On islands, all natural sources of freshwater come from rainfall received within their limited land areas. Piping water from neighboring states is not an option, making islands uniquely vulnerable to climate-driven variations and changes in rainfall, rates of evaporation, and water use by plants. The reliability of precipitation is a key determinant of ecosystem health, agricultural sustainability, and human habitability.

Severe droughts are common, making water shortage one of the most important climate-related risks in the region.⁵ In water emergencies, some islands rely on temporary water desalination systems or have water sent by ship, both of which are costly but life-saving measures (Figure 27.4).⁵⁶ Droughts occur naturally in this region and are often associated with El Niño events. Rainfall in Hawai‘i and the U.S.-Affiliated Pacific Islands (USAPI) is strongly affected by seasonal movement of the intertropical convergence zone and ENSO (see Box 27.1). Similarly, other patterns of climate variability, such as the Pacific Decadal Oscillation, produce strings of wet or dry years lasting decades in the region. Because of this natural variability, including dry seasons and frequent dry years, Pacific islands are highly vulnerable to any climate shifts that reduce rainfall and increase the duration and severity of droughts.

Compounding the direct effects of climate change, such as changing rainfall patterns, are the impacts of sea level rise on groundwater and groundwater-fed surface environments, such as wetlands and open lakes and ponds in low islands. For atoll islands, residents depend on shallow aquifers for some of their domestic water needs and for food production.⁵⁷ Rising sea level leads to a higher frequency of overwash events,⁵⁸ during which seawater inundates large parts of the islands and contaminates freshwater aquifers, wetlands, and other aquifer-fed environments. Overwash events already periodically occur during unusually high tides as a result of storm-driven waves or because of tsunamis. Rising sea level greatly increases the risk of groundwater contamination when these events occur.

Climate shifts have already been observed in the region, with increases in temperature and changes in rainfall. In Hawai‘i, temperature has risen by 0.76°F over the past 100 years (Figure 27.5),⁵⁹ and 2015 and 2016 were the warmest years on record. Higher temperatures increase evaporation, reducing water supply and increasing water demand. Hawai‘i rainfall has been trending downward for decades, with the period since 2008 being particularly dry.⁶⁰ These declines have occurred in both the wet and dry seasons and have affected all the major islands (Figure 27.6). In Micronesia, rainfall has generally decreased in the east, remained steady for some islands in the west (for example, Guam), and increased for other islands in the west.^23,32,61,62

The set of global and regional climate model outputs available for the U.S. Pacific Islands region shows a range of possible future precipitation changes, with implications for economic and policy choices. In Hawaiʻi, end-of-century rainfall projections under a higher scenario (RCP8.5) range from small increases to increases of to up to 30% in wet areas, and from small decreases to decreases of up to 60% in dry areas.^34,35

Using global climate model results for the lower scenario (RCP4.5) (see the Scenario Products section of App. 3), rainfall in Micronesia is projected to become as much as 10% lower to as much as 20% higher than at present within the next several decades, changes that are within the range of natural variability.⁶³ Changes are projected to be slightly greater by the end of the century but still within the −10% to +20% range for Micronesia.⁶³ In American Sāmoa, rainfall is projected to increase by up to 10% by mid-century compared with the present, with additional slight increases by the end of the century.

While rainfall in Hawai‘i generally has been decreasing, it is also becoming more extreme.^64,65 Both extreme heavy rainfall events (causing increased runoff, erosion, and flooding) and droughts (causing water shortages) have become more common.⁶⁶ The number of consecutive wet days and the number of consecutive dry days are both increasing in Hawaiʻi.⁶⁶ In American Sāmoa, drought magnitude and duration have minimal decreasing trends.²³

Higher rates of evaporation can strongly affect water resources by reducing the amount of water available (water supply) and by increasing the amount of water needed for irrigation and outdoor residential uses (water demand). Increasing temperatures throughout the Hawai‘i–USAPI region and decreased cloud cover in some areas will cause increases in rates of evaporation. These increases will worsen effects of reduced rainfall by further reducing water supply and simultaneously increasing water demand.

Streamflow in Hawai‘i has declined over approximately the past 100 years, consistent with observed decreases in rainfall.⁶ Trends showing low flows becoming lower indicate declining groundwater levels. On islands such as O‘ahu, water supply is mainly derived from groundwater.⁶⁷ If these declines continue due to further reductions in rainfall and/or increases in evaporation, groundwater availability will be impaired. Chronic water shortages are possible as rainfall decreases and both evaporation and the water requirements of a growing human population increase.

Given the small land areas and isolation of islands, and the current high level of year-to-year climate variability, even small changes in average climate are likely to cause extreme hardship. In the USAPI, subsistence-based agriculture persists, but the cultural and economic conditions that provided resilience have been eroded by the effects of colonization and globalization.⁶⁸ Hence, especially severe impacts of climate shifts are expected in these communities. Decreases in precipitation, together with saltwater contamination of groundwater systems due to sea level rise, threaten water and food security in some locations.^18,69,70

Adaptation. Impacts and risks from climate change will vary due to differences in hydrological characteristics and the governance and adaptive capacity of each island. To address ongoing and future impacts of these changes, adaptive capacity can be enhanced by enabling individual island communities to identify and prioritize climate-related risks.⁷¹ In Hawai‘i, adaptation to address water shortages is already taking place through successful water conservation programs (see Case Study “Planning for Climate Impacts on Infrastructure”), watershed protection (Watershed Partnerships), drought planning (Commission on Water Resource Management), and changes in plumbing codes and policies (Fresh Water Initiative) to enhance groundwater recharge and wastewater reuse.^72,73

In the USAPI, potential adaptation measures include development or improvement of emergency water shortage planning, including portable desalination systems and rapid-response drinking water shipments, although the high costs would prohibit larger desalination plants on most islands and atolls without international aid or other finance mechanisms.^74,75 Island communities can also improve their resilience to water shortages by increasing both rooftop water catchment and storage system capacity and by adopting drought-resistant and salt-tolerant crop varieties.

