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Assessing the Vulnerability of Pacific Atolls to Climate Change

Collaboration by USGS, National Oceanic and Atmospheric Administration, and University of Hawai‘i serendipitously captures a significant overwash event

By Peter Swarzenski March / April 2014

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Pacific atolls and the people who live on them are well known to be among the most vulnerable to the impacts of future climate change and sea-level rise. The U.S. Geological Survey (USGS) is leading a multiagency project to assess the impacts of sea-level rise and storm-wave inundation on small Pacific atoll islets and their freshwater resources under various sea-level rise and climatic scenarios. In March 2014, instruments deployed by the project unexpectedly recorded an event that demonstrates the work’s importance: a combination of unusually high tides and large swells that flooded many areas within the Republic of the Marshall Islands.

Above: During the March 2, 2014, overwash in the Republic of the Marshall Islands, seawater regularly topped the manmade perimeter berm on the island of Roi-Namur and covered large areas of the adjacent land surface. Inset shows location of photograph, taken by Peter Swarzenski, USGS. [larger version]

Funded by the Department of Defense (DoD) Strategic Environmental Research and Development Program (SERDP), the project “The Impact of Sea-Level Rise and Climate Change on Department of Defense Installations on Atolls in the Pacific Ocean” was initiated to help DoD develop climate-change adaptation plans for U.S. installations in the Pacific. Its findings will have broad application to atolls worldwide and will be useful to Pacific island nations already threatened by sea-level rise and changing climate.

Above Left: USGS technician Cordell Johnson installing a temporary groundwater well on the beach at site C (see labeled square on aerial view, below) on Roi-Namur in November 2103. Instruments within the well continuously measure groundwater levels, salinity, temperature, and barometric pressure. USGS photograph by Peter Swarzenski. [larger version] Above Right: USGS technician Cordell Johnson taking notes during a multichannel electrical resistivity survey behind the beach at site B (see labeled square on aerial view, below) on Roi-Namur in November 2013. By temporarily inducing a weak electrical current into the ground, the researchers gain information about the nature and location of the freshwater/saltwater interface. USGS photograph by Peter Swarzenski. [larger version]

The island of Roi-Namur on Kwajalein Atoll in the Republic of the Marshall Islands was chosen as the model atoll islet for this project. USGS scientists and their collaborators in the National Oceanic and Atmospheric Administration (NOAA) and the University of Hawai‘i are using a combination of data gathering and computer modeling to assess future vulnerabilities and to help make this and other Pacific atoll islets more resilient. Components of the project include oceanographic observations and modeling, coastal hydrologic observations and modeling, global climate modeling, and benthic substrate characterization.

Above: Map of the western Pacific Ocean, showing location of Kwajalein Atoll (circled in red) in the Republic of the Marshall Islands. Inset: Aerial view of Roi-Namur, an islet in the north part of Kwajalein Atoll. Squares A, B, and C mark sites where temporary wells were installed to collect groundwater data. Aerial view courtesy of Google Maps. [larger version]

A suite of time-series instruments (which take measurements at regular intervals over an extended period of time) were deployed on Roi-Namur during an October–November 2013 field campaign by USGS personnel Steven Gingerich (USGS-Pacific Islands Water Science Center [PIWSC]), Cordell Johnson (USGS-Pacific Coastal and Marine Science Center [PCMSC]), Josh Logan (PCMSC), Sarah Rosa (PIWSC), Kurt Rosenberger (PCMSC), Curt Stolazzi (PCMSC), and Peter Swarzenski (PCMSC). The instruments were installed on the beach face, the reef flat (shallow area between shoreline and reef crest), and the fore reef (which slopes down from the reef crest toward the open ocean), as well as in groundwater wells across the island. Still in place, they are continuously measuring parameters such as waves, tides, currents, run-up levels (distance inland reached by tides and waves), salinity, and temperature. In combination, these various time-series instruments probably make Kwajalein the most well instrumented atoll in the world!

