Coral islands defy sea-level rise over the past century: Records from a central Pacific atoll (original) (raw)
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The future of low-lying reef islands has been the subject of international concern, scientific debate, and media interest in the last decade. As a result of sea-level rise, atoll islands are expected to become increasingly unstable and to be susceptible to potential depopulation by the end of the 21st century. Some have suggested that sea-level rise has already resulted in widespread erosion and inundation of atoll islands. Here, we analyze the physical changes in over 200 islands on 12 atolls in the central and western Pacific in the past few decades when sea level in the region increased at rates three to four times the global average. Results show little evidence of heightened erosion or reduction in island size. Instead island shores have adjusted their position and morphology in response to human impacts such as seawall construction and to variations in climate–ocean processes. These changes are reviewed and the role of sea-level rise is evaluated. The implications of this analysis are addressed in two parts. First, we consider the proposition that future sea-level rise will destabilize atoll islands to such an extent that their inhabitants will be forced to migrate offshore. And second, we identify a series of new challenges relating to risk reduction and adaptation policy for atoll island governments, international agencies, and island communities. These require a substantial shift away from the present adaptation paradigm of external migration and focus on the persistence of atoll islands and in-country solutions.
Interactions between sea-level rise and wave exposure on reef island dynamics in the Solomon Islands
Environmental Research Letters, 2016
Low-lying reef islands in the Solomon Islands provide a valuable window into the future impacts of global sea-level rise. Sea-level rise has been predicted to cause widespread erosion and inundation of low-lying atolls in the central Pacific. However, the limited research on reef islands in the western Pacific indicates the majority of shoreline changes and inundation to date result from extreme events, seawalls and inappropriate development rather than sea-level rise alone. Here, we present the first analysis of coastal dynamics from a sea-level rise hotspot in the Solomon Islands. Using time series aerial and satellite imagery from 1947 to 2014 of 33 islands, along with historical insight from local knowledge, we have identified five vegetated reef islands that have vanished over this time period and a further six islands experiencing severe shoreline recession. Shoreline recession at two sites has destroyed villages that have existed since at least 1935, leading to community relocations. Rates of shoreline recession are substantially higher in areas exposed to high wave energy, indicating a synergistic interaction between sea-level rise and waves. Understanding these local factors that increase the susceptibility of islands to coastal erosion is critical to guide adaptation responses for these remote Pacific communities.
Role of Future Reef Growth on Morphological Response of Coral Reef Islands to Sea-Level Rise
2020
Plain Language Summary Coral reef islands are particularly exposed to the impacts of sea-level rise. They are usually fronted by "living" coral reef platforms that protect the island shoreline from energetic wave action. Healthy reef platforms grow vertically and can keep up with rising sea level, maintaining a constant water depth in front of the island. It is therefore suggested that future reef growth may be a critical factor in reducing the vulnerability of coral reef islands to sea-level rise. We use a computer model to simulate the response of coral reef islands to sea-level rise with and without future reef growth. We find that as sea level rises, the islands evolve by retreating, while at the same time building up vertically. Island build-up is accomplished by waves overwashing the island and depositing sediment on the top of the island. According to our model results, vulnerability of the reef islands to sealevel rise is not dependent on whether the reef platform grows or not. In both cases, islands are regularly overwashed, but this is necessary for islands to grow vertically. Island accretion by overwash offers hope for uninhabited and sparely populated islands, but will negatively impact infrastructure and assets on urbanized islands. MASSELINK ET AL.
Many Atolls May be Uninhabitable Within Decades Due to Climate Change
Scientific reports, 2015
Observations show global sea level is rising due to climate change, with the highest rates in the tropical Pacific Ocean where many of the world's low-lying atolls are located. Sea-level rise is particularly critical for low-lying carbonate reef-lined atoll islands; these islands have limited land and water available for human habitation, water and food sources, and ecosystems that are vulnerable to inundation from sea-level rise. Here we demonstrate that sea-level rise will result in larger waves and higher wave-driven water levels along atoll islands' shorelines than at present. Numerical model results reveal waves will synergistically interact with sea-level rise, causing twice as much land forecast to be flooded for a given value of sea-level rise than currently predicted by current models that do not take wave-driven water levels into account. Atolls with islands close to the shallow reef crest are more likely to be subjected to greater wave-induced run-up and flooding ...
