Climate change impacts in the Pacific Islands are expected to amplify existing risks and lead to compounding economic, environmental, social, and cultural costs (likely, medium confidence). In some locations, climate change impacts on ecological and social systems are projected to result in severe disruptions to livelihoods (likely, high confidence) that increase the risk of human conflict or compel the need for migration. Early interventions, already occurring in some places across the region, can prevent costly and lengthy rebuilding of communities and livelihoods and minimize displacement and relocation (likely, high 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 call 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:

For Atlantic and eastern North Pacific hurricanes and western North Pacific typhoons, increases are projected in precipitation rates and intensity. The frequency of the most intense of these storms is projected to increase in the western North Pacific and in the eastern North Pacific (see also Key Message 3).52ce1b63-1b04-4728-9f1b-daee39af665e Studies indicate that Hawaiʻi will see an increased frequency of tropical cyclones (TCs) due to storm tracks shifting northward in the central North Pacific.9082a92d-e1be-4346-8657-7b172a8f91bc^,2eff8dd3-b0da-474d-b7be-ba223baa8396

The Climate Science Special Report (CSSR) summarizes extensive evidence that is documented in the climate science literature and is similar to statements made in NCA3 and internationalf03117be-ccfe-4f88-b70a-ffd4351b8190 assessments.75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 More recent downscaling studies have further supported these assessments,4f1e7aa1-0c36-4220-ac77-7d55bcb33061 though pointing out that the changes (future increased intensity and TC precipitation rates) will not necessarily occur in all basins.^{52ce1b63-1b04-4728-9f1b-daee39af665e}

Damage from TCs is significant. Tropical Cyclone Evan struck Sāmoa in December 2012 and caused damage and losses of approximately $210 million dollars (dollar year not reported), representing 30% of its annual gross domestic product (GDP). Tropical Cyclone Pam struck Vanuatu, Tuvalu, and Kiribati in 2015; in Vanuatu, it killed 11 people and caused approximately $450 million (dollar year not reported) in damages and losses, equal to 64% of GDP.^{08f6548b-e879-45a6-97df-3790e804e73e}

In the CSSR, future relative sea level rise as shown for the 3.3-feet (1 m) Interagency scenario in 2100 indicates that, because they are far from all glaciers and ice sheets, relative sea level rise in Hawai‘i and other Pacific islands due to any source of melting land ice is amplified by the static-equilibrium effects. Static-equilibrium effects on sea level are produced by the gravitational, elastic, and rotational effects of mass redistribution resulting from ice loss.3bae2310-7572-47e2-99a4-9e4276764934

Sea level rise across Hawaiʻi is projected to rise another 1–3 feet by the end of this century. Sea level rise has caused an increase in high tide floods associated with nuisance-level impacts. High tide floods are events in which water levels exceed the local threshold (set by the National Oceanic and Atmospheric Administration’s National Weather Service) for minor impacts. These events can damage infrastructure, cause road closures, and overwhelm storm drains. Along the Hawaiian coastline, the number of tidal flood days (all days exceeding the nuisance-level threshold) has also increased, with the greatest number occurring in 2002–2003. Continued sea level rise will present major challenges to Hawaiʻi’s coastline through coastal inundation and erosion. Seventy percent of Hawaiʻi’s beaches have already been eroded over the past century, with more than 13 miles of beach completely lost. Sea level rise will also affect Hawai‘i’s coastal storm water and wastewater management systems and is expected to cause extensive economic impacts through ecosystem damage and losses in property, tourism, and agriculture.2eff8dd3-b0da-474d-b7be-ba223baa8396

In the Pacific Islands region, population, urban centers, and critical infrastructure are concentrated along the coasts. This results in significant damages during inundation events. In December 2008, wind waves generated by extratropical cyclones, exacerbated by sea level rise, caused a series of inundation events in five Pacific island nations.7717dd13-7f6b-4b7c-ab84-571d50f7b8da An area of approximately 3,000 km in diameter was affected, impacting approximately 100,000 people. Across the islands, major infrastructure damage and crop destruction resulted, costing millions of dollars and impacting livelihoods, food security, and freshwater resources.

