing inequities.18 At the same time, each person carries multiple identities and plays multiple roles, creating intersectional vulnerabilities that can intensify direct impacts of climate change and indirect impacts of climate actions by more privileged groups. For instance, in rural Alaska, climate change is rendering more difficult the lives of Iñupiat women as they care for multiple generations, maintain land-based food and cultural systems, and endure gender-based violence,76 while young, single men working in the oil and gas industry are experiencing the energy transition very differently.
Increased exposure to flooding provides an example of how compounding and cascading interactions and intersectional vulnerabilities can amplify harm in already-burdened communities. Legacies of inequitable access to residential home loans, municipal incorporation to isolate wealth in suburbs, and infrastructure investments that privileged certain neighborhoods and municipalities over others have concentrated low-income people, African Americans, and other frontline communities in places with high flood risk.77,78,79,80 Federal flood risk response programs privilege predominantly White and wealthier communities by giving them more funding for levees or seawalls.81 These measures worsen downstream or down-coast flooding in places that might not be able to afford such infrastructure.80 Federal programs also disproportionately fund predominantly White communities to voluntarily relocate from floodplains (Figure 20.3),82,83,84 while low-income people, including renters, receive less assistance and have to move farther away to places with fewer amenities.85 Box 18.2 shows a different example of how cascading and compounding events can worsen long-standing inequities. It discusses how a wildfire exacerbated social conflict over land, housing, and infrastructure in California. Accordingly, people’s cumulative vulnerability can be understood and addressed only through multiple scales of analysis (e.g., Turek-Hankins et al. 202086) and through integrated strategies for housing, planning, social services, lending, and racial justice.
Models of how climate impacts affect complex systems and how societies can respond usually do not account for people’s diverse roles, identities, and lived experiences nor the social, political, and governance characteristics of decision-making.3,87,88,89,90 For example, political polarization, in addition to structural inequities, increasingly shapes individual and government responses to disasters and long-term planning.91,92 This can lead to a patchwork of mitigation and adaptation efforts across a region, which can weaken overall system function.93,94,95 Data gaps exacerbate the modeling uncertainties described in Key Message 18.1, especially for smaller, less studied urban, suburban, and rural areas and Indigenous communities (KMs 11.3, 16.2).69,96,97 Research on the linkages between places, such as how impacts on agriculture systems affect urban food security, migration, and housing demand, is also limited (KM 11.2).98 The absence of research on the lived experiences, climate impacts and risks, and implementation outcomes for overburdened communities often leads to their underrepresentation in decision-making.99 Accounting for people’s diverse roles and identities, modeling social and political responses to climate change, and improving data availability can improve the modeling of complex systems and the inclusivity of decision-making tools.
Responding effectively to complex climate challenges benefits from integrated frameworks and modeling approaches that incorporate diverse types of knowledges suited to specific contexts and needs (high confidence). Participatory and collaborative approaches and tools bring together diverse knowledge holders and improve the generation and use of actionable knowledge for complex-systems decision-making (medium confidence). These collaborative approaches help navigate complex challenges, such as competing perspectives and knowledge uncertainties, thereby improving climate responses (low confidence).
