Fisheries, coral reefs, and the livelihoods they support are threatened by higher ocean temperatures and ocean acidification (very likely, high confidence).Widespread coral reef bleaching and mortality have been occurring more frequently, and by mid-century these events are projected to occur annually, especially if current trends in emissions continue (likely, medium confidence). Bleaching and acidification will result in loss of reef structure, leading to lower fisheries yields, and loss of coastal protection and habitat (very likely, very high confidence). Declines in oceanic fishery productivity of up to 15% and 50% of current levels are projected by mid-century and 2100, respectively, under the higher scenario (RCP8.5; likely, medium confidence).

This finding is from chapter 27 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

To frame this chapter, the regional leads wanted to maximize inclusiveness and represent the key sectoral interests of communities and researchers. To select sectors and a full author team, the coordinating lead author and regional chapter lead author distributed an online Google survey from September to October 2016. The survey received 136 responses representing Hawaiʻi and all the U.S.-Affiliated Pacific Islands (USAPI) jurisdictions; respondents identified which of the National Climate Assessment (NCA) sectors they were most interested in learning about with respect to climate change in the Pacific Islands and suggested representative case studies.884675c9-3e31-483d-b6b9-fd53b99875ae The five top sectors were picked as the focus of the chapter, and a total of eight lead authors with expertise in those sectors were invited to join the regional team. To solicit additional participation from potential technical contributors across the region, two informational webinars spanning convenient time zones across the Pacific were held; 35 people joined in. The webinars outlined the NCA history and process, as well as past regional reports and ways to participate in this Fourth National Climate Assessment (NCA4).

A critical part of outlining the chapter and gathering literature published since the Third National Climate Assessment (NCA3)dd5b893d-4462-4bb3-9205-67b532919566 was done by inviting technical experts in the key sectors to participate in a half-day workshop led by each of the lead authors. A larger workshop centered on adaptation best practices was convened with participants from all sectors, as well as regional decision-makers. In all, 75 participants, including some virtual attendees, took part in the sectoral workshops on March 6 and 13, 2017. Finally, to include public concerns and interests, two town hall discussion events on March 6 and April 19, 2017, were held in Honolulu, Hawaiʻi, and Tumon, Guam, respectively. Approximately 100 participants attended the town halls. Throughout the refining of the Key Messages and narrative sections, authors met weekly both via conference calls and in person to discuss the chapter and carefully review evidence and findings. Technical contributors were given multiple opportunities to respond to and edit sections. The process was coordinated by the regional chapter lead and coordinating lead authors, as well as the Pacific Islands sustained assessment specialist.

Description of evidence base:

The Key Message was developed based on input from an expert working group convened at the outset of this section development and supported by extensive literature.

Ocean warming: NCA3 documented historical increases in sea surface temperature (SST), and current levels in much of the region have now exceeded the upper range of background natural variation.081bdbe7-f95f-4708-b18c-e7bc797effa7^,dd11662f-f1a1-4a3b-a34f-295e5364e5ed Future increases are projected even under lower-than-current emissions rates.^{fa5d3ea3-a0ad-4418-b516-20c748528b2f,dd11662f-f1a1-4a3b-a34f-295e5364e5ed}

Ocean acidification: Atmospheric carbon dioxide levels recorded at Mauna Loa, Hawaiʻi, have recently exceeded 400 parts per million, and oceanic pH levels measured off Oʻahu have steadily declined from an annual average of about 8.11 to 8.07 over the past 25 years (data from Hawaiʻi Ocean Time Series, SOEST, University of Hawaiʻi) and are projected to decrease to 7.8 by 2100.fa5d3ea3-a0ad-4418-b516-20c748528b2f As pH declines, it lowers the saturation level of aragonite (the form of calcium carbonate used by corals and many other marine organisms), reducing coral and shell growth.7ab1d9e1-75a1-48c5-8d85-02258496f919 By the end of the century, aragonite saturation is projected to decline from a current level of 3.9 to 2.4, representing extremely marginal conditions for coral reef growth.^{081bdbe7-f95f-4708-b18c-e7bc797effa7,fa5d3ea3-a0ad-4418-b516-20c748528b2f,cfdaea11-95e2-4789-914b-74901b2f26b0,5d518479-27a0-47b4-b30b-4b84f25fe4d2}

Bleaching events: These continue to occur—most recently over successive years—with widespread impacts.be538e70-7c97-4680-a580-7ee398361090^,e8acff77-3b2a-4b04-8bb4-6c8a50bbdc3e Sea surface temperature time series from a suite of Climate Model Intercomparison Project 5 outputs that are statistically downscaled to 4 km resolution are used to project the year when coral reefs will begin to experience annual bleaching under the higher scenario (RCP8.5).994e6b3d-5b20-4f51-9cc2-a3a523349078 These data forecast that bleaching will be an annual event for the region starting in about 2035.^{994e6b3d-5b20-4f51-9cc2-a3a523349078}

Mortality: During the 2014–2015 bleaching events, coral mortality in western Hawaiʻi was estimated at 50%be538e70-7c97-4680-a580-7ee398361090 and over 90% at the pristine equatorial Jarvis Atoll.^{71443a23-b42f-4435-93ac-78d32df5bd30}

