The ocean supports diverse and productive marine ecosystems that provide innumerable benefits to the United States. Fishing, recreation and tourism, energy, shipping, and transportation in the ocean and Great Lakes (see Ch. 24) sustain a marine economy that contributed over $781 billion (in 2022 dollars) to the US economy in 2021.1 Ocean resources support human health and well-being in communities throughout the US, and sustained connections to the ocean are foundational to cultures and identities. This chapter assesses climate impacts and risks to US marine ecosystems, and to the communities and industries that depend on them, as well as ocean-based measures for climate change adaptation and mitigation.
Mills, K.E., E.B. Osborne, R.J. Bell, C.S. Colgan, S.R. Cooley, M.C. Goldstein, R.B. Griffis, K. Holsman, M. Jacox, and F. Micheli, 2023: Ch. 10. Ocean ecosystems and marine resources. 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.CH10
Download citation: BibTeX | RIS
Across the globe, climate change is altering marine ecosystems and connected social systems at a scale and pace that is unprecedented in recent millennia. The combination of long-term changes in physical ocean conditions—such as warming, sea ice loss, acidification, and deoxygenation (KMs 2.1, 3.3)—and short-term extreme events (KM 2.2) such as marine heatwaves threatens marine ecosystems and human communities (Focus on Compound Events). Numerous marine species, from phytoplankton to whales, are altering their distribution, seasonal activities, and behaviors to align with suitable ocean conditions. These changes ripple through the food web, affecting species interactions, ecosystem functions, and biodiversity, as well as conservation, management, and uses of valuable ocean resources.2 Climate-driven changes to marine ecosystems significantly affect ocean-dependent livelihoods and, in some communities, threaten food supplies and ways of life.3
In affected communities, the magnitude of climate impacts and levels of adaptive capacity vary with marine resource dependence, socioeconomic status, and historical and institutionalized inequities.4,5,6 Some individuals, communities, and industries are adapting to changes, largely through reactionary responses and, in some cases, through coordinated resilience planning.7,8,9 However, responses are uneven across communities and sectors, and they remain insufficient to meet mounting challenges and costs.9,10 Global policy choices regarding greenhouse gas (GHG) mitigation govern the intensity and trajectory of future climate impacts and the diversity and effectiveness of adaptation options. Mitigation and adaptation efforts require explicit accountability in social equity, sustainability goals, and fairness in governance and finance to address entrenched inequities that increase climate change risks and adaptation burdens.5,11
This chapter draws on global insights to address climate-related changes and challenges in US marine areas. It largely focuses on continental shelf waters, with some discussion of topics that extend shoreward to intertidal areas, and it complements Chapter 9 (Coastal Effects), which extensively covers the topic of sea level rise. The chapter builds upon the climate-related physical oceanographic changes discussed in Chapters 2 (Climate Trends) and 3 (Earth Systems Processes) to highlight some of the unprecedented ecological changes taking place in US marine waters and their impacts on social, economic, and governance systems. Policy directions, planning efforts, and investment decisions being made now will affect mitigation and adaptation options and timelines and will determine the future of our ocean and social and economic systems that rely on it.
Close
Climate change is significantly altering US marine ecosystems at a pace, magnitude, and extent that is unprecedented over millennia (very high confidence). Changes in species locations, productivity, and seasonal timing are cascading through ecosystems, threatening critical connections between people and the ocean (high confidence), especially for Indigenous Peoples (very high confidence). Risks to marine ecosystems and the people connected to them will be greater under higher scenarios (likely, very high confidence) and will depend on the ability of ecological and social systems to adapt to the pace of climate change (very high confidence). Continued climate change, particularly under higher scenarios, is projected to push many systems toward novel conditions and critical tipping points (very high confidence), beyond which the risk of significant impacts to marine ecosystems, including collapse, is high, adaptation may be insufficient, and human well-being is threatened (high confidence).
Climate-driven changes are altering marine ecosystems via complex physical, biological, and socioeconomic interactions (Figure 10.1). Many ocean characteristics, such as the timing and length of seasonal cycles, extent and duration of sea ice, oxygen content, and severity of extreme events are exhibiting major divergences from historical patterns (Box 10.1; KMs 2.2, 3.1; Figure A4.11).10,12,13,14 Changes in distributions, population productivity, and timing of life events are widely documented for marine species and are increasing in prevalence and magnitude (Figure A4.12).15,16,17,18,19
Critical habitats such as coral reefs, seagrass beds, and kelp forests have experienced large-scale degradation due to climate-related stressors, threatening their ability to support commercially and ecologically important fish, shellfish, turtles, and marine mammals.20,21,22,23 Degradation of nursery habitats, spawning areas, and other essential habitats has the potential to affect the productivity and distribution of species.21,24
Marine species are shifting their geographic distributions even faster than terrestrial species25 and are changing the timing of seasonal activities.16 As changes cascade from microbes to top predators across food webs, these shifts are decoupling some predator–prey relationships26,27 and amplifying others.28 For example, shifts in species have reduced prey availability for seabirds, driving large-scale starvation events and breeding-colony failures.29,30,31
While warming has benefitted some marine resources in poleward portions of their range (such as an increased abundance of American lobster in the Gulf of Maine32), many species—especially those that are cold-adapted, fixed in place, or have complex life histories—have been negatively affected.33,34,35 Protected and endangered species with limited population resilience, including multiple species of coral, salmon, and whales, are particularly vulnerable to impacts of unfavorable physical and ecosystem conditions (KM 8.2).22,36,37,38
Alyse DietelFragility(2023, pen, watercolor, acrylic)Artist’s statement: “Fragility” expresses the delicateness of our natural world and ecosystems. The vintage Heisey teacup, historically owned by the wealthy, represents how people have affected the environment for generations and continue to do so. Two of the whales are humpbacks, which in some cases are still endangered. The third whale is a North Atlantic right whale. North Atlantic right whales are critically endangered and are expected to become functionally extinct in just a few decades. Above the whales float several icebergs, which are spilling over the edge of the cup bit by bit. The earth is losing approximately 1.2 trillion tons of ice per year due to climate change.View the full Art × Climate gallery.Artworks and artists’ statements are not official Assessment products.
