Rising sea levels, coupled with high water levels caused by tropical and extra-tropical storms, will incrementally increase coastal flooding and erosion, damaging coastal ecosystems, infrastructure, and agriculture, and negatively affecting tourism.

This finding is from chapter 23 of Climate Change Impacts in the United States: The Third National Climate Assessment.

Process for developing key messages: A central component of the assessment process was convening three focus area workshops as part of the Pacific Islands Regional Climate Assessment (PIRCA). The PIRCA is a collaborative effort aimed at assessing the state of climate knowledge, impacts, and adaptive capacity in Hawai‘i and the U.S. Affiliated Pacific Islands. These workshops included representatives from the U.S. federal agencies, universities, as well as international participants from other national agencies and regional organizations. The workshops led to the formulation of a foundational Technical Input Report (TIR).7350d7b3-6e95-4375-ba23-26756b441fc2 The report consists of nearly 140 pages, with almost 300 references, and was organized into 5 chapters by 11 authors. The chapter author team engaged in multiple technical discussions via regular teleconferences that permitted a careful review of the foundational TIR7350d7b3-6e95-4375-ba23-26756b441fc2 and of approximately 23 additional technical inputs provided by the public, as well as the other published literature, and professional judgment. These discussions included a face-to-face meeting held on July 9, 2012. These discussions were supported by targeted consultation among the lead and contributing authors of each message. There were several iterations of review and comment on draft key messages and associated content.

Description of evidence base: Description of evidence base All of the scientific approaches to detecting sea level rise come to the conclusion that a warming planet will result in higher sea levels. Recent studies give higher sea level rise projections than those projected in 2007 by the Intergovernmental Panel on Climate Changef83b5613-7609-4799-ab8c-c2a41bdc924c for the rest of this century (Ch. 2: Our Changing Climate, Key Message 10).7b7ffcb0-766c-43b3-ac22-db29fbffef71 Sea level is rising and is expected to continue to rise. Over the past few decades, global mean sea level, as measured by satellite altimetry, has been rising at an average rate of twice the estimated rate for the previous century, based on tide gauge measurements,7b7ffcb0-766c-43b3-ac22-db29fbffef71 with models suggesting that global sea level will rise significantly over the course of this century. Regionally, the highest increases have been observed in the western tropical Pacific.cc032910-1557-4eb7-ac24-d183ad14a8da cef33c87-edde-4bbd-bd8c-325ae895cace However, the current high rates of regional sea level rise in the western tropical Pacific are not expected to persist, as regional sea level will fall in response to a change in phase of natural variability.6fd7abfe-17d7-49a9-bc90-bf85fa4041d3 Regional variations in sea level at interannual and interdecadal time scales are generally attributed to changes in prevailing wind patterns associated with El Niño-Southern Oscillation (ENSO) as well as the Pacific Decadal Oscillation (PDO) and low frequency components of the Southern Oscillation Index (SOI).1c00ae8a-ba2d-464d-9f3a-a353404c4baf 0fd3c22b-8b49-472d-959c-92c4742ac794 ca7d7630-6134-49a5-b933-7de9417ace1b For the region, extreme sea level events generally occur when high tides combine with some non-tidal residual change in water level. In the major typhoon zones (Guam and Commonwealth of the Northern Mariana Islands), storm-driven surges can cause coastal flooding and erosion regardless of tidal state. Wave-driven inundation events are a major concern for all islands in the region. At present, trends in extreme levels tend to follow trends in mean sea level. Increasing mean water levels and the possibility of more frequent extreme water level events, and their manifestation as flooding and erosion, will threaten coastal structures and property, groundwater reservoirs, harbor operations, airports, wastewater systems, sandy beaches, coral reef ecosystems, and other social and economic resources. Impacts will vary with location, depending on how natural sea level variability combines with modest increases of mean levels.6fd7abfe-17d7-49a9-bc90-bf85fa4041d3 On low-lying atolls, critical public facilities and infrastructure as well as private commercial and residential property are especially vulnerable.6fd7abfe-17d7-49a9-bc90-bf85fa4041d3 Agricultural activity will also be affected, as sea level rise decreases the land area available for farming6aa21d2e-f2dc-45b6-9815-bf1132eba02c and episodic inundation increases salinity of groundwater resources. Impacts to the built environment on low-lying portions of high islands will be much the same as those experienced on low islands. Islands with more developed built infrastructure will experience more economic impacts from tourism loss. One report stated: “Our analyses estimate that nearly $2.0 billion in overall visitor expenditures could be lost annually due to a complete erosion of Waikīkī Beach.”a02dcc41-9b79-4564-89af-fcdad699e5d3 Coastal and nearshore environments (sandy beaches, shallow coral reefs, seagrass beds, intertidal flats, and mangrove forests) and the vegetation and terrestrial animals in these systems will progressively be affected as sea level rise and high wave events alter atoll island size and shape and reduce habitat features necessary for survival. Based on extrapolation from results in American Samoa, sea level rise could cause future reductions of 10%–20% of total regional mangrove area over the next century.da5a93c8-9c73-45f7-966f-5b1970fec7a4 Further, atoll-breeding Pacific seabirds will lose large segments of their breeding populations

