The Nation’s valuable ocean ecosystems are being disrupted by increasing global temperatures through the loss of iconic and highly valued habitats and changes in species composition and food web structure. Ecosystem disruption will intensify as ocean warming, acidification, deoxygenation, and other aspects of climate change increase. In the absence of significant reductions in carbon emissions, transformative impacts on ocean ecosystems cannot be avoided.
Marine fisheries and fishing communities are at high risk from climate-driven changes in the distribution, timing, and productivity of fishery-related species. Ocean warming, acidification, and deoxygenation are projected to increase these changes in fishery-related species, reduce catches in some areas, and challenge effective management of marine fisheries and protected species. Fisheries management that incorporates climate knowledge can help reduce impacts, promote resilience, and increase the value of marine resources in the face of changing ocean conditions.
Marine ecosystems and the coastal communities that depend on them are at risk of significant impacts from extreme events with combinations of very high temperatures, very low oxygen levels, or very acidified conditions. These unusual events are projected to become more common and more severe in the future, and they expose vulnerabilities that can motivate change, including technological innovations to detect, forecast, and mitigate adverse conditions.
The Nation’s valuable ocean ecosystems are being disrupted by increasing global temperatures through the loss of iconic and highly valued habitats and changes in species composition and food web structure. Ecosystem disruption will intensify as ocean warming, acidification, deoxygenation, and other aspects of climate change increase. In the absence of significant reductions in carbon emissions, transformative impacts on ocean ecosystems cannot be avoided.
Marine fisheries and fishing communities are at high risk from climate-driven changes in the distribution, timing, and productivity of fishery-related species. Ocean warming, acidification, and deoxygenation are projected to increase these changes in fishery-related species, reduce catches in some areas, and challenge effective management of marine fisheries and protected species. Fisheries management that incorporates climate knowledge can help reduce impacts, promote resilience, and increase the value of marine resources in the face of changing ocean conditions.
Marine ecosystems and the coastal communities that depend on them are at risk of significant impacts from extreme events with combinations of very high temperatures, very low oxygen levels, or very acidified conditions. These unusual events are projected to become more common and more severe in the future, and they expose vulnerabilities that can motivate change, including technological innovations to detect, forecast, and mitigate adverse conditions.
Virtually Certain | Extremely Likely | Very Likely | Likely | About as Likely as Not | Unlikely | Very Unikely | Extremely Unlikely | Exceptionally Unlikely |
---|---|---|---|---|---|---|---|---|
99%–100% | 95%–100% | 90%–100% | 66%-100% | 33%-66% | 0%-33% | 0%-10% | 0%-5% | 0%-1% |
Very High | Hoch | Medium | Niedrig |
---|---|---|---|
Strong evidence (established theory, multiple sources, consistent results, well documented and accepted methods, etc.), high consensus | Moderate evidence (several sources, some consistency, methods vary and/or documentation limited, etc.), medium consensus | Suggestive evidence (a few sources, limited consistency, models incomplete, methods emerging, etc.), competing schools of thought | Inconclusive evidence (limited sources, extrapolations, inconsistent findings, poor documentation and/or methods not tested, etc.), disagreement or lack of opinions among experts |
Documenting Uncertainty: This assessment relies on two metrics to communicate the degree of certainty in Key Findings. See Guide to this Report for more on assessments of likelihood and confidence.
Americans rely on ocean ecosystems for food, jobs, recreation, energy, and other vital services. Increased atmospheric carbon dioxide levels change ocean conditions through three main factors: warming seas, ocean acidification, and deoxygenation. These factors are transforming ocean ecosystems, and these transformations are already impacting the U.S. economy and coastal communities, cultures, and businesses.