Ocean ecosystems are complex and interconnected, making it challenging to fully understand and anticipate climate-induced changes. Climate impacts are less well documented for certain ecosystem components, even ubiquitous organisms such as microbes39 and pathogens.40,41 Additionally, climate drivers impacting ocean ecosystems often act in complex ways and, in some cases, can originate on land. For example, altered precipitation patterns over the continental US have both reduced river flow in the Pacific Northwest and increased flooding on the Mississippi, inducing population declines in iconic species such as Chinook salmon42 and Gulf of Mexico oysters,43 respectively. Coastal “blue carbon” ecosystems, including coral reefs, seagrass and seaweed beds, mangrove forests, and tidal marshes, are also impacted by interactions between land- and ocean-based changes,44,45 and these effects can extend to deep-sea ecosystems (Focus on Blue Carbon).46,47 While US coastal and shelf ecosystems are relatively well studied, the deep ocean (below 650 feet) remains poorly studied.48,49 The deep ocean stores and absorbs a vast quantity of carbon and heat, buffering the impacts of climate change but also resulting in warming and changes to biogeochemistry (such as deoxygenation) in this portion of the ocean (KMs 2.1, 3.4),13,49,50 potentially impairing the health of deep-sea ecosystems and the capacity for carbon sequestration.51
Ocean climate impacts affect many communities, from coastal inhabitants who make a living from ocean industries to people who live far from the shore and eat fish in the US Midwest or vacation at Gulf Coast beaches. For example, harmful algal blooms (HABs) and increases in pathogens, such as Vibrio species, have become more prevalent in some regions, resulting in beach and fishery closures and impacting people’s health and livelihoods.52,53 Effects are amplified for Indigenous Peoples whose long-standing social, cultural, and spiritual connections to the ocean are being altered.54,55 Subsistence harvests that are critical for food and nutritional security have been disrupted by shifts in species distributions, sea ice loss that limits access to resources, and HABs that make food sources such as razor clams, Pacific walruses, and bowhead whales unsafe for human consumption (KMs 16.1, 29.5).56,57,58,59,60,61,62,63 Cumulatively, these changes threaten to break vital social and cultural connections by undermining food security and the mental and physical health and well-being of marine resource users.59,64,65,66
As impacts of climate change mount, species and people are beginning to adapt. Examples of observed adaptations include species shifting distributions as they track preferred temperatures67 and subsistence harvesters changing what, where, and when they harvest.68 The largest and fastest adaptation responses have followed climate impacts that occur as extreme events (e.g., heatwaves, HABs, hypoxia) or that amplify background risks and pressures (e.g., habitat degradation, resource overexploitation; Box 10.1).69,70
Cumulative GHG emissions will continue to affect marine ecological and social systems over the coming decades. Changes in physical and biogeochemical conditions, including temperature, stratification, upwelling, and ocean chemistry, are projected to become stronger and more widespread, particularly for higher scenarios (KM 2.3),71 and interactions with chronic stressors such as habitat degradation or overfishing will amplify ecosystem impacts.10 Shifts in the distribution and biomass of marine species, changes in food web structure and ecosystem functions, and increases in HABs and pathogens would be more pronounced under very high scenarios (RCP8.5, SSP5-8.5).72,73,74,75 Climate change will drive physical and biological systems toward critical tipping points, triggering feedbacks that may threaten biodiversity, undermine system stability, permanently alter ecosystem functions and services, and limit adaptation options.76,77,78,79
Continued climate-driven changes pose challenges for social, economic, and governance systems, particularly those based on expectations that historical conditions will persist into the future. Shifting fish distributions are creating jurisdictional challenges for area-based management, undermining commercial fishery management approaches,80 and jeopardizing treaty resources such as Tribes’ rights to “usual and accustomed” fishing grounds.81 The severity of impacts to marine social–ecological systems will depend on peoples’ ability to adapt at the pace of climate change, which will require participatory governance systems that can effectively and equitably adjust to shifting circumstances.
Climate change poses a substantial risk to ocean-related industries and economic activities such as fisheries, tourism, recreation, transportation, and energy (high confidence). As climate change continues, economic and cultural impacts are expected to become larger and more widespread, especially under higher scenarios and in communities that are highly dependent on ocean resources (very high confidence). A range of approaches can facilitate adaptation to some degree of climate change (medium confidence), but higher levels of climate change will limit the success of adaptation measures and markedly increase climate risk to marine-related economic activities (high confidence).