Throughout the region, the number of climate and water resources monitoring stations has declined,^23,76,77 reducing the ability of researchers to project future changes in climate. Restoring and enhancing monitoring of rainfall, evaporation-related climate variables (net radiation, air temperature, humidity, and wind speed), soil moisture, streamflow, and groundwater levels—critically important information for understanding, planning, and assessing adaptation actions—are prerequisites to building adaptive capacity to address the impacts of climate change on water resources.

Island landscapes and climates differ dramatically over short distances, producing a wide variety of ecological habitats and profoundly influencing the abundance and distribution of organisms, many of which have evolved to live in very specific environments and in close association with other species. Invasive species, landscape change, habitat alteration, and reduced resilience have resulted in extinctions and diminished ecosystem function (see Ch. 7: Ecosystems, KM 1).

The Hawaiian Islands illustrate the challenges the broader Pacific region is facing. Ninety percent of the terrestrial species native to Hawaiʻi are endemic (unique to the region). New, and potentially invasive, species are arriving much more frequently than in the past.^80,81 Hawaiʻi is home to 31% of the Nation’s plants and animals listed as threatened or endangered, and less than half of the landscape on the islands is still dominated by native plants.⁸² A similar picture describes most of the USAPI, as well. For example, Guam is well known for the decimation of its birds by the accidental introduction of the brown tree snake.

Nesting seabirds, turtles and seals, and coastal plants in low-lying areas are expected to experience some of the most severe impacts of sea level rise.⁸³ As detailed in the following section, rising sea levels will both directly inundate areas near shorelines and cause low-lying areas to flood due to the upward displacement of shallow aquifers. Rising sea levels also increase the tendency of large waves to wash inland and flood areas with saltwater, making the soil unsuitable for many plants and contaminating the underlying aquifer so that the water is not fit for drinking or crop irrigation.

Atolls are projected to be inundated, impacting existing on-island ecosystems.¹⁸ Atoll communities that depend on subsistence agriculture already experience loss of arable land for food crops such as taro and breadfruit,⁷⁰ along with the degradation of aquifers from sea level variability and extreme weather. Without dramatic adaptation steps, the challenges of sea level rise will likely make it impossible for some atolls to support permanent human residence. Wildlife that relies on coastal habitats will likely also be severely impacted. More than half of the global populations of several seabird species nest in the atolls and low islands of Northwestern Hawaiian Islands. In addition to the direct impact from the loss and degradation of habitat, Key Message 4 describes how these species are at risk from changes in prey availability and increasing land surface temperatures.⁸⁴

On many Pacific islands, native mangroves are highly productive coastal resources that provide a number of ecosystem services, including storm protection and food and building materials for Indigenous and local communities. Mangroves also serve as fish nursery areas, trap land-based sediment that would otherwise flow to coral reefs,⁸⁵ and provide habitat for many species. They are important reservoirs of organic carbon, providing yet another ecosystem service.⁸⁶ Mangroves are already under threat from coastal development and logging. Climate change, particularly sea level rise, will likely add additional stress.^87,88

The planning and economic implications for biodiversity management are substantial. The main islands of Hawai‘i have more than 1,000 native plant species,⁸⁹ and many of these are vulnerable to future climate shifts. Projections under a higher scenario (RCP 8.5) suggest that by the end of the century, the current distributions of more than 350 native species will no longer be in their optimal growing climate range.⁹⁰ For example, 18 of 29 native species studied within Hawaiʻi Volcanoes National Park are projected to shrink in range, such that most of the high-priority areas managed to protect biodiversity are projected to lose a majority of the studied native species.⁹¹ Approximately $2 million is spent annually to manage these areas (dollar year not reported),⁹² so climate-driven changes in plant distribution would have significant consequences on the allocation of funds. A global analysis suggests that the displacement of native species would provide increased opportunities for the establishment and spread of invasive species and that biodiversity would decrease as a result.^93,94

Throughout the Pacific, climate change will likely alter ecosystem services provided by agroforestry (the intentional integration of trees and shrubs into crop and animal farming systems to create environmental, economic, and social benefits). In American Sāmoa, the Republic of the Marshall Islands, and the Federated States of Micronesia, upland or inland forest services include substantial acreage in mixed agroforests (forests with various trees, lower shrubs, and row crops used for food, building, and cultural practices).^95,96 Agroforest production is impacted by drought, flooding, soil and water salinization (increased salt content in low-lying areas), wind, disease, pests, and clearing for development.⁷⁰ Climate change is projected to exacerbate these impacts in complex patterns related to the stressors present in specific locations.

