finding 23.2 : freshwater-supplies-more-limited

Freshwater supplies are already constrained and will become more limited on many islands. Saltwater intrusion associated with sea level rise will reduce the quantity and quality of freshwater in coastal aquifers, especially on low islands. In areas where precipitation does not increase, freshwater supplies will be adversely affected as air temperature rises.



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: There is abundant and definitive evidence that air temperature has increased and is projected to continue to increase over the entire region,a639a8f6-3355-43d7-ab21-ef29364db75a c70e33c2-49f5-499b-ac5a-a63cbf83b698 d28411c8-f40c-446a-b6a9-2dd1c844deea dde62a65-2752-4101-bd95-cfcc0e89dc66 as there is globally (Ch. 2: Our Changing Climate, Key Message 3). In Hawai‘i and the Central North Pacific (CNP), projected annual surface air temperature increases are 1.0°F to 2.5°F by 2035, relative to 1971-2000.634424c9-14a9-4133-9ed6-8948faa0b11e eff5ce0d-c403-455b-9358-98be45351c7b In the Western North Pacific (WNP), the projected increases are 2.0°F to 2.3°F by 2030, 6.1°F to 8.5°F by 2055, and 4.9°F to 9.2°F by 2090.a639a8f6-3355-43d7-ab21-ef29364db75a In the central South Pacific (CSP), projected annual surface air temperature increases are 1.1°F to 1.3°F by 2030, 1.8°F to 2.5°F by 2055, and 2.5°F to 4.9°F by 2090.a639a8f6-3355-43d7-ab21-ef29364db75a (Please note that the islands that comprise the U.S. Pacific Islands Region are shown in Figure 23.1). In Hawai‘i, mean precipitation, average stream discharge, and stream baseflow have been trending downward for nearly a century, especially in recent decades and with high variability related to El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO).8e3247e0-fd15-4c29-8ed4-4aafd9c8660f 9a5c2112-9d46-45f0-b97a-64a8a8c574b1 a7bc5b58-4952-4499-9285-af61da992262 For the WNP, a decline of 15% in annual rainfall has been observed in the eastern-most islands in the Micronesia region and slight upward trends in precipitation have been seen for the western-most islands, with high ENSO-related variability.a639a8f6-3355-43d7-ab21-ef29364db75a In American Samoa, no trends in average rainfall are apparent based on the very limited available data.a639a8f6-3355-43d7-ab21-ef29364db75a ab312e6f-70ad-4653-ac9f-78ce77e4ded7 For the region as a whole, models disagree about projected changes in precipitation. Mostly models predict increases in mean annual rainfall and suggest a slight dry season decrease and wet season increase in precipitation.a639a8f6-3355-43d7-ab21-ef29364db75a However, based on statistical downscaling, one study2aea4790-7e38-4922-bc92-d20c06d23b7d projected a 5% to 10% reduction in precipitation for the wet season and a 5% increase in the dry season for Hawai‘i by the end of this century. On most islands, increased temperatures coupled with decreased rainfall and increased drought will reduce the amount of freshwater for drinking and crop irrigation.6eb1ee99-0b99-452a-a8cb-48b961fd4ac5 ca2a1adf-0f5a-4838-aa81-fc0c44f50369 c600849a-ed32-4109-a103-1571e136f0b6 Atolls will be particularly vulnerable due to their low elevation, small land mass, geographic isolation, and limited potable water sources and agricultural resources.15a85bcf-1235-4258-860c-24948df66935 The situation will also be exacerbated by the increased incidence of intrusion of saltwater from the ocean during storms as the mean sea level rises over time (Key Message 4, this chapter; Ch. 2: Our Changing Climate, Key Message 10).7350d7b3-6e95-4375-ba23-26756b441fc2

New information and remaining uncertainties: Climate change impacts on freshwater resources in the Pacific Islands region will vary because of differing island size and height, which affect water storage capability and susceptibility to coastal inundation. The impacts will also vary because of natural phase variability (for example, ENSO and PDO) in precipitation and storminess (tropical and extra-tropical storms) as well as long-term trends, both strongly influenced by geographic location. Climate model simulations produce conflicting assessments as to how the tropical Pacific atmospheric circulation will respond in the future to climate change.

Assessment of confidence based on evidence: Freshwater systems are inherently fragile in many Pacific Islands. Historical observations show strong evidence of a decreasing trend for rainfall in Hawai‘i and many other Pacific Islands (Ch. 2: Our Changing Climate).7350d7b3-6e95-4375-ba23-26756b441fc2 There is abundant and definitive evidence that air temperature has increased and will continue to increase. All of the scientific approaches to detecting sea level rise come to the conclusion that a warming planet will result in higher sea levels. Based on the evidence base and remaining uncertainties, we have high confidence in the key message.

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