From energy to fisheries to tourism, the ocean economy is deeply intertwined with the economic health of the United States (Figure 10.3). Populations in shore-adjacent counties grew 5.3% from 2010 to 2019, with employment increasing three times as fast (16.3%). From 2005 to 2019, the ocean-related GDP grew by nearly 60% (in constant dollars), representing a total of 3.5 million jobs.89 Ocean-based activities and industries are being affected by climate change,90,91 and future impacts may slow growth of the ocean economy. Limiting global warming to 1.5°C (2.7°F) above preindustrial levels confers clear social and economic advantages compared to higher scenarios.92,93
Climate change has impacted commercial marine fisheries in every region of the US by altering the availability and quality of harvested species, destabilizing fisheries-related revenue and employment, and inducing new management challenges.15,69,85,96,97 The large-scale redistribution of highly valuable Bering Sea (Alaska) Pacific cod and snow crab and subsequent declines in multiple stocks, including closure of the snow crab fishery in 2022, followed low sea ice conditions and protracted warm bottom temperatures across the region (Box 10.1).98,99,100,101,102 On the East Coast, the northern shrimp fishery collapsed and a fishing moratorium was imposed following a marine heatwave in 2012,103 and the highest-valued single-species fishery in the US, American lobster, has seen the southern portion of its population decline to very low levels with warming waters.32 Disaster declarations for commercial fisheries increased markedly from 1994 to 2019, with more than 84% of fishery disasters linked to extreme environmental events, totaling $3.4 billion of lost revenue and $2.3 billion (in 2022 dollars) in federal funding for disaster relief.104 Recent climate-related fishery declines have been widespread,85,105,106 although a few stocks have increased with ocean warming and heatwaves (e.g., regional increases in the northern stock of American lobster, market squid, and sablefish32,107,108). While climate is not the sole driver impacting fish populations, it is an added stressor that exacerbates other negative impacts.33
Over the next century, climate change is expected to reduce catch in all US regions,92 including some of the highest-valued fisheries (e.g., Bering Sea snow crab, walleye pollock, Pacific cod, American lobster, and Atlantic sea scallops32,109,110,111,112). For 16 species that represent more than half of commercial fisheries revenue, climate-induced changes are projected to result in billions of dollars of economic losses by 2100, with losses twice as high under a very high scenario (RCP8.5) than an intermediate scenario (RCP4.5).113 Many species will continue moving northward and deeper, reducing accessibility for subsistence harvesters and smaller vessels and complicating management policies and quota allocations.114,115,116,117,118 Severe storms and sea level rise will increasingly threaten shoreside infrastructure and transportation networks that are critical for harvesting and distributing seafood products (KM 9.1).4,119
Climate impacts are not distributed equally across all fisheries and can be compounded by non-climate factors, including fisheries management, market conditions, socioeconomic conditions, and external shocks (e.g., COVID-19).4,86,115,120 Impacts are generally greater for small-scale coastal harvesters who are less able to follow shifts in fish distribution or who have access to a limited number of fish stocks, while those with larger vessels and more diverse harvest portfolios are generally more resilient.85,86,118,121 Commercial fisheries and subsistence harvesters are adapting to these changes through short-term incremental measures, business investments appropriate for changing conditions, and management efforts supporting climate-ready fisheries (Figure 10.4).96,122,123
The effectiveness of future adaptation responses may be limited by the magnitude of change and factors like inequities in finance and governance across communities, costs of equipment or infrastructure, and access to fishing permits (Focus on Risks to Supply Chains).80,97,115,124 As fisheries adapt, community initiatives such as permit banks and seafood cooperatives that plan for climate change can enhance equitable opportunities and socioeconomic benefits (Figure 10.4).125,126 Diversifying harvest and livelihoods, including expanding into marine aquaculture (KM 11.1), can also help stabilize income or buffer risk. Tools that predict species distribution changes can help avoid bycatch, reduce costs, and increase yield.127 Further, ecosystem-based and climate-informed management can align harvest limits with population productivity to maintain sustainable fishing levels.109,110,128
Ocean-based tourism and recreation—the largest sector in the ocean economy, representing $274.5 billion of economic activity in 2021 (in 2022 dollars)1—is both positively and negatively impacted by climate change.129 Warming temperatures extend the coastal tourism season, yet sea level rise threatens shoreside facilities (KM 9.2) and will change nearshore wave dynamics in ways that reduce or eliminate some surfing opportunities.130 In the Gulf of Mexico and Caribbean, worsening HABs131 and blooms of macroalgae (e.g., Sargassum132) due to climate and local non-climate stressors have raised human health concerns (KM 23.1) that have disrupted tourism and fishing.133,134 Recreational fisheries are experiencing climate-related changes in anglers’ participation, location choices, and expenditures.135 As warming continues, angler participation may decline by up to 15%, with losses as high as $413 million annually (in 2022 dollars) along the Atlantic and Gulf of Mexico Coasts; however, warming is increasing participation in some areas (e.g., New England).136
Similarly, tourism impacts are different across regions. Arctic sea ice loss is creating tourism opportunities by allowing “last-chance” cruise ship tourists to see ecosystems before they are further altered by climate change.137 However, coral reef tourism—valued at nearly $3 billion annually for 2008–2012 (in 2022 dollars) in Hawai‘i and Florida—is threatened by bleaching and disease that deter divers and snorkelers (KMs 23.3, 30.4).138 At a more localized scale, the loss of endangered southern resident orca whales in Washington’s Puget Sound due to climate-driven declines in food would result in annual losses of $39 million in economic activity (in 2022 dollars).36,139
Climate change is already affecting marine transportation. Sea ice loss and longer open-water seasons have enabled transit between the Atlantic and Pacific via the Arctic, with ship traffic in the Arctic increasing threefold between 1990 and 2015,140,141 and Arctic-routed shipping continues to be considered.142 With 3.6°F (2°C) of warming above preindustrial levels, ships are projected to be able to reliably navigate the Northwest Passage and Arctic Bridge trade routes in summer.143 These routes may reduce carbon emissions and shipping costs, but concerns exist about impacts to marine species and local communities, as well as about black carbon emissions.54,144,145,146
Commercial vessel emissions have increased over time, as has the sector’s proportional contribution to global emissions (KM 13.1),147 but emissions from recreational boats in the US declined between 1990 to 2021.148 The shipping sector is initiating further measures to reduce its GHG emissions by powering docked vessels with electricity,149 increasing vessel efficiency to reduce global shipping emissions by 50% by 2050,147 and planning for some zero-emissions maritime routes by 2025.150