Adaptation to Sea Level Rise On Low Coral Islands: Lessons from Recent Events
Ocean & Coastal Management , 2019
In the past two decades there have been fears that many low-lying atoll islands around the world could disappear as a consequence of climate change and sea level rise, leading to mass migration and threatening the existence of several island nations. Here we show how sea level rise does not inevitably lead to coastal areas becoming uninhabitable, and that humans have an innate and often underestimated capacity to adapt to changes in their environment. To do so we showcase three instances of human-and earthquake-induced land subsidence that have taken place in the 21 st century, where the coastal/island areas are still inhabited despite the challenge of living with higher water levels: the Tohoku coastline following the 2011 Tohoku Earthquake Tsunami (subsidence ~0.4-1.0 m), the present day situation of coastal areas in Jakarta due to ground water extraction (>5.0 m), and the islands of Tubigon, Bohol in central Philippines after the 2013 Bohol Earthquake (~1.0 m). Humans are able to adapt and arrive at solutions even when confronted with cases of rapid rises in water levels, and thus it is likely that in the future vulnerable coastlines will be engineered and largely remain at present day locations, particularly in densely populated areas. If anything, around densely populated areas it is more likely that humans will continue to encroach on the sea rather than the reverse. We caution, however, that small islands are not homogeneous, and many are unlikely to respond to rising sea levels in the manner that atolls do (in fact, many might just resort to build at higher elevations). Where engineering and other adaptation responses become necessary, the financial and human resource requirements may well be beyond capacity of some small islands, which could lead to impoverishment and associated challenges in many communities.
Drivers of shoreline change in atoll reef islands of the Tuamotu Archipelago, French Polynesia
A B S T R A C T This paper increases by around 30% the sample of atoll reef islands studied from a shoreline change perspective, and covers an under-studied geographical area, i.e. the French Tuamotu Archipelago. It brings new irrefutable evidences on the persistence of reef islands over the last decades, as 77% of the 111 study islands exhibited areal stability while 15% and 8% showed expansion and contraction, respectively. This paper also addresses a key research gap by interpreting the major local drivers controlling recent shoreline and island change, i.e. tropical cyclones and seasonal swells, sediment supply by coral reefs and human activities. The 1983 tropical cyclones had contrasting impacts, depending on the shoreline indicator considered. While they generally caused a marked retreat of the stability line, the base of the beach advanced at some locations, as a result of either sediment reworking or fresh sediment inputs. The post-cyclone fair weather period was characterised by reversed trends indicating island morphological readjustment. Cyclonic waves contributed to island upwards growth, which reached up to 1 m in places, through the transfer of sediments up onto the island surface. However, the steep outer slopes of atolls limited sediment transfers to the reef flat and island system. We found that 57% of the study islands are disturbed by human activities, including 'rural' and uninhabited islands. Twenty-six percent of these islands have lost the capacity to respond to ocean-climate related pressures, including the 'capital' islands concentrating atolls' population, infrastructures and economic activities, which is preoccupying under climate change.
Earth Surface Processes and Landforms, 1995
Coral islands formed of largely unconsolidated sands only a few metres above sea level are thought to be particularly vulnerable to sea-level rise consequent upon global warming. However, scenarios which predict catastrophic flooding and loss of island area need reassessment, particularly in the light of the continued downwards revision of projected rates of future sea-level rise. Revised questions concern the interactions between reef growth and sea-level change, biophysical constraints on coral growth, and the importance to reef systems of potential changes in the magnitude, frequency and location of tropical cyclones and hurricanes. It is clear that most reefs have the growth potential to meet even the highest of future sea-level rise scenarios, but too little is known about physiological and physical constraints to reef growth to adequately evaluate the importance of these two factors in constraining this potential at the present time. Future sea-level rise in the tropical oceans, and coral reef responses, will take place against a backdrop of interregional differences in Holocene sea levels, resulting from the varying interaction of eustatic and hydro-isostatic processes. These differences have generated varying constraints on the development of modern reefs and varying inherited topographies upon which future sea-level changes will be superimposed. These controls are particularly important in assessing differences in vulnerability to future sea-level rise for reef islands in the Pacific Ocean and the Caribbean Sea. KEY WORDS Coral reefs