While climate-driven ecosystem changes are pervasive in the ocean, the most apparent impacts are occurring in tropical and polar ecosystems, where ocean warming is causing the loss of two vulnerable habitats: coral reef and sea ice ecosystems. The extent of sea ice in the Arctic is decreasing, which represents a direct loss of important habitat for animals like polar bears and ringed seals that use it for hunting, shelter, migration, and reproduction, causing their abundances to decline (Ch. 26: Alaska, KM 1). Warming has led to mass bleaching and/or outbreaks of coral diseases off the coastlines of Puerto Rico, the U.S. Virgin Islands, Florida, Hawai‘i, and the U.S.-Affiliated Pacific Islands (Ch. 20: U.S. Caribbean, KM 2; Ch. 27: Hawaiʻi & Pacific Islands, KM 4) that threaten reef ecosystems and the people who depend on them. The loss of the recreational benefits alone from coral reefs in the United States is expected to reach $140 billion (discounted at 3% in 2015 dollars) by 2100. Reducing greenhouse gas emissions (for example, under RCP4.5) (see the Scenario Products section of App. 3 for more on scenarios) could reduce these cumulative losses by as much as $5.4 billion but will not avoid many ecological and economic impacts.
Ocean warming, acidification, and deoxygenation are leading to changes in productivity, recruitment, survivorship, and, in some cases, active movements of species to track their preferred temperature conditions, with most moving northward or into deeper water with warming oceans. These changes are impacting the distribution and availability of many commercially and recreationally valuable fish and invertebrates. The effects of ocean warming, acidification, and deoxygenation on marine species will interact with fishery management decisions, from seasonal and spatial closures to annual quota setting, allocations, and fish stock rebuilding plans. Accounting for these factors is the cornerstone of climate-ready fishery management. Even without directly accounting for climate effects, precautionary fishery management and better incentives can increase economic benefits and improve resilience.
Short-term changes in weather or ocean circulation can combine with long-term climate trends to produce periods of very unusual ocean conditions that can have significant impacts on coastal communities. Two such events have been particularly well documented: the 2012 marine heat wave in the northwestern Atlantic Ocean and the sequence of warm ocean events between 2014 and 2016 in the northeastern Pacific Ocean, including a large, persistent area of very warm water referred to as "the Blob." Ecosystems within these regions experienced very warm conditions (more than 3.6°F [2°C] above the normal range) that persisted for several months or more. Extreme events in the oceans other than those related to temperature, including ocean acidification and low-oxygen events, can lead to significant disruptions to ecosystems and people, but they can also motivate preparedness and adaptation.
<b>Pershing</b>, A.J., R.B. Griffis, E.B. Jewett, C.T. Armstrong, J.F. Bruno, D.S. Busch, A.C. Haynie, S.A. Siedlecki, and D. Tommasi, 2018: Oceans and Marine Resources. In <i>Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II</i> [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 353–390. doi: 10.7930/NCA4.2018.CH9
From tropical waters in Hawai‘i and Florida, to temperate waters in New England and the Pacific Northwest, to cold Arctic seas off of Alaska, the United States has some of the most diverse and productive ocean ecosystems in the world. Americans rely on ocean ecosystems for food, jobs, recreation, energy, and other vital services, and coastal counties of the United States are home to over 123 million people, or 39% of the U.S. population (Ch. 8: Coastal).8 The fishing sector alone contributes more than $200 billion in economic activity each year and supports 1.6 million jobs.9 Coastal ecosystems like coral and oyster reefs, kelp forests, mangroves, and salt marshes provide habitat for many species and shoreline protection from storms, and they have the capacity to sequester carbon.10,11,12,13
The oceans play a pivotal role in the global climate system by absorbing and redistributing both heat and carbon dioxide.14,15 Since the Third National Climate Assessment (NCA3),16 understanding of the physical, chemical, and biological conditions in the oceans has increased, allowing for improved detection, attribution, and projection of the influence of human-caused carbon emissions on oceans and marine resources.
