- Federal Coordinating Lead Authors:
- Roger B. Griffis, National Oceanic and Atmospheric Administration
- Elizabeth B. Jewett, National Oceanic and Atmospheric Administration
- Chapter Lead:
- Andrew J. Pershing, Gulf of Maine Research Institute
- Chapter Authors:
- C. Taylor Armstrong, National Oceanic and Atmospheric Administration
- John F. Bruno, University of North Carolina at Chapel Hill
- D. Shallin Busch, National Oceanic and Atmospheric Administration
- Alan C. Haynie, National Oceanic and Atmospheric Administration
- Samantha A. Siedlecki, University of Washington (now at University of Connecticut)
- Desiree Tommasi, University of California, Santa Cruz
- Review Editor:
- Sarah R. Cooley, Ocean Conservancy
- Technical Contributor:
- Vicky W. Y. Lam, University of British Columbia
- USGCRP Coordinators:
- Fredric Lipschultz, Senior Scientist and Regional Coordinator
- Apurva Dave, International Coordinator and Senior Analyst
<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
Oceans and Marine Resources
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.
Key Message 1: Ocean Ecosystems
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 (very high confidence). Ecosystem disruption will intensify as ocean warming, acidification, deoxygenation, and other aspects of climate change increase (very likely, very high confidence). In the absence of significant reductions in carbon emissions, transformative impacts on ocean ecosystems cannot be avoided (very high confidence).
Description of evidence base
Ocean warming has already impacted biogenically built habitats. Declines in mussel beds, kelp forests, mangroves, and seagrass beds, which provide habitat for many other species, have been linked to ocean warming and interactions of warming with changes in oxygen levels or other stressors (see Ch. 27: Hawaiʻi & Pacific Islands, Key Message 4 for impacts on mangrove systems in the Pacific Islands).155,156,157,158 Sea level rise will continue to reduce the extent of many estuarine and coastal habitats (for example, salt marshes, seagrass beds, and shallow coral reefs) in locations where they fail to accrete quickly enough to outpace rising seas.159,160 The composition and timing of phytoplankton blooms are shifting, and dominant algal species are changing, which can cause bottom-up changes in food web structure.17,18,161
Some of the most apparent ecosystem changes are occurring in the warmest and coldest ocean environments, in coral reef and sea ice ecosystems. Live coral cover in coral reef ecosystems around the world has declined from a baseline of about 50%–75% to only 15%–20% (the current average for most regions; see Bruno & Valdivia 2016; Eddy et al. 201869,162), primarily due to ocean warming.163,164 Exposure to water temperatures just a few degrees warmer than normal for a given reef can cause corals to bleach; bleached corals have expelled their colorful symbiotic dinoflagellate algae, and the lack of algae can partially or wholly kill coral colonies.165 Over the past four decades, warming has caused annual average Arctic sea ice extent to decrease between 3.5% and 4.1% per decade; sea ice melting now begins at least 15 days earlier than it did historically (Ch. 26: Alaska, KM 1).166,167,168 Several studies have shown that sea ice loss has changed food web dynamics, caused diet shifts, and contributed to a continued decline of some Arctic seabird and mammal populations.49,169,170,171,172 For instance, polar bear litter sizes have already declined and are projected to decline further; models suggest that sea ice breaking up two months earlier than the historical normal will decrease polar bear pregnancy success in Huntington Bay by 55%–100%.173,174
Species differ in their response to warming, acidification, and deoxygenation. This imbalance in sensitivity will lead to ecosystem reorganization, as confirmed by a number of recent ecosystem models focused on phytoplankton17,175,176 and on entire food webs.40,68,177,178,179,180 Local extinction and range shifts of marine species due to changes in environmental conditions have already been well documented, as have the corresponding effects on community structure.32,81
Global-scale coral bleaching events in 1987, 1998, 2005, and 2015–2016 have caused a rapid and dramatic reduction of living coral cover; as the regularity of these events increases, their effects on ecosystem integrity may also increase.7,164,181,182 Warming increases the likelihood of coral disease outbreaks and reduces coral calcification, reproductive output, and a number of other biological processes related to fitness.183,184 Under the higher scenario (RCP 8.5), all shallow tropical coral reefs will be surrounded by water with Ω < 3 by the end of this century.59 Laboratory research finds that many coral species are negatively impacted by exposure to high CO2 conditions,185,186,187 and field research conducted near geologic CO2 vents have found that exposure to high CO2 conditions changes some, but not all, coral communities.188,189,190,191 Sea ice loss in the Arctic is expected to continue through this century, very likely resulting in nearly sea ice-free late summers by the middle of the century (Ch. 26: Alaska, KM 1).166 Ice-free summers will result in the loss of habitats in, on, and under the ice and the emergence of a novel ecosystem in the Arctic.51 Arctic waters are also acidifying faster than expected, in part due to sea ice loss.192