Ocean-based energy production in the US is in a period of transition. Ocean-based energy has been almost exclusively derived from hydrocarbon extraction, which generated $96.4 billion in 2021 (in 2022 dollars).1 Globally, nearly 30% of commercially recoverable oil and gas assets are found in areas at high risk for climate impacts.151 In the US, stronger hurricanes, increasing wave heights, and sea level rise will threaten offshore facilities and associated coastal structures, such as underwater pipelines and refineries (KM 9.2).152,153 Facilities may increasingly require adaptive responses, such as raising the height of oil and gas platforms in the Gulf of Mexico to reduce hurricane damage.154,155
Renewable energy sources are expected to increase as part of the ocean-based energy mix over the next several decades. The first US facilities to generate electricity from ocean wind are in place off the Atlantic Coast, and in 2021, the US set a goal of installing more than 30 gigawatts of capacity by 2030, enough to power about 10 million homes.156 States have set additional goals for offshore wind energy development that may further advance this capacity. Through 2022, more than two million acres of ocean bottom have been leased for wind energy, with more leases anticipated by 2025.157 The growth of ocean-based renewable energy is expected to bring jobs and economic benefits to certain coastal communities, but its ecosystem impacts are still being determined, and its development may constrain other ocean uses, including fishing, transportation, and aesthetic preferences.158,159,160,161,162
Future risks to marine ecosystems, ocean resources, and people will be substantially reduced by implementing adaptation and mitigation actions now (very high confidence). Responding swiftly to climate change will improve outcomes, reduce costs, promote resilience and equity, and allow the widest possible suite of adaptation solutions (very high confidence). Impacts will continue to be uneven across communities, with more harmful outcomes in communities that are highly ocean-reliant and historically marginalized, unless equitable adaptation and mitigation efforts are implemented (high confidence).
Although substantial climate-driven changes in ocean ecosystems are inevitable over the coming decades (KM 2.3), the future of these systems, their valuable services, and the businesses, communities, and economies that depend on them will be determined by the choices we make now on mitigation of GHG emissions and investment in adaptation measures (Figure 10.5). Proactive, coordinated, large-scale approaches to planning, financing, and implementing adaptation measures are necessary to achieve effective and equitable outcomes (KM 31.2).10 Reactive actions to cope with climate impacts are occurring at individual, business, and community scales but are largely uncoordinated and sometimes ineffective (Figure 10.4). Adaptive capacity is not the same across communities or groups; communities that are highly reliant on ocean resources may face the greatest risks and be constrained by socioeconomic factors, historical and ongoing inequities, and access to governance systems or financing.4,5,163 Promising proactive adaptation-planning measures are starting to emerge in various regions and sectors. For example, some state and federal fishery-management bodies and stakeholder communities have prioritized climate preparedness and are developing information, tools, plans, and processes to address future changes and uncertainty in marine resources and fisheries.164,165 Certain municipalities and Tribal communities are pursuing integrative climate resilience planning that considers adaptation needs across multiple ocean-related sectors (e.g., Cities of Portland and South Portland 2021; Takak et al. 2021166,167).
Scarlett W.Youth Entry, Grade 12Polychromatic Cast(2023, watercolor)Artist’s statement: My piece depicts fish traveling in a school. I'm trying to express positive energy moving forward. The fish move against currents and through waves, though it is not meant to be scary. The waves capture the light of the sun, which shines onto the fish. In a future that is cast with an environmental water crisis--with rising sea levels, floods, and droughts--the only way to go on is forward. The future should not be scary, as we are together and can only make a change as a collective.View the full Art × Climate gallery.Artworks and artists’ statements are not official Assessment products.
Various ocean-based mitigation approaches are also advancing.168 Measures to protect and restore marine ecosystems that capture and store carbon dioxide—such as mangroves, seagrasses, and kelp forests—are underway and offer additional benefits like wave energy dissipation and fisheries enhancement, but carbon mitigation benefits may be modest and variable (Focus on Blue Carbon).10,169 Public- and private-funded projects are evaluating technical, economic, and social dimensions of ocean-based carbon dioxide removal techniques (KM 32.3; Focus on Blue Carbon).170,171,172,173,174 Ocean-based wind energy is being implemented (KM 10.2) and wave energy conversion is being developed, especially in the Pacific basin.175 Electric and hybrid engines for small boats176 and expanded production of aquatic foods with lower GHG emissions177 also support ocean-based mitigation. Estimates suggest that fully scaled-up ocean-based mitigation measures would provide about a quarter of the atmospheric GHG reduction required to meet global pledges by 2050.172,176,178
Immediate implementation of ambitious mitigation and adaptation measures offers the greatest chance of maintaining ocean ecosystems and their benefits to people, as well as supporting equitable human development.10 Carbon emissions peak in the mid-2020s in scenarios that limit warming to less than 3.6°F (2°C), above which risks and impacts are projected to rapidly increase across sectors and regions.179 Without carbon mitigation, estimates indicate that a critical global warming threshold of 2.7°F (1.5°C) will be crossed in the 2030s.179
Coordinated adaptation planning is essential to ensure that strategies across sectors, communities, and regions are complementary and achieve equitable outcomes. Adaptation and mitigation options tend to be most successful if they are based on sound information, developed in collaboration with local communities and diverse actors, and designed to lower ecosystem and community risk.10,180,181 Although adaptation measures are already being taken in some areas (KM 10.2), the ability to adapt is uneven across groups, communities, and sectors (KM 31.2). People with socioeconomic assets such as strong social connections, alternative livelihood options, and economic wealth are more resilient to climate disruptions,6,85,86,182 and those with greater access to information and other resources will be better positioned to engage in adaptation efforts. Participatory planning, financial, and governance processes designed to account for divergent power dynamics and institutionalized discrimination can engage a broad array of community members in co-producing climate solutions.181 Deliberately incorporating local knowledge, perspectives, and values can help determine efficient, enduring, and equitable adaptation and mitigation solutions.8,180,181
Effective climate change adaptation in marine systems also depends on implementation of carbon mitigation. Without emissions reductions, the range of possible adaptation options decreases substantially. For example, adaptation options in coral and mangrove ecosystems include reducing non-climate stressors such as pollution and prioritizing effective harvest management and habitat restoration. With increased emissions, these measures will become insufficient to maintain coral and mangroves due to warmer, more acidic conditions, and adaptations will be limited to more expensive, higher-risk options such as active translocation of species, assisted adaptation, or reef shading.183,184 As emissions increase and the range of options for maintaining these habitats decreases, the risks of losing services they provide, such as coastal protection, livelihoods, food security, cultural identity, and tourism, are magnified.10