During February–March 2014, Swarzenski returned to Roi-Namur to download data and reprogram the groundwater instruments. On March 2, 2014, the last day of his field operations, a large swell with 5-meter (16 foot)-high waves arriving approximately every 15 seconds from the north-northeast struck the Republic of the Marshall Islands. Calm weather and sea conditions implied that the swell must have originated from afar. On the same day, a perigean spring high tide occurred in the mid-afternoon. ("A perigean spring tide happens when the moon is either new or full and closest to Earth"; a popular term for an extreme tide is “king tide.”) The combination of the unusually high tide and swell produced a dramatic overwash event throughout the Republic of the Marshall Islands. During this overwash, which lasted approximately 3 hours, seawater regularly topped the protective perimeter berm on Roi-Namur and covered large areas of the adjacent land surface (see photographs at top and below). On nearby Majuro, the capital of the Marshall Islands, as many as 70 homes were directly affected by this overwash event, and a state of emergency was declared by President Christopher Loeak on March 5, 2014.

Above: Waves overtopping the perimeter berm on the north shore of Roi-Namur during the March 2, 2014, overwash event in the Republic of the Marshall Islands. Inset shows location of photograph, taken by Peter Swarzenski, USGS. [larger version]

A press release (scroll to press release dated March 7, 2014) about the March 2014 event from the Republic of the Marshall Islands government stated that “This week’s king tides were the worst that the Marshall Islands has experienced in over 30 years, and the third time the capital Majuro has flooded in the last year alone.” Such events used to occur once every 10 to 20 years; today such events appear to occur once a year or more, due to rates of sea-level rise in the region exceeding 1 centimeter (a third of an inch) per year over the past 20 years.

Above: Large swells from the north-northeast with heights up to 5 meters (16 feet) combined with unusually high tides to inundate much of the Republic of the Marshall Islands on March 2, 2014. USGS photograph by Peter Swarzenski. [larger version]

The March overwash event occurred just hours after Swarzenski had finished downloading data from the groundwater instruments and programming them to collect another several months’ worth of data. When the researchers next retrieve data from these instruments, and from the oceanographic instruments installed offshore, they will see evidence of the overwash, both its causes and effects, in data such as tide levels, wave heights, and changes in groundwater levels and salinity.

Above: Pre-overwash time-series data from a groundwater well at site A (see labeled square on aerial view, above). Over this four-month time span, groundwater levels (in blue) expectedly responded to tidal fluctuations (in red). Specific conductivity (a proxy of salinity) in micro-siemens (µS) per centimeter is shown in green. These data were downloaded on March 2, 2014, and do not record the overwash that occurred later that day; the effects of that event will be evident in the next set of data, scheduled for download in several months. [larger version]

Although as scientists we consider ourselves lucky to have been able to capture such a dramatic overwash event with our instruments, the destructive power and dire implications are very sobering.

To learn more about USGS coral reef studies in the Pacific, visit http://coralreefs.wr.usgs.gov/.

To learn more about USGS coastal groundwater studies in the Pacific, visit http://walrus.wr.usgs.gov/sgd/.

USGS Pacific Coral Reefs Website USGS

Submarine Groundwater Discharge USGS

Strategic Environmental Research and Development Program SERDP

Perigean Spring Tide NOAA

Office of the President Republic of Marshall Islands

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March / April 2014

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in this issue:

cover story:
Assessing Climate Change Vulnerability of Pacific Atolls

Fire Island Oceanographic Study Update

Linking Coastal Processes and Vulnerability in Assateague Island Region

Recent Hires Assist USGS Barrier Island and Estuarine Studies

EDEN and EVE—Getting the Water Right in Paradise

"Marathon" Bird May Plan Flights Based on Weather Across the Pacific

Warmer Conditions Create New Goose Habitat in Arctic Alaska

25 Years After the Exxon Valdez, Sea Otter Populations at Pre-Spill Levels

USGS Intern Teaches Kids about Ocean Acidification

USGS Scientists Support the National Ocean Science Bowl’s Spoonbill Bowl

Communications Awards Recognize Ocean Chemistry Topics

Three USGS Volunteers in Florida Working on Ocean Acidification

USGS Employee in Florida Recognized for Service on Science Museum Board

New Kid on the Web: USGS CMGP Redesigned Website Goes Live

March / April Publications