Conservation measures, such as ecosystem-based fisheries management (Key Message 2) and marine-protected areas that reduce or respond to these other stressors, can increase resilience;66,67 however, these approaches have limits and can only slow the impact of climate change and ocean acidification.68 Ocean warming, acidification, and deoxygenation, among other indirect stressors, will lead to alterations in species distribution, the decline of some species’ calcification, and mismatched timing of prey–predator abundance that cannot be fully avoided with management strategies.33,193 Coral bleaching occurs on remote reefs, suggesting that even pristine reefs will be impacted in a warmer, more acidified ocean.69,70 Without substantial reductions in CO2 emissions, massive and sometimes irreversible impacts are very likely to occur in marine ecosystems, including those vital to coastal communities.57
Further research is necessary to fully understand how multiple stressors, such as temperature, ocean acidification, and deoxygenation, will concurrently alter marine ecosystems in U.S. waters. More research on the interaction of multiple stressors and in scaling results from individual to population or community levels is needed.27,194,195,196
Most species have some capacity to acclimate to changes in thermal and chemical conditions, depending on the rate and magnitude at which conditions change, and there may be enough genetic variation in some populations to allow for evolution.73,197,198,199 Some research suggests that only microbes have the ability to acclimate to the expected anthropogenic temperature and pH changes, suggesting a reduction in the diversity and abundance of key species and a change in trophic energy transfer, which underpin ecosystem function of the modern ocean.33
Description of confidence and likelihood
The amount of research and agreement among laboratory results, field observations, and model projections demonstrate very high confidence that ecosystem disruption has occurred due to climate change, particularly in tropical coral reef and sea ice-associated ecosystems due to the global increase of ocean temperatures. It is very likely that ecosystem disruption will intensify later this century under continued carbon emissions, as there is very high confidence that warming, acidification, deoxygenation, and other aspects of climate change will accelerate. While conservation and management practices can build resilience in some ecosystems, there is very high confidence that only reductions in carbon emissions can avoid significant ecosystem disruption, especially in coral reef and sea ice ecosystems.
Key Message 2: Marine Fisheries
Marine fisheries and fishing communities are at high risk from climate-driven changes in the distribution, timing, and productivity of fishery-related species (likely, high confidence). 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 (warming: very likely, very high confidence; acidification and deoxygenation: likely, high confidence). 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.
Description of evidence base
Most evidence of the impacts of climate variability on U.S. living marine resources comes from numerous studies examining the response of these species to variability in ocean temperature. There is strong evidence that fluctuations in ocean temperature, either directly or indirectly via impacts to food web structure, are associated with changes in the distribution,31,79,80,81 productivity,74,75,76,77,200,201,202 and timing of key life-history events, such as the spawning1,31,88,89 of fish and invertebrates in U.S. waters. These temperature-driven changes in the dynamics of living marine resources in turn affect commercial fisheries catch quantity,79 composition,203 and fisher behavior.1,83,204,205 Beyond temperature, there is robust evidence from experimental studies demonstrating the impacts of oxygen and pH variability on the productivity of marine fish and invertebrates.55,117,206 However, studies linking changes in oxygen or pH to variations in fisheries and aquaculture dynamics in the field are few and are mainly regional and/or specific to localized deoxygenation or acidification events.71,128,207
These observational and experimental studies have provided the foundation for the development of models projecting future impacts of changing climate and ocean conditions on fisheries. Global and regional applications of such models provide strong evidence that changes in future ocean warming will alter fisheries catches in U.S. waters.64,100,103,208,209,210 The projected decrease in catch potential in the tropics and the projected increase in high-latitude regions under both RCP4.5 and RCP8.5 scenarios are robust to model structural uncertainty103 and are consistent across modeling approaches.100,103,209,210 In addition, there is moderate evidence from regional ecosystem and single-species models of reduced future catch in specific U.S. regions from future ocean acidification.40,95,177,179,211