Increases in air temperature are projected to have severe negative impacts on the range of Hawaiian forest birds. Avian malaria currently threatens this iconic fauna except at high elevations, where lower temperatures prevent its spread. However, as temperatures rise, these high-elevation sites will become more suitable for malaria. Model projections suggest that even under moderate warming, 10 of 21 existing forest bird species across the state will lose more than 50% of their range by 2100 (Figure 27.7). Of those, 3 are expected to lose their entire ranges and 3 others are expected to lose more than 90% of their ranges,^43,97 making them of high concern for extinction.

Streams on U.S. Pacific Islands are also home to native fauna that are unique and typically restricted to specific island groups such as the Mariana, Sāmoan, and Hawaiian archipelagos. A model of streamflow and habitat on the Island of Maui suggests that physical habitat for stream animals will decrease by as much as 26% in some streams under a higher scenario (RCP8.5), but the overall forecast is for habitat changes of less than 5% by 2100.⁹⁸ Throughout Hawaiʻi, elevated stream water temperatures from urbanization and a warming climate will likely reduce available habitat for temperature-sensitive species. Additionally, the larvae of native Hawaiian stream animals develop in the ocean, and exposure to ocean acidification puts them at risk of physiochemical changes resulting in lower reproductive success.⁹⁹

Adaptation. Adapting to the impacts of climate change on terrestrial ecosystems is challenging. Management measures can take years to design and fund. Currently, understanding specific impacts of climate change on a particular ecosystem is confounded by other stressors (such as land development and invasive species) and clouded by a lack of precision in forecasting how sea level, rainfall, and air temperatures will change at the ecosystem or habitat level. A recent report summarizes both vulnerabilities and potential adaptations across all Hawaiian Islands and ecosystem types.¹⁰⁰ Through research and collaboration with Indigenous communities and land managers, ecosystem resilience to climate change can be enhanced and the most severe climate change effects on biodiversity decreased.¹⁰¹ Many Pacific island communities view the protection and management of native biodiversity as ways to reduce climate change impacts. For example, a watershed model of the windward side of Hawaiʻi Island suggested that control of an invasive tree with high water demand would partially offset decreases in streamflow that might be caused by a drier climate.⁴⁴ In another example, resource managers are now keenly aware that climate change represents a serious long-term threat to Hawaiian forest birds. As a result, discussions involving multiple federal, state, and nongovernmental organization stakeholders are underway regarding a range of management responses, such as shifting protected areas, landscape-level control of avian malaria, and captive breeding and propagation. Some of these discussions are focused on adaptation to many aspects of climate change, whereas others address the broad range of threats to Hawaiian forest birds. Preparedness and planning can strengthen the resilience of native species and ecosystems to drought, wildfire, and storm damage, which will help them to avoid extinction due to climate change.

The rate of global average sea level rise has accelerated^102,103 and has become very damaging in the region (Figure 27.8). Impacts include coastal erosion,^7,8 episodic flooding,^9,10 permanent inundation,¹¹ heightened exposure to marine hazards,¹² and saltwater intrusion to surface water and groundwater systems.^13,14 Already apparent on many shorelines, these problems endanger human communities by negatively impacting basic societal needs, such as food and freshwater availability, housing, energy and transportation infrastructure, and access to government services.¹⁰⁴

Sea level could rise by as much as 1 foot by 2050 and by as much as 4 feet by 2100. Emerging science suggests that, for the Extreme sea level rise scenario, sea level rise of more than 8 feet by 2100 is physically possible. It is extremely likely that sea level rise will continue beyond 2100.^17,105

Communities in Hawaiʻi and the USAPI typically live in low-lying settings clustered around the coastal zone. Whether on high volcanic islands or low reef islands (atolls), exposure to marine hazards and dependency on global trade mean escalating vulnerability to climate change (Ch. 16: International, KM 1).¹⁸

Until recently, global sea level rise of about 3 feet by the end of the century was considered a worst-case scenario, becoming more likely without reductions in global greenhouse gas emissions.¹⁰⁶ However, new understanding about melting in Antarctica,^107,108,109 Greenland,¹¹⁰ and alpine ice systems;¹¹¹ the rate of ocean heating;^112,113 and historical sea level trends¹⁰³ indicates that it is physically possible to see more than double this amount this century (see also Ch. 2: Climate, KM 4).^17,114

The Intermediate sea level rise scenario predicts up to 3.2 feet of global sea level rise by 2100; however, recent observations and projections suggest that this magnitude of sea level rise is possible as early as 2060 in a worst-case scenario.¹⁷ Studies in Hawaiʻi show that the value of all structures and land projected to be flooded by 3.2 feet of sea level rise amounts to more than $19 billion (in 2013 dollars; $19.6 billion in 2015 dollars) statewide (Figure 27.9).⁴² Across the state, nearly 550 Hawaiian cultural sites would be flooded or eroded, 38 miles of major roads would be chronically flooded, and more than 6,500 structures and 25,800 acres of land located near the shoreline would be unusable or lost, resulting in approximately 20,000 displaced residents in need of homes.⁴²

Owing to global gravitational effects, sea level rise will disproportionately affect the tropical Pacific¹⁵ and potentially exceed the global average.¹⁶ This, plus sea level variability internal to the Pacific Basin (see Figure 27.3), means that parts of the region are likely to experience the highest rates of sea level rise on the planet.¹¹⁵ Scientific understanding of the timing and magnitude of future global sea level rise continues to improve,^116,117 making regular updates of management plans and engineering codes an important activity for island communities.

