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Sea level is rising faster than projected in the western Pacific, so understanding how wave-driven coastal flooding will affect inhabited, low-lying islands—most notably, the familiar ring-shaped atolls—as well as the low-elevation areas of high islands in the Pacific Ocean, is critical for decision-makers in protecting infrastructure or relocating resources and people.
In March 2014, USGS instruments recorded an unexpected combination of unusually high tides and large 5-meter swells that flooded many areas within the Republic of the Marshall Islands. The Marshall Islands president issued a state of emergency and press release: “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 historically occurred every few decades—but now they are occurring multiple times a decade. Saltwater can flood natural inland depressions, such as freshwater ponds where taro grows. USGS research hydrologist Stephen Gingerich has seen islanders frantically digging long trenches out to the ocean to attempt to drain the seawater before it kills their crops.
The rise in sea level has far surpassed the 2007 estimate from the Intergovernmental Panel on Climate Change and could reach 2 meters within this century. Human populations are generally concentrated along coastlines, and people on low-lying islands in the Pacific are at particular risk since they cannot move to higher elevations. With a cultural history going back hundreds of years in these islands, inhabitants would not want simply to leave.
Some of these islands have average elevations of only 2 meters above sea level and are exposed to waves as high as 5 to 7 meters most winters. Coral reefs surrounding these islands provide an important natural barrier that dissipates the destructive energy from large waves, but this protection will decline as sea-level rise outpaces reef growth. The effects of storm waves coupled with sea-level rise will exacerbate flooding problems, but these effects have generally not been incorporated into climate change projections Furthermore, the islands’ shallow, freshwater aquifers can be contaminated by a rise in sea level and subsequent saltwater flooding, which can also destroy most of the agricultural and habitable lands located in low-lying areas.
Knowing what is possible to protect can save time and money for individuals, planners, and government officials. Many islands in the Pacific Ocean are part of the U.S (such as Hawaii), are U.S. territories (for example, Guam, Johnston Atoll, Jarvis Island, American Samoa), or fall under the umbrella of the U.S. Department of the Interior (U.S. Office of Insular Affairs) and the Department of Defense because they are part of the Compact of Free Association (for example, Republic of the Marshall Islands, Federated States of Micronesia). The Republic of the Marshall Islands has collected meteorological and oceanographic data on rising seas for several decades—data that can help researchers determine what might happen to other islands around the world. Which islands are immediately threatened? Which have more time to plan for sea-level rise? USGS research can tackle those questions that ultimately help world leaders set priorities for their own nations and understand the potential consequences of an influx of climate change refugees from other nations.
An increase in wave-driven flooding is expected to affect areas of human habitation and agriculture on islands in the Pacific. To determine how climate change will alter the size and direction of ocean waves, and how far the waves might travel over coral reefs to flood inland areas and infiltrate freshwater aquifers, the USGS is studying 25 Pacific islands. The data they collect will help validate oceanographic models of future wind and wave action in the studied regions.
USGS has deployed instruments on the beach, along various parts of the reef, and in groundwater wells on the island of Roi-Namur, on Kwajalein Atoll in the Republic of the Marshall Islands, to record wave-driven flood events over a long period of time. Kwajalein Atoll is one of the largest atolls in the world, and the U.S. Army’s Reagan Test Site located here will be a test case for modeling flood scenarios for other Pacific islands.
USGS researchers measure the waves, tides, currents, temperature, salinity, and run-up levels (distance inland that waves travel). An important feature vulnerable to waves is an island’s fresh groundwater “lens.” Normally, this underground freshwater floats on top of denser seawater. Large storm waves can wash over the island, leaving saltwater on top of the freshwater lens. Researchers want to know: How long will it take for the saltwater to percolate downward and contaminate the island’s freshwater supply? How long before rainfall can replenish and purify the freshwater?
Studying the island’s groundwater can help researchers isolate and project the effects of large storms. Analyzing the water’s geochemistry can provide insights into the groundwater’s “age,” contaminants, and how quickly water flushes through the system. Because saltwater conducts electricity better than freshwater, USGS researchers also measure the electrical resistivity of the water in groundwater wells to better understand how the “lens” reacts to tides and wave-driven flooding.
Creating new, high-resolution bathymetric maps from satellite imagery, and topographic maps from terrestrial lidar data, has enabled USGS to generate maps to a resolution of a centimeter or less. Mapping an island with high-precision tools reveals more details about what happens as waves move from deep water to the shallow regions of the coast and up onto the land, and allows for more accurate modeling of future climatic effects on atolls.
Modeling by USGS researchers and colleagues at the University of California, Santa Cruz, shows that climate changes during the 21st century will alter the strength and direction of the highest waves and strongest winds across U.S. and U.S.-affiliated Pacific Islands. For example, extreme wave heights will increase until the middle of the 21st century and then decrease toward the end of the century. A recent USGS report gives detailed projections that will help communities, engineers, and natural-resource managers prepare for resulting shifts in flooding threats to their particular areas.
Modeling by USGS scientists and colleagues at the Deltares institute in the Netherlands shows that climate change may reduce the ability of coral reefs to protect tropical islands against wave attack, erosion, and saltwater contamination of freshwater resources. Healthy coral reefs have rough surfaces and complex structures that slow incoming waves. But climate-change effects, including ocean acidification, coral bleaching, and smothering by sediment stirred up by waves, threaten reefs. As coral reefs decay, they become smoother, inhibiting their ability to dissipate wave energy. It is expected that smoother reefs combined with rising sea level will lead to increased flooding on land.
Another modeling effort by scientists with the USGS, Deltares, and the University of Hawai‘i at Hilo forecasts that waves will interact synergistically with sea-level rise, causing twice as much land to flood for a given future sea level than currently predicted by models that do not take wave-driven water levels into account. These changes mean that many atolls could be flooded every year and their freshwater supplies contaminated—forcing some inhabitants to abandon their homes in decades, rather than in centuries as previously thought.
This research is part of the USGS project titled, “Coastal Climate Impacts.”Explore other research topics associated with this project, below.
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