Human-caused carbon emissions influence ocean ecosystems through three main processes: ocean warming, acidification, and deoxygenation. Warming is the most obvious and well-documented impact of climate change on the ocean. Ocean surface waters have warmed on average 1.3° ± 0.1°F (0.7° ± 0.08°C) per century globally between 1900 and 2016, and more than 90% of the extra heat linked to carbon emissions is contained in the ocean.15 This warming impacts sea levels, ocean circulation, stratification (density contrast between the surface and deeper waters), productivity, and, ultimately, entire ecosystems. Changes in temperature in the ocean and in the atmosphere alter ocean currents and wind patterns, which influence the seasonality, abundance, and diversity of phytoplankton and zooplankton communities that support ocean food webs.17,18
In addition to warming, excess carbon dioxide (CO2) in the atmosphere has a direct and independent effect on the chemistry of the ocean. When CO2 dissolves in seawater, it changes three aspects of ocean chemistry.15,19,20,21 First, it increases dissolved CO2 and bicarbonate ions, which are used by algae and plants as the fuel for photosynthesis, potentially benefiting many of these species. Second, it increases the concentration of hydrogen ions, acidifying the water. Acidity is measured with the pH scale, with lower values indicating more acidic conditions. Third, it reduces the concentration of carbonate ions. Carbonate is a critical component of calcium carbonate, which is used by many marine organisms to form their shells or skeletons. The saturation state of calcium carbonate is expressed as the term Ω. When the concentration of carbonate ions in ocean water is low enough to yield Ω < 1 (referred to as undersaturated conditions), exposed calcium carbonate structures begin to dissolve. For simplicity, the terms ocean acidification and acidifying will refer to the suite of chemical changes discussed above.
Increased CO2 levels in the atmosphere are also causing a decline in ocean oxygen concentrations.15 Deoxygenation is linked to ocean warming through the direct influence of temperature on oxygen solubility (warm water holds less oxygen). Warming of the ocean surface creates an enhanced vertical density contrast, which reduces the transfer of oxygen below the surface. Ecosystem changes related to temperature and stratification further influence oxygen dynamics by altering photosynthesis and respiration.22,23
All three of these processes—warming, acidification, and deoxygenation—interact with one another and with other stressors in the ocean environment. For example, nitrogen fertilizer running off the land and entering the Gulf of Mexico through the Mississippi River stimulates algal blooms that eventually decay, creating a large dead zone of water with very low oxygen24,25 and, simultaneously, low pH.26 Warmer conditions at the surface slow down the rate at which oxygen is replenished, magnifying the impact of the dead zone. Changes in temperature in the ocean and in the atmosphere affect ocean currents and wind patterns that can alter the dynamics of phytoplankton blooms,17 which then drive low-oxygen and low-pH events in coastal waters.
Transformations in ocean ecosystems are already impacting the U.S. economy and the coastal communities, cultures, and businesses that depend on ocean ecosystems (Key Message 1). Fisheries provide the most tangible economic benefit of the ocean. While the impact of warming on fish stocks is becoming more severe, there has also been progress in adapting fisheries management to a changing climate (Key Message 2). Finally, the ability for climate-related changes in ocean conditions to impact the United States was made especially clear by major marine heat wave events that occurred along the Northeast Coast in 2012 and along the entire West Coast in 2014–2016 (Key Message 3). During these events, the regions experienced high ocean temperatures similar to the average conditions expected later this century under future climate scenarios. Ecosystem changes included the appearance of warm-water species, increased mortality of marine mammals, and an unprecedented harmful algal bloom, and these factors combined to produce economic stress in some of the Nation’s most valuable fisheries.