Because of accelerating sea level rise, coastal communities are projected to experience saltwater intrusion of aquifers and agricultural resources. As sea level rise continues in coming decades, freshwater sources will become increasingly at risk in communities dependent on restricted groundwater supplies.⁶⁹ Saltwater intrusion, which is amplified by climate variability and changing precipitation patterns (see Key Message 1),¹² is difficult to prevent, and, once damaged, water and food resources are challenging to restore.¹³

Future changes in global and regional precipitation vary among current climate models,^34,35,118 but the potential for changes in precipitation and the projected impacts of saltwater intrusion cast uncertainty over the sustainability of freshwater resources throughout the region. Because many island groups are very isolated, severe drought punctuated by saltwater intrusion can displace communities and produce feedback effects, such as failure of cultural, health, education, and economic systems (Ch. 17: Complex Systems).¹¹⁹ However, strategic planning for the inevitability of these events can greatly reduce their impact.

In many areas, Pacific island coastal populations already exist on the edge of sustainability. Urban areas typically cluster around port facilities, as nearly all Pacific communities are tied to goods and services delivered by cargo ships. As the world’s most isolated chain of islands, Hawaiʻi imports nearly 90% of its food at a cost of more than $3 billion per year (in 2004–2005 dollars),¹²⁰ resulting in government programs focused on food security.¹²¹ Without adaptation measures, the additional stress on sustainable practices related to sea level rise is likely to drive islanders to leave the region and make new homes in less threatened locations (see Key Message 6 and the Case Study “Bridging Climate Science and Traditional Culture”).¹²²

Away from urban areas, many island communities rely on food gathered from the ocean and land. Populations on remote reef islands throughout Micronesia depend on water, food, and medical assistance that are often in question and are a source of persistent community stress. Extreme water levels accompanied by high waves have swept over remote atoll communities and destroyed taro patches, contaminated fragile aquifer systems, and deeply eroded island shores.^9,10,58

In 2007, extreme tides coupled with high waves flooded the Federated States of Micronesia and triggered a national emergency. Food, water, and medical supplies had to be immediately delivered to dozens of communities in widely distributed locations to prevent famine (see Key Message 1)(see also Ch. 14: Human Health, KM 1).⁵⁷ It is likely that events of this type will increase in frequency as sea level rise accelerates in the future.

Rising sea surface temperatures are shifting the location of fisheries (Ch. 9: Oceans, KM 2).¹²³ Ocean warming ¹²⁴ and acidification,^125,126 coupled with damaging watershed¹²⁷ and reef practices,¹²⁸ converge on island shores to increasingly limit the food resources that can be gathered from the sea (see Key Message 4).¹²⁹ Growing exposure to coastal hazards, such as storm surges,¹³⁰ compounds this threat to sustainability.

The Pacific Ocean is highly variable; fundamental characteristics of ENSO (see Box 27.1) appear to be changing.¹³¹ Both El Niño and La Niña episodes are projected to increase in frequency and magnitude as the world warms.^3,52 Patterns of variability are complex,^132,133 and as climate changes over the long term, the oceanic and atmospheric forces that cause shorter-term climate variability (such as ENSO) also will be changing. Model projections indicate changing future wave conditions that will vary in complex ways spatially, by season, and with shoreline exposure and orientation.^37,38,134 These changes will challenge community efforts to define adaptation plans and policies.

The 2015–2016 El Niño was a Pacific-wide event with widespread impacts.¹³⁵ As warm water shifted from west to east, Palau, Yap, and other western Pacific communities experienced deep drought, requiring water rationing, as well as falling sea level that exposed shallow coral reefs.¹³⁶ In the central Pacific, Hawaiʻi experienced 11 days of record-setting rainfall that produced severe urban flooding,¹³⁷ while American Sāmoa faced long-term dry conditions punctuated by episodic rain events. Honolulu experienced 24 days of record-setting heat that compelled the local energy utility to issue emergency public service announcements to curtail escalating air conditioning use that threatened the electrical grid (Ch. 4: Energy, KM 1).¹³⁸ Nine months of drought stressed local food production, and a record tropical cyclone season saw Hawaiʻi monitoring three simultaneous hurricanes at one point.¹³⁹

There is great uncertainty about how Pacific variability occurring on shorter timescales (for example, El Niño and La Niña) will combine with multidecadal changes in temperature, waves, rainfall, and other physical factors. This variability affects sea level extremes, which are likely to become more frequent this century,^4,12 along with changes in precipitation,¹⁴⁰ ocean temperature,¹¹³ and winds.¹⁴¹ These, in turn, drive difficult-to-forecast stressors that challenge the sustainability of coastal communities.

