Coastal flooding by tropical cyclones and sea-level rise

Abstract

The future impacts of climate change on landfalling tropical cyclones are unclear. Regardless of this uncertainty, flooding by tropical cyclones will increase as a result of accelerated sea-level rise. Under similar rates of rapid sea-level rise during the early Holocene epoch most low-lying sedimentary coastlines were generally much less resilient to storm impacts. Society must learn to live with a rapidly evolving shoreline that is increasingly prone to flooding from tropical cyclones. These impacts can be mitigated partly with adaptive strategies, which include careful stewardship of sediments and reductions in human-induced land subsidence.

Full text

1Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA. 2Civil and Environmental Engineering, Virginia Tech, Blacksburg 24061, Virginia, USA. 3Lamont–
Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA.
F
looding in the context of future storm variability, sea-level rise and
shoreline change is one of the most important issues facing coastal
populations today. In the regions they affect, tropical cyclones are
often the most damaging storms and, therefore, of primary importance
when assessing flood risk. It is clear that coastal populations are becom-
ing more prone to extreme flooding from tropical cyclones
1
. There is
also growing evidence for a future shift in the average global intensity of
tropical cyclones towards stronger storms2. Although both of these two
points are probably true, most researchers would agree that linking the
two in terms of cause and effect is in many ways incorrect.
First, significant uncertainty exists as to how tropical cyclone activ-
ity will vary regionally, particularly with respect to landfalling storms.
Second, the level of regional tropical cyclone activity is just one of the
factors that drives the magnitude and frequency of tropical cyclone
flooding. For example, the Western North Pacific has been the most pro-
lific tropical cyclone basin over the instrumental record, both in terms of
the overall number of tropical cyclones (30% of global activity) and peak
tropical cyclone wind intensities (Fig.1). However, in recent decades
this basin accounted for neither the majority of economic nor human
losses from tropical cyclones. These records have been held by two of
the least active tropical cyclone basins, the North Atlantic (10% of global
tropical cyclone activity) and North Indian Ocean (5% of global tropical
cyclone activity), respectively. Since 1970, around 65% of all lives lost as
a result of tropical cyclones occurred within the North Indian Ocean —
equivalent to more than half a million deaths
3
. Over this same period,
more than 60% of all economic losses from tropical cyclones took place
in the North Atlantic — amounting to around US$400billion3.
Although tropical cyclone activity is relatively low in the North Indian
Ocean and the North Atlantic, the frequency of coastal flooding is not.
Extreme flooding is prevalent mainly on low-gradient shores, includ-
ing barrier and deltaic systems; these areas have often also attracted the
development of dense population centres. Low-lying coasts are typi-
cally composed of soft sediments and are particularly dynamic, with
geometries that greatly enhance storm impacts. For these evolving
shores, storms provide the dominant mechanism of extreme flooding
and erosion — although in this Review we discuss how it is often sea-
level rise (SLR) that is the underlying cause of both increasing rates of
long-term shoreline retreat and flood frequency. Human factors are of
equal importance in terms of influencing coastal impacts by tropical
cyclones
1,4,5
, but this topic is beyond the scope of this Review. However,
at the root of these human factors is the flood-prone landscape on which
coastal populations have developed. In these settings, joint considera-
tion of tropical cyclone climatology, relative SLR and shoreline change
is crucial for accurate assessments of future flood risks. We focus this
Review on these three physical factors, highlighting that rising sea levels
will become a dominant driver of increased tropical cyclone flooding
irrespective of changes in tropical cyclone activity. We point to popu-
lation centres most at risk of tropical cyclone impacts — those that
are mainly located along dynamic and subsiding sedimentary coasts
that will serve to further enhance the impact of future tropical cyclone
floods. Finally, we discuss managing risk in the context of an almost
certain increase in tropical cyclone flood frequency, and the importance
of using a holistic approach to manage coastal systems.
Tropical cyclone climatology
On average, about 90tropical cyclones occur worldwide per year,
with the annual distribution of these events varying among the vari-
ous tropical cyclone basins
6
. Only about one-fifth of tropical cyclones
make landfall with the intensity of a hurricane (defined by wind speeds
≥ 33 ms
−1
), but coastal impacts by tropical cyclones are due largely to
this important subset of storms
7
. Accumulated cyclone energy (ACE)
is a common metric for comparing the overall tropical cyclone activity
of different tropical cyclone regions; it is calculated by taking the sum
of each tropical cyclone’s maximum wind speed squared for all storms
passing through a selected area. Storm surge is also related to wind speed
squared (discussed later), thus ACE is a useful measure of both tropical
cyclone activity and tropical cyclone surge potential, all else being equal
(for example, ignoring the configuration of a coastline and bathymetry).
Spatial variability in ACE highlights anomalously high levels of tropical
cyclone activity in the North Pacific, relative to the substantially lower
levels of activity within the other tropical cyclone regions (Fig.1a).
Environmental influences
A warm upper ocean, represented by sea surface temperature (SST),
is one of the requirements for tropical cyclone formation and inten-
sification, as is evident by the modulation of tropical cyclone activity
in response to the seasons
8
. All else being equal, SST directly relates to
the theoretical maximum wind speed that tropical cyclones can reach
under specific local environmental conditions
9
. This theoretical maxi-
mum wind speed, or potential intensity (PI), is also inversely related to
The future impacts of climate change on landfalling tropical cyclones are unclear. Regardless of this uncertainty, flooding
by tropical cyclones will increase as a result of accelerated sea-level rise. Under similar rates of rapid sea-level rise during
the early Holocene epoch most low-lying sedimentary coastlines were generally much less resilient to storm impacts.
Society must learn to live with a rapidly evolving shoreline that is increasingly prone to flooding from tropical cyclones.
These impacts can be mitigated partly with adaptive strategies, which include careful stewardship of sediments and
reductions in human-induced land subsidence.
Coastal flooding by tropical
cyclones and sea-level rise
Jonathan D. Woodruff1, Jennifer L. Irish2 & Suzana J. Camargo3
44 | NATURE | VOL 504 | 5 DECEMBER 2013
REVIEW doi:10.1038/nature12855
© 2013 Macmillan Publishers Limited. All rights reserved

