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The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal
analysis of island change in the Central Pacific
Arthur P. Webb
a
, Paul S. Kench
b,
⁎
a
Pacific Islands Applied Geoscience Commission, SOPAC, Fiji
b
School of Environment, The University of Auckland, Private Bag 92019, Auckland, New Zealand
abstractarticle info
Article history:
Received 22 February 2010
Accepted 13 May 2010
Available online xxxx
Keywords:
Atoll island
sea-level rise
erosion
island migration
Pacific Ocean
Low-lying atoll islands are widely perceived to erode in response to measured and future sea-level rise.
Using historical aerial photography and satellite images this study presents the first quantitative analysis of
physical changes in 27 atoll islands in the central Pacific over a 19 to 61 yr period. This period of analysis
corresponds with instrumental records that show a rate of sea-level rise of 2.0 mm yr
−1
in the Pacific.
Results show that 86% of islands remained stable (43%) or increased in area (43%) over the timeframe of
analysis. Largest decadal rates of increase in island area range between 0.1 to 5.6 ha. Only 14% of study
islands exhibited a net reduction in island area. Despite small net changes in area, islands exhibited larger
gross changes. This was expressed as changes in the planform configuration and position of islands on reef
platforms. Modes of island change included: ocean shoreline displacement toward the lagoon; lagoon
shoreline progradation; and, extension of the ends of elongate islands. Collectively these adjustments
represent net lagoonward migration of islands in 65% of cases. Results contradict existing paradigms of island
response and have significant implications for the consideration of island stability under ongoing sea-level
rise in the central Pacific. First, islands are geomorphologically persistent features on atoll reef platforms and
can increase in island area despite sea-level change. Second, islands are dynamic landforms that undergo a
range of physical adjustments in responses to changing boundary conditions, of which sea level is just one
factor. Third, erosion of island shorelines must be reconsidered in the context of physical adjustments of the
entire island shoreline as erosion may be balanced by progradation on other sectors of shorelines. Results
indicate that the style and magnitude of geomorphic change will vary between islands. Therefore, island
nations must place a high priority on resolving the precise styles and rates of change that will occur over the
next century and reconsider the implications for adaption.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Coral reef islands are low-lying accumulations of unconsolidated, or
poorly lithified, carbonate sand and gravel deposited on coral reef
platforms by the focussing effect of waves and currents (Stoddart and
Steers, 1977). Coral reef islands are commonly found in barrier reef
systems (e.g. Great Barrier Reef); open reef seas (e.g. Torres Strait) or in
mid-ocean atolls. In atoll nations such as Tuvalu, Kiribati and the
Maldives reef islands provide the only habitable area, which can carry
very high population densities (e.g. 8300 people/km
2
on Fongafale,
Tuvalu and 47,400 people/km
2
on Male, Maldives). These low-lying reef
islands and their populations are considered physically vulnerable to a
range of climate change impacts including: sea-level rise; changing
weather and oceanographic wave regimes, and increased cyclone
frequency and intensity (Church et al., 2006; Mimura et al., 2007).
Under current scenarios of global climate-induced sea-level rise of 0.48
to 0.98 m by 2100 it is widely anticipated that low-lying reef islands will
become physically unstable and be unable to support human popula-
tions over the coming century (Leatherman, 1997; Connell, 1999). The
most anticipated physical impacts of sea-level rise on islands are
shoreline erosion, inundation, flooding, salinity intrusion, and reduced
resilience of coastal ecosystems (Leatherman, 1997; Mimura, 1999;
Khan et al., 2002; Yamano et al., 2007). It is also widely perceived that
island erosion will become so widespread that entire atoll nations will
disappear rendering their inhabitants among the first environmental
refugees of climate change (Connell, 2003, 2004).
Attempts to resolve future island morphological response to global
climate change can be divided into two broad groups. First a number
of studies have examined the Holocene formation of islands as
analogues of future response. Such studies have attempted to resolve
critical linkages between reef growth, sea level and timing of island
formation in order to project future morphological behaviour (e.g. Roy
and Connell, 1991; Woodroffe and McLean, 1992, 1993; Dickinson,
1999; Kench et al., 2005, 2009a). Inevitably such assessments focus on
Global and Planetary Change xxx (2010) xxx–xxx
⁎Corresponding author. Tel.: +649 3737599x88440; fax: +649 3737435.
E-mail addresses: arthur@sopac.org (A.P. Webb), p.kench@auckland.ac.nz
(P.S. Kench).
GLOBAL-01591; No of Pages 13
0921-8181/$ –see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.gloplacha.2010.05.003
Contents lists available at ScienceDirect
Global and Planetary Change
journal homepage: www.elsevier.com/locate/gloplacha
Please cite this article as: Webb, A.P., Kench, P.S., The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis
of island change in the Central Pacific, Global and Planetary Change (2010), doi:10.1016/j.gloplacha.2010.05.003
the sea-level reef growth linkage as a critical boundary control of
island adjustment but do not recognise that such adjustments
typically occur at timescales at least an order of magnitude greater
(centennial to millennial) than timescales relevant to island morpho-
logical adjustment in the near-future (Kench et al., 2009b). Second, a
number of studies have relied on the extant geomorphic properties of
islands to argue varying levels of resilience or resistance (Woodroffe
and McLean, 1993). For example, Woodroffe (2008) noted that the
susceptibility of islands to morphological change can be expected to
vary considerably depending on subtle differences in island topogra-
phy and geomorphic characteristics such as reef elevation and
whether island material is partially lithified. Typically, these studies
treat islands as static landforms. In particular, studies of future
flooding and inundation have made projections of future flood risk
based on static landform boundaries (Mimura, 1999; Khan et al.,
2002; Yamano et al., 2007).
Collectively, these approaches provide insights into the past
development and existing morphological attributes of reef islands
with which to infer potential morphological adjustment or resilience.
However, such approaches have not incorporated a full appreciation
of the contemporary morphodynamics of landforms nor considered
the style and magnitude of changes that may be expected in the
future. Reef islands are dynamic landforms that are able to reorganise
their sediment reservoir in response to changing boundary conditions
(wind, waves and sea-level, Kench et al., 2009b). An increasing
number of studies have shown that reef islands exhibit a high degree
of morphological variability with respect to location and planform
configuration on reef surfaces, in response to changing wind and wave
patterns (Flood, 1986; Kench and Brander, 2006). Extreme events
(cyclones and tsunami) have also been shown to have promoted both
island erosion (Stoddart, 1963, 1971; Flood and Jell, 1977; Harmelin-
Vivien, 1994) and accretion signatures (Maragos et al., 1973; Webb,
2006; Kench et al., 2006) depending on the calibre of sediment
comprising islands and whether islands are located in storm or non-
storm environments (Bayliss-Smith, 1988). Of note, these studies
have shown differing modes of island shoreline adjustment that
include horizontal displacement, and washover sedimentation that
can vertically build island surfaces (e.g. Kench and Cowell, 2001;
Kench et al., 2006, 2009b).