Marine species are sensitive to the physical and chemical conditions of the ocean; thus, warming, acidification, deoxygenation, and other climate-related changes can directly affect their physiology and performance.27,28,29 Differences in how species respond to physical conditions lead to changes in their relative abundance within an ecosystem as species decline or increase in abundance, colonize new locations, or leave places where conditions are no longer favorable.30,31,32,33 Such reorganization of species in marine communities can result in some species losing resources they depend on for their survival (such as prey or shelter). Other species may be exposed to predators, competitors, and diseases they have rarely encountered before and to which they have not evolved behavioral responses or other defenses.34,35,36 Climate change is creating communities that are ecologically different from those that currently exist in ocean ecosystems. Reorganization of these communities would change the ecosystem services provided by marine ecosystems in ways that influence regional economies, fisheries harvest, aquaculture, cultural heritage, and shoreline protection (Figure 9.1) (see also Ch. 7: Ecosystems, KM 1; Ch. 8: Coastal, KM 2).37,38,39,40
While climate-driven ecosystem changes are pervasive, the most apparent impacts are occurring in tropical and polar ecosystems, where ocean warming is causing the loss of two vulnerable habitats: coral reef and sea ice ecosystems.41,42 Warming is leading to an increase in coral bleaching events around the globe,7 and mass bleaching and/or outbreaks of coral diseases have occurred off the coastlines of Puerto Rico, the U.S. Virgin Islands, Florida, Hawai‘i, and the U.S.-Affiliated Pacific Islands.43,44 Loss of reef-building corals alters the entire reef ecosystem, leading to changes in the communities of fish and invertebrates that inhabit reefs.45,46 These changes directly impact coastal communities that depend on reefs for food, income, storm protection, and other services (Figure 9.1) (see also Ch. 27: Hawaiʻi & Pacific Islands, KM 4).
The extent of sea ice in the Arctic is decreasing, further exacerbating temperature changes and increasing corrosiveness in the Arctic Ocean (Ch. 26: Alaska, KM 1).15 The decline in sea ice represents a direct loss of important habitat for animals like polar bears and ringed seals that use ice for hunting, shelter, migration, and reproduction, causing their abundances to decline.47,48,49 The Arctic Ocean food web is fueled by intense blooms of algae that occur at the ice edge. Loss of sea ice is also shifting the location and timing of these blooms, impacting the food web up to fisheries and top predators like killer whales (Ch. 26: Alaska, Figure 26.4).50,51,52 Surface waters around Alaska have or will soon become permanently undersaturated with respect to calcium carbonate, further stressing these ecosystems (Ch. 26: Alaska, Figure 26.3).
The majority of marine ecosystems in the United States and around the world now experience acidified conditions that are entirely different from conditions prior to the industrial revolution (Ch. 7: Ecosystems).14,53,54 Models estimate that by 2050 under the higher emissions scenario (RCP8.5) (see the Scenario Products section of Appendix 3 for more on scenarios) most ecosystems (86%) will experience combinations of temperature and pH that have never before been experienced by modern species.54 Regions of the ocean with low oxygen concentrations are expected to expand and to increasingly impinge on coastal ecosystems.15,55,56 Warming and ocean acidification pose very high risks for many marine organisms, including seagrasses, warm water corals, pteropods, bivalves, and krill over the next 85 years.57 Ocean acidification and hypoxia (low oxygen levels) that co-occur in coastal zones will likely pose a greater risk than if species were experiencing either independently.58 Furthermore, under the higher scenario (RCP8.5), by the end of this century, nearly all coral reefs are projected to be surrounded by acidified seawater that will challenge coral growth.59
Changes in biodiversity in the ocean are underway, and over the next few decades will likely transform marine ecosystems.33 The species diversity of temperate ecosystems is expected to increase as traditional collections of species are replaced by more diverse communities similar to those found in warmer water.60 Diversity is expected to decline in the warmest ecosystems; for example, one study projects that nearly all existing species will be excluded from tropical reef communities by 2115 under the higher scenario (RCP8.5).61
Climate-induced disruption to ocean ecosystems is projected to lead to reductions in important ecosystem services, such as aquaculture and fishery productivity (Key Message 2) and recreational opportunities (Figure 9.1) (Ch. 7: Ecosystems, KM 1). Eelgrass, saltmarsh, and coral reef ecosystems also help protect coastlines from coastal erosion by dissipating the energy in ocean waves (Ch. 8: Coastal, KM 2). The loss of the recreational benefits alone from coral reefs in the United States is expected to reach $140 billion by 2100 (discounted at 3% in 2015 dollars).62 Reducing greenhouse gas emissions (for example, under RCP4.5) could reduce these cumulative losses by as much as $5.4 billion but will not avoid many ecological and economic impacts.62