the outflow temperature where rising air exits a tropical cyclone. The
difference between the observed distribution and intensity of tropical
cyclone activity (Fig.1a, b) and PI (Fig.1c) is due to other environ-
mental factors that are also important in determining tropical cyclone
frequency10,11 . For example, in the South Atlantic tropical cyclones are
scarce (Fig.1b), despite having a relatively high PI (Fig.1c). Wind speed
in the South Atlantic varies greatly with height in the troposphere (high
values of vertical wind shear), which is one important reason for tropical
cyclone scarcity in the basin. High vertical wind shear is also a central
mechanism for inhibiting tropical cyclone frequency and intensity in
the other tropical cyclone regions10,12,13. The amount of humidity in
the atmosphere and the presence of pre-existing disturbances, in the
form of atmospheric waves and storms that are precursors for tropical
cyclone formation also have an important influence on tropical cyclone
frequency. All of these factors should be taken into account in future
tropical cyclone projections.
Future projections
At the end of the twenty-first century there will probably be fewer, but
stronger, storms globally
2
. However, the magnitude range for these pre-
dicted changes is still wide, because the different models used to make
these projections exhibit different sensitivities to climate change. For
example, projections for changes in the number of tropical cyclones
range from −6 to −34% globally, with increases in mean tropical cyclone
global wind speed ranging between 2to11% by the end of the twenty-
first century
2
. Significantly greater uncertainty exists with respect to
how tropical cyclone activity will vary regionally, with projected changes
up to ±50% in the number of tropical cyclones in individual tropical
cyclone basins by the end of the twenty-first century2. Similarly, not all
ocean basins may experience an increase in tropical cyclone intensity.
A statistical downscaling of the tropical cyclone projections of the Cou-
pled Model Intercomparison Project Phase 5 (CMIP5) shows a probable
increase in tropical cyclone frequency in the first half of the twenty-
first century in the North Atlantic, but the trends in North Atlantic
tropical cyclone frequency by the end of the twenty-first century are
still uncertain
14
. By contrast, North Atlantic tropical cyclone intensity
is projected to increase in all climate scenarios by the end of the twenty-
first century15.
Modes of climate variability such as the El Niño–Southern Oscillation
(ENSO) and the Madden–Julian Oscillation (MJO) can have a strong
regional influence on tropical cyclone frequency and intensity
10,12,16,17
,
and current uncertainties in ENSO and MJO have contributed to
the difficulties in obtaining robust global-specific and basin-specific
projections18,19. A number of other natural climate modes on various
Figure 1 | Global tropical cyclone activity for the period 1981–2010. a,
Accumulated cyclone energy (ACE). In the Northern Hemisphere, ACE is
highest in the western and eastern North Pacific, with lower values in the
North Atlantic and Indian Oceans. In the Southern Hemisphere, ACE is
highest in the South Indian Ocean. b, Historical tropical cyclone tracks.
Tracks of intense tropical cyclones concentrate in the western and eastern
North Pacific regions, with fewer occurring in the North Atlantic and
Southern Hemisphere. Colour scale refers to intensities of tropical cyclone
tracks. c, Potential intensity for the western North Atlantic and eastern
North Pacific87, western North and South Pacific and Indian Ocean88, and
South Atlantic89. Colour scale is the same as in b and refers to potential
intensity wind speed contours. In the North Atlantic and eastern North
Pacific, tropical cyclones with maximum 1minute sustained wind speeds in
excess of 33 ms−1 are classified as hurricanes, whereas in the western North
Pacific storms meeting this same criterion are called typhoons, and in the
Southern Hemisphere they are called severe tropical cyclones. Hurricanes
with wind speeds in excess of 50 ms−1 are defined as major hurricanes
(Categories 3–5).
c
40º S
20º S
0
20º N
40º N
140º E 100º E 60º E 180º W 140º W 100º W 60º W 20º W 20º E
40º S
20º S
0
20º N
40º N
a
b
40º S
20º S
0
20º N
40º N
0.5
0.4
0.3
0.2
0.1
0
×104
0
18
33
43
50
58
70
Wind speed (ms–1)
ACE (ms–1)2
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timescales also influence tropical cyclones in different regions