Complex-systems responses to climate change require diverse types of knowledge, incorporating different ways of thinking about climate change and complexity, and often arise through participatory and collaborative processes (KMs 12.4, 20.2, 31.3, 31.4).115,116 While much research focuses on analyses, modeling, or projections specific to individual sectors, regions, or actors, a richer diversity of information is needed to fully understand complexity.116,117 Decision-making for complex systems benefits from knowledge of the interdependence of human–natural systems; the venues where decisions are made; the actors participating and impacted by decisions, politics and ideologies; and the values, attitudes, and beliefs of people and institutions (Figure 18.3).117 Such knowledge stems from diverse disciplines as well as interdisciplinary and transdisciplinary endeavors. Interdisciplinary work integrates knowledge from different disciplines, and transdisciplinary approaches often additionally integrate knowledge from researchers and nonacademic partners such as communities or decision-makers.118,119,120,121,122 For instance, social science and humanities research that examines the distribution of inequities of climate change has helped to highlight the overlapping and often compounding impacts on overburdened groups and can be used to ensure the inclusion of those communities in climate response decision-making (KM 18.2). Transdisciplinary knowledge that draws from both academic and nonacademic actors can help clarify systemic feedbacks and path dependencies that might go unnoticed if viewed from siloed disciplinary perspectives alone (Figures 18.3, 29.16).123,124 Overall, complex-systems responses necessitate moving beyond traditional siloed knowledge production processes to integrated approaches that include diverse types of knowledge and actors.125,126,127
The diverse types of actionable knowledge needed for responding to complex climate risks include data and modeling, decision support tools, case studies, art, and lived experiences (KMs 4.3, 13.2, 17.1; Box 19.1). Different types of knowledge can be critical depending on the context and needs. For instance, qualitative forms of knowledge, such as oral histories or ethnographies, provide rich, place-specific understandings of how complex systems function and how climate-related experiences influence behavior.128 Narratives and crowdsourced data of extreme events, including how people cope with them, have improved the integration of climate knowledge into social and cultural life.129 Stories have advanced climate and energy solutions by allowing exploration of the intersection of nature, humanity, and technology.130
Integrated frameworks that bring together different knowledge types across various sectoral and regional contexts are essential for holistic analyses of complex systems. For example, recent developments in coupled human–natural systems modeling have not only examined cascading impacts across sectors and scales but have also incorporated the feedback relationships from social and political systems back into the models.131 The Water Utility Climate Alliance has used a “chain of models” approach to link global climate models (GCMs) to hydrological models and then water utility decision-making tools.132 Connections between GCMs and urban tree canopy models have supported analysis of heat management strategies across multiple dimensions, including air quality, irrigation demand, and greenhouse gas emissions.133,134,135 Food–Energy–Water (FEW) systems studies have developed frameworks126,136 to better understand regional to global dynamics within cross-sectoral FEW systems.131,137
These advanced integrated frameworks draw on knowledge that stems not just from academics and researchers, but also from a wide range of experts, including practitioners, decision-makers, and local and Indigenous Peoples.116,127,138,139,140 There are increasing examples of community and citizen scientists monitoring specific environmental indicators at temporal and spatial scales otherwise infeasible (see Table 12.2).141,142,143,144 The long-term environmental knowledge and socioecological memory held by Indigenous Peoples has helped to detect, understand, and predict complex changes in climate systems.138,145 Further, land management approaches based on Indigenous Knowledge have long emphasized flexibility and diversity of resources and serve as successful examples of resilient practices under complex environmental changes (e.g., Box 27.3).138
Connecting different knowledge holders and knowledges requires a different type of scientific practice that is more collaborative, participatory, or community-engaged.116,146,147,148,149 Approaches such as coproduction, which bring together diverse knowledge holders with potential knowledge users, have proven to be successful in developing actionable knowledge for complex systems (Box 18.3; see Key Message 20.2 for more on engaging diverse stakeholders).150,151,152 Such approaches allow for iteration and deliberation about multiple worldviews and have been effective in improving credibility, relevance, and trust in knowledge.115,139,153,154 They also enable better understanding and management of knowledge uncertainties.154,155,156 For example, participatory modeling of complex socioecological systems has included experience-based practitioner knowledge to improve identification of system boundaries, elicit realistic management alternatives, and increase the decision-relevance of outputs.21,132,140,157 The county of Los Angeles has used participatory modeling to identify cascading impacts across infrastructural systems, their downstream effects for residents, and key intervention points.158 Collaborative decision-making under deep uncertainty (DMDU) approaches, such as flexible adaptation pathways, scenario planning, and decision scaling, seek to develop robust climate responses for multiple potential futures rather than planning for a single best-estimate future, and they have also been positively received by decision-makers (see Figure 18.4).159,160,161 The US National Park Service has used collaborative scenario planning, bringing together natural and cultural resource managers and subject-matter experts, to negotiate social and scientific uncertainties associated with climate change and inform the setting of conservation and resource stewardship goals.162,163,164,165
Effectively implementing and sustaining such collaborative processes requires specialized boundary organizations and climate services agencies (Box 18.3).166,167,168 These boundary agencies facilitate, translate, and mediate engagements between scientists and different actor groups. NOAA’s Climate Adaptation Partnerships, USGS’s Climate Adaptation Science Centers, USDA’s Climate Hubs, and university agricultural extension services have emerged as leading boundary agencies for the production and use of climate information through trust- and partnership-building with various knowledge-holder and user groups.169,170
Evaluation of effectiveness and equity is an important aspect of efforts to use climate-relevant knowledge in complex-systems decision-making. While there is increasing evidence that many collaborative approaches (such as coproduction, collaborative scenario planning, and DMDU tools) have been effective in improving the production and use of complex-systems knowledge in decision-making (Box 18.3), there has been limited examination of the long-term outcomes and equity of these processes and tools.151,152,171 Additionally, the relative advantages and disadvantages of when and where different types of knowledges are most useful is also not well understood.119,172 Overall, despite preliminary successes, understanding of the transferability, impact, and equity of different collaboratively generated knowledge and tools is still emerging.