Adaptation measures with co-benefits for mitigation are especially promising. These include reef and marsh restoration, seaweed aquaculture, ecosystem-based management, and marine spatial planning. Nature-based solutions have the potential to be cost-effective and self-reinforcing over time, and if implemented at scale, they may impart climate, societal, and ecological benefits for adaptation and carbon mitigation.185,186,187 Solutions that include equity and diversity targets and are designed through inclusive and participatory approaches have the greatest potential to both address ongoing injustices and impart benefits for marine resource users and Indigenous communities.180,188
Emerging technologies could further expand ocean-based mitigation, but significant uncertainties must be resolved. Research projects are exploring the design, manufacturing, and grid integration of wave, thermal, and tidal energy-capturing devices.189 Electricity and scalable zero-emission fuels such as hydrogen are being evaluated for decarbonizing oceangoing vessels.190 All ocean-based carbon dioxide removal techniques still require substantial research on scalability, durability of carbon storage, environmental and social impacts, governance, and financing, as well as development of suitable regulatory frameworks.169,172,191
Trade-offs among adaptation and mitigation activities, ecosystems, and social systems may become more challenging as more options are deployed. Ocean-based mitigation measures such as offshore wind or carbon dioxide removal could have environmental and economic impacts.161,192 Mitigation infrastructure may affect existing activities, including fishing, boating, and shipping—which are themselves adapting to climate change.193 Decision-making about mitigation and adaptation choices—for example, those around disproportionate environmental burdens borne by historically marginalized racial and ethnic groups or communities with fewer economic resources—also poses ethical challenges.194 These ethical challenges may be greater for actions related to the ocean, given its complex governance systems.195
Ocean-related efforts to mitigate and adapt to climate change generally lag terrestrial efforts for several reasons, including gaps in ocean observations, lack of robust forecasts and projections, and limitations in mechanistic understandings of underlying climate-related changes. The ocean sector also faces challenges related to missing tools and services for adaptation, sector-specific (or siloed) management and governance, insufficient financing, and divergent stakeholder goals.10 Providing equitable access to information from scientific research and local knowledge, promoting evidence-based planning and adaptive management, and implementing actions to address near- and long-term risks can help prepare for climate impacts to marine ecosystems and resources.
Effective and cost-sensitive responses to changing oceans entail tracking changes in social–ecological systems and using that information to address risks. Strategic expansion and coordination of ocean observations and long-term monitoring programs (inclusive of community science) are necessary to document changes across marine ecosystems.196,197,198,199 Indigenous and local knowledge of ecosystem changes can be more fully integrated with other knowledge sources to support decision-making for ocean ecosystems.180,200,201 Key limitations remain in tracking, understanding, and projecting changes in marine ecosystems and impacts on people and economies. In particular, limited data are available in the US Caribbean and US-Affiliated Pacific Islands. Moreover, few coordinated monitoring and information-development efforts span regional or international boundaries.202
Climate-relevant economic and social data are not available at temporal and geographic scales necessary for tracking how climate change impacts on the ocean affect people. Socioeconomic data, such as the number of people using the ocean for recreation, are lacking or exist only at large geographic scales that do not support analyses of local impacts or evaluation of the effectiveness of adaptation strategies.203,204 Further, the lack of socioeconomic data precludes efforts to understand disparate impacts of and responses to climate change on communities of different sizes and income levels.
Responses to climate impacts are most successful when they incorporate robust scientific information into decisions, which can be supported by research and products that are designed with end users.205 Increased data accessibility and technical expertise focused on interpreting climate impacts and adaptation effectiveness will facilitate novel research and help deliver information that is relevant to decision-makers and stakeholders. Continued advances in near-term to decadal forecasts are urgently needed to provide decision-makers with early warnings and shape options that are incorporated into response plans, particularly for extreme events such as marine heatwaves, coral bleaching, HABs, or fish population changes.206,207,208,209,210 Mid- and longer-term projections of changes in ocean ecosystems are necessary to support risk assessments and strategic planning.211,212 Development of operational ocean modeling and decision support systems is a promising step to provide decision-makers with science-based information to implement adaptation measures.122,210
The extent of future climate impacts will depend both on the nature and magnitude of climate-related changes and on the degree to which individuals, businesses, communities, and governments can adapt to those changes.213 The pace, scale, and scope of expected climate impacts on ocean ecosystems necessitates assessing the ability of existing governance and management frameworks to effectively respond. There is also a need for financial incentives to develop and implement mitigation and adaptation actions, including support for community and sectoral adaptation. Adaptation and mitigation choices inevitably result in trade-offs that affect possible outcomes, implementation costs, and entities that bear the costs or receive the benefits.168 Inclusive and participatory frameworks for evaluating these trade-offs will support equitable deliberations about potential outcomes and uncertainties surrounding specific options. Such processes are especially critical for Indigenous communities with strong sociocultural connections to marine ecosystems and subsistence harvesters who rely on marine resources for food, nutritional, and economic security.5,214 Adaptive governance systems and cross-sector, cross-scale coordinating mechanisms can help advance actions that are acceptable to multiple stakeholders.213
Read more
Chapter leadership considered suggestions from the Federal Register Notice process and their own networks to identify authors with topical expertise, geographic familiarity, and disciplinary perspectives that span many issues relevant to the chapter. The goal was to build a diverse team in terms of racial, ethnic, and gender diversity; career stage; involvement in past climate assessments; and representation from the academic, governmental, and nongovernmental sectors. Seventeen invitations were issued, from which a team of eight authors was assembled, including physical scientists, marine ecologists, fishery scientists, economists, and policy analysts with experience assessing climate impacts on marine ecosystems, fisheries, marine economies, and coastal communities. Authors also have expertise in conservation approaches, adaptation strategies, and management measures that may buffer climate change impacts, and several authors are engaged in research and policy analysis related to ocean-based climate mitigation options.