At inter-annual to decadal timescales studies have also identified
changes in island size and position on reefs (Taylor, 1924; Stoddart
et al., 1978; Flood, 1986; Aston, 1995). Umbgrove (1947) and
Verstappen (1954) were the first to develop a causal relationship
between climate and reef island behaviour invoking medium-term
(decadal) shifts in prevailing wind direction and strength and its
influence on wave energy as a control on morphological adjustment of
islands in Djakarta Bay, Indonesia. Flood (1986) also related decadal
changes in wind to progressive shifts in reef island planform in the
Great Barrier Reef whereas, Stoddart et al. (1982) found that decadal
change on islands within the Belize barrier reef system resulted from
hurricane activity. There are a number of conspicuous features of
these medium-term studies that are relevant to the issue of future
island change. First, they have focused either on islands in fringing or
barrier reef settings. Studies of atoll island change are scarce (Kench
and Harvey, 2003). Second, these decadal-scale studies have generally
linked island morphological change to shifts in climate (wind). Third,
short and medium-term analogues of morphological change have not
been adopted to project near-future changes in reef islands, despite
the fact that climate has been implicated as the driver of morpholog-
ical change. Fourth, these studies generally occurred prior to concerns
over accelerated sea-level rise and no contemporary study exists that
has attempted to examine decadal-scale island adjustments in
response to variations in sea level.
In contrast to studies of physical island change, there has been
considerable scientific effort in reconstructing past and present sea-
level behaviour. The global dataset on sea-level trends over the past
130 yr shows an increase in global averaged mean sea level of
approximately 200 mm (Fig. 1A). Analysis of available sea-level data
from the northeast, central and western Pacific show significant
regional differences in sea-level behaviour over the past century but
generally agree with the large-scale global trend (Fig. 1B). However,
numerous studies have noted the lack of long-term sea-level records
from the Pacific Ocean, with this region being under-represented in
analyses of global sea-level change (Milne et al., 2009). Over the past
20 yr there has been an increase in the number of high quality water
level gauges deployed in islands in the southwest and central Pacific
(South Pacific Sea Level and Climate Monitoring Project), with which
to resolve high resolution sea-level behaviour (Church et al., 2006;
Fig. 2). Furthermore, satellite altimeter data (TOPEX/Poseidon and
Jason-1) captured over the past 17 yr has provided near-global maps
of absolute sea level generated at 10-day intervals and has permitted
sea-level trends to be identified for the world's oceans (Milne et al.,
2009).
Current consensus of regional sea-level patterns in the central and
southwest Pacific over the past 50 to 100 yr indicates sea level is subject
to large inter-annual variations of ±0.45 m driven by ENSO cycles
(Church et al.,2006). Superimposed on these short-term oscillations isa
long-term trend of sea-level rise on the order of 1.6mm yr
−1
, which is
consistent with global projections (Church et al., 2006; Milne et al.,
2009; Woodworth et al., 2009). However, data also show considerable
Fig. 1. A) The monthly global mean sea level time series derived from tide-gauge data
1870–2002, source data (Church and White, 2006). Sea level was reconstructed as
described in Church et al., 2004. B) Sea level curves derived from tide-gauge data using
the ‘virtual station’method. Each time series has been offset along the y axis by an
arbitrary amount to avoid overlap (from Milne et al., 2009).
2A.P. Webb, P.S. Kench / Global and Planetary Change xxx (2010) xxx–xxx
Please cite this article as: Webb, A.P., Kench, P.S., The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis
of island change in the Central Pacific, Global and Planetary Change (2010), doi:10.1016/j.gloplacha.2010.05.003
within region variation. Most r ecords show an increase over a 50 yr t ime
horizon ranging from 0.5 mm yr
−1
at Malakal, Palau to 2.1 mm yr
−1
at
Majuro atoll, Marshall Islands (Church et al., 2004). Of note, sea-level
rise at Majuro and Funafuti in the central Pacific is 2.1 and 1.6 mm yr
−1
respectively althoughthere is considerable uncertainty in the recordson
the order of ±0.3 mm yr
−1
. In addition there is tentative evidence that
sea-level rise is accelerating throughout the tropical Indian and Pacific
Oceans (Church and White, 2006; Woodworth et al., 2009).
Despite assertions of island vulnerability to sea level and climatic
change there have been few studies that have quantified island
morphological change at the same temporal scale as detailed sea level
records. Indeed, no systematic monitoring programme exists to
document detailed reef island morphological change (Kench and
Harvey, 2003). The lack of monitoring seems a gross oversight given
the international concern over small island stability and pressing
concerns of island communities to manage island landscapes. Further-
more, the lack of island shoreline monitoring represents a missed
opportunity to couple morphological change data with the detailed sea-
level records that have been accruing over the past 18 yr (Kench and
Harvey, 2003).
This study presents new data on decadal-scale atoll island land-
form dynamics in the central Pacific Ocean. It addresses the question
whether atoll islands have shown any consistent trends in morpho-
logical stability as a consequence of documented increases in sea level
over the past half century. Specific objectives are to evaluate the net
changes in reef island planform configuration over the past 20–60 yr,
and examine gross changes in island planform adjustment over
decadal timeframes. Significantly, the temporal scale of analysis
overlaps the period in which sea level records have been analysed to
establish rates of sea level change on the order of 2.0 mm yr
−1
in the
central Pacific Ocean. Consequently, our results are used to evaluate
assumptions that increased sea level will destabilise and cause net
erosion of atoll islands.
2. Field setting
This study examines the planform morphological change of 27
atoll islands located in the central Pacific(Figs. 2 and 3). The islands
are located in three Pacific countries, in four atolls, and span 15° of
latitude from Mokil atoll in the north (6°41.04′N) to Funafuti in the
South (8°30.59′S). The atolls vary significantly in terms of size,
structure and number of islands distributed on the atoll rim. The atolls
also vary in potential exposure to tropical cyclones. Whereas the
Federated States of Micronesia (FSM) and Tuvalu can be affected by
cyclones, equatorial Kiribati has no record of direct cyclone impact. All
27 islands in the study are located on atoll reef rims of Holocene age.
Therefore, the islands are all also of Holocene age (McLean and
Hosking, 1991; Dickinson, 1999).
Two atolls were examined from the Federated States of Micronesia
(Fig. 3C andD). Mokil and Pingelapatolls are both small with a totalarea
of approximately 8 and 12 km
2
respectively and with continuous atoll
reef rims that enclose a central lagoon. Both atolls have three islands on
their reef platforms found on all exposures of the atoll rim. The islands
are of varying size and shape with the longest axes ranging from 0.42 to
3.54 km and width ranging from 0.18 to 0.48 km (Table 1).
