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Research

Climate Change Reduces Coral Reefs' Ability to Protect Coasts



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Climate change may reduce the ability of coral reefs to protect tropical islands against wave attack, erosion, and salinization of the drinking-water resources that help to sustain life on those islands. A new paper by researchers from the Dutch independent institute for applied research Deltares and the U.S. Geological Survey (USGS) gives guidance to coastal managers to assess how climate change will affect a coral reef's ability to mitigate coastal hazards.

About 30 million people live on low-lying coral islands and atolls where they depend on coral reefs for protection against waves. Healthy coral reefs have rough surfaces and complex structures that slow incoming waves. Coral reef health, however, is threatened by the effects of climate change, including ocean acidification caused by increasing carbon dioxide in the atmosphere and ocean, coral bleaching caused by warming ocean temperatures, and smothering and light reduction caused by sediment stirred up by large waves that will become more common as sea-level rise outstrips coral reef growth.

Aerial photograph of Kwajalein Atoll in the Republic of the Marshall Islands, showing its low-lying islands and coral reefs.
Above: Aerial photograph of Kwajalein Atoll in the Republic of the Marshall Islands, showing its low-lying islands and coral reefs. The line of breaking waves on the left marks the reef crest, where much of the waves' energy is dissipated. Additional energy is lost through friction as the water flows shoreward over the rough surface of the healthy reef flat. USGS photograph taken May 2015 by Tom Reiss. [larger version]

At present, some low-lying tropical islands are flooded by wave events a few times per decade. It is expected that this rate of flooding will increase as sea level rises and coral reefs decay, because the remaining dead corals are generally smoother in structure and do less to dissipate wave energy. Loss of coral cover not only causes increased shoreline erosion but also affects the sparse drinking-water resources on these islands by allowing saltwater to wash inland. Deterioration of freshwater resources may eventually make some islands uninhabitable.

In order to prevent or mitigate these impacts, coastal managers need know to what extent their reef system may lose its protective function so that they can take action. The new paper, titled "The influence of coral reefs and climate change on wave-driven flooding of tropical coastlines," gives guidance on a local reef's sensitivity to change. It was published 4 August 2015 in Geophysical Research Letters, a journal of the American Geophysical Union, at http://dx.doi.org/10.1002/2015GL064861.

reef diagram
Above: Many coral reefs consist of nearshore inner reef flats that slope to deeper water fore reefs farther offshore. The reef crest, between the inner reef flat and outer fore reef, lies in extremely shallow water and may be exposed during the lowest tides. Waves commonly crash against or break on the reef crest. From "U.S. Coral Reefs—Imperiled National Treasures." [larger version]

To gain insight into effects of changing conditions on coral reefs, the study authors used Xbeach, an open-source wave model. The computer model was first validated with field measurements obtained on the Kwajalein Atoll in the Republic of the Marshall Islands in the Pacific Ocean, and was then used to investigate how changes in certain reef properties would affect water levels, waves, and wave-driven runup (the maximum height above sea level reached by the uprushing wave). Reef roughness, steepness, width, and the water depth above the reef platform are all important factors for coastal managers to consider when planning mitigating measures.

The results suggest that coasts fronted by relatively narrow reefs with steep faces and deeper, smoother reef flats are expected to experience the highest wave runup and thus the greatest potential for island flooding. Wave runup increases for higher water levels (expected with sea-level rise), higher waves, and lower bed roughness (as coral degrades and becomes smoother), which are all expected effects of climate change. The authors conclude that rising sea levels and climate change will have a significant negative impact on the ability of coral reefs to mitigate the effects of coastal hazards in the future.

In March 2014, a combination of unusually high tides and large swells flooded many areas within the Republic of the Marshall Islands
Above: In March 2014, a combination of unusually high tides and large swells flooded many areas within the Republic of the Marshall Islands. During this event, seawater regularly topped the manmade perimeter berm on the island of Roi-Namur and covered large areas of the adjacent land surface (see related Sound Waves story, "Assessing the Vulnerability of Pacific Atolls to Climate Change"). Such events are likely to occur more frequently as sea level continues to rise. Inset shows location of photograph, taken March 2, 2014, by Peter Swarzenski, USGS. The paper provides details about the wave data collected during this event. [larger version]

The research paper, published as an open-access paper, is available online at http://dx.doi.org/10.1002/2015GL064861.

The complete citation is:
Quataert, E., Storlazzi, C., van Rooijen, A., Cheriton, O., and van Dongeren, A., 2015, The influence of coral reefs and climate change on wave-driven flooding of tropical coastlines, Geophysical Research Letters, 42, doi: 10.1002/2015GL064861 [http://dx.doi.org/10.1002/2015GL064861].
Learn more about USGS coral reef studies in the Pacific at http://coralreefs.wr.usgs.gov/. Learn more about Deltares, an independent institute for applied research in the field of water and subsurface, at https://www.deltares.nl/en/.

Related Sound Waves Stories
Assessing the Vulnerability of Pacific Atolls to Climate Change
March / April 2014

Related Websites
The influence of coral reefs and climate change on wave-driven flooding of tropical coastlines
Geophysical Research Letters
U.S. Coral Reefs—Imperiled National Treasures
USGS Fact Sheet 205–02
XBeach
Delteres
Future Wave and Wind Projections for United States and United States-Affiliated Pacific Islands
USGS Open-File Report 2015–1001

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in this issue:

Research
cover story:
Northern Alaska Coastal Erosion Threatens Habitat and Infrastructure

Climate Change Reduces Coral Reefs' Ability to Protect Coasts

Polar Bears Forced on Shore by Sea-Ice Loss Are Unlikely to Thrive on Land-Based Foods

Many Dry Tortugas Loggerheads Actually Bahamas Residents

Fieldwork
USGS Oceanographer Participating on Collaborative U.S. and Canadian Research Cruise

Spotlight on Sandy
Detailed Flood Information Key to More Reliable Coastal Storm Impact Estimates

Oureach
Coral Photo Selected as Popular Photography's "Photo of the Day"

Staff
New Marine Facility Chief for the Pacific Coastal and Marine Science Center

George Tate Retires from the Pacific Coastal and Marine Science Center

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