Fifth National Climate Assessment
18. Sector Interactions, Multiple Stressors, and Complex Systems

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.

URL

Alternative text

Climate-related experiences and actions connect with many other activities and contexts.

Figure 18.1. Interacting and overlapping features of human–natural systems shape actors’ capacities to respond to climate change. The effectiveness of decisions and actions—taken or not (teal center of the figure)—is shaped by multiple other features (outer rings). Ways of knowing (light blue ring), such as local, Indigenous, or scientific knowledge, are a foundation that determines how climate risks and responses are perceived and understood. Moving outward, interactions across sectors (e.g., water and energy) and geographic scales (e.g., from local neighborhoods to the Nation as a whole) determine risks and responses under both climate-related hazards (e.g., flooding, drought, wildfire, and heat) and non-climate influences (e.g., social inequities and cultures). Ultimately, these features together define the complexity inherent to climate risks and responses. Adapted from Brelsford and Jones 2021.⁵⁹

Close

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.

URL

Alternative text

Intersecting social and environmental factors privilege some people’s ability to respond to climate change.

Figure 18.2. Climate impacts and societal responses exacerbate intersectional vulnerabilities. People’s gender, class, ethnicity, age, race, and ability form their intersectional identity (left). Intersectional vulnerabilities emerge when intersectional identities interact with inequities in complex systems, as outlined in Figure 18.1 (right). In the face of climate risks and responses, these intersectional vulnerabilities can result in unequal exposure, exclusion from benefits, and cascading impacts that further impact already-overburdened groups. Societal responses to climate change—including uneven existing resources across municipalities, decisions about where to allocate investments, and noninclusive ways of knowing—can exacerbate existing harms and generate new ones. Adapted with permission from Box TS.4, Figure 1 of Field et al. 2014.¹¹⁴

Close

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 frameworks^126,136 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.

URL

Alternative text

Climate responses, ranging from the individual to global scale, interact with and draw from diverse knowledges.

Figure 18.3. Activities of government at multiple scales (green) affect climate risk (red) via multiple pathways, by decreasing or increasing greenhouse gas emissions (gray) and supporting adaptive (or maladaptive) actions (pink, purple). Moreover, government actions are influenced by the opinion and interests of residents, businesses, and other organizations (orange), and they in turn shape the actions of households and businesses (pink, blue). Interacting actors are nested within multiple spatial scales (left) and also rely on different knowledge sources for decision-making (right). Adapted from Moore et al. 2022.⁸⁸

Close

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

URL

Alternative text

Governing complex systems involves pathways of decision-making across time.

Figure 18.4. Adaptively managing complex systems involves a series of choices and actions through time. There are path dependencies shaped by the past and present, opportunities for maintaining flexibility under deep uncertainties, and needs for iterative learning through time. Governance of complex systems is more effective when it is flexible, inclusive, integrated, collaborative, and adaptive. At each decision point in time (circles with arrows), diverse decision-makers evaluate possible solutions available at that time and choose a path forward (people in gray considering benefits and trade-offs of different possible pathways). Given these responses, alternate pathways (red, orange, yellow, and green) may remain possible for the future or become closed off. At each decision point, an important consideration is how actions taken may expand or contract the options available in the future. Adapted with permission from Figure SPM.6 in IPCC 2023.²⁴¹

Close