Climate change presents challenges for managing risks and responses across different levels of government, the private sector, and civil society. Current governance entities and their existing jurisdictional authorities are often unable to resolve conflicts posed by the wide-ranging and unprecedented interactions and complexities of climate risks and more localized compounding stressors (high confidence). Local and regional governments have experimented with alternative institutional arrangements, funding mechanisms, and decision coordination (medium confidence). Thus far, however, there is only preliminary evidence of their effectiveness (low confidence). These pilots and other innovations developed for climate mitigation and adaptation may well present opportunities for replication and broader successes in other locations and different local contexts (medium confidence).
Climate responses and management practices are expanding to address complexity in coevolving human–natural systems. The need for science-informed, inclusive decision-making around complex climate risks is immense. The form and quality of governance of individual and interacting systems are shaped by government agencies, civil-sector actors, and private-sector entities.182 The actors and their roles vary across jurisdictions and are dependent on constitutional authorities, modes of control (e.g., proprietary, regulatory, budgetary), geographic territories, climate-related functions, technical capacities, budgets, access to financial and intellectual resources, and institutional or political power. The current state of these factors depends on institutional legacies and their momentum or inertia in response to changing climate conditions.183
There are inherent challenges in governing complex systems in response to climate change (KM 31.3). Relevant jurisdictions tend to be highly defined and siloed despite overlapping social or environmental system interactions.184 Structural inequities established in past governance institutions are often inextricable from current decision-making processes (KM 20.1). Market failures are sometimes ignored (e.g., disclosure of a property’s climate risks) or exacerbated by public policy (e.g., securitizing mortgages for risky properties, paying out insurance claims for repeat losses, or decreasing housing affordability as a result of mitigation efforts). Access to financial and intellectual resources has historically been weak at the local level and highly variable across geographies and urban to rural gradients.185 Governance of complex systems requires flexibility beyond the formal governance of any one system.186 Alternative processes for decision-making have been proposed or are in preliminary levels of implementation. These include an increased and explicit reliance on available scientific, engineering, and social-science evidence to inform policy. Iterative, participatory planning and deliberation inclusive of all residents or stakeholders are also possible. Similarly, transparency and accountability in public information and deliberations are increasingly important. Improved coordination strategies between governing entities—adjusting to evolving conditions and improving information—include decentralization and de-siloing in the public sector across agencies or jurisdictions, effectively reorganizing government. Governing entities are also looking at making longer-term commitments in public works and social programming and in longer-term visioning scenarios than their current budgetary and planning terms have traditionally prescribed (KM 12.4). Finally, governing institutions increasingly recognize the distribution of risks, path dependencies, and costs and burdens across communities.18,187
The state of evidence on effective governance approaches varies for each characteristic of complex-systems management:
Complex systems are often characterized by deep uncertainty—the presence of systemic structural uncertainties or unknowns without objective probabilities. Climate risks and possible scenarios exacerbate this uncertainty (KM 18.1). Stakeholders such as private-sector interests, public-sector officials, and civil-sector representatives may disagree about the likelihood of future scenarios or consequences of decisions. Effective governance approaches include transparency in system frameworks, integration of contingencies and redundancies, and flexible planning and operations to avoid decisions that are costly or impossible to reverse (Figure 18.4).188,189,190,191 Actors are already revisiting their crisis management plans to avoid worst-case outcomes and reduce short-term losses.