Chapter authors reviewed the Fourth National Climate Assessment (NCA4) “Oceans and Marine Resources” chapter215 and brainstormed topics for NCA5 that had emerged since then or were not well covered in NCA4. The chapter lead identified additional topics from the US Global Change Research Program (USGCRP) assessment review document and public comments. The importance of certain topics was reinforced and additional topics were identified during three public engagement workshops organized by USGCRP (January 25, 2022), the American Fisheries Society (February 1, 2022), and the Ocean Sciences Meeting (February 24, 2022). Initial topics were subsequently honed through agency review and public input. The author team routinely reevaluated the literature to incorporate scientific advances into the assessment and prioritize topics that could be covered within the space limitations.
The chapter team held biweekly to weekly videoconferences to hone the chapter’s topics, Key Messages, and supporting information based on discussions of the state of the science. Small groups of authors developed text associated with each Key Message based on their expertise, literature review, and stakeholder input. The full author team reviewed each Key Message and its supporting information, and revisions were made until the team was satisfied with the text. The lead author administered a survey to elicit detailed input from each author on the high-level Key Message statements and the associated confidence and likelihood ratings. Differences in phrasing and ratings were discussed among the author team, and revisions were made until the group reached consensus on the content of those statements.
Read about Confidence and Likelihood
A robust body of evidence shows that climate change is having major impacts on US marine ecosystems. Changes in physical and chemical ocean conditions (Chs. 2, 3)10,12 affect species through distribution shifts, productivity changes, and phenology alterations.15,16,18,19 In shallow-water habitats (coral reefs, seagrass beds, and kelp forests), climate-related declines have been documented,20,22 but limited evidence is available to assess impacts in the deep sea.49 In certain places (e.g., Arctic and coral reef habitats), thresholds are being reached, beyond which ecosystem functions will be eroded and systems will be permanently altered.22,58
Evidence documenting climate impacts on marine-dependent human communities and alterations in cultural and social interconnections, economies, and livelihoods is growing. Climate-driven ecosystem changes threaten critical social couplings that underpin the well-being, subsistence, and economic and cultural identities of many communities with strong ties to the ocean, particularly coastal and island-based Indigenous communities (KMs 29.5, 30.5).54,163 Climate change has profoundly impacted Indigenous harvest of marine species, including those critical for subsistence.54,56,60,62 Indigenous Knowledge continues to reveal the breadth of climate impacts on human health, ecosystems, and subsistence resources, as well as the effectiveness of adaptation measures.57,62,63
Evidence of future ecological and social impacts draws on climate projections to extrapolate contemporary responses into the future. Model projections show consensus on the direction of many physical and chemical changes (e.g., warming temperature, declining pH; KM 2.1).179 Based on observed responses of species to environmental conditions and known physiological limits, populations and distributions are expected to be substantially altered by climate change.10,72,75,78 Impacts of marine ecosystem changes on humans are expected to increase as the conditions depart further from past conditions, although the magnitude depends on the rate of change and capacity for adaptation.213
While overall physical and biological trends are well characterized and projected to continue, the exact scale, timing, and location of future impacts are uncertain. Uncertainty in the scale of impacts derives primarily from unclear future socioeconomic pathways (including greenhouse gas [GHG] emissions). Spread among models on the sensitivity of Earth’s climate to socioeconomic futures (KM 2.1) and, in some cases, inadequate model resolution to forecast local-scale effects also contribute to uncertainty.212 While the severity of extreme events will increase as natural variability occurs on top of a changing baseline (KM 2.2), we do not know exactly when or where extreme events will occur. Thus, the continued development of prediction systems is a priority to extend the lead time of extreme event warnings (e.g., Tommasi et al. 2017; Jacox et al. 2022206,210).
Biological and ecological impacts of climate change, such as shifts in species distributions, can be assessed based on past observations. However, many existing observation systems were not deployed until recent decades,199 with the deep ocean remaining particularly under-observed.216 There is uncertainty associated with models of physical–biological relationships and challenges in scaling climate change impacts at the individual level to population dynamics, community interactions, or ecosystem functions. Data and studies of ecosystems and coupled social–ecological systems become scarce at large or complex scales.
Research gaps increase across the spectrum of complexity, from physical changes to system-level ecological and human impacts.10 Baseline studies vary widely for ocean ecosystems and regions. For example, coastal ecosystems are much better observed and studied than the deep ocean, and US regions with the strongest climate signals or occurrence of extreme events (e.g., Alaska, Northeast, West Coast) have been more extensively studied than other regions. Few studies are available for assessing climate impacts to marine ecosystems, resources, and communities in non-continental US regions, such as Hawai‘i and the US-Affiliated Pacific Islands and the US Caribbean.