Tarawa atoll in the Republic of Kiribati (1° 26.2′N and 172° 58.8′E) is
broadly triangular in shape with dimensions of approximately 40 km in
length and 25 km in maximum width (Fig. 3B). An o pen submerged reef
system characterises the western atoll rim with a single deeper passage
connecting the lagoon and open ocean. Islands form a near-continuous
chain along the eastern and southern sections of the atoll reef rim
(Fig. 3B). This study examined four islands in Tarawa atoll; three islands
from SouthTarawa and the most northern island Buariki(Fig. 3B). These
islands differ significantly in energy exposure, size and level of
development. The three islands located on the southern atoll rim are
all inhabited and are part of the urban precinct South Tarawa. Nanikai is
the smallest island measuring 0.82 km in length and only 0.11 km in
width (total area of 6.4 ha). In contrast Betio measures 4.4 km in length
and 0.36 km in mean width (area of 120 ha). These islands have a range
of structures on the ocean and lagoon shorelines. The largest island
studied is located in North Tarawa. Buariki has a dimension of 6.6 km in
length and up to 0.54 km in width (area of 338 ha, Table 1).
Funafuti atoll in Tuvalu (8° 30.6′S and 179° 6.9′E) measures
approximately 20 km in length and up to 15 km in width (Fig. 3A).
The atoll has a near-continuous reef rim that surrounds the lagoon,
with a small number of deep passages that connect the lagoon to open
ocean. There are only a few small islands on the western leeward reef
rim (Fig. 3A). Islands are present on the northeast to southern sections
of the atoll reef rim. Eighteen islands spanning the northern to
southern extent of the atoll were selected for analysis (Table 1). The
Fig. 2. Location diagram of the southwest Pacific Ocean showing network of seaframe water level gauges (triangles) and atolls in this study (grey boxes).
3A.P. Webb, P.S. Kench / Global and Planetary Change xxx (2010) xxx–xxx
Please cite this article as: Webb, A.P., Kench, P.S., The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis
of island change in the Central Pacific, Global and Planetary Change (2010), doi:10.1016/j.gloplacha.2010.05.003
islands vary significantly in dimension and overall size. Tengasu in the
southwest of the atoll is the smallest island with dimensions of
0.07× 0.08 km in length and width with a total area of 0.68 ha.
Funafara in the southeast of the atoll has dimensions of 2.2 km
(length) and 0.11 km (width) with an area of 22.9 ha.
Sea level records from the nearest sea level recorder to each atoll
are presented in Fig. 4 for the past 20 to 30 yr. These records show large
inter-annual variations and are in general agreement with regional
patterns and rates of sea level rise, in the order of 2.0 mm yr
−1
.
Consequently, the study islands have all experienced increase in sea
level over the past 20 yr.
3. Methodology
A total of 27 islands were examined using comparative analysis of
historical aerial photography and remotely sensed images. Historical
aerial photographs were either scanned from hard copies or negatives
at minimum resolution of 900 dpi. The aerial photographs used all had
Fig. 3. Location of study islands in four atolls selected for analysis. Islands examined in this study are labelled.
4A.P. Webb, P.S. Kench / Global and Planetary Change xxx (2010) xxx–xxx
Please cite this article as: Webb, A.P., Kench, P.S., The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis
of island change in the Central Pacific, Global and Planetary Change (2010), doi:10.1016/j.gloplacha.2010.05.003
a scale of b1:25,000. Once scanned these images were enhanced to
maximise contrast of features, orientated to grid North and in some
cases cropped to avoid excessive overlap.
The timeframe of analysis is different between atolls and islands
depending on aerial photograph coverage and availability (Table 1).
The minimum time period is 19 yr for islands in Funafuti with the
maximum timespan of 61 yr for Mokil and Pingelap. Georectification
and referencing of the historical aerial photographs was accomplished
using ERDAS Imagine 8.4 software using georeferenced (UTM WGS
84) IKONOS and/or Quickbird satellite imagery as the source of
ground control points. Once corrected the historical images were
saved as geotif image files with WGS 84 co-ordinate system
embedded. Truthing of each image was achieved by comparison of
reliable ground control points between the georeferenced satellite
images and the georectified historical images (Thieler and Danforth,
1994; Graham and Koh, 2003). Each historical image was subse-
quently re-corrected using the same process until error between the
satellite and historical images was due to issues of resolution rather
than systematic error in position of control points (Moore, 2000).
Due to the isolated and undeveloped nature of many of these islands
there were considerable challenges in identifying conventional perma-
nent reference points (such as surveyed datum points) which can be
commonly found in images from differing time periods (Thieler and
Danforth, 1994). Additionally, features such as sealed roads and
permanent buildings are often restricted to only small areas on any
particular island and are almost absent in the pre 1960's historical
images. Consequently, a range of anthropogenic (e.g. ancient stone fish
traps) andnatural geomorphic featuresthat have temporalstability (e.g.
beach rock and conglomerate outcrops) were used for rectification.
A further limitation in the analysis of aerial photographs is the
differing resolution or quality of images (Anders and Byrnes, 1991).
Aerial photographs generally have better resolution than satellite images
but older air photos may be similarly limited due to the state of
technology at that time and/or the poor condition of negatives. It should
also be recognised that historical images were seldom acquired with the
specific intention of use as a coastal management tool and as such, the
Table 1
Physical attributes of study islands and timespan of aerial imagery.