The increases in the frequency and intensity of climate change hazards, including cyclones, wind, rainfall, and flooding, pose an immediate danger to the Pacific Islands region. A decrease in the return times of extreme events, which will reduce the ability of systems to recover, will likely cause long-term declines in welfare.dd36c0e3-b849-46c9-8685-dd76a465223a For small islands states, the damage costs of sea level rise are large in relation to the size of their economies.^{2e8e659c-1150-4516-b2c2-fd3176f9c641,494a5b2a-b76c-4c14-8654-c0ce218d2387}

The social science research on climate and conflict suggests a possible association between climate variability and change and conflict. Consensus or conclusive evidence of a causal link remains elusive. Hsiang et al. (2013)6013994a-8717-4a99-935a-8a13800fcdc5 find strong causal evidence linking climatic events to human conflict across a range of spatial scales and time periods and across all major regions of the world. They further demonstrate that the magnitude of climate influence is substantial.6013994a-8717-4a99-935a-8a13800fcdc5 Specifically, large deviations from average precipitation and mild temperatures systematically increase the risk of many types of conflict (intergroup to interpersonal), often substantially. Hsiang and Burke (2014)e3a15302-b1ec-4bfe-9ac3-3a2cf23d3303 describe their detailed meta-analysis, examining 50 rigorous quantitative studies, and find consistent support for a causal association between climatological changes and various conflict outcomes.e3a15302-b1ec-4bfe-9ac3-3a2cf23d3303 They note, however, that multiple mechanisms can explain this association and that the literature is currently unable to decisively exclude any proposed pathway between climatic change and human conflict.^{6013994a-8717-4a99-935a-8a13800fcdc5}

Evidence of the impact of climate on livelihoods is also well established. Barnett and Adger (2003, 2007)0dc9b00c-1fc7-43ee-bcb6-4c8e783e88f1^,15a85bcf-1235-4258-860c-24948df66935 are among a range of studies that conclude that climate change poses risks to livelihoods, communities, and cultures.15a85bcf-1235-4258-860c-24948df66935 These risks can influence human migration. The United Nations Environment Programme finds that the degree to which climatic stressors affect decisions to migrate depend on a household’s vulnerability and sensitivity to climatic factors.2fafa6e9-39fd-45bf-8a22-b6c84528e6b0

New information and remaining uncertainties:

A key uncertainty remains the lack of a supporting, detectable anthropogenic signal in the historical data to add further confidence to some regional projections. As such, confidence in the projections is based on agreement among different modeling studies. Additional uncertainty stems from uncertainty in both the projected pattern and magnitude of future sea surface temperatures.^{75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1,9082a92d-e1be-4346-8657-7b172a8f91bc,4f1e7aa1-0c36-4220-ac77-7d55bcb33061}

One study projects an increase in tropical cyclone frequency (TCF) of occurrence around the Hawaiian Islands but stipulates that TCF around the Hawaiian Islands is still very low in a warmed climate, so that a quantitative evaluation of the future change involves significant uncertainties.9082a92d-e1be-4346-8657-7b172a8f91bc

Uncertainties in reconstructed global mean sea level (GMSL) change relate to the sparsity of tide gauge records, particularly before the middle of the twentieth century, and to the use of a variety of statistical approaches to estimate GMSL change from these sparse records. Uncertainties in reconstructed GMSL change before the 20th century also relate to the lack of geological proxies (preserved physical characteristics of the past environment that can stand in for direct measurement) for sea level change, the interpretation of these proxies, and the dating of these proxies. Uncertainty in attribution relates to the reconstruction of past changes and the magnitude of natural variability in the climate.

Since NCA3, multiple approaches have been used to generate probabilistic projections of GMSL rise. These approaches are in general agreement. However, emerging results indicate that marine portions of the Antarctic ice sheet are more unstable than previously thought. The rate of ice sheet mass changes remains challenging to project.

