Sea Level Rise and the Ongoing Battle of Tarawa

Eos, Vol. 93, No. 17, 24 April 2012 VOLUME 93 NUMBER 17 24 APRIL 2012 EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION PAGES 169–176 Sea Level Rise and the Ongoing Natural Variability and Sea Level Much like the weather, the local sea level and the shoreline vary over multi- Battle of Tarawa ple time scales. In addition to the diurnal, weekly, and annual cycles of the astro- PAGES 169–170 activities on coastal processes. As with climate warming, the state of an individual nomical tides, the local sea level is sensi- At dawn on 20 November 1943, U.S. shoreline or the extent of flooding on a tive to the weather and to ocean dynamics. marines launched an assault on Tarawa, given day is not proof of a sea level trend, On an hourly to weekly time scale the sea a Japanese- held atoll in the British Protec- nor is a global sea level trend a good predic- level can rise during periods of low atmo- torate of the Gilbert and Ellice Islands. The tor of individual flooding or erosion events. spheric pressure (e.g., the “storm surge” water in the lagoon was only 3 feet deep that morning, less than the 4–5 feet required by a fully loaded personnel carrier to navi- gate the waters. As a result, marines saddled with equipment were forced to wade almost a mile across the lagoon under heavy Japa- nese fire. The United States won the Battle of Tar- awa, but it proved to be one of the bloodi- est battles of the war; almost 6000 Japanese, Americans, and Gilbertese were killed in just 3 days of fighting. Of the American fatalities, almost half occurred because U.S. military planners ignored warnings about the local tides on the morning of the assault [e.g., Wright, 2000]. Today, Tarawa Atoll (Figure 1, top left), capital of the independent Republic of Kiribati and home to 40% of the country’s 103,500 people, receives public and political attention because of a very dif- ferent battle. Along with the Maldives and Tuvalu, Kiribati is broadly considered to be one of the countries most threatened by sea level rise. In the growing discourse on the effects of sea level rise on places like Kiribati, scientists and educators can learn from the Battle of Tarawa: Like the weather, the sea and the shoreline are con- stantly changing. The distinction between weather— what you “get”—and climate—what you “expect”—is consistently confusing to the public. As scientists often explain in the media, the weather at a particular location on a particular day is not proof of a global climate trend, nor is a global climate trend a good predictor of the local weather at a particular location on a particular day. This same nuance is important in discourse about sea level rise. As the global average sea level rises, the response of any one location at any given time will depend on the natural variability in regional sea level and the effects of local human Fig. 1. (counterclockwise from top left) Map of Tarawa Atoll; monthly mean sea level at Betio, Tarawa, from Australian Tidal Facility data; inundation behind a failed sea wall in South Tarawa; BY SIMON DONNER and flooding on the lagoon shoreline of Bikenibeu during a 2005 storm. Eos, Vol. 93, No. 17, 24 April 2012 driven by a tropical cyclone). On weekly to the temperature and moisture budget of a Betio- Bairiki causeway, which redirects sedi- monthly time scales the local sea level can region by altering the surface albedo, sur- ment flow Solomon[ and Forbes, 1999]. vary because of a tmosphere- ocean oscilla- face roughness, and atmospheric compo- tions like the El Niño–Southern Oscillation sition, human modification of the coastal Communicating About Sea Level Rise (ENSO). environment can affect flood magnitude, The importance of natural variability in flood frequency, and even island shape by The failure to consider the contribution of sea level on shorelines is most evident in altering hydrodynamics and sediment sup- natural variability and direct human modifi- central equatorial Pacific atolls like Tarawa ply. Together, these practices can create the cations can lead to misattribution of flood- (Figure 1, bottom). During El Niño events, geomorphic equivalent of an “urban heat ing events or shoreline changes to sea level slowdown or reversal of the trade winds island” effect, where instead of the urban rise. Tarawa, the most easily accessible atoll and the South Equatorial Current raises sea environment altering the local temperature, in Kiribati, is a popular destination for jour- surface temperatures and the sea level in it alters the coastline. nalists and activists interested in observing the central and eastern equatorial Pacific. Three types of shoreline modification that and communicating the impacts of sea level For example, the monthly mean sea level are typical in low- lying island nations have rise on a low- lying nation. For example, a dropped by 45 centimeters from March 1997 altered sediment supply and island shape in