finding 27.1 : key-message-27-1

Dependable and safe water supplies for Pacific island communities and ecosystems are threatened by rising temperatures (very high confidence), changing rainfall patterns (low confidence), sea level rise (very high confidence), and increased risk of extreme drought and flooding (medium confidence). Islands are already experiencing saltwater contamination due to sea level rise, which is expected to catastrophically impact food and water security, especially on low-lying atolls (medium confidence). Resilience to future threats relies on active monitoring and management of watersheds and freshwater systems.

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:

Vulnerability of water supplies to climate change: With their isolation and limited land areas, Hawai‘i and the USAPI are vulnerable to the effects of climate change on water supplies.97ae6e5b-3482-4760-a4f4-9e00ee6337b6,63d934ca-be24-4c42-97e2-ac3ed3ff1b6f Ongoing and projected changes in temperature and precipitation will have negative effects on water supplies in Hawai‘i and some parts of the USAPI. For example, stream low flow and base flow in Hawai‘i decreased significantly over the period 1913–2008, which is at least partly explained by a decline in precipitation.

Temperature change: In Hawai‘i, air temperature increased by 0.76°F (0.42°C) over the past 100 years. The year 2015 was the warmest on record at 1.43°F (0.79°C) above the 100-year average. Mean and minimum (nighttime) temperatures both show long-term, statistically significant increasing trends, while the diurnal temperature range (the average difference between daily minimum and maximum temperature) shows a long-term, statistically significant decreasing trend.7621e6f5-4234-4a44-bb1d-8278429deb2b Estimates of historical temperature changes in Hawai‘i are based on the relatively few observing stations with long records and represent the best available data. Further temperature increases in the Hawai‘i–USAPI region are highly likely. Northern tropical Pacific (including Micronesia) sea level air temperatures are expected to increase by 2.2°–2.7°F (1.2°–1.5°C) by mid-century and by 2.7°–5.9°F (1.5°–3.3°C) by 2100.1fca63fb-3033-445e-99ba-1136da451058 Southern tropical Pacific (including American Sāmoa) sea level air temperatures are expected to increase by 1.8°–3.1°F (1.0°–1.7°C) by mid-century and by 2.5°–5.8°F (1.4°–3.2°C) by 2100.1fca63fb-3033-445e-99ba-1136da451058 Increasing temperatures throughout the Hawai‘i–USAPI region might cause increases in potential evapotranspiration,bdbb00c1-6c40-4e03-9120-2df759b580a7 with consequent negative impacts on water supplies.

Precipitation change: While Hawai‘i precipitation has experienced upward and downward changes across a range of timescales, more than 90% of the state had a net downward rainfall trend during 1920–2012.e6d1098e-93cc-4285-9d16-6e52c5e302f7 Projections of future precipitation changes in Hawai‘i are still uncertain. Using a dynamical downscaling approach to project climate changes in Hawai‘i for the 20-year period at the end of the this century under a middle-of-the-road scenario (SRES A1B) resulted in increases in mean annual rainfall of up to 30% in the wet windward areas of Hawaiʻi and Maui Islands and decreases of 40% in some of the dry leeward and high-elevation interior areas.f60beed2-efd0-40c8-8f94-b81d3e9f1509 Somewhat different results were obtained using an independent statistical downscaling method.f60beed2-efd0-40c8-8f94-b81d3e9f1509 For the lower scenario (RCP4.5), mean annual rainfall in Hawai‘i is projected by statistical downscaling to have only small changes in windward areas of Hawai‘i and Maui Islands, to decrease by 10%–20% in windward areas of the other islands, and to decrease by up to 60% in leeward areas for the period 2041–2070. For the same scenario, the late-century (2071–2100) projection is similar to the 2041–2070 projection, except that a larger portion of the leeward areas will experience reductions of 20%–60%. For the higher scenario (RCP8.5), windward areas of Hawai‘i and Maui Islands will see changes between +10% and −10%, and rainfall in leeward areas will decrease by 10% to more than 60% by the 2041–2070 period. By the late-century period (2071–2100), windward areas of Hawai‘i and Maui Islands will see increases of up to 20%, windward areas on other islands will have decreases of 10% to 30%, and leeward areas will have decreases of 10% to more than 60%. The number of climate and water resources monitoring stations has declined across the region,7350d7b3-6e95-4375-ba23-26756b441fc2,2d45df75-a007-48ce-8c6e-35ed7b31ca1b,92dce0e4-b003-4a91-8965-7b79ddeccbb8 reducing the ability of researchers to project future changes in climate.

Trends in hydrological extremes in Hawai‘i: Increasing trends in extreme 30-day rainfall and the lengths of consecutive dry-day and consecutive wet-day periodsf8ccfb79-1bed-462d-9172-c56a71b542b9 indicate that Hawai‘i’s rainfall is becoming more extreme and suggest that both droughts and floods are becoming more frequent in Hawai‘i. With the addition of more years of observed data, and a more detailed spatiotemporal analysis from a grid-box level down to the island level, this contrasts with the earlier findings of a decreasing trend in the number of extreme rainfall events in Hawai‘i.39d80273-7b1f-4342-8c9d-439e262dea4f

Saltwater contamination due to sea level rise: Sea level rise exacerbates the existing vulnerability of groundwater lenses on small coral islands to contamination by saltwater intrusion by amplifying the impacts of freshwater lens-shrinking droughts and storm-related overwash events.dc90f15e-2420-461e-ba38-f65717485591

New information and remaining uncertainties:

Effects of warming on evapotranspiration: There are uncertainties in how warming will affect cloud cover, solar radiation, humidity, and wind speed. All of these affect potential evapotranspiration and changes in soil moisture, and the effects will differ by region.44466960-d3a9-4374-b1cf-893bb8a476f0

Future precipitation changes: Global models differ in their projections of precipitation changes for the Hawai‘i–USAPI region.1fca63fb-3033-445e-99ba-1136da451058 For Hawai‘i, downscaled projections differ according to the choice of global model time horizon, emissions scenario, and downscaling method.9171ec97-c44b-48eb-80b3-2756ba8a14a3

Assessment of confidence based on evidence:

There is very high confidence in further increases in temperature in the region, based on the consistent results of global climate models showing continued significant increases in temperature in the Hawai‘i–USAPI region for all plausible emissions scenarios.

There is low confidence regarding projected changes in precipitation patterns, stemming from the divergent results of global models and downscaling approaches and from uncertainties around future emissions. However, for leeward areas of Hawai‘i and the eastern part of the Federated States of Micronesia (FSM), future decreases in precipitation are somewhat more likely, based on greater agreement between downscaling approaches for Hawai‘i and greater agreement among global models for eastern FSM.

There is very high confidence in future increases in sea level, based on widely accepted evidence that warming will increase global sea level, with amplified effects in the low latitudes.

There is medium confidence in the increasing risk of both drought and flood extremes patterns, based on both observed changes (for example, increasing lengths of wet and dry periods) and projected effects of warming on extreme weather globally.

There is medium confidence in possible future catastrophic impacts on food and water security resulting from saltwater contamination in low atolls due to sea level rise; this is based on very high confidence in continuing sea level rise, the known effects of saltwater contamination on water supply and agriculture, and uncertainty regarding the effectiveness of adaptation measures.

References :

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