Fisheries management in the United States has become increasingly effective at setting sustainable harvest levels, and the number of U.S. fisheries that are overfished or subjected to overfishing has declined in most regions.212 Science-informed management in general has been shown to be effective in improving ecosystem status107 and has been projected to greatly improve the benefits from marine resources.65 Climate change presents new challenges to management systems, as some species move across management boundaries and away from traditional fishing grounds and as productivity patterns shift. Management approaches that do not consider climate-driven ecosystem changes can lead to overfishing when the environment shifts rapidly.76,213 Some measures have been proposed to make the fisheries management system more climate ready.84,105,214 In many cases, these management strategies will include measures to allow for greater flexibility for harvesters to adapt to changing distributions and quantities of target species. Some preliminary evidence suggests that the use of climate-informed harvest rules can improve fishery sustainability in a variable environment,102 but at present, few fisheries management decisions integrate climate-related environmental information.215 The North Pacific Fishery Management Council is currently examining a strategic, multispecies, climate-enhanced model that informs managers how climate change and variation are expected to impact key stocks.106
While shifts in the productivity and distribution of living marine resources and ecosystem structure are expected to change catch potential and catch composition in U.S. regions, many uncertainties exist. Projections of catch potential have largely been performed using dynamical bioclimatic envelope models (e.g., Cheung et al.103). In these models, the spatial population dynamics of fish stocks are forced by temperature (with additional net primary productivity effects on carrying capacity and pH and oxygen effects on growth) and do not include the potential for major changes in species interactions, as has previously occurred with warming events (e.g., Vergés et al32) and food web structure (e.g., Fay et al.179). Furthermore, recent studies indicate that zooplankton and export production may serve as better indicators of carrying capacity for fisheries than net primary productivity.210,216 Net primary productivity trends will likely be amplified by higher trophic levels, such as zooplankton and ultimately fish; thus, trends in catch potential projected from primary productivity alone may underestimate future changes.210 These models also do not consider the potential for evolutionary adaptation of marine species. Uncertainties in projections are particularly high for primary productivity, oxygen, and pH, especially at regional and coastal scales,217,218,219 but these uncertainties are not typically incorporated into projected catch trends. In terms of the economic impacts on consumers, there is also uncertainty about how potential decreases in the catch of some species will impact net revenues, as lower quantities will be compensated in some cases by increased prices paid by consumers (e.g., Seung and Ianelli94). Fish prices are expected to increase very modestly over the next decade, yet there are great uncertainties in longer-term prices based on uncertainty about climate, economic growth, and the effectiveness of management in fisheries around the world.220
In addition, climate change is only one of many stressors affecting fish dynamics. Future fish distribution, abundance, and productivity will depend on the interaction between these stressors, including fishing and climate-related stressors. Conceptually and empirically, it is clear that fishers are responding to a wide diversity of factors and may not narrowly follow shifting fish populations.83,221,222 The development of management measures that respond rapidly to dramatic shifts in environmental factors that impact recruitment, productivity, and distribution will also reduce the potential impacts of climate change by avoiding overfishing in times of environmental stress.
Description of confidence and likelihood
There is high confidence that climate change-driven alterations in the distribution, timing, and productivity of fishery-related species will likely lead to increased risk to the Nation’s valuable marine fisheries and fishing communities. There is very high confidence that future ocean warming will very likely increase these changes in fishery-related species, reduce catches in some areas, and challenge effective management of marine resources. There is high confidence that ocean acidification and deoxygenation will likely reduce catches in some areas, which will challenge effective management of marine fisheries and protected species.
Key Message 3: Extreme Events
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 (very likely, very high confidence), and they expose vulnerabilities that can motivate change, including technological innovations to detect, forecast, and mitigate adverse conditions.