New information and remaining uncertainties: Sea levels in the Pacific Ocean will continue to rise with global sea level. Models provide a range of predictions, with some suggesting that global warming may raise global sea level considerably over the course of this century. The range of predictions is large due in part to unresolved physical understanding of various processes, notably ice sheet dynamics. Changes in prevailing wind patterns associated with natural climate cycles such as ENSO and the PDO affect regional variations in sea level at interannual and interdecadal time scales. Sea level at specific locales will continue to respond to changes in phase of these natural climate cycles. The current high rates of regional sea level rise in the western tropical Pacific are not expected to persist over time, falling once the trade winds begin to weaken. Future wind wave conditions are difficult to project with confidence given the uncertainties regarding future storm conditions.

Assessment of confidence based on evidence: Evidence for global sea level rise is strong (Ch. 25: Coasts; Ch. 2: Our Changing Climate). Confidence is therefore very high. Modeling studies have yielded conflicting results as to how ENSO and other climate modes will vary in the future. As a result, there is low confidence in the prediction of future climate states and their subsequent influence on regional sea level.6fd7abfe-17d7-49a9-bc90-bf85fa4041d3 Recent assessments of future extreme conditions generally place low confidence on region-specific projections of future storminess.5138b20c-7049-433e-a1ec-24417cccd3c2 For aspects of the key message concerning impacts, confidence is high.

References :

• Regional sea level trends due to a Pacific trade wind intensification (1c00ae8a)
• Sea level variations at tropical Pacific islands since 1950 (cc032910)
• Climate Change and Pacific Islands: Indicators and Impacts. Report for the 2012 Pacific Islands Regional Climate Assessment (PIRCA) (6fd7abfe)
• Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation. A Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change (5138b20c)
• Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (f83b5613)
• Tropical Pacific spatial trend patterns in observed sea level: internal variability and/or anthropogenic signature? (ca7d7630)
• Estimating Mean Sea Level Change from the TOPEX and Jason Altimeter Missions (7b7ffcb0)
• Economic Impact Analysis of the Potential Erosion of WaikÄ«kÄ« Beach (a02dcc41)
• Threats to mangroves from climate change and adaptation options: A review (da5a93c8)
• Wind Effects on Past and Future Regional Sea Level Trends in the Southern Indo-Pacific (cef33c87)
• Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (6aa21d2e)
• Multidecadal sea level anomalies and trends in the western tropical Pacific (0fd3c22b)
• Climate Change and Pacific Islands: Indicators and Impacts. Report for the 2012 Pacific Islands Regional Climate Assessment (PIRCA) (7350d7b3)