Warming, acidification, and reduced oxygen conditions will interact with other non-climate-related stressors such as pollution or overfishing (Key Message 2). Conservation measures such as efforts to protect older individuals within species,63,64 maintain healthy fish stocks (Key Message 2),65 and establish marine protected areas can increase resilience to climate impacts.66,67,68 However, these approaches are inherently limited, as they do not address the root cause of warming, acidification, or deoxygenation. There is growing evidence that many ecosystem changes can be avoided only with substantial reductions in the global average atmospheric CO2 concentration.57,69,70
Species can adapt or acclimatize to changing physical and chemical conditions, but little is known about species’ adaptive capacity and whether the rate of adaptation is fast enough to keep up with the unprecedented rate of change to the environment.71,72,73 Furthermore, ocean ecosystems are becoming increasingly novel, meaning that knowledge of current ecosystems will be a less reliable guide for future decision-making (Ch. 28: Adaptation, KM 2). Continued monitoring to measure the effects of warming, acidification, and deoxygenation on marine ecosystems, combined with laboratory and field experiments to understand the mechanisms of change, will enable improved projections of future change and identification of effective conservation strategies for changing ocean ecosystems.
Variability in ocean conditions can have significant impacts on the distribution and productivity (growth, survival, and reproductive success) of fisheries species.74,75 For stocks near the warm end of their range (such as cod in the Gulf of Maine),76 increases in temperature generally lead to productivity declines; in contrast, warming can enhance the productivity of stocks at the cold end of their range (such as Atlantic croaker).77 These changes in productivity have direct economic and social impacts. For example, warming water temperatures in the Gulf of Maine exacerbated overfishing of Gulf of Maine cod, and the subsequent low quotas have resulted in socioeconomic stress in New England.76 Reductions in the abundance of Pacific cod associated with the recent heat wave in the Gulf of Alaska led to an inability of the fishery to harvest the Pacific cod quota in 2016 and 2017, and to an approximately 80% reduction in the allowable quota in 2018.78
Changes in productivity, recruitment, survivorship, and, in some cases, active movements of target species to track their preferred temperature conditions are leading to shifts in the distribution of many commercially and recreationally valuable fish and invertebrates, with most moving poleward or into deeper water with warming oceans.31,79,80,81,82 Shifts in fish stock distributions can have significant implications for fisheries management, fisheries, and fishing-dependent communities. Fishers may be expected to move with their target species; however, fishing costs, port locations, regulations, and other factors can constrain the ability of the fishing industry to closely track changes in the ocean.83 Shifts across governance boundaries are already creating management challenges in some regions and can become trans-boundary issues for fish stocks near national borders (Ch. 16: International, KM 4).84
Changes in the timing of seasonal biological events can also impact the timing and location of fisheries activities. The timing of peak phytoplankton and zooplankton biomass is influenced by oceanographic conditions (such as stratification and temperature).85,86 Since juvenile fish survival and growth are dependent on food availability, variability in the timing of plankton blooms affects fish productivity (e.g., Malick et al. 201587). Migration and spawning, events that often depend on temperature conditions, are also changing.1,88,89,90 For example, management of the Chesapeake Bay striped bass fishery is based on a fixed fishing season that is meant to avoid catching large egg-bearing females migrating early in the season. As temperatures rise, more females will spawn early in the season, reducing their availability to fishers.89 The location and size of coastal hypoxic zones (which are likely exacerbated by temperature and ocean acidification)56 can affect the spatial dynamics of fisheries, such as the Gulf of Mexico shrimp fishery, with potential economic repercussions.91
