The Pacific Islands face a range of challenges from sea-level rise. Higher average water levels and increasingly frequent extreme water-level events threaten infrastructure, property, groundwater, waste water systems, sandy beaches, and coral reef ecosystems. Atolls and low islands are especially vulnerable, and it may become difficult to for populations live in low-lying areas.
Rising sea levels will increase the likelihood of coastal flooding and erosion,
damaging coastal infrastructure and agriculture, negatively impacting tourism,
reducing habitat for endangered species, and threatening shallow reef systems.
Extreme water levels will occur when sea-level rise related to longer-term
climate change combines with seasonal high tides, interannual and
interdecadal sea-level variations (e.g., ENSO, Pacific Decadal Oscillation,
mesoscale eddy events), and surge and/or high run-up associated with storms.
- Sea-level rise is happening. The globally averaged rate of sea-level rise since the mid-1800s has been greater than the rate over the previous two millennia. Since the early 1990s, the rate of globally averaged sea-level rise has increased to 3.4 mm (a little more than a tenth of an inch) per year. This is twice the estimated rate for the 20th century as a whole (IPCC, 2013).
- At present, extreme sea-level events are increasing, following trends in average sea level. Extreme sea-level events generally occur when high tides combine with some nontidal residual change in water level. Storm-driven surges can cause coastal flooding and erosion regardless of the tidal state. Wave-driven inundation events are also a major concern for all islands in the region.
- A number of efforts are under way in the region to identify vulnerable assets, assess potential impacts, and determine appropriate adaptive responses to sea-level rise and coastal inundation.
Central North Pacific: Hawai‘i
- In Hawai‘i, rates of sea-level rise across the islands vary based on the distance from volcanic activity on Hawai‘i Island, which is contributing to local sea-level rise. Rates of sea-level rise ranged 0.6 inches (1.4 cm) per decade on O‘ahu and Kaua‘i, to 1.2 inches (3.0 cm) per decade on Hawai‘i Island over the last century (NOAA CO-OPS, 2016).
- The shorelines of Hawai‘i are experiencing erosion and retreat, with a statewide average retreat of 1 foot per year. Erosion has resulted in loss of beaches, cliff collapse, and the landward migration of wetlands. An estimated 13 miles of beach has been lost to erosion across the state (Fletcher, et al., 2012). Sea-level rise is likely to be a driving factor in the shoreline erosion trend (Romine et al., 2013, Romine et al., 2016).
Western North Pacific (Guam, Palau, Federated States of Micronesia, Marshall Islands, Northern Mariana Islands) and Central South Pacific (American Samoa)
- In the past two decades (since 1993), the highest rates of sea-level rise globally have occurred in the western tropical Pacific, an area that includes all of the U.S.-Affiliated Pacific Islands. The average rate of sea-level rise in the western tropical Pacific for this period is nearly three times the global average. Sea level in a region fluctuates, mostly due to wind-driven redistributions of water. The current high rates of regional sea-level rise in the western tropical Pacific are not expected to persist, as trade winds are expected to weaken in a natural shift (Merrifield, 2012).
Future Projections (updated June 2016)
- Global mean sea level will continue to rise over the course of this century. At the current rate of greenhouse gas production, global mean sea level is likely to rise 1 foot by 2050 and 2 to 3 feet (0.52 to 0.98 m) by the end of the 21st century (medium confidence, IPCC, 2013).
- Modeling suggests Hawai‘i and other central Pacific Islands are likely to experience slightly greater-than-average sea-level rise by 2100 (Kopp et al., 2014).
- With increasing average water levels, it is very likely that extreme high sea-level events will become more frequent and more extreme, causing flooding and erosion (IPCC, 2013). High sea-level events will threaten coastal structures and property, groundwater reservoirs, harbor operations, airports, waste water systems, sandy beaches, coral reef ecosystems, and other social and economic resources. Low islands are especially vulnerable over the near to mid-term (next 25 to 50 years). Local sea level rise will vary site-to-site due to varying non-climatic uplift or subsidence, oceanographic effects, and variability in processes such as shrinking land ice.
- How El Niño Southern Oscillation (ENSO) and other climate modes will vary in the future is less certain. As a result, there is uncertainty about the influence of future climate modes on regional sea level.
- Tropical cyclone (hurricane/typhoon) intensity (maximum wind speeds and rain rates) is likely to increase on average globally. There is lower confidence in regional projections for the frequency and intensity of tropical cyclones (IPCC, 2013).
- An increase in the most intense tropical cyclones is more likely than not in the Western North Pacific (Micronesia) by late in the 21st Century (IPCC, 2013; Emmanuel, 2013).
- In the Central North Pacific (Hawai‘i), cyclone storm tracks are expected to shift northward, and modeling studies indicate tropical cyclone activity is likely to increase in Hawai‘i over the 21st century as a result (Murakami et al., 2013; Li et al., 2010).
The preceding text (excluding updated projections) is excerpted and abridged from this chapter, which should be cited as:
Marra, J. J., Merrifield, M. A., & Sweet, W. V. (2012). Sea Level and Coastal Inundation on Pacific Islands. In V. W. Keener, J. J. Marra, M. L. Finucane, D. Spooner, & M. H. Smith (Eds.), Climate Change and Pacific Islands: Indicators and Impacts. Report for the 2012 Pacific Islands Regional Climate Assessment (PIRCA). Washington, DC: Island Press.