Atoll / Island Coordinates Atoll rim
location
Island physical characteristics Time span of imagery
(yr)
Latitude Longitude Length
(km)
Width
(km)
Area
(ha)
Funafuti Atoll 8° 30.592′S 179° 6.932′E
Paava Island 8° 25.651′S 179° 7.002′E North 0.24 0.08 1.48 1984–2003
Fualifeke Island 8° 25.649′S 179° 7.350′E North 0.50 0.17 6.85 1984–2003
Mulitefala Island 8° 26.062′S 179° 10.016′E Northeast 0.75 0.11 2.33 1984–2003
Amatuku
b
8° 26.301′S 179° 10.277′E Northeast 0.70 0.11 6.13 1984–2003
Fatato 8° 32.865′S 179° 9.732′E Southeast 0.85 0.07 5.11 1984–2003
Funagongo 8° 33.478′S 179° 8.778′E Southeast 1.11 0.13 10.66 1984–2003
Funamanu 8° 33.918′S 179° 8.012′E Southeast 0.55 0.08 2.99 1984–2003
Falefatu 8° 34.904′S 179° 6.980′E Southeast 0.62 0.06 3.23 1984–2003
Mateiko 8° 36.133′S 179° 6.006′E Southeast 0.81 0.08 4.25 1984–2003
Luamotu 8° 36.562′S 179° 5.948′E Southeast 0.43 0.05 1.80 1984–2003
Funafara 8° 37.451′S 179° 5.961′E Southeast 2.2 0.11 22.95 1984–2003
Telele 8° 38.131 ′S 179° 5.731′E South 1.34 0.05 8.83 1984–2003
Motungie 8° 38.503′S 179° 5.155′E South 0.86 0.05 4.97 1984–2003
Avalau/Teafuafou 8° 38.277′S 179° 4.463′E Southwest 0.71 0.16 12.14 1984–2003
Tengasu 8° 37.983′S 179° 4.547′E Southwest 0.07 0.08 0.68 1984–2003
Tutanga 8° 37.651′S 179° 4.689′E Southwest 0.13 0.15 1.66 1984–2003
Falaoingo 8° 37.504′S 179° 4.749′E Southwest 0.17 0.06 1.31 1984–2003
Tarawa Atoll 1° 26.178′N 172° 58.779′E
Betio
a
1° 21.356′N 172° 55.901′E Southwest 4.4 0.36 120.03 1943–2004
Bairiki
a
1° 19.773′N 172° 58.674′E South 1.78 0.28 35.46 1969–2004
Nanikai
a
1° 19.814′N 172° 59.851′E South 0.82 0.11 6.40 1969–2004
Buariki
b
1° 36.634′N 172° 57.787′E Northeast 6.58 0.54 338.30 1943–2004
Pingelap Atoll 6° 13.031′N 160° 42.169′E
Deke 6° 13.672′N 160° 41.824′E North 1.31 0.40 59.90 1944–2006
Sukoru 6° 13.217′N 160° 41.575′E West 0.21 0.42 5.84 1944–2006
Pingelap
b
6° 12.439′N 160° 42.348′E East 3.54 0.48 127.00 1944–2006
Mokil Atoll 6° 41.044′N 159° 45.476′E
Mwandohn 6° 41.496′N 159° 45.155′E West 1.11 0.24 26.19 1944–2006
Kahlap
b
6° 41.192′N 159° 45.891′E East 2.97 0.18 55.90 1944–2006
Uhrek 6° 39.044′N 159° 45.476′E South 1.19 0.48 51.50 1944–2006
Island length is the longest axis of the island parallel to the reef rim.
Island width represents the mean width of the island perpendicular to the reef rim.
Island area calculated from the earliest aerial imagery.
a
Denote densely populated and urbanised islands.
b
Denote islands with small rural villages.
Fig. 4. Historical sea level observations for study atolls Tarawa and Funafuti and nearest
record for Pingelap and Mokil (Pohnpei, Federated States of Micronesia).
5A.P. Webb, P.S. Kench / Global and Planetary Change xxx (2010) xxx–xxx
Please cite this article as: Webb, A.P., Kench, P.S., The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis
of island change in the Central Pacific, Global and Planetary Change (2010), doi:10.1016/j.gloplacha.2010.05.003
flight angles and paths, exposure, coverage and elevation are often not
optimum. Nevertheless, these photographic rec ords when compared as a
time series, offer the best opportunity available to determine accurate
rates and patterns of coastal change and processes over time.
Georectification and analysis is reliant on an intimate knowledge
of land, shoreline and shallow marine forms and structures in these
atoll environments. Ground-truthing and community discussion of
the changes perceived from this work was also undertaken between
2004 and 2006 and there was consistent agreement with the findings
of the study. Error is largely determined by the resolution of satellite
imagery (Funafuti —4 m IKONOS and Tarawa, Pingelap and Mokil —
0.6 m Quickbird; Crowell et al., 1991). Due to the accuracies of aerial
photographs measured changes in shoreline position within ±3%
were not considered significant and reflect relative stability of islands.
Once images were rectified analysis involved the overlay of the
historical time series for each island. Areas of accretion and erosion
were subsequently identified. Changes in island area were calculated
and compared to establish change through time. Observations of
changes in the configuration and position of the island on reef
platforms were also made.
4. Results
Summary data on changes in island area are presented in Table 2
and selected examples of changes in the planform configuration of
islands over the time interval of analysis are shown in Figs. 5–8.
On Funafuti islands exhibited differing physical adjustments over
the 19 yr of analysis. Six of the islands have undergone little change in
area (b±3%). Seven islands have increased in area by more than 3%.
Maximum increases have occurred on Funamanu (28.2%), Falefatu
(13.3%) and Paava (10.1%). In contrast, four islands decreased in area
by more than 3%. The largest decrease in area is in Tengasu (−14.7%)
although it should be noted that this was the smallest island
examined. The remaining three islands all decreased in area by less
than 4% in area.
Plannimetric changes are illustrated for selected islands in Fig. 5.
Notable planform adjustments include: expansion (progradation) of
lagoon shorelines on Fualifeke, Mulitefala and Funamanu; extension
of the ends of the elongate island Funamanu; and erosion of ocean
shorelines on a number of islands (Fig. 5;Table 2). As illustrated by
Mulitefala, erosion of the ocean shoreline and expansion of lagoon
shorelines results in net displacement of the island in a lagoonward
direction across the reef. However, on the leeward reef, islands exhibit
lagoonal erosion and either expansion or stability of oceanside
coastlines (e.g. Falaoingo and Tutanga, Fig. 5E).
Analysis of island change over a 35–61 yr timeframe on Tarawa atoll
shows that all four islands exhibited an increase in island area. Notably
the three urbanised islands of Betio, Bairiki and Nanikaiincreased in area
by 30, 16.3 and 12.5% respectively. Buariki in the north of the atoll
exhibited an increase of 2%. This was the largest island examined and
represents an increase in area of 10.1 ha (Table 2).
Changes in the planform configuration of Betio and Bairiki show an
expansion in the island footprint on both ocean and lagoon shorelines
(Fig. 6E and D).Nanikai displays oceanside erosion, embaymentinfilling
and eastward extension by up to 300 m (Fig. 6C). Buariki exhibited
localised embayment infilling on the exposed ocean shoreline and
Table 2
Summary of island change characteristics.