Wide-ranging actors within complex systems may have multiple, often competing objectives regarding climate actions. In response, effective governance approaches include establishing consistent criteria to evaluate alternative actions, sometimes using consensus-based principles. Inclusive governance, stakeholder consultation, and explicit consideration of multiple criteria are other strategies for addressing cross-system conflicts. However, interactions among actors across overlapping governance structures and systems, combined with strategic behavior for individual decisions, can complicate these processes.192,193,194,195,196
Complex systems are often characterized by broadly distributed knowledge and power—with diverse actors controlling knowledge about different parts of the system. Approaches in these polycentric governance systems include processes for data sharing, decision coordination and deliberation, and de-siloing of decision-making in related areas.117,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212 Mechanisms such as market-based approaches can sometimes allow for interaction of diverse actors via price signals, reducing the need for direct, centralized regulations, although these efforts may not account for inequitable financial access and outcomes across stakeholders, particularly overburdened communities.213,214
Governance of complex systems tends to be geographically and functionally nested—controlled by multiple jurisdictions such as municipal, state, and federal authorities (Figure 31.4). Coordination across actors in different jurisdictions, together with improved clarity and boundary definitions, can therefore be valuable (Figure 18.3). The primary tools of nested governance structures—that is, regulation, statute, program rules, and coordinated budget transfers from multiple sources—can be negotiated through coordination rather than preemption.202,215,216,217,218,219,220,221,222,223,224
Finally, feedbacks and path dependencies are common in complex systems. Choices at one point in time, such as the capacity and siting of physical infrastructure projects, can either expand or limit options later on (Figure 18.4). As a result, the full effects of management actions can be difficult to anticipate, particularly in the longer run. Path dependencies and policy feedbacks can lead to immediate commitments that restrict later change. Effective governance strategies include adaptive management approaches that repeatedly monitor, evaluate, and amend actions, although such governance strategies are not well developed.225,226,227,228,229,230,231,232,233,234,235
Overall, there is only preliminary evidence of the effects of alternative governance structures on climate mitigation and adaptation actions or their outcomes. Existing governance structures largely predate contemporary public climate responses and extend histories of US federalism, home rule, privatization, and ad hoc regional collaborations or special districts created in response to temporal needs or crises. These structures have generally not been reconfigured in the face of complex climate challenges, with some exceptions (KM 31.3). The governance structure between the Federal Government and state governments across and between most systems is constitutionally defined. The resulting flows of resources and other forms of assistance are therefore statutorily defined by program authorizations and appropriations. Existing governance entities such as state government agencies have attempted broad internal coordination among sub-state governments, although adjustments have met with varied success and contested leadership.236
There are more examples of alternative governance structures between and within subnational governments. Examples of the former include the cross-state Regional Greenhouse Gas Initiative for climate mitigation actions or the cross-county Southeast Florida Regional Climate Change Compact (Box 18.3) for climate adaptation; these efforts attempt to manage multiple systems across different jurisdictions.237,238 Other regional coordinating entities have been created across states, counties within states, or cities within counties, including those affiliated with the Alliance of Regional Collaboratives for Climate Adaptation. But these efforts are typically preempted or bounded by the constitutional authorities allowed by the superseding level of government (for example, a state over counties). There is preliminary evidence about the effectiveness of individual climate professionals (e.g., chief resilience officers and related boundary-spanners).203,239 However, the long-term outcomes from these efforts have not been conclusively measured.240
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The scope of this chapter was first developed by considering 1) the corresponding chapter in the Fourth National Climate Assessment (NCA4) as a starting point; 2) new emphases on topics relevant to complex systems, intersectoral interactions, and multiple stressors across NCA5; and 3) evolving areas of research and practice. The chapter lead identified important areas of expertise for the author team, including complex-systems methods, engagement methods, and topical expertise (e.g., energy–water–land, coastal, urban). Potential authors were identified from the nominations database, literature searches by the chapter lead and coordinating lead author, and the authors of the NCA4 chapter.