To date, tropical cyclone frequency and intensity have not been observed to change in the region of the USAPI. Trade winds and monsoon wind characteristics are expected to change in the future, but projections for specific geographic locations are unclear.¹⁴² Under scenarios with more warming (for example, SRES A1B),¹⁴³ wind speeds are projected to decrease in the western Pacific and increase in the South Pacific;¹⁴² central Pacific tropical cyclone frequency and intensity are expected to increase;^40,142 and in the western and South Pacific, tropical cyclone frequency is projected to decrease, while cyclone intensity is projected to increase.¹⁴² Combined with continued accelerations in sea level rise, storm surge associated with a tropical cyclone has the potential to deliver a profound shock to a community beyond any ability to meaningfully recover.

Adaptation. Despite these threats, many Pacific communities are growing more resilient with renewed focus on conservation,¹⁴⁴ sustainably managing natural resources,¹⁴⁵ adapting to climate change,¹⁴⁶ and building more resilient systems.¹⁴⁷ Pacific island governments are taking steps to anticipate marine flooding (securing food and water resources) and doing so in the context of environmental conservation. Islanders throughout the USAPI are committing to demonstrate climate leadership, identifying sector vulnerabilities, and calling on their international partners to support their implementation of climate change resilience and adaptation actions.^{55,148,149,150,151,152}

The ocean around Hawaiʻi and the USAPI supports highly diverse marine ecosystems that provide critical ecosystem services.¹²³ Coral reef ecosystems are vitally important for local subsistence, tourism, and coastal protection. The pelagic (open ocean) ecosystem supports protected species, including sea turtles, sea birds, and marine mammals, as well as economically valuable fisheries for tunas and other pelagic fishes. In Hawaiʻi, for example, coral reefs inject an estimated $364 million in goods and services annually (in 2001 dollars) into the local economy,¹⁹ while the landings from the pelagic longline fisheries is worth over $100 million annually (in 2012–2013 dollars).¹⁵³

Climate change is already being observed in the Pacific Ocean. Sea surface temperatures and ocean pH, an indicator of acidity, are now beyond levels seen in the instrument record.¹⁵⁴ Additionally, oxygen levels in the subtropical Pacific have been declining over the past five decades, negatively impacting fishes that draw oxygen from the water.¹⁵⁵ Impacts from sea level rise on coastal habitats and infrastructure have already occurred in the region, and the rate of sea level rise is projected to accelerate (see Key Message 3).

Widespread coral bleaching and mortality occurred during the summers of 2014 and 2015 in Hawaiʻi and during 2013, 2014, and 2016 in Guam and the Commonwealth of the Northern Mariana Islands. Impacts varied by location and species, but the 2015 bleaching event resulted in an average mortality of 50% of the coral cover in western Hawaiʻi.⁴⁵ Coral losses exceeded 90% at the remote and pristine equatorial reef of Jarvis Island.¹⁵⁶In response to the prolonged and widespread bleaching, the State of Hawaiʻi convened an expert working group to generate management recommendations to promote reef recovery.¹⁵⁷

Under projected warming of approximately 0.5°F per decade, coral reefs will experience annual bleaching beginning in about 2035 in the Mariana Archipelago, in about 2040 in American Sāmoa and the Hawaiian Islands, and in about 2045 at other equatorial reefs (Figure 27.10).⁴⁶ Warming reductions on the order of the aims of the 2015 Paris Agreement are projected to delay the onset of annual severe bleaching by 11 years on average.⁴⁶ Because some coral species are more resilient to thermal stress than others, low levels of thermal stress are expected to only alter the types of corals present. However, at high levels of thermal stress, most coral species experience some bleaching and mortality.¹⁵⁸ Ocean acidification reduces the ability of corals to build and maintain reefs,^125,159 while land-based nutrient input can substantially exacerbate acidification and reef erosion.¹⁶⁰ Under the higher scenario (RCP8.5), by the end of the century, virtually all coral reefs are projected to experience an ocean acidification level that will severely compromise their ability to grow.^125,161 Loss of coral reef structure results in a decline in fish abundance and biodiversity, negatively impacting tourism, fisheries, and coastal protection.¹²³ In the Hawaiian Archipelago under the higher scenario (RCP8.5), coral reef cover is projected to decline from the present level of 38% to 11% in 2050 and to less than 1% by the end of the century. This coral reef loss is projected to result in a total economic loss of $1.3 billion per year in 2050 (in 2015 dollars, undiscounted) and $1.9 billion per year in 2090 (in 2015 dollars, undiscounted). In 2090, the lower scenario (RCP4.5) would avoid 16% of coral cover loss and $470 million in damages per year (in 2015 dollars, undiscounted) compared to the higher scenario (RCP8.5).¹⁶² In the central and western Pacific, coral reef cover is projected to decline by 2050 from a present-day average of 40% to 10%–20%, and coral reef fish production is expected to decline by 20% under a high emissions scenario (SRES A2).¹²³ 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 (EEZs) of most islands in the central and western Pacific.¹⁶³ A key uncertainty is the extent to which corals can develop resilience to the rapidly changing ocean conditions.^164,165 Changing ocean temperature and acidification will impact many other organisms that will likely alter the functioning of marine ecosystems.