Atoll/island Time
period
(Yr)
Initial
area
(Ha)
Final
area
(Ha)
Net island
change
Decadal rate
of change
Geomorphic change in island planform characteristics
(Ha) (%) (Ha) (%) Ocean
shore
Lagoon
shore
Dominant style of island planform adjustment
Funafuti Atoll
Paava Island 19 1.48 1.63 0.15 10.0 0.08 5.26 Accretion Erosion Ocean migration and contraction of eastern end of island
Fualifeke Island 19 6.85 6.61 −0.24 −3.5 −0.13 −1.84 Erosion Accretion Lagoon migration of N and E shorelines. Island tip extension
Mulitefala Island 19 2.33 2.35 0.02 0.8 0.01 0.42 Erosion Accretion Lagoon migration. Contraction NW end of island
Amatuku
b
19 6.13 6.42 0.29 4.6 0.15 2.42 Accretion Accretion Island expansion, lagoon progradation. Contraction NW end
of island
Fatato 19 5.11 5.54 0.44 8.6 0.23 4.53 Accretion Accretion Lagoon migration N end. Extension of S and N ends of island
Funagongo 19 10.66 10.76 0.10 1.0 0.06 0.53 Erosion Accretion Lagoon migration NE end
Funamanu 19 2.99 3.83 0.84 28.2 0.44 14.84 Stable Stable Lagoon migration, extension of W and E ends of island
Falefatu 19 3.23 3.66 0.43 13.3 0.73 7.00 Erosion Accretion Lagoon migration. SW end stable, Lagoon migration central
and N end
Mateiko 19 4.25 4.51 0.26 6.1 0.14 3.21 Erosion Accretion Lagoon migration
Luamotu 19 1.80 1.74 −0.06 −3.3 −0.03 −1.74 Erosion Accretion Lagoon migration. S end contracted, N end extension
Funafara 19 22.95 23.78 0.83 3.6 0.43 1.89 Accretion Accretion Lagoonward deposition in S, spit growth and embayment infilling
Telele 19 8.83 8.87 0.04 0.5 0.02 0.26 Erosion Accretion Lagoonward migration
Motungie 19 4.97 5.03 0.05 1.0 0.03 0.53 Erosion Accretion Lagoon migration. SW tip extension ∼100 m.
Avalau/Teafuafou 19 12.14 11.89 −0.25 −2.1 −0.14 −1.11 Erosion Erosion Contraction. Localised embayment sedimentation
Tengasu 19 0.68 0.59 −0.10 −14.7 −0.05 −7.74 Stable Erosion Contraction of lagoon shoreline
Tutanga 19 1.66 1.60 −0.06 −3.6 −0.03 −1.89 Stable Erosion Contraction of lagoon shoreline
Falaoingo 19 1.31 1.31 0.00 0.0 0.00 0.00 Stable Erosion Contraction of lagoon shoreline
Tarawa Atoll
Betio
a
61 120.03 156.0 36.0 30.0 5.81 4.84 Accretion Accretion Expansion of island footprint, localised areas of erosion
Bairiki
a
35 35.46 41.25 5.79 16.3 1.65 4.66 Accretion Accretion Expansion of island footprint, localised areas of erosion
Nanikai
a
35 6.40 7.20 0.80 12.5 0.23 3.57 Accretion Accretion Lagoon expansion, embayment infilling
Buariki
b
61 338.30 348.40 10.1 2.9 1.62 0.48 Accretion Accretion Lagoon expansion of cuspate shoreline. Embayment deposition
Pingelap Atoll
Deke 62 59.90 60.61 0.70 1.2 0.11 0.19 Accretion Erosion General stability slight northward movement.
Sukoru 62 5.84 5.74 0.10 −1.7 −0.02 −0.27 Erosion Accretion Lagoonward migration. Spit extension into lagoon
Pingelap
b
62 127.00 125.0 2.00 −1.2 −0.24 −0.19 Erosion Stable Accretion NW tip and north coast
Mokil Atoll
Mwandohn 62 26.19 27.40 1.20 4.6 0.19 0.74 Stable Accretion Lagoonward expansion. Pronounced extension of N and S end
of island
Kahlap
b
62 55.90 57.80 1.90 1.6 0.15 0.26 Erosion Accretion Lagoonward migration. Extension of N and S end of island
Uhrek 62 51.50 52.90 1.40 2.7 0.23 0.44 Erosion Accretion Lagoonward migration. Movement of NE end, embayment infilling
a
Denote densely populated and urbanised islands.
b
Denote islands with small rural villages.
6A.P. Webb, P.S. Kench / Global and Planetary Change xxx (2010) xxx–xxx
Please cite this article as: Webb, A.P., Kench, P.S., The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis
of island change in the Central Pacific, Global and Planetary Change (2010), doi:10.1016/j.gloplacha.2010.05.003
minor erosion/migration on the northern and western (lagoon) points.
Localised accretion was detected on the lagoon shore (Fig. 6B).
The islands of Pingelap atoll have remained relatively stable over
the 62 yr of analysis with changes in island area all less than 2%
(Table 2,Fig. 7). There is evidence that the islands of Pingelap (main
island) and Sukoru both exhibited erosion of the ocean shoreline and
accretion of the lagoon coastline (Fig. 7C and D). In contrast, lagoon
erosion and oceanside accretion is evident on Deke (Fig. 7B).
Over a similar 62 yr window of analysis the islands on Mokil also
show a minor amount of change. However, in each case the change is
an increase in island area from 1.6% on Kahlap to 4.6% on Mwandohn
(Fig. 8B and C). While the percentages are small they represent the net
addition of more than 1 ha of land on each island (Table 2). The
islands show evidence of localised erosion of the ocean shoreline.
Mwandohn and Uhrek both show lagoon accretion while Kahlap and
Mwandohn also exhibit extension of the eastern and southern ends of
islands respectively (Fig. 8C and B).
5. Discussion
Results show that all islands have undergone physical change over
the respective timeframes of analysis and over the period in which the
instrumental records indicate an increase in sea level. The data
indicate that islands have undergone contrasting morphological
Fig. 5. Changes in reef island planform characteristics 1984–2003 for selected study islands Funafuti atoll, Tuvalu.
7A.P. Webb, P.S. Kench / Global and Planetary Change xxx (2010) xxx–xxx
Please cite this article as: Webb, A.P., Kench, P.S., The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis
of island change in the Central Pacific, Global and Planetary Change (2010), doi:10.1016/j.gloplacha.2010.05.003
adjustments over the period of analysis. Furthermore, the magnitude
and styles of island change show considerable variation both within
and between atolls in the study.
5.1. Net change in island area
The total change in area of reef islands (aggregated for all islands in
the study) is an increase in land area of 63 ha representing 7% of the
total land area of all islands studied. The majority of islands appear to
have either remained stable or increased in planform area (86%).
Forty-three percent of islands have remained relatively stable (b±3%
change) over the period of analysis. A further 43% of islands (12 in
total) have increased in area by more than 3%. The remaining 15% of
islands underwent net reduction in island area of more than 3%.
Of the islands that show a net increase in island area six have
increased by more than 10% of their original planform area. Three of
these islands were in Funafuti; Funamanu increased by 28.2%, Falefatu
13.3% and Paava Island by 10% (Table 2). The remaining three islands
are in Tarawa atoll with Betio, Bairiki and Nanikai increasing by 30%,
16.3% and 12.5% respectively over the 60 yr period of analysis
(Table 2). Of note, the large percentage change on Betio represents
an increase of more than 36 ha.
There appears to be no relationship between island area and the
direction and magnitude of island change (Fig. 9). Islands with
increases of more than 10% in area are all greater than 1 ha, while the
island with the largest increase (Betio) had an initial area of
approximately 120 ha. Consequently the percentage increase repre-
sents a large absolute increase in land area.