Sea level Coral bleaching Cyclones Cook Islands Global environmental change INTRODUCTION Coral reefs are unique amongst shallow marine ecosystems in being dependent upon their ability to produce sedimentary frameworks and infills to maintain themselves at, or very near, sea level. Ever since Darwin's (1842) The Structure and Distribution of Coral Reefs, which marked the beginning of comprehensive scientific studies into coral reef development, reef scientists have recognized the key role of sea-level change, and its relation to tectonic and oceanographic conditions, in influencing the vertical accretion of reef structures and shaping the subaerial landsurfaces which veneer such structures. These land areas-typically reef-marginal elongate islands of coral rubble and gravels with associated beach and dune sands, often known as motus within the Pacific Ocean, and smaller, more mobile sand cays of carbonate sands and gravels (Stoddart and Steers, 1'977)-are widely regarded as being particularly susceptible to potential future sea-level rise. An attempt in the 1980s to compare rates of reef growth to expected rates of sea-level rise was pessimistic about the ability of coral reef structures to keep pace with expected sea-level change (Buddemeier and Smith, 1988); several scenarios at this time predicted catastrophic flooding and extensive island erosion. However, the downward revision of projected rates of sea-level rise to 2100, from best estimates of 100-150cm in the mid-1980s to the IPCC's recent estimate of < 50cm (Wigley and Raper, 1992; Gornitz, 1995), requires a reassessment of the likely impacts of future sea-level rise on coral atoll ecosystems and their associated landforms. Rather than simple in situ drowning and loss of island area, reefs and reef islands may show more complex responses to environmental change. Consequently, the relative rates of different reef-forming processes and the net
Evidence for coral island formation during rising sea level in the central Pacific Ocean
Geophysical Research Letters, 2014
The timing and evolution of Jabat Island, Marshall Islands, was investigated using morphostratigraphic analysis and radiometric dating. Results show the first evidence of island building in the Pacific during latter stages of Holocene sea level rise. A three-phase model of development of Jabat is presented. Initially, rapid accumulation of coarse sediments on Jabat occurred 4800-4000 years B.P. across a reef flat higher than present level, as sea level continued to rise. During the highstand, island margins and particularly the western margin accreted vertically to 2.5-3.0 m above contemporary ridge elevations. This accumulation phase was dominated by sand-size sediments. Phase three involved deposition of gravel ridges on the northern reef, as sea level fell to present position. Jabat has remained geomorphically stable for the past 2000 years. Findings suggest reef platforms may accommodate the oldest reef islands in atoll systems, which may have profound implications for questions of prehistoric migration through Pacific archipelagos.
Keep up or drown: adjustment of western Pacific coral reefs to sea-level rise in the 21st century
Royal Society Open Science, 2015
Since the Mid-Holocene, some 5000 years ago, coral reefs in the Pacific Ocean have been vertically constrained by sea level. Contemporary sea-level rise is releasing these constraints, providing accommodation space for vertical reef expansion. Here, we show thatPoritesmicroatolls, from reef-flat environments in Palau (western Pacific Ocean), are ‘keeping up’ with contemporary sea-level rise. Measurements of 570 reef-flatPoritesmicroatolls at 10 locations around Palau revealed recent vertical skeletal extension (78±13 mm) over the last 6–8 years, which is consistent with the timing of the recent increase in sea level. We modelled whether microatoll growth rates will potentially ‘keep up’ with predicted sea-level rise in the near future, based upon average growth, and assuming a decline in growth for every 1°C increase in temperature. We then compared these estimated extension rates with rates of sea-level rise under four Representative Concentration Pathways (RCPs). Our model suggest...
Pathways to sustain atolls under rising sea levels through land claim and island raising
Environmental Research: Climate
Low-lying atoll nations (e.g. the Maldives, Kiribati, Tuvalu, Marshall Islands) are highly vulnerable to climate change, especially sea-level rise (SLR). Stringent climate change mitigation will slow but not stop SLR, which will continue for centuries, mandating additional long-term adaptation. At the same time, urbanisation is concentrating population in a few centres, especially around capital islands which creates additional pressure as most atoll nations are ‘land-poor’. This paper demonstrates how structural adaptation using land claim and island raising can be utilised within an adaptation pathway approach to sustain enough islands and land area above rising sea levels to satisfy societal and economic needs over multiple centuries. This approach is illustrated using the Maldives, especially around the capital and its environs (Greater Malé). Raising, expanding and connecting ‘urban’ islands can provide multiple benefits. Significant developments have already occurred in Greate...