For most elements of this Key Message, the authors have decided not to assign likelihood ratings, as quantitative projections of the impacts discussed are typically focused on a specific species, process, or ecoregion. Scaling likelihoods from these focused studies up to a general message is difficult. Statements of likelihood are scenario-dependent, and studies may not use the same scenarios or compare scenarios, which limits a consistent evaluation of likelihood.
The large and growing literature on climate impacts in marine ecosystems, coupled with attribution studies demonstrating that human-caused climate change is driving ocean conditions beyond the envelope of historical variability, give very high confidence that we have entered an unprecedented period of climate-driven marine ecosystem change.179,213 Studies of biological responses to climate change are widespread, but there is somewhat less research documenting how the cascading impacts of physical and ecological ocean changes affect human communities. The available information indicates that impacts are predominantly neutral or negative, leading to high confidence in our understanding of impacts to livelihoods, cultures, food supplies, and other human-ocean connections.10 A number of studies have focused on impacts to Indigenous Peoples, indicating very high confidence that climate change is altering ways of life, cultural traditions, and connections to the ocean for many Indigenous groups.54,58,68 The scientific literature overwhelmingly projects that climate-driven changes in social–ecological systems will become more frequent and intense as human-caused climate change emerges further from natural climate variability, with the greatest impacts under high or very high scenarios.72,75 Because a large evidence base consistently projects higher risks to marine ecosystems under higher scenarios, this outcome is considered likely, with very high confidence. While there is uncertainty about the pace and effectiveness of adaptation in social–ecological systems, there is very high confidence that the risks will be elevated if the pace of adaptation does not match or exceed the pace of climate change. The existence of ecological tipping points is supported by theory and empirical evidence (e.g., Hoegh-Guldberg et al. 2019; Stewart-Sinclair et al. 2020; Penn and Deutsch 202277,78,79), and the authors have very high confidence that many systems are moving toward tipping points and that some will be crossed in the future. This confidence is highest in ecosystems such as coral reefs that are experiencing frequent bleaching events and die-offs10 and in the Arctic, where declining sea ice is altering the ecosystem and social–ecological connections (KM 29.5). The authors have high confidence that as ecosystems move toward tipping points, interconnected social systems will be fundamentally changed in ways that threaten the well-being of people and communities.76
Studies characterize the observed and projected impact of climate change on US commercial marine fisheries. These include temperature impacts on productivity and redistribution of species and dependent fisheries, communities, supply chains, markets, and fisheries management.15,69,85,96,97,100,118,121 Commercial fisheries and subsistence harvesters are adapting to these changes through shifts in fishing locations, target species, harvest diversification, and other strategies, yet adaptive capacity varies across different types of harvesters and communities.68,85,118,163 Projections of how climate change will affect fisheries are available for many of the largest US commercial fisheries (e.g., Rheuban et al. 2017; Le Bris et al. 2018; Holsman et al. 2020; Moore et al. 202132,110,111,113). However, the magnitude of impacts differs across models that vary in resolution, complexity, and inclusion of regional management measures.109,217
Several studies characterize temperature- and weather-driven changes to human behavior around tourism and recreation,130,135,136 as well as direct impacts to resources that drive tourism.36,138,139 The data needed to quantify impacts and benefits of mitigation efforts in the transportation sector are more limited, although there has been a strong focus on the Arctic.54,60,140,143 Sea level rise will also threaten ocean transportation and shoreside infrastructure (KM 9.2).
Studies have determined that climate change, particularly sea level rise and stronger storms, poses a direct threat to ocean-based oil and gas infrastructure,155 as well as an indirect increase in the risk of oil spills due to climate change.154 For offshore wind, studies have estimated production capacity and variability off the US coasts and described potential impacts to surrounding ecosystems161,192 and existing ocean uses.158,160,162
Scientific literature associated with climate impacts and adaptive responses in ocean-based industries is developed and growing for commercial fisheries but is limited for many other sectors. Although tourism represents the largest sector of the ocean economy, there are relatively few studies that project climate impacts to the US ocean tourism sector at regional to national scales. Those that are available focus on specific industries in specific locations, such as cruising in the Arctic,137 coral reef tourism in Florida and Hawai‘i,138 and whale watching in Puget Sound.139 Studies of climate impacts on marine recreational fisheries are also limited,135,136 particularly compared to extensive studies of commercial fisheries. Efforts to reduce GHG emissions from vessels, ports, and shipping are developing,147,149 but limited data availability makes it difficult to track associated implementation progress and emission outcomes. Syntheses of the state of knowledge related to ecological, economic, and community impacts of the development of ocean-based renewable energy are just recently becoming available.159
The greatest limitation in understanding economic and social impacts of climate change on marine-dependent livelihoods stems from the lack of publicly available economic and social data—specifically at spatial and temporal scales necessary to track changes, measure impacts, and make projections for marine economic sectors.218 This gap constrains efforts to quantify the magnitude of impacts, effectiveness of adaptation strategies, and differential impacts and responses across distinct groups. A nascent area of study concerns the interacting and compounding ecological, social, economic, and cultural impacts of changes on the social–ecological systems with which marine industries interact.