The productivity, distribution, and phenology of fisheries species will continue to change as oceans warm and acidify. These changes will challenge the ability of existing U.S. and international frameworks to effectively manage fisheries resources and will have a variety of impacts on fisheries and fishing-dependent sectors and communities. Projected increases in ocean temperature are expected to lead to declines in maximum catch potential under a higher scenario (RCP8.5) in all U.S. regions except Alaska (Figure 9.2).92 Because tropical regions are already some of the warmest, there are few species available to replace species that move to cooler water.61 This means that fishing communities in Hawai‘i and the Pacific Islands, the Caribbean, and the Gulf of Mexico are particularly vulnerable to climate-driven changes in fish populations. Declines of 10%–47% in fish catch potential in these warm regions, as compared to the 1950–1969 level, are expected with a 6.3°F (3.5°C) increase in global atmospheric surface temperature relative to preindustrial levels (reached by 2085 under RCP8.5).92 In contrast, total fish catch potential in the Gulf of Alaska is projected to increase by approximately 10%, while Bering Sea catch potential may increase by 46%.92 However, species-specific work suggests that catches of Bering Sea pollock, one of the largest fisheries in the United States, are expected to decline,93 although price increases may mitigate some of the economic impacts.94 Similarly, abundance of the most valuable fishery in the United States, American lobster, is projected to decline under RCP8.5.64 Ocean acidification is expected to reduce harvests of U.S. shellfish, such as the Atlantic sea scallop;95 while future work will better refine impacts, cumulative consumer losses of $230 million (in 2015 dollars) across all U.S. shellfish fisheries are anticipated by 2099 under the higher scenario (RCP8.5).62
The implications of the projected changes in fisheries dynamics on revenue94,96 and small-scale Indigenous fisheries remain uncertain.97 Indigenous peoples depend on salmon and other fishery resources for both food and cultural value, and reductions in these species would pose significant challenges to some communities (e.g., Krueger and Zimmerman 200998) (Ch. 15: Tribes, KM 2; Ch. 24: Northwest). Additionally, western Alaska communities receive a significant share of the revenues generated by Alaska groundfish fisheries through the Western Alaska Community Development Quota program.99 This program provides an important source of fishery-derived income for these communities. Where there is strong reliance of fish stocks on specific habitats, shifts may lead to fish becoming more concentrated when water temperature or other changes in ocean conditions push species against a physical boundary such as ice or the ocean bottom.83 Alternatively, shifts in species distributions are likely to drive vessels farther from port, increasing fishing costs and potentially impacting vessel safety.100 Under such conditions, there will also be new opportunities that result from species becoming more abundant or spatially available. Advance knowledge and projections of anticipated changes allow seafood producers to develop new markets and harvesters the ability to adapt their gear and fishing behavior to take advantage of new opportunities.84,101,102
A substantial reduction of greenhouse gas emissions would reduce climate-driven ocean changes and significantly reduce risk to fisheries.103 Warming, acidification, and deoxygenation interact with fishery management decisions, from seasonal and spatial closures to annual quota setting, allocations, and fish stock rebuilding plans. Accounting for these factors is the cornerstone of climate-ready fishery management.84,104,105 Modeling studies show that climate-ready, ecosystem-based fisheries management can help reduce the impacts of some anticipated changes and increase resilience under changing conditions.93,106,107 There is now a national strategy for integrating climate information into fishery decision-making,105 and the North Pacific Fishery Management Council is now directly incorporating ocean conditions and climate projections in its planning and decision-making.108,109
National and regional efforts have been underway to characterize community vulnerability to climate change and ocean acidification.38,110,111 The development of climate-ready fisheries will be particularly important for coastal communities, especially those that are highly dependent on fish stocks for food and for income. Targeting and participating in an increased diversity of fisheries with more species can improve economic resilience of harvesters and fishing communities.112,113,114 Current policies can create barriers that impede diversification,112 but more dynamic management can enable better adaptation.115 Even without directly accounting for climate effects, precautionary fishery management and better incentives can increase economic benefits and improve resilience.64,65,116
Many studies have documented the impact of temperature on fish distribution and productivity, enabling initial projections of species distribution, productivity, and fishery catch potential under future warming (e.g., Cheung 2016103). While laboratory studies have shown that ocean acidification can impact fish and their prey,117 there have been no studies demonstrating that acidification is currently limiting the productivity of wild fish stocks. Acidification will become an increasingly important driver of ocean ecosystem change.39 It is likely that the primarily temperature-based projections described above are underestimating the total magnitude of future changes in fisheries. More work would be required to understand how management and climate change are likely to interact.105,118 Climate vulnerability assessments (e.g., Hare et al.119) estimate which fisheries are most vulnerable in a changing climate and could be used to develop adaptation strategies and prioritize research efforts.