In sea level rise projections, Antarctic contributions are amplified along U.S. coastlines, while Greenland contributions are dampened; regional sea level is projected to be higher than if driven by a more extreme Greenland contribution and a somewhat less extreme Antarctic contribution.^{c66bf5a9-a6d7-4043-ad99-db0ae6ae562c}

The degree to which climate variability and change impact conflict, and related causal pathways, remains uncertain. This is compounded by the fact that different types of conflict—social, political, civil, or violent—are conflated.657d9028-d5b7-4e0c-980e-dd71138c8bd7^,b3626f1d-9cfa-469a-b52d-dce3c6a0dff0 Violent conflict can describe interpersonal-, intergroup-, and international-level disputes. Some researchers contend that systematic research on climate change and armed conflict has not revealed a direct connection.9d9049c5-28b9-4029-a6b7-a18838dcdc69 Gemenne et al. (2014)32ad430a-4769-4e16-8ece-c28d123504b0 argue that there is a lack of convincing empirical evidence or theories that explain the causal connection between climate change and security. They do, however, note that there is some evidence for statistical correlation between climatic changes and conflict, broadly referenced.

Gemenne et al. (2014)32ad430a-4769-4e16-8ece-c28d123504b0 also note that the relationship between climate change and security comes from observation of past patterns and that present and projected climate change have no historical precedent. In effect, understanding past crises and adaptation strategies will no longer be able to help us understand future crises in a time of significant climate change.

The degree to which climate variability and change affect migration decisions made today also remains uncertain. This is in part due to the diverse scenarios that comprise climate migration, which themselves result from multiple drivers of migration.b3626f1d-9cfa-469a-b52d-dce3c6a0dff0 Burrows and Kinney (2016)b3626f1d-9cfa-469a-b52d-dce3c6a0dff0 detail examples of climate extremes leading to migration conflicts since 2000, yet they note that there are surprisingly few case studies on recent climate extremes that lead to migration and conflict specifically, despite an increasing body of literature on the theory.

While researchers disagree as to the degree to which climate change drives conflict and migration and the causal pathways that connect them, there is agreement that further research is needed. Buhaug (2015)9d9049c5-28b9-4029-a6b7-a18838dcdc69 and Gemenne et al. (2014)32ad430a-4769-4e16-8ece-c28d123504b0 argue for research to develop a more refined theoretical understanding of possible indirect and conditional causal connections between climate change and, specifically, violent conflict.9d9049c5-28b9-4029-a6b7-a18838dcdc69 Hsiang and Burke (2014)e3a15302-b1ec-4bfe-9ac3-3a2cf23d3303 would like additional research that reduces the number of competing hypotheses that attempt to explain the overwhelming evidence that climatic variables are one of many important causal factors in human conflict.^{e3a15302-b1ec-4bfe-9ac3-3a2cf23d3303} Burrows and Kinney (2016)b3626f1d-9cfa-469a-b52d-dce3c6a0dff0 explore the potential pathways linking climate change, migration, and increased risk of conflict and argue that future research should focus on other pathways by which climate variability and change are related to conflict, in addition to the climate–migration–conflict pathway. Kallis and Zografos (2014)657d9028-d5b7-4e0c-980e-dd71138c8bd7 seek greater understanding of the potential harm of certain climate change adaptation measures that have the potential to result in maladaptation by spurring conflict.

Assessment of confidence based on evidence:

There is medium confidence that climate change will yield compounding economic, environmental, social, and cultural costs. There is greater evidence of these compounding costs resulting from extreme events that are exacerbated by climate change.

There is high confidence that food and water insecurity will result in severe disruptions to livelihoods, including the displacement and relocation of island communities.

It is likely that the absence of interventions will result in the costly and lengthy rebuilding of communities and livelihoods and more displacement and relocation. Events have played out repeatedly across the region and have resulted in damage, disruptions, and displacements.