Greenpeace slide show within an explana- to February 1998, according to the Austra- South Tarawa [Webb, 2005]. First, land rec- tion of what sea level rise means that depicts lian Tidal Facility gauge, due to a switch lamation, accomplished by infilling behind the 2005 flooding remains among the top from El Niño to La Niña conditions (Fig- a constructed sea wall, has increased land responses to an Internet query of “Kiribati” ure 1, bottom). Moreover, during the three area in some locations but exacerbated ero- and “sea level rise.” These common images most recent El Niños centered in the central sion and inundation in others. The shore- of flooded homes and waves crashing across Pacific (2002–2003, 2004–2005, and 2009– line of islets like Bairiki has been extended the causeways—collected during an anom- 2010) [Lee and McPhaden, 2010] the peak lagoonward through the construction of gov- alous event on islets susceptible to flood- monthly mean sea level at Tarawa averaged ernment facilities, landfills, maneabas (com- ing due in part to local modifications to the 15 centimeters above the long-term gauge munity meeting houses), and individual environment—can provide the false impres- mean. This spike during these El Niño events homes [Webb and Kench, 2010]. At the same sion that Tarawa is subject to constant flood- is equivalent to 50 years of global sea level time, poor engineering of sea walls has led ing because of sea level rise. rise at the rate observed since 2000 of 3 mil- to erosion at the airport and the hospital The attribution problem is further magni- limeters per year [Nicholls and Cazenave, [Webb, 2005] on the islet of Bikenibeu and fied by the political situation. The Kiribati 2010]. also led to inundation of reclaimed lands government faces the difficult challenge The combination of natural weather- and along the lagoon shoreline in Abarao (Fig- of raising international awareness about climate- driven variability in sea level and ure 1, middle right) and other islets. the local impacts of climate change to sup- the astronomical tidal cycle can lead to Second, the practice of mining of beaches port adaptation and mitigation efforts. Inter- flooding and erosion events, particularly in and barrier reefs for construction materials, preting the causes of shoreline changes or sand- dominated systems like atolls and bar- common in Kiribati, Tuvalu, and other atoll flood events, as well as predicting the local rier islands. For example, the 2004–2005 nations, can make the shoreline more vul- impacts of sea level rise, is challenging for ENSO event contributed to two major flood- nerable to tidal extremes and storms [Webb, a developing country with limited resources ing events in Tarawa. During a “king” tide 2005]. Almost three quarters of the house- for scientific investigations. Many individual on 10 February 2005, water flooded sev- holds in South Tarawa mine sand, gravel, observations of erosion, flooding, or ground- eral causeways between the islets in South and reef rock from the lagoon or the ocean water salinization, recorded in community Tarawa and damaged the hospital in the reef, with one third doing so more than once consultations for internationally funded cli- town of Betio. A second flooding event a week [Greer Consulting Services, 2007]. mate change adaptation programs, are thus occurred 2 weeks later, despite the lower Although the effect of beach mining on the attributed to climate change without scien- daily tidal range, because of record high shoreline is difficult to distinguish from that tific analysis [e.g., Mackenzie, 2004]. These winds (47 knots at Betio) and record- low of other coastal processes, concern is suffi- events are presented as examples of cli- surface level pressure (999.2 hectopascals). cient to warrant European Union investment mate change impacts in promotional mate- Even though the maximum gauge height in a midlagoon dredging project to provide rials and at international events (e.g., “Our was 25 centimeters below that reached an alternative source of fill. Road to Copenhagen,” a Kiribati side event 2 weeks earlier, the northwest winds gener- Last, the construction of causeways at COP15 in Copenhagen), without any men- ated lagoon waves that again breached sea between islets has altered islet evolution. tion of ENSO- driven natural variability or walls, flooded causeways, and damaged Unlike a bridge, a solid, hard-topped cause- local shoreline modification. homes and public infrastructure (Figure 1, way limits or blocks the natural flow of sedi- Such unverified attribution can inflame top right). ment between the ocean and the lagoon. or invite skepticism of the scientific evi- These flooding events, though statistically Causeway construction allowed nearshore dence for a human- caused increase in the more likely to happen as global average currents to deposit sediment along the global sea level.

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