The first two Key Messages focused on the impacts of long-term climate trends. Ocean conditions also vary on a range of timescales, with month-to-month and year-to-year changes aligning with many biological processes in the ocean. The interaction between long-term climate change and shorter-term variations creates the potential for extreme conditions—abrupt increases in temperature, acidity, or deoxygenation (Figure 9.3). Recent extreme events in U.S. waters demonstrated that these events can be highly disruptive to marine ecosystems and to the communities that depend on them. Furthermore, these events provide a window into the conditions and challenges likely to become the norm in the future.
Two recent events have been particularly well documented: the 2012 marine heat wave in the northwestern Atlantic Ocean (Ch. 18: Northeast, Box 18.1) and an event occurring between 2014 and 2016 in the northeastern Pacific Ocean, nicknamed the Blob (Figure 9.3) (Ch. 24: Northwest, KM 1; Ch. 25: Southwest, KM 3; Ch. 26: Alaska, KM 1). Ecosystems within these regions experienced very warm conditions (greater than 3.6°F [2°C] above the normal range) that persisted for several months or more.1,2,3 Additionally, the very warm temperatures during the 2015–2016 El Niño led to widespread coral bleaching, including reefs off of American Sāmoa, the Marianas, Guam, Hawaiʻi, Florida, and Puerto Rico (Ch. 20: U.S. Caribbean, KM 2; Ch. 27: Hawaiʻi & Pacific Islands, KM 4).7
Coastal communities are especially susceptible to changes in the marine environment,110,111 and the interaction between people and the ecosystem can amplify the impacts and increase the potential for surprises (Ch. 17: Complex Systems, KM 1). In the Gulf of Maine in 2012, warm temperatures caused lobster catches to peak 3–4 weeks earlier than usual. The supply chain was not prepared for the early influx of lobsters, leading to a severe drop in price.1 The North Pacific event, centered in 2015, featured an extensive bloom of the toxic algae Pseudo-nitzschia4,120 that led to mass mortalities of sea lions and whales and the closure of the Dungeness crab fishery.121,122 The crab fishery then reopened in the spring of 2016, normally a time when fishing effort is low. The shift in timing led to increased fishing activity during the spring migration of humpback and gray whales and thus an elevated incidence of whales becoming entangled in crab fishing gear.122 Continued warm temperatures in the Gulf of Alaska during 20165 led to reduced catch of Pacific cod.78
Extreme events other than those related to temperature can also occur in the oceans. Short-term periods of low-oxygen, low-pH (acidified) waters have occurred more frequently along the Pacific coast during intense upwelling events.15,123,124,125,126 The acidified waters were corrosive (Ω < 1) and reduced the survival of larval Pacific oysters (Crassostrea gigas) in commercial hatcheries that support oyster aquaculture127,128 and increased dissolution of the shells of pteropods, a type of planktonic snail important in many ocean ecosystems.129,130,131,132
The extreme temperatures experienced during both recent heat waves exposed ecosystems to conditions not expected for 50 or more years into the future, providing a window into how future warming may impact these ecosystems. In both regions, southerly species moved northward, and warmer conditions in the spring shifted the timing of biological events earlier in the year.1,133
In the future, the same natural patterns of climate variability associated with the heat waves in both ocean basins3,134,135,136,137 will continue to occur on top of changing trends in average conditions, leading to more extreme events relative to current averages.138
Human-caused climate change likely already contributed to the events observed in 2012 and 2015, helping drive temperatures to record levels.139,140 Ocean acidification events such as those described along the Pacific coast are already increasing and are projected to become more intense, longer, and increasingly common.53,141 The increase in intensity and frequency of toxic algal blooms has been linked to warm events and increasing temperatures in both the Atlantic and Pacific Oceans.4,120,142 Changes resulting from human activities, especially increased nutrient loads, accelerate the development of hypoxic events in many areas of the world’s coastal ocean.15,143