Description of evidence base
Marine heat waves have been described as regions of large-scale and persistent positive sea surface temperature anomalies that can vary in size, distribution, timing, and intensity akin to their terrestrial counterparts.137,223 Well-documented marine heat waves have recently occurred in the northwest Atlantic in 20121,134,151 and the North Pacific in 2014–2016.2,6
Each of these events resulted in documented impacts to ecosystems and, in many cases, to the human communities to which they were connected. The recent major events in the U.S. northwest Atlantic and North Pacific led to economic challenges in the American lobster, Dungeness crab, and Gulf of Alaska Pacific cod fisheries.1,2,78,224
Abrupt warming can induce other ecosystem-level impacts. The North Pacific event featured an extensive bloom of the harmful algae Pseudo-nitzschia4,120 that led to mass mortalities of sea lions and whales and the closure of the Dungeness crab fishery. The increase in intensity and occurrence of these toxic algal blooms has been linked to warm events in both the Atlantic and the Pacific.4,120,142 Abrupt warming was inferred to trigger the expansion of the North Pacific oxygen minimum zone through reduced oxygen solubility and increased marine productivity.225
Extreme events with corrosive (Ω < 1) and/or low oxygen conditions can occur when deep waters, which are generally corrosive and have low oxygen levels, are brought into the coastal area during upwelling. They can also occur in response to the delivery of corrosive freshwater from the landscape, ice melting, and storms. These conditions now occur more frequently in coastal waters of the Pacific coast of the United States.39,126,131,226,227,228,229,230,231 Such events have led to the elevated mortality of coastal shellfish in hatcheries128 and die-offs of crabs and other animals living on the ocean bottom.123
Heat wave, high-acidity, and low-oxygen events are all produced by variability in the system occurring on timescales ranging from days to years. For example, recent marine heat waves have been linked to natural climate modes such as the North Atlantic Oscillation, Atlantic Multidecadal Oscillation, Pacific Decadal Oscillation, or North Pacific Gyre Oscillation, which change over several years.3,137 Persistent weather patterns lasting several months can further amplify conditions in the ocean, leading to extreme conditions.2,134,151 These climate modes and atmospheric conditions occur on top of the long-term trends caused by global climate change. Thus, as climate change progresses, events with temperatures above a certain level, oxygen below a certain level, or pH below a specified level will occur more frequently and will last longer.56,141,146,232
The intensity of corrosive events along the upwelling margin of the Pacific coast of the United States is increasing due to more intense winds over the past decade and ocean acidification.15,53,123,125 In Alaska waters, these events are associated with freshwater inputs and storm events that may also have a link to climate change.226,227,228,229,230,233
There is ample evidence that extreme events motivate adaptive change in human systems. For example, Hurricane Katrina and Superstorm Sandy motivated communities near the affected areas to expand planning against future storms.234,235 The 2012 North Atlantic heat wave prompted the development of a forecast system to help Maine’s lobster fishery avoid future supply chain disruptions (Ch. 18: Northeast).150 The impact of corrosive waters on shellfish hatcheries in the Pacific Northwest motivated the development of new technology to monitor and manage water chemistry in shellfish hatcheries.128
The description above assumes that natural modes of climate variability remain the same and can be simply added to baseline conditions set by the global climate. There is evidence that some natural climate modes may change in the future. As mentioned in the narrative, the climate oscillations linked to the 2014–2016 event in the North Pacific increase in amplitude in climate model projections.3,135,236 This suggests that extreme events will be more likely in the future, even without accounting for the shift to a warmer temperature baseline. Declines in Arctic sea ice are also hypothesized to impact future climate variability by causing the atmospheric jet stream to get stuck in place for days and weeks (e.g., Overland et al. 2016, Vavrus et al. 2017, but see Cohen 2016152,153,154). This has the potential to create persistent warm (where the jet stream is displaced to the north) and cold (where the jet stream moves south) weather conditions over North America.152,153 These conditions are similar to the precursors to both the northwestern Atlantic and North Pacific heat waves.2,134
For biogeochemistry, other factors may amplify the global changes at the regional level as well, especially in the coastal environment. These factors include local nutrient runoff, freshwater input, glacial runoff, spatial variability in retentive mechanisms, variability in upwelling strength, cloud cover, and stability of sedimentary deposits (for example, methane).15,125,143,151,231,233 Most of the factors will amplify the global trends toward lower oxygen and pH, leaving these estimates to be conservative. In addition, temperature, oxygen, and pH have synergistic effects that provide some uncertainties in the projected events.56
Description of confidence and likelihood
Because there is very high confidence and very high likelihood that oceans will get warmer, more acidified, and have lower oxygen content in response to elevated atmospheric carbon dioxide levels,15 it is very likely and there is very high confidence that extreme events will occur with increased intensity and frequency in the future.6,138,141,232,237
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