The First Order Draft, Second Order Draft (2OD), Third Order Draft (3OD), and Fourth Order Draft (4OD) were developed from the Zero Order Draft (ZOD) narrative outline through a multistage process. First, feedback on the framework and topics of the chapter was solicited via a public engagement workshop. Discussions focused on the NCA5 chapter framework for complexity, lived experiences, and the management of complex interactions. Second, chapter authors discussed the outline, and a lead author, along with accompanying contributors, was identified for each Key Message, figure, and box. Third, drafting proceeded through iterative processes of full-team and subgroup conversations on the scope and approach for each chapter element, literature reviews, and preparation of text and graphics, as well as identifying emerging assessment findings, areas of overlap across chapter sections, and potential directions for refinement in subsequent drafting stages. Fourth, cross-chapter discussions and the ZOD public review comments were used in the revision of the 2OD. These themes were further prioritized as the chapter was revised in the development of 3OD text and figures, also incorporating the review comments on the 2OD. The 4OD was then developed on the basis of public review and NASEM (National Academies of Sciences, Engineering, and Medicine) comments, with monitoring by the chapter review editor.
Read about Confidence and Likelihood
Key Message 18.1 material draws heavily from the recent assessment conducted in development of the Multisector Dynamics (MSD) vision report,3 which was an in-depth assessment by a diverse research community directly building from NCA4, as well as recent literature outside the scope of that report. In addition to citing the report itself, Key Message 18.1 directly cites underlying literature from the MSD vision report and other fields such as socio-ecological-technical systems, sociohydrology, complex systems, exploratory modeling, and decision-making under deep uncertainty.5,12,13,14,15,16,17,18,25,54,56,242 Given this foundation, the Key Message draws from an expansive evidence base, for which agreement in the literature pertains both to areas of agreement across studies and to deep uncertainties that remain.
Figure 18.1 is a combination of new and existing work. The underlying source was a conceptual diagram of complexity in urban systems and the lenses through which it can be organized and understood. Here, the figure has been adapted for complex systems as a whole, pursued through an actor-oriented lens and reflecting the authors’ review of figures that explain different aspects of complexity in socioecological systems or coupled human–natural systems under a changing climate.5,29,243,244,245,246,247,248,249 The MSD vision report executive summary Figure 1 was also a source of inspiration, yet the figure included in this Assessment centers actors and their capacities to respond. This adjustment reflects increasing recognition of the role of people in complex systems and associated climate risks and responses.
Major uncertainties are explicitly acknowledged in the discussion of deep uncertainties inherent to complex systems. Based on the available literature, we provide assessment of emerging approaches relevant to complex systems under such uncertainties. There are deep uncertainties in predicting complex interconnections among human and natural systems over long time periods, which are particularly exacerbated by a lack of human systems research that focuses on the interactions between human and natural systems.12,14,22 Exploratory systems modeling52,53,54 is developing approaches to address these deep uncertainties.
Given the robust evidence underpinning this foundational section of the chapter, along with high agreement about that evidence (see especially the foundational assessment by Reed et al. 2022b3) Key Message 18.1 is assessed with high confidence overall. The first confidence-assigned statement in the Key Message describes the increasing vulnerability of interconnected human–natural systems to risks from climate change. Given the high degree of agreement among voluminous sources of evidence,1,3,5,12,14,15,16,20,25,26,28,29,30,35,242,248,250, we assign high confidence to this statement. The second statement describes how these vulnerabilities depend on human responses and other compounding stressors. Given the high degree of agreement among many sources of evidence (see above), we assign high confidence to this statement. The final statement describes the diverse and sometimes competing objectives that decision-makers will need to navigate in managing climate risks. Given the high degree of agreement among ample sources of evidence (see above), we assign high confidence to this statement.