Across numerous studies, there is high agreement and robust evidence, and therefore high confidence, that climate change poses significant risk to marine economic sectors and activities. This evidence includes various determinations of climate change risk from recent assessment reports with focused chapters on marine sectors and communities (e.g., Constable et al. 2022; Cooley et al. 2022; Hicke et al. 20227,10,219). Multiple studies have evaluated risk over time under contrasting future carbon mitigation scenarios; high agreement in results yields very high confidence that climate change impacts increase over time and with higher levels of global warming, posing higher risks to communities and groups that have fewer economic alternatives and lower adaptive capacity.179,213 The impacts of ocean-based climate change depend on the effectiveness and feasibility of adaptation measures that remain largely nascent,10 leading to medium confidence that adaptation measures can help reduce the impacts of climate change. Adaptation options narrow and challenges of adaptation increase with greater magnitude and complexity of impacts,7,10 giving high confidence in the limits of adaptation under higher scenarios. Quantitative projections of climate impacts on marine economy sectors are few and location-specific, and they do not use multiple or comparable climate scenarios; as such, the authors have decided not to apply likelihood ratings.
There is abundant evidence that the severity and rate of future climate impacts on ocean systems and ocean-reliant human communities will vary based on GHG trajectories, which are the outcomes of societal choices (KMs 2.3, 3.1). Local, regional, and sectoral impacts will be influenced by the pace and effectiveness of adaptation efforts.10
An increasing number of efforts indicate the potential for ocean-based mitigation and adaptation solutions. Advances in mitigation are being realized through the production of ocean-based renewable energy,156,157 decarbonization of the maritime shipping industry (KM 13.2),147 and expansion of aquatic food systems with lower overall emissions (KM 11.1).177 In addition, nature-based solutions such as the preservation and restoration of blue carbon ecosystems (Focus on Blue Carbon)220 and carbon dioxide removal techniques that leverage ocean systems and enhance the ocean’s natural carbon sink (KM 32.3)172 may offer cost-efficient and effective approaches that support carbon mitigation, climate adaptation, and biodiversity.185,186,221 However, the benefits for adaptation and biodiversity are presently more clear than long-term benefits for mitigation (e.g., carbon sequestration), and more research would be needed to understand scales at which mitigation benefits are realized, rates of benefit growth over time, and the effectiveness of specific measures relative to other mitigation options.187,222
Even with swift and ambitious reductions in GHG emissions, climate impacts to oceans will continue.179 Existing studies document how ocean users, economic sectors, and communities in the US are reacting to climate impacts with a variety of strategies, including business changes, early warning systems, evidence-based management, resilience planning, governance adjustments, and technological innovations.10,69,96,122 Disparities are expected because not all individuals and communities are equally able to adapt, yet few studies exist to understand the types of disparities that are arising, how they are distributed among different communities and groups, and the extent to which they are mediated by factors such as social connectivity, wealth, or the diversity of available livelihood options.4,86,182
There is increasing evidence that adaptation strategies that are highly coordinated, planned in advance, and applied to larger scales lead to more durable, equitable outcomes.7,10 Regardless of the adaptation approach, there is strong and abundant evidence that with continued increases in emissions, the number of effective adaptation options will decrease.213 Plentiful and diverse evidence from the US and worldwide indicates that future conditions will make it more difficult to maintain ecological, social, cultural, and economic interconnections related to ocean ecosystems.213
A greater understanding of the relative benefits and risks of adaptation strategies, conditions that influence effectiveness, feasibility of uptake by different groups of people, and implementation costs is needed. Limited data and research are available to quantify the socioeconomic impacts of climate change and how they vary among communities or groups or to evaluate how social conditions and interactions (e.g., economic, governance, or social coordination) influence choices, implementation, and effectiveness of adaptation options.10 Limits to adaptation are not yet well known for ecosystems, individuals, and communities. Whether certain conditions, such as social connectivity, flexibility, socioeconomic assets, and livelihood diversity,4,182 insulate marine resource users from climate impacts and how they can be enhanced are still emerging areas of understanding.
How ocean-based mitigation solutions would affect marine ecosystems, existing uses of the ocean, and marine-dependent human communities is not yet well understood. The GHG-reduction potential and costs of many ocean-based mitigation options are still highly uncertain, and more information is needed to fully assess their effectiveness, scalability, and affordability.169,176 There is an emerging body of information about how offshore wind development may affect the surrounding physical and natural system,159 and some of these insights may apply to techniques under development, such as ocean carbon dioxide removal. Development of new ocean uses is expected to alter access for other activities, but it is unclear how adaptation strategies and mitigation measures may influence ocean use patterns and the types of users who may be advantaged or disadvantaged by these changes.193
The strong relationship between ambitious mitigation and a larger portfolio of effective adaptations is recognized across many ecosystems and sectors. However, data exist for only a limited number of ocean ecosystems, such as warm-water coral reefs and mangroves,10 and additional ocean-focused studies would improve understanding. There is relatively little information on trade-offs among adaptation choices or interactions between ocean-focused adaptation, mitigation, and prevailing social conditions. These connections are mainly derived from analogy with coastal and terrestrial systems, where evidence about human–natural system decision-making tends to be more available.
Across a range of studies, climate change impacts are affecting marine social–ecological systems, and risks are projected to increase in the future.213 Projections consistently indicate that risks to marine social–ecological systems are lower under climate scenarios that achieve high mitigation and adaptation and that are implemented sooner, yielding very high confidence in this pattern of outcomes.10 A broader array of adaptation options will be preserved if they are implemented sooner and keep pace with the rate of climate change impacts,7,10 giving very high confidence that earlier adaptation will enhance outcomes and reduce costs. Impacts are being observed in communities that heavily depend on marine resources and have limited capacity to adapt, including Indigenous communities, resource-dependent economies, and smaller-scale fisheries.58,68,86,118,163 These studies give high confidence that impacts are uneven and that intentional considerations that promote equitable mitigation and adaptation are required to reduce disproportionate impacts.