Coral reef ecosystem impacts: Coral reef cover around the Pacific Islands region is projected to decline from the current average level of about 40% to 15%–30% by 2035 and 10%–20% by 2050.fa5d3ea3-a0ad-4418-b516-20c748528b2f The loss of coral reef habitat is projected to reduce fish abundance and fisheries yields by 20%.fa5d3ea3-a0ad-4418-b516-20c748528b2f Loss of coral reefs will result in increased coastal erosion.7350d7b3-6e95-4375-ba23-26756b441fc2^,246e0108-088f-481b-9251-75855df86d97 Tourism is the major economic engine in Hawai‘i, and healthy coral reef ecosystems are critical to this economy. Under the higher scenario (RCP8.5), coral reef loss is projected to result in a total economic loss of $1.3 billion per year in 2050 and $1.9 billion per year in 2090 (in 2015 dollars, undiscounted). In 2090, a lower scenario (RCP4.5) would avoid 16% of coral cover loss and $470 million per year (in 2015 dollars, undiscounted) compared to the higher scenario.0b30f1ab-e4c4-4837-aa8b-0e19faccdb94 The confidence intervals around these loss estimates under RCP8.5 for 2050 range from a gain of $240 million to a loss of $1.9 billion, and for 2090 range from a loss of $1.7 billion to $1.9 billion (in 2015 dollars, undiscounted).0b30f1ab-e4c4-4837-aa8b-0e19faccdb94

Insular fisheries: Insular fishes, including both coral reef fishes and more mobile, coastal pelagics (species such as mahi mahi and wahoo), are impacted both from declines in carrying capacity and loss from migration in response to temperature change. Taken together, declines in maximum catch potential exceeding 50% from late 20th century levels under the higher scenario are projected by 2100 for the exclusive economic zones of most islands in the central and western Pacific.faaa3555-cab7-44a6-a71e-dc269d1b67ce

Oceanic fisheries: A number of studies have projected that ocean warming will result in lower primary productivity due to increased vertical stratification and loss of biodiversity as organisms move poleward.d055c0df-2c85-4ee1-a3c6-8e6c79e425bd^,a34935d6-874d-4cef-ab70-e5d1e3d7e8ca^,41f0be78-707a-4169-8838-2446d6587a79 Estimates of up to a 50% decline in fisheries yields are projected with two different modeling approaches.d055c0df-2c85-4ee1-a3c6-8e6c79e425bd^,41f0be78-707a-4169-8838-2446d6587a79 The impact of climate change specifically on fisheries targeting bigeye, yellowfin, and skipjack tunas in the western and central equatorial Pacific has been explored with fisheries models.^{fa5d3ea3-a0ad-4418-b516-20c748528b2f,890b5b14-3c3c-4a52-b57a-69a54bffb0a2,1365f267-545d-472b-874d-9b2bb73ff327} However, there is considerable uncertainty in the projections of population trends, given our lack of understanding of how the various life stages of these species will respond and the sensitivity of the projections to the specific model used.^{1365f267-545d-472b-874d-9b2bb73ff327,891f6f9e-d8be-4069-b2ec-69b005beee1e}

New information and remaining uncertainties:

A major uncertainty for coral reefs is whether they can evolve rapidly enough to keep up with the changing temperature and pH.06b7a4b5-9c7e-42de-8dee-c54d443a2af3^,3febdda6-8289-4929-a0f6-2184ff560ff8 In the oceanic ecosystem, the impacts of changing ocean chemistry on the entire food web are not well understood but are expected to result in shifts in the composition of the species or functional groups, altering the energy flow to top trophic levels.b53e94f9-9fd9-4de6-8617-49470b98dacc^,3b556038-4a34-49f5-9dfa-1e20fdb33514 For example, a shift in the micronekton community composition (squids, jellyfishes, fishes, and crustaceans) was projected to alter the abundance of food available to fishes at the top of the food web.b53e94f9-9fd9-4de6-8617-49470b98dacc The impact of climate change on the intensity and frequency of interannual and decadal modes of climate variability (such as El Niño–Southern Oscillation and Pacific Decadal Oscillation) is not well known but has very important consequences.^{1a46c6a2-4b5f-408d-b3d0-21ebdd4f960b}

Assessment of confidence based on evidence:

There is high confidence that fisheries and the livelihoods they support are threatened by warmer ocean temperatures and ocean acidification. Widespread and multiyear coral reef bleaching and mortality are already occurring. It is likely, based on modeled SST projections, that by mid-century, bleaching will occur annually with associated mortality.

There is medium confidence in the projection of annual bleaching by mid-century, as it does not take into account any adaptation in corals.

There is high confidence that bleaching and rising seawater acidity will result in loss of reef structure, leading to lower fisheries yields and loss of coastal protection. This is deemed very likely because significant coral mortality has recently been observed in western Hawaiian coral reefs that suffered from the 2015 bleaching event. Further, the positive relationship between fish density and coral reef cover is well established. The magnitude of this impact depends on the extent that coral species exhibit adaptive or resilience capacity.

There is medium confidence that declines in oceanic fishery productivity of up to 15% and 50% are likely by mid-century and 2100, respectively. These declines are considered likely because we have seen related linkages between climate variability such as ENSO and the Pacific Decadal Oscillation and fisheries yields that provide an analog in some ways to global warming impacts. The uncertainty lies in our limited understanding of the linkages and feedbacks in the very complex oceanic food web. As temperate habitats warm, they will likely gain some tropical species, while the tropical habitats will likely only lose species.

Provenance

This finding was derived from scenario rcp_4_5
This finding was derived from scenario rcp_8_5