Evidence of the intersectional impacts of natural hazards, decarbonization efforts, and climate adaptation is well documented. Quantitative assessments of program outcomes and post-disaster outcomes, with a large number of data points, consistently show that more rural or smaller municipalities are less able to attract resources for adaptation (e.g., Mach et al. 201983). Numerous case studies and spatial and quantitative assessments provide evidence of the displacement tendencies of climate-exacerbated disasters and climate mitigation and resilience initiatives.73,85,109,251,252,253 In-depth qualitative research and longitudinal studies of overburdened communities have studied how cyclical disasters erode community response capacity254,255,256 and trigger cascading infrastructure and housing effects in cities receiving groups migrating from or displaced by climate-related events and trends.103 Research on the climate transition from fossil fuel economies shows that these efforts intersect with social, cultural, and political challenges to create intersectional vulnerabilities and complexities.257,258
Just as important as what we know about distributive impacts in complex systems is what we do not know or measure. Most research assessing complex systems is highly technocratic, focusing on specific events, discrete risks, and the impacts of cascading infrastructure systems, demographic change, and natural resource systems. However, complexity studies do not tend to encompass the large body of social and humanities research—increasingly focusing on climate change—related to cultural, psychological, and effective responses to shocks; racial, gender-based, and equity-oriented studies of vulnerabilities, risks, and societal responses; or the roles of political ideology and violent conflict (e.g., Palmer and Smith 2014; Beckage et al. 2020; Moore et al. 2022; Reed et al. 2022, 2022; Rising et al. 20223,87,88,89,90,242). These issues can help explain political decision-making, protests, and post-disaster conflicts and are important for understanding interactions across social and physical systems in the near and long term. There is a resulting lack of tools and communities of practice to integrate disciplinary divides in support of complex-systems research.
Limited knowledge about climate-exacerbated migration and how receiving cities are responding results in uncertainties about how climate migration differs from other forms of migration, what migration means for overall vulnerability trends, and how it will impact other interlinked physical and natural systems. Information on urban–rural linkages is limited, especially in forms that can support complex-systems modeling. Trade-offs between mitigation efforts and adaptation strategies with competing impacts on individuals’ well-being are also understudied.
There is high confidence in each statement of Key Message 18.2. First, there is high agreement in the literature that climate change disproportionately impacts already-overburdened groups (statement 1), as recently assessed in the comprehensive Sixth Assessment Report of the Intergovernmental Panel on Climate Change, Working Group II.69 Second, multiple studies with high agreement across them have documented the lack of social and political dynamics in complex-systems models and limitations resulting from sectoral, regional and jurisdictional, and disciplinary silos of vulnerability and risk assessments (statement 2).3,86,87,88,89,90,242 Some complex-systems analyses have made first steps in incorporating social and political dynamics (e.g., Moore et al. 202288), but these are emerging efforts, and statement 2 is therefore made with high confidence. Third, data about how complex systems affect frontline communities is lacking for hard-to-reach populations, and this can lead to disproportionate climate impacts (statement 3).97,99 This statement is made with high confidence given the large evidence basis on inequalities in data availability and quality across communities and the focus of the literature on data-rich places (e.g., Friel et al. 2011; Pörtner et al. 2022; Reed et al. 20223,69,98).
Key Message 18.3 and corresponding text draw from a review of various types of literature on knowledge for complex systems, several author team meetings and deliberations over written documents, and takeaways from the public engagement workshop. The write-up was also coordinated with authors of other Key Messages, especially Key Messages 18.2 and 18.4. Literature reviewed for this Key Message spanned several topical areas. The authors reviewed literature describing integrated frameworks and modeling for complex systems,3,123,126,127,131,136 as well as characteristics of actionable or usable knowledge.115,116,120,153,154 Authors also focused on literature showcasing the role of qualitative knowledge such as narratives in managing complex systems.128,129,130 In terms of knowledge production processes, the team reviewed the latest papers on co-production of knowledge and collaborative research approaches, as well as on interdisciplinary and transdisciplinary research.115,124,147,259 The authors also reviewed literature and case studies on Indigenous and Traditional Knowledges and their role in complex systems, as well as the role of citizen science.141,142,143,144 Studies on participatory modeling,21,140,156,157 as well as collaborative decision-support tools for complex systems such as decision-making under deep uncertainty and scenario planning, were also reviewed. Finally, research on climate services and boundary agencies was also reviewed.
Box 18.3 centers on complex coastal governance, drawing from evaluations of climate responses in Metropolitan regions including the San Francisco Bay area, Los Angeles County, Southeast Florida, and metropolitan New York; a specific case example of the Little River Adaptation Action Area is discussed here. Multiple recent studies have examined the science–policy–practice processes and partnerships, along with the evolving governance systems, inherent to complex coastal climate responses (e.g., Treuer et al. 2017; Kim 2019; Solecki et al. 2021; Tedesco et al. 2021; Troxler et al. 2021; Lubell and Robbins 2022178,179,260,261,262,263). Direct references are provided to relevant regional strategies and decision support forums (e.g., Miami-Dade County 2021; Miami-Dade County 2021; SFWMD 2021175,176,177).