Only one island has shown a net reduction in island area greater
than 10%. Tengasu is located on the southwest atoll rim of Funafuti
and decreased in area by 14% over the 19 yr period of analysis.
However, closer examination of the Tengasu data shows that it was
the smallest island in the study sample (0.68 ha) and the absolute
change in island area was 0.1 ha, which represents a substantial
proportion of the total island area. Of note, approximately 78% of
islands exhibited changes in island area greater than 0.1 ha (Table 2).
Fig. 6. Changes in reef island planform characteristics for selected study islands on Tarawa atoll, Kiribati. B) Buariki and E) Betio 1943–2004. C) Nanikai and D) Bairiki 1969–2004.
8A.P. Webb, P.S. Kench / Global and Planetary Change xxx (2010) xxx–xxx
Please cite this article as: Webb, A.P., Kench, P.S., The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis
of island change in the Central Pacific, Global and Planetary Change (2010), doi:10.1016/j.gloplacha.2010.05.003
5.2. Net vs gross island planform change
The net changes in island area mask larger gross changes in island
planform configuration and location on the reef platform that have
occurred over the time periods of analysis. For example, on Fualifeke
in Funafuti (Fig. 5B), the eastern half of the island has migrated south
indicating up to 30% of island materials have been reworked over the
19 yr window of analysis. In another example, on the island of
Mulitefala (Funafuti, Fig. 5D) erosion on the oceanside of the island
has been compensated by progradation on the lagoon shoreline.
5.3. Styles of island planform change
Examination of gross changes in island planform across the
analysis period indicates that entire shorelines of islands may have
undergone positional adjustment. When individual shoreline changes
are aggregated to the island scale they represent adjustments in the
nodal position of islands on their reef surfaces. A number of styles of
island planform adjustment are evident and are summarised in
Table 3.
5.3.1. Ocean shoreline adjustments
Erosion of shorelines facing the ocean reef was detected in 50% of
islands examined. While in most cases this represented marginal
trimming of the island shoreline (often localised) in some cases it
produced up to 5–10 m of shoreline displacement. In the majority of
examples ocean shoreline erosion occurred on islands on the
windward margin of the atoll, which receives maximum oceanic
swell energy. Accretion of ocean shorelines was apparent on 30% of
islands examined. In nearly every instance such accretion occurred on
the leeward (non-exposed) margins of the atoll (e.g. Paava in
Funafuti, Fig. 5B; Deke in Pingelap, Fig. 7B).
5.3.2. Lagoon shoreline adjustments
Accretion of lagoon shorelines was detected in 70% of the islands
examined (Table 3). While accretion was generally on the order of 5–
Fig. 7. Changes in reef island planform characteristics for selected study islands 1944–2006 on Pingelap atoll, Federated States of Micronesia.
9A.P. Webb, P.S. Kench / Global and Planetary Change xxx (2010) xxx–xxx
Please cite this article as: Webb, A.P., Kench, P.S., The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis
of island change in the Central Pacific, Global and Planetary Change (2010), doi:10.1016/j.gloplacha.2010.05.003
10 m in some cases maximum lagoonward accretion was on the order
of 20–40 m (e.g. Fualifeke and Mulitefala, Fig. 5B and D; all islands in
Tarawa, Fig. 6).
5.3.3. Island migration
The aggregated effect of ocean shoreline displacement and lagoon
progradation is a shift in the nodal position of islands on reef surfaces.
Such movement, while small in magnitude, represents net lagoon-
ward migration of islands and was observed in 65% of islands studied.
This response is most evident on the windward margins of the atolls.
In only one case was an island found to have migrated toward the reef
edge (Paava, Funafuti, Fig. 5B). In this case the island was located on
the leeward reef rim of the atoll.
5.3.4. Contraction, expansion and extension
Table 3 identifies a number of other styles of island adjustment
which was observed in only a small number of islands. The extension
of the ends of elongate islands was observed in a number of islands
(∼33%). This is most clearly observed on Funamanu in which gravel
spits extended more than 100 m in 19 yr (Funafuti, Fig. 5C). A small
number of islands also exhibited expansion (accretion on all
shorelines) and contraction (erosion on all shorelines) in island area.
Fig. 8. Changes in reef island planform characteristics for selected study islands 1944–2006 on Mokil atoll, Federated States of Micronesia.
10 A.P. Webb, P.S. Kench / Global and Planetary Change xxx (2010) xxx–xxx
Please cite this article as: Webb, A.P., Kench, P.S., The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis
of island change in the Central Pacific, Global and Planetary Change (2010), doi:10.1016/j.gloplacha.2010.05.003
5.4. Mechanisms driving change
There are a number of mechanisms that may account for the
observed changes in atoll island planform configuration.
5.4.1. Change in boundary conditions: sea level and climate
Sea level rise has been implicated as a primary mechanism that
may promote erosion and complete destabilisation and loss of islands
in atoll environments (Dickinson, 1999; Barnett and Adger, 2003;
Kahn et al., 2002). Such assertions invoke sea level as the primary
control on island stability and persistence. In this model increased sea
level is expected to raise mean water depths across reef surfaces
allowing higher wave energy to propagate onto reef surfaces,
impacting and eroding island shorelines (Sheppard et al., 2005). The
projected morphological response is erosion of the ocean shoreline.
Such an adjustment assumes that atoll island shorelines are
positioned at an equilibrium distance across the reef flat surface
which reflects co-adjustment between relative water depth over the
reef, incident wave energy, reef width and sediment calibre (Kench
and Cowell, 2001; Kench et al., 2009b).
Resultsof this study show thata significant number of islands exhibit
ocean shoreline erosion (50%, Tables 2 and 3) which may reflect shore
readjustment to measured increased sea levels over the study period
(Fig. 4) and potentially increased wave energy incident at shorelines.
However, it is important to stress that such movement does not
necessarily result in a netreduction in island area.While ocean shoreline
erosion was prevalent (50% of islands) most islands (86%) displayed
either no change in area or an increase in area. These observations
suggest that in cases where there was no significant change in island
area, shoreline erosion and lagoon progradation was balanced via
reworking of the finite reservoir of sediment containedwithin islands. In
cases with significant increase in area, ocean shoreline erosion is likely
to have been compensated by larger lagoon progradation, which must
have occurred through additional inputs of sediment to the island
system. In both instances the islands migrated lagoonward on their reef
platforms. These observations are consistent with thoseof Stoddart et al.
(1982) who suggested that reef islands in Belize also displayed
lagoonward migration in response to rising sea level.
Similar migration of islands on reef surfaces has been identified by
Verstappen (1954) in the Indonesian seas and Flood (1986) in the
Great Barrier Reef. In these examples, decadal-scale changes in
prevailing wind systems and their influence on wave propagation
(direction and energy) have been implicated in island migration.
Indeed, Solomon and Forbes (1999) implicate inter-annual El-Nino
Southern Oscillation variations and their control on the wind and
wave regime as a control on erosion and accretion patterns in Kiribati.