Mangroves provide coastal protection and nursery habitat for fishes and, in some cases, protect coral reefs from sediment and enhance the density of coral reef fishes.¹⁶⁶ Sea level rise has caused the loss of mangrove areas at sites in American Sāmoa⁸⁷ and is projected to further reduce mangrove area in the Pacific Islands region by 2100.^87,88

In the open ocean, warming is projected to reduce the mixing of deep nutrients into the surface zone. Under the higher scenario (RCP8.5), increasing temperatures and declining nutrients are projected to reduce tuna and billfish species’ richness and abundance in the central and western Pacific Ocean, resulting in declines in maximum fisheries yields by 2%–5% per decade.^{129,167,168,169} Climate change is also projected to result in overall smaller fish sizes, further adding to the fishing impact (Ch. 9: Oceans, KM 2).¹⁷⁰

Tuna habitat in the equatorial region is projected to shift eastward with changing temperatures, so that by the end of the century the availability of skipjack tuna within the EEZs of Micronesian countries will likely be 10%–40% lower than current levels.¹²³

On low-lying islands and atolls, sea level rise is projected to result in the loss of resting and nesting habitat for sea birds and sea turtles and the loss of beach and pupping habitat for Hawaiian monk seals. Modeling exercises that take wave height into account project much greater habitat flooding than sea level rise alone would suggest.^18,38,171 For example, sea level rise of about 6 feet combined with both storm wave run-up and concurrent groundwater rise is projected to wash out 60% of the albatross nests across the U.S. Marine National Monuments each breeding season.⁸³

Adaptation. Management actions that remove other stressors on corals (such as those recommended in Hawaiʻi, Guam, and the Commonwealth of the Northern Mariana Islands after the recent bleaching events) have been proposed as strategies to enhance the resilience of corals to moderate levels of thermal stress and to aid their recovery.¹⁵⁷ However, experience from the 2016 extreme bleaching on the Great Barrier Reef found that water quality and fishing pressure had minimal effect on the unprecedented bleaching, suggesting that local reef protection measures afford little or no defense against extreme heat.¹⁵⁸ This suggests that more active intervention is necessary, such as incorporating assisted evolution and selectively breeding corals, to enhance their resilience to rapidly rising ocean temperatures and acidification,¹⁷² as is being tested in Hawai‘i. In the case of the pelagic ecosystem, fishing and climate change work together to reduce the abundance of tunas and billfishes targeted by the fishery.^170,173 Thus, an ecosystem-based approach to international management of open ocean fisheries in the Pacific that incorporates climate-informed catch limits is expected to produce more realistic future harvest levels and enhance ecosystem resilience.²⁰ Lastly, relocations of seabirds to nesting sites on higher islands have been proposed to mitigate lost nesting habitat on low-lying islands and atolls.⁸³

Indigenous communities of the Pacific have an inseparable connection to and derive their sense of identity from the lands, territories, and resources of their islands. This connection is traditionally documented in genealogical chants and stories transmitted through oral history.¹⁴⁶ The rich cultural heritage of Pacific island communities comprises spiritual, relational, and ancestral interconnectedness with the environment¹⁷⁴ and provides land security, water and energy security, livelihood security, habitat security,¹⁷⁵ and cultural food security.¹⁷⁶ Climate change threatens this familial relationship with ancestral resources¹⁷⁷ and is disrupting the continuity that is required for the health and well-being of these communities (this experience is common to many tribal and Indigenous communities across the United States) (see Ch. 15: Tribes, KM 2).^176,177

Sea level rise imperils Indigenous communities of the Pacific. The sea that surrounds Pacific island communities continues to rise at a rate faster than the global average,¹¹⁵ with documented impacts on agriculture, coastal infrastructure, food security, livelihoods, and disaster management in the Republic of Palau¹⁴⁹ and the Republic of the Marshall Islands.¹⁴⁷

In Hawaiʻi, sea level rise impacts on traditional and customary practices (including fishpond maintenance, cultivation of salt, and gathering from the nearshore fisheries) have been observed (Figure 27.11).¹⁷⁷ Since 2014, Indigenous practitioners have had limited access to the land where salt is traditionally cultivated and harvested due to flooding and sea level rise. Detachment from traditional lands has a negative effect on the spiritual and mental health of the people (Ch. 14: Human Health, KM 1; Ch. 15: Tribes, KM 2).¹⁷⁶

Ocean acidification and drought, in combination with pollution and development, are negatively affecting fisheries and ecosystems (which are drivers of tourism), directly impacting the livelihood security of Pacific communities. For example, across all Pacific island countries and territories, industrial tuna fisheries account for half of all exports, 25,000 jobs, and 11% of economic production.¹⁷⁸ In Hawaiʻi, between 2011 and 2015, an annual average of 37,386 Native Hawaiians worked in tourism-intensive industries; based on the 2013 U.S. census, this number represents 12.5% of the Native Hawaiian population residing in Hawaiʻi.