Despite the promise and potential of collaborative knowledge production processes and collaborative decision tools, long-term monitoring and evaluation of these processes and tools are largely lacking; hence, the long-term impact and effectiveness of such collaboratively generated knowledge in responding to complex climate risks in complex systems are not well understood.147,151,172,264 In addition, the impact of collaboratively generated actionable knowledge on long-term management of complex climate-impacted systems, particularly as it relates to procedural or distributive justice in outcomes, is not well understood.265
There is high agreement in the literature, from both theoretical120,124,127 and empirical studies,117,123,131,261 that effective complex systems responses benefit from integrated frameworks that bring together diverse and context-specific knowledge. Therefore, there is high confidence in the first statement in this Key Message. A growing number of studies provide evidence that participatory and collaborative approaches that bring together diverse actors have improved the actionability of knowledge for managing complex systems,150,151,152 but more evidence is needed on the extent to which this actionable knowledge has actually been used in complex-systems decision-making.266,267,268 Hence, there is medium confidence in the second statement. Although the theoretical literature suggests that collaborative approaches help to navigate competing perspectives of different actors and knowledge uncertainties,43,117,119 there are very few studies that have evaluated the extent to which these approaches have successfully led to improved climate responses in the long term.171,267,269,270 Hence, there is low confidence in the third statement.
The subgroup of authors for Key Message 18.4 pulled from a multidisciplinary set of research products at the intersection of governance, regional planning, systems theory, and jurisdictional authority in relation to both climate mitigation and adaptation planning, actions, outputs, and outcomes.202 The subgroup identified key peer-reviewed research products based on the themes categorized by subgroup members’ expertise, as well as the themes that surfaced in the public engagement workshop; essentially, the identification of sources was expansive, but the themes from which the sources were identified and classified were necessarily limited. Further, the subgroup sought to identify documentation of alternative governance successes and failures in relation to climate mitigation and adaptation in order to expand the pool of sources to other disciplines that may not explicitly use governance terms.191 For each of the themes identified, there is a vast literature from which to pull findings. Identifying these was a straightforward process, and there was minimal disagreement on either the themes or the sources.
In slight contrast, there was significant discussion regarding the level of confidence around the synthesis of sources’ findings, given their range of empirical inquiry and methodological rigor. The vast majority of evidentiary sources rely on either singular governance case studies (or, less often, a few cases) or theoretical exploration in either qualitative or quantitative ways.195,199 Fewer studies include sample sizes of governance cases that are powerful enough—or that employ comparison groups of any design—to produce conclusive and generalizable findings.203,221 Consequently, assessments of uncertainties and confidence are based largely on the variable rigor of these sources.
Governance, by definition, varies by geographic, social, and political landscapes, among many contextual factors. The first factor—geography—poses a particular challenge for the development of conclusive evidence of governance actions and effects.188 Essentially, each place has a unique governance framework.117 Although rigorous qualitative and quantitative studies have been conducted regarding governance interventions and their outcomes in specific places, the sample sizes of individual monographs and consistent application of terms across them have consequently produced only preliminary evidence to date. As with urban studies and political geography, there are few multisite studies of complex-systems governance at any level of quasi-experimental rigor, or even basic outcome evaluations, that allow for replicable responses to the fundamental question posed in the public engagement workshop: What is “good” climate governance?
This fundamental methodological challenge, then, prohibits an assessment of high confidence for all complex-systems governance themes—with the sole exception of their current inability to produce consistently positive outcomes. Further study with larger sample sizes and consistent terms of governance frameworks—including their inputs, activities, outputs, and outcomes—is necessary to assess higher levels of confidence. Statements in the Key Message regarding evaluation/learning, effectiveness, and replicability are therefore made with low and medium confidence. The quantity of evidence around climate governance implementation and outcomes is too small to conclude that their findings are externally valid and replicable.203 Policy and governance literature for other subject domains beyond climate mitigation and adaptation for which there has been more extensive study, however, suggests that pilot governance efforts’ success may not be insignificant, implying a higher confidence that the implementation of current climate pilots may also yield positive outcomes.200,224