Kench and Brander (2006) also identified rapid morphological
adjustment of reef island shorelines, and consequently island location
on reef platforms in the Maldives in response to monsoonal variations
in incident wind and wave energy.
Storms and hurricanes have also been shown to have both
constructional and erosional impacts on reef sedimentary landforms
with the contrasting responses reflectingdifferences in storm frequency
and texture of island building materials (Bayliss-Smith, 1988). In
settings with low storm frequency, landforms are typically composed of
sand-size sediments, which are susceptible to erosion during extreme
events (Stoddart, 1963). However, in reef settings with high storm
frequency, islands are commonly composed of rubble on their exposed
margins. In such settings, large volumes of rubble can be generated in
single events from coral communities on theouter reef and contributeto
island accretion (Chivas et al., 1986; Hayne and Chappell, 2001). In
Tuvalu, Hurricane Bebe in 1972 deposited an extensive storm rubble
rampart onto the reef flat of Funafuti atoll (Maragos et al., 1973).
Subsequent storms have reworked this rampart onto island shorelines
(as described by Baines and McLean, 1976) accounting for the increases
in island area along the eastern reef rim of Funafuti (Table 2;Fig. 5). In
particular, extension of the ends ofthe elongate islandsis a consequence
of onshore and alongshore transport of sediment (Fig. 5C).
5.4.2. Anthropogenic modification
As identified in Table 2 a number of the study islands contain human
settlements. Those islands with small villages have exhibited small
variations in island area. However, the most densely settled islands in
the study arelocated in South Tarawa (Betio, Bairiki and Nanikai). These
islands have all experienced an increase in island area greater than 10%
over the period of analysis with a decadalrate of increase of between 3.5
and 4.8%. Indeed Betio increased in area by 30% (36 ha) and Bairiki by
16.3% (5.8 ha). This expansion in island area has occurred over a time
period in which the shoreline has undergone significant modification
and change in coastal processes. The shoreline has numerous coastal
structures including seawalls, groynes and minor reclamations that all
promote disruption to coastal processes. Causeways have also been
inserted between islands along the southern atoll rim. The causeways
block ocean to lagoon fluxes of water and sediment. Consequently,
nearshore current and littoral drift processes have been altered
promoting extension of the shoreline along causeways and increasing
island area. Anthropogenic modification of coral reefs and island
shorelines have been identified in a number of studies and are generally
associated with negative environmental outcomes with regard reef
health and shoreline erosion (Brown and Dunne, 1988; Maragos, 1993;
Fig. 9. Relationship between decadal change in reef islands (%) and reef island area for
study islands.
Table 3
Summary of physical adjustments of reef island shorelines.
Atoll Shoreline adjustment Island adjustment
[No. of islands] Ocean erosion Ocean accretion Lagoon erosion Lagoon accretion Spit extension Contraction Expansion Lagoon migration Ocean migration
Funafuti [17] 9 4 5 11 3 4 –12 1
Tarawa [4] –4–43–4––
Mokil [3] 2 –– 31––3–
Pingelap [3] 2 1 1 1 2 ––2–
Total 13 9 6 19 9 4 4 17 1
11A.P. Webb, P.S. Kench / Global and Planetary Change xxx (2010) xxx–xxx
Please cite this article as: Webb, A.P., Kench, P.S., The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis
of island change in the Central Pacific, Global and Planetary Change (2010), doi:10.1016/j.gloplacha.2010.05.003
Sheppard et al., 2005). However, results presented in this study show
that anthropogenically modified shoreline processes can also contribute
to land building in atoll environments.
5.5. Implications for vulnerability assessments
The findings challenge the conventional frame of reference for
considering shoreline adjustment in small reef island settings and for
approaches to evaluate vulnerability. Typically assessments of
shoreline change adopt 2-dimensional across-shore methods that
are applied to single locations on island shorelines. In particular, the
Bruun Rule is commonly advocated as an appropriate tool to assess
coastal change (UNEP). This simple geometric profile model implies
coastlines will migrate landward due to erosion and the relative
extent of erosion is a direct function of the magnitude of sea-level rise
and gradient of the coast. Whilst numerous studies have critiqued the
use of this model for predicting shoreline change in response to sea
level rise on sandy shorelines (e.g. Cooper and Pilkey, 2004) its
continued use and advocacy at an international level is perplexing and
ultimately misleading.
The physical characteristics of reef island coastlines confound the
assumptions of the Bruun rule and render any results highly
questionable (Cowell and Kench, 2001). First, reef islands are typically
low in elevation and experience wave overtopping during storms and
high energy wave events. Second, the presence of horizontal and non-
erodable reef flat surfaces truncates the active beach. Third, atoll
island shorelines have a 360° perimeter rather than a linear planform
configuration as is common in siliciclastic settings. Consequently,
alongshore sediment transport processes dominate shoreline change.
Results presented in this study show that the entire footprint of
islands are able to change so that erosion at the local scale (on one
aspect of an island) may be offset by accretion on other parts of the
coastline. This change was recognised by Kench and Brander (2006)
who suggested that the ‘sweepzone’(that demarcates the envelope of
coastal change) on reef islands occurs through alongshore reorganisa-
tion of sediment as opposed to the across-shore exchange of sediment
that characterises siliciclastic shorelines. Recognition of the along-
shore adjustment of island shorelines suggests that local scale analysis
of two dimensional shoreline adjustment using tools such as the
Bruun Rule are subject to significant error when extrapolated to the
island scale. Consequently, assessments of island change must be
evaluated at the ‘whole island’scale involving analysis of potential
change in the entire island perimeter.
6. Conclusions
The future persistence of low-lying reef islands has been the
subject of considerable international concern and scientific debate.
Current rates of sea level rise are widely believed to have destabilised
islands promoting widespread erosion and threatening the existence
of atoll nations. This study presents analysis of the physical change in
27 atoll islands located in the central Pacific Ocean over the past 20 to
60 yr, a period over which instrumental records indicate an increase
in sea level of the order of 2.0 mm yr
−1
.
The results show that island area has remained largely stable or
increased over the timeframe of analysis. Forty-three percent of
islands increased in area by more than 3% with the largest increases of
30% on Betio (Tarawa atoll) and 28.3% on Funamanu (Funafuti atoll).
There is no evidence of large-scale reduction in island area despite the
upward trend in sea level. Consequently, islands have predominantly
been persistent or expanded in area on atoll rims for the past 20 to
60 yr.
Persistence of reef islands does not necessarily equate to geo-
morphic stability and the results also show that despite small net
changes in island area most islands have experienced larger gross
changes. The results show that reef islands are morphologically
dynamic features that can change their position on reef platforms (e.g.
lagoon migration) at a range of timescales. Characteristic planform
adjustments in islands include: ocean shoreline erosion, lagoon
progradation and, lateral extension of elongate islands. Mechanisms
driving these observed changes are varied and can include a
combination of sea-level rise, decadal-scale variations in wind and
wave climate and anthropogenic impacts. Aggregated to the island
scale these shoreline changes indicate that islands have adjusted their
nodal position on reef surfaces. Over 65% of islands examined have
migrated toward the lagoon (away from the reef edge) across the
period of analysis.