Climate change is impacting subsistence^{18,70,95,123,175} and cultural food security^70,176 of Pacific island communities. Subsistence food security is essential for the survival of Indigenous peoples of the world and is valued socially, culturally, and spiritually.¹⁷⁵ Cultural food security refers to the provision of food that is a necessary part of a community’s regular diet and sustains the connection with cultural and social practices and traditions.¹⁷⁶ Taro and fish are two examples of cultural foods important to the livelihoods of Pacific island communities and to economic development for the community and government.¹²³

In Hawaiʻi, climate change impacts, such as reduced streamflow, sea level rise, saltwater intrusion, and long periods of drought, threaten the ongoing cultivation of taro and other traditional crops.¹⁷⁷ Identifying and developing climate-resilient taro and other crops are critical for their continued existence.¹⁷⁹ In Yap, taro is a key element of the diet. Groundwater salinization has resulted in smaller corms (underground tubers), causing declines in harvest yield.¹⁸⁰ In American Sāmoa, the Republic of the Marshall Islands, and the Federated States of Micronesia, crops grown in mixed agroforests provide important sources of nutrition, meet subsistence needs, supplement household incomes through sales at local farmers’ markets, and support commercial production.^95,96 These crops include breadfruit, mango, and coconut as overstory components; citrus, coffee, cacao, kava, and betel nut as perennial components; and banana, yams, and taro. Climate change is expected to result in changes in farming methods and cultivars (Figure 27.13). Consequently, these changes will likely impact the relationship between communities and the land. These kinds of climate impacts lead to an increased dependence on imported food that is of little nutritional value.¹⁸¹ This is a public health concern for Hawaiʻi and the USAPI, as Indigenous Pacific Islanders have the highest rates of obesity and chronic diseases, such as diabetes, in the region.¹⁸²

The rich body of traditional knowledge is place-based and localized²¹ and is useful in adaptation because it builds on intergenerational sharing of observations²² of changes in climate-related weather patterns, ocean phenomena, and phenology (the study of cyclic and seasonal natural phenomena, especially in relation to climate and plant and animal life). These observations, gathered over millennia, are useful in defining baselines and informing adaptive strategies.¹⁸³ Indigenous cultures are resilient, and their resilience has empowered Pacific island communities to survive several millennia on islands.¹⁸⁰ These communities have survived extreme events and responded to change through adaptive mechanisms based on traditional knowledge that has evolved over many generations.¹⁸⁴

Women play a vital role in ensuring that adaptation planning and action in the Pacific draw on traditional knowledge and new technologies.¹⁸⁴ The role of women in Indigenous communities includes maintaining crop diversity as collectors, savers, and managers of seeds and thus enhancing livelihood security for the community.¹⁸⁵ Indigenous women are also central in teaching, practicing, protecting, and transmitting traditional knowledge and practices.¹⁸⁵ Women have also been identified as a more vulnerable population to regional climate risks due to the role they have in terms of economic activities, safety, health, and their livelihoods.¹⁴⁷ For example, in Palau, as in the broader region, the central role of Indigenous women as lead project participants is key to the success of any project.

In Pacific island cultures, lunar calendars are tools used to identify baselines of an environment, track changes (kilo, in Hawaiian), and record seasonality, migration patterns, and weather.¹⁸³ In Hawaiʻi, use of the traditional lunar calendar (kaulana mahina) and kilo in climate change adaptation assists communities with decision-making that allows for the best survival techniques.¹⁸³ In Moʻomomi, Molokai, an intact coastal sand dune ecosystem in the main Hawaiian Islands, kaulana mahina has proven to be a useful tool that has enhanced the resilience of this coastline.^186,187 Similarly, a calendar for traditional Marshallese agroforestry crops recently was adapted to account for ENSO and climate conditions (see Figure 27.14).¹⁸⁸

Emerging issues for Indigenous communities of the Pacific include the resilience of marine-managed areas and climate-induced human migration from their traditional lands, territories, and resources. Marine-managed areas, such as those designated under the Micronesia Challenge and the Papahānaumokuākea Marine National Monument in Hawaiʻi, demonstrate a commitment by multiple partners to conserve marine resources. Over time, monitoring the ability of Indigenous peoples to continue to experience kinship and maintain traditional practices can help to preserve the cultural heritage associated with these protected areas. Documenting the kinds of governance structures or decision-making hierarchies created for their management can serve as a learning tool that can be shared with other island communities.

Sectoral impacts act together to compound environmental, social, cultural, and economic costs. Pacific islands are particularly vulnerable to climate change impacts due to their exposure and isolation, small size, low elevation (in the case of atolls), and concentration of infrastructure and economy along the coasts. The interconnectedness of people in island communities and the interdependence between human activities and the natural environment¹¹⁹ mean that extreme events cause multiple, layered impacts, intensifying their effects (see Ch.17: Complex Systems). While each of these impacts presents challenges, when combined, the environmental, social, cultural, and economic impacts will have compounding costs. In addition, as some types of extreme events become more frequent, recovery from those events will prove increasingly difficult for isolated, resource-challenged islands,¹⁸⁹ resulting in long-term declines in people’s welfare.^190,191

Coastal flooding is a widely recognized threat to low-lying areas (see Key Message 3).⁷ Extreme sea level events—created by combinations of factors such as storm-generated waves, storm surges, king tides, and ENSO-related sea level changes (see Box 27.1), combined with ongoing sea level rise—pose multiple challenges to habitability; on atolls, they are a clear threat to communities’ existence (Figures 27.15, 27.16, 27.17). In 2005, when Cyclone Percy hit the Northern Cook Islands, waves swept across the atoll from both the ocean and the lagoon sides. Fresh food supplies were destroyed due to saltwater intrusion into taro fields, 640 people were left homeless, and freshwater wells were polluted, posing a risk to public health. Saltwater contamination of the freshwater lenses lasted 11 months or longer.¹³ In Tokelau, Cyclone Percy scattered human waste, trash, and other debris into the ocean and across the island. Tokelau’s three atolls lost most of their staple crops, while fish habitats were destroyed.¹⁹² The islands suffered beach erosion, and many live coral formations were covered by sand and debris. In addition, the storm damaged many of the hospitals, making treatment of the injured or displaced difficult.¹⁹³ Lack of technology and resources limits small islands’ ability to adapt to these complex threats. The cascading effects on infrastructure, health, food security, and the environment result in significant economic costs.^194,195