Extreme events in the oceans can lead to significant disruptions to ecosystems and people, but they can also drive technological adaptation. Several corrosive events along the Pacific Northwest coast prompted the Pacific Coast Shellfish Growers Association to work with scientists to test new observing instruments and develop management procedures.128 The hatcheries now monitor pH and pCO2 (partial pressure of carbon dioxide) in real time and adjust seawater intake to reduce acidity. Similar practices are being employed on the East Coast to adapt shellfish hatcheries to the increasing frequency of low-pH events associated with increased precipitation and runoff.144
Similarly, the need to forecast El Niño events led to the development of seasonal climate forecast systems.145 Current modeling systems make it possible to forecast temperature, pH, and oxygen conditions several months into the future.101,102,146,147,148 Operational forecasts are also being developed for harmful algal blooms149 and for the timing of Maine’s lobster fishery.150 Further engagement with users would improve the utility of these emerging forecasts.101,148
The recent extreme events in U.S. ocean waters were the result of the interaction between natural cycles and long-term climate trends. As carbon emissions drive average temperatures higher and increase ocean acidification, natural climate cycles will occur on top of ocean conditions that are warmer, acidified, and have generally lower oxygen levels. A major uncertainty is whether these natural cycles will function in the same way in an altered climate. For example, the natural patterns of climate variability that contributed to the formation of the Blob show increasing variability in climate model projections.3 This suggests that similar temperature events in the North Pacific may be more likely. Unusually persistent periods of warm weather led to the formation of both the North Atlantic and North Pacific heat waves.2,134,151 Observational and modeling studies suggest that the loss of Arctic sea ice may disrupt mid-latitude atmospheric circulation patterns, making extreme weather conditions more likely (e.g., Overland et al. 2016, Vavrus et al. 2017, but see Cohen 2016152,153,154). This mechanism suggests that extremes in the ocean may be more extreme in the future, even after accounting for climate trends.
Ocean ecosystems provide economic, recreational, and cultural opportunities for all Americans. Increasing temperatures, ocean acidification, and deoxygenation are likely to alter marine ecosystems and the important benefits and services they provide. There has been progress in developing management strategies and technological improvements that can improve resilience in the face of long-term changes and abrupt events. However, many impacts, including losses of unique coral reef and sea ice ecosystems, can only be avoided by reducing carbon dioxide emissions.
The goal when building the writing team for the Oceans and Marine Resources chapter was to assemble a group of scientists who have experience across the range of marine ecosystems (such as coral reefs and temperate fisheries) that are important to the United States and with expertise on the main drivers of ocean ecosystem change (temperature, deoxygenation, and acidification). We also sought geographic balance and wanted a team that included early-career and senior scientists.
We provided two main opportunities for stakeholders to provide guidance for our chapter. This included a town hall meeting at the annual meeting of the Association for the Sciences of Limnology and Oceanography and a broadly advertised webinar hosted by the National Oceanic and Atmospheric Administration. Participants included academic and government scientists, as well as members of the fisheries and coastal resource management communities. We also set up a website to collect feedback from people who were not able to participate in the town hall or the webinar.
An important consideration in our chapter was what topics we would cover and at what depth. We also worked closely with the authors of Chapter 8: Coastal to decide which processes and ecosystems to include in which chapter. This led to their decision to focus on the climate-related physical changes coming from the ocean, especially sea level rise, while our chapter focused on marine resources, including intertidal ecosystems such as salt marshes. We also decided that an important goal of our chapter was to make the case that changing ocean conditions have a broad impact on the people of the United States. This led to an emphasis on ecosystem services, notably fisheries and tourism, which are easier to quantify in terms of economic impacts.