Of significance, the results of this study on atoll islands are
applicable to islands in other reef settings, as the boundary controls on
island formation and change are comparable. Results of this study
contradict widespread perceptions that all reef islands are eroding in
response to recent sea level rise. Importantly, the results suggest that
reef islands are geomorphically resilient landforms that thus far have
predominantly remained stable or grown in area over the last 20–
60yr. Given this positive trend, reef islands may not disappear from
atoll rims and other coral reefs in the near-future as speculated.
However, islands will undergo continued geomorphic change. Based
on the evidence presented in this study it can be expected that the
pace of geomorphic change may increase with future accelerated sea
level rise. Results do not suggest that erosion will not occur. Indeed, as
found in 15% of the islands in this study, erosion may occur on some
islands. Rather, island erosion should be considered as one of a
spectrum of geomorphic changes that have been highlighted in this
study and which also include: lagoon shoreline progradation; island
migration on reef platforms; island expansion and island extension.
The specific mode and magnitude of geomorphic change is likely to
vary between islands. Therefore, island nations must better under-
stand the pace and diversity of island morphological changes and
consider the implications of island persistence and morphodynamics
for future adaptation.
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13A.P. Webb, P.S. Kench / Global and Planetary Change xxx (2010) xxx–xxx
Please cite this article as: Webb, A.P., Kench, P.S., The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis
of island change in the Central Pacific, Global and Planetary Change (2010), doi:10.1016/j.gloplacha.2010.05.003
A brief on the peer reviewed, scientific publication.
The dynamic response of reef islands to sea level rise: evidence from multi-decadal
analysis of island change in the Central Pacific.
Webb & Kench, 2010. Global & Planetary Change (accepted).
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Background and Methods.
This research paper describes the results of investigation into historical trends of atoll
island shoreline change using historical aerial photography compared over time - the
paper does not predict future response of shorelines and only documents past response.
Depending on available records the studies look back in time from 20 to 60 years and
covers atoll islands from Federated States of Micronesia, Kiribati and Tuvalu. It is the first
research in the world to address, from a holistic empirical perspective, how atoll shorelines
have been responding to sea level rise over the past half centaury.
Records and estimations of the rate of sealevel rise are available in the literature but there
is much variability. The best estimates for the central pacific which correspond to the
period covered by the historical images suggest there has been approximately 120mm of
sealevel rise over this period. Whilst rates of sea level rise have accelerated over the last
centaury, the rates reviewed in this paper are in agreement with historical rates considered
by IPCC 2007.
Historical aerial photographs exist for some Pacific Islands from the early 1940’s to the
present and these offer an invaluable opportunity to look back in time to understand how
islands have responded to historical sea level rise up until present times. The historical
photos are compared using highly accurate satellite images which when processed and
combined in computer mapping systems (GIS), allow measurement of change in shoreline
position with accuracy and confidence. Where possible, field trips were also undertaken to
confirm these results.
The Results.
The results tell a remarkable story of atoll island shoreline stability and resilience over the
last 20 to 60 years and this was unexpected given accompanying sea level rise of around
120mm. To understand if the total land area of each island was getting larger or smaller in
area, the entire island area was measured at different historical times. The analysis
included all erosive, accreting (building) and stable beaches (not just individual beaches).
The results highlighted examples of both island accretion (islands increasing in area) and
erosion (islands getting smaller in area), however when all of the data was combined the
average rate of island area change showed that 86% of islands had remained relatively
stable or increased in area and only 14% of islands had eroded or become smaller in total
land area.
There was also some evidence that showed islands are tending to erode more on their
ocean side shores and are accreting more on their lagoon side shores however this was
very variable. It is not understood if this possible pattern of shifting away from the ocean
and towards the lagoon may be a response to sea level rise but it is possible. It will be
extremely important to continue monitoring this possible trend as such information would
be a critical component of adaptation response and planning on these islands (at least in
the immediate term). The islands studied also included both inhabited and uninhabited
islands to try and reveal any relationship between human activities and natural processes;
urbanised islands tended to have large increases in area which was partly due to
engineering however some of the largest increases in area also occurred in uninhabited
islands with no engineering.
The future?
Whilst the shorelines of these atolls appear to have responded to historical rates of sea
level rise and the average change in island land area has so far been positive, it is not
known how long this trend will continue or if this pattern is similar across the whole Pacific
region. There are a range of other climate change stresses which may disrupt shoreline
processes; e.g. sea surface temperature increase, ocean acidification, accelerating rates
of sea level rise and possible changes in storm frequency or intensity. Any of these or a
combination may result in overwhelming the present resilience of these shorelines and if
such a “tipping point” is reached, these atoll shorelines could become rapidly erosive.
Now that these images have been processed they form the baseline (starting point) for on-
going monitoring. Every time new aerial photography or satellite imagery is acquired or
available for any of these established islands it can be added to this system to provide up
dates on how shorelines are continuing to respond. We will be able to detect if there are
dramatic changes or other trends such as a gradual shifting of the island towards the
lagoon (i.e. eroding on the ocean side coast and accretion (building) on the lagoon side.
This form of monitoring can also highlight urban impacts such as beach mining or
inappropriate coastal engineering which tends to cause intense localised erosion or
disturbance. It can also highlight unstable areas informing development planning.
Does this information change the vulnerability of the atolls to sea level rise?
This study did not measure vertical growth of the island surface nor does it suggest there
is any change in the height of the islands. Since land height has not changed the
vulnerability of the greater part of the land area of each island to submergence due to sea
level rise is also unchanged and these low-lying atolls remain immediately and extremely
vulnerable to inundation or sea water flooding.
Why is the data useful?
This information and data is crucially important for Governments and atoll communities,
technical authorities, advisers and others who implement shoreline adaptation or
development projects in the pacific atolls. Accurate information informs appropriate
adaptation responses and planning in the shoreline zone. Previous assumptions that all
atoll shorelines were erosive or alternatively that all erosion problems are caused by sea
level rise are inaccurate and cripple our ability at a technical level to effectively design and
implement appropriate shoreline protection and adaptation responses.
This study underlines the importance of protecting resilient shorelines from other human
impacts; e.g. beach sand and gravel mining, poor engineering, reclamation, etc. Since we
still have many functional shoreline systems, priority should be given to their protection
from disturbance so that can continue to provide better protection from waves for a long as
possible.
These shoreline monitoring efforts are ongoing; they are currently the only sustained
efforts of their type in the world providing accurate information on shoreline response to a
range of stresses in central pacific atolls.
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