Sea level rise, the deterioration of coral reef and mangrove ecosystems (see Key Message 4), and the increased concentration of economic activity will make coastal areas more vulnerable to storms (see Key Message 3).¹⁹⁶ Pacific Islands already face underlying economic vulnerabilities and stresses caused by unsustainable development, such as the use of beaches for building materials that results in coastal erosion or the waste disposal on mangroves and reefs that undermines critical ecological functions. The compounding impacts of climate change put the long-term habitability of coral atolls at risk, introducing issues of sovereignty, human and national security,¹⁹⁷ and equity,^198,199,200 a subject of discussion at the international level.

An increase in the incidence of vector-borne diseases such as malaria and dengue in the Pacific Islands has been linked to climate variability and is expected to increase further as a result of climate change (see Ch. 14: Human Health, KM 1).^201,202 For example, in late 2013 and early 2014, Fiji experienced the largest outbreak of dengue in its history, with approximately 28,000 reported cases.²⁰³

Climate change impacts on ecological and social systems are already negatively affecting livelihoods^204,205,206 and undermining human security.^191,207 In some cases, changes in climate increase the risk of human conflict (see Ch. 16: International, KM 3).^191,207,208 However, exactly how and when these changes can lead to conflict needs further study.²⁰⁸ Climate change poses a threat to human security through direct impacts on economies and livelihoods that aggravate the likelihood of conflict and risk social well-being.²⁰⁹ For example, climate change puts ongoing disputes over freshwater in Hawaiʻi at risk of intensifying in the absence of policy tools to help resolve conflicts.²³ Human conflict in the Asia Pacific region are expected to increase as unequal resource distribution combines with climate impacts to affect communities that are heavily dependent on agriculture, forestry, and fishing industries.²¹⁰

Climate change is already contributing to migration of individuals and communities.^211,212 In March 2015, Marshall Islands Bikinian people gathered to discuss resettlement because of increased flooding from high tides and storms that was making the atoll of Kili uninhabitable (see Case Study “Understanding the Effect of Climate Change on the Migration of Marshallese Islanders”).²¹³

Climate change induced community relocation, a recognized adaptation measure, results in disruption to society–land relationships and loss of community identity.²¹⁴ Resettlement has resulted in people facing landlessness, homelessness, unemployment, social marginalization, food insecurity, and increased levels of disease.¹²²

Inaction to address climate-related hazards is projected to lead to high economic costs that are preventable.²⁰⁵ Remote island communities that are unprepared for extreme events would face disruptions of goods and services that threaten lives and livelihoods. Rebuilding is expensive and lengthy.^{13,218,219,220} Further, due to the special connections Indigenous people have to ancestral lands and territories, any loss of these resources is a cultural loss (see Key Message 5).²²¹

Early intervention, occurring already in some places across the region, can prevent costly and lengthy rebuilding of communities and livelihoods and minimize displacement and relocation (see Ch. 28: Adaptation, KM 4). Early intervention includes taking steps now to protect infrastructure, as is being done by the Honolulu Board of Water Supply (see Case Study “Planning for Climate Impacts on Infrastructure”), such as redesigning areas to allow for periodic inundation and flooding, reverting natural areas to facilitate a return to original drainage patterns, and building social networks to take immediate actions and plan future responses.²²² Policymakers prefer approaches that are low cost, yield benefits even in the absence of climate change, are reversible and flexible, and build safety margins into new investments to accommodate uncertain future changes.¹⁹⁶ Examples of safety margins include more climate-adapted housing, provisions to expand rainwater storage capacity in water tanks, reverse osmosis capabilities for removing salt from water (Figure 27.4), development of saline-tolerant crop varieties (Figure 27.13), and implementation of more effective early warning systems for typhoons, king tides, and coastal storms.

Across the region, groups are coming together to minimize damage and disruption from coastal flooding and inundation, as well as other climate-related impacts. In some cases, the focus is on taking preventive measures to remove exposure to hazards, rather than focusing on protection and impact reduction (for example, through relocation or increased protection of threatened infrastructure). On Kosrae, the Federated States of Micronesia, for example, the Kosrae Island Resource Management Authority has laid out a strategy to redirect development inland (such as repositioning the main access road away from the shoreline to higher ground).⁷

Social cohesion is already strong in many communities in the region, making it possible to work together to take action. Stakeholders representing academia, resource managers, and government came together across the State of Hawai‘i to summarize ecosystem-specific vulnerabilities and prioritize potential adaptations at the island scale.¹⁰⁰ In Molokai, a community-led effort is underway to prepare traditional fishponds for climate change (see Case Study “Bridging Climate Science and Traditional Culture”). One of the core benefits of this effort is the strengthening of relationships between the diverse people who will benefit from collaborating to address future climate change impacts on the island.

Where successful, early intervention can lower economic, environmental, social, and cultural costs and reduce or prevent conflict and displacement from ancestral land and resources.