20
, and
are a source of additional uncertainty. Furthermore, future changes
in hybrid storm frequency, including tropical cyclones that undergo
extratropical transition, such as Hurricane Sandy in 2012, are largely
unknown21.
Currently, modes of climate variability, including ENSO and MJO,
explain roughly 30–45% of tropical cyclone activity variance within
the instrumental historical record6. The percentage is much less,
however, when considering only storms that make landfall. Further-
more, although these modes of climate variability modulate landfall
probabilities in large regions, exact landfall locations are determined
by storm tracks, and there is significant variability in tracks both
season-to-season and within a single season
22
. Landfall probabilities
are often described as a stochastic process given the high uncertainty
associated with local tropical cyclone activity, particularly on shorter
timescales (Box1).
Sea-level rise and tropical cyclone flooding
Global sea level is expected to rise in the upcoming centuries, with a
mean global increase that could approach or exceed 1m by 2100 (ref.23).
SLR is also expected to continue to accelerate through the twenty-first
century. Relative SLR at individual sites will vary from this global aver-
age
24
; however, in general, densely populated regions affected by coastal
flooding from tropical cyclones have experienced a rate of SLR near or
greater than the global average over the instrumental record (Fig.2).
Before the satellite era, instrumental records of SLR are mostly
derived from tide gauges, which record long-term sea-level trends,
as well as the sudden rise in water level associated with storm events.
Analyses of these time series indicate an increase in extreme high water
levels worldwide since 1970, with this increase due almost exclusively
to SLR rather than changes in storm climatology25. Longer tide-gauge
records along the East Coast of the United States reveal similar results26.
However, tide gauge data alone is generally too short to obtain mean-
ingful extreme value statistics
27
, with derived probabilities that do not
account for future, potentially higher, magnitude changes in both sea-
level and tropical cyclone activity.
Controls on flooding
Storm surge induced by tropical cyclones depends greatly on coastal
geometries, including topography, local shoreline configurations and
depth, and individual tropical cyclone characteristics — predominantly
the wind speed, storm size and landfall location. The storm’s forward
motion, angle of approach, and atmospheric pressure drop also influ-
ence surge generation. Tidal range and storm timing with the tide; the
increase in water level, owing to the presence and local behaviour of
shoaling waves; and river discharge and rainfall-driven runoff also
contribute to flooding. However, in coastal regions that experience
the most extreme tropical cyclone flooding, the greatest elevated water
levels are largely due to wind-driven storm surge. Using a linearized
momentum conservation argument, for which bottom friction and
other external forces are neglected, it can be shown that wind surge is
proportiona l to:
where U is wind speed, W is the distance over which the wind blows in
the same direction, and h is the mean depth over the region where the
wind blows28. As equation (1) indicates, wind-driven surge is mainly
generated in relatively shallow depths, and where shallow waters extend
far offshore. Thus, areas with a relatively broad and shallow continental
shelf, such as the western North Atlantic, generally have larger wind-
driven surge than areas where offshore slopes are steep, such as the
mountainous islands of the western North Pacific and the Caribbean
(Fig.3). However, deltaic low-lying coasts along otherwise steep, less
habitable terrain are also particularly susceptible to enhanced tropical
cyclone flooding — for example, many of the large population centres
in the Bay of Bengal, and sites of growing vulnerability in the western
North Pacific29,30 (Fig.3b).
Equation (1) also shows that storm surge is expected to increase with
the square of tropical cyclone wind speed. As an example, if tropical
cyclone wind intensity for a given tropical cyclone increases by 4% for
each degree Celsius of SST warming31,32, from equation (1) we can expect
wind surge to increase by 8% for each degree Celsius of SST warming.
Damage from storm winds is related to the wind speed cubed, thus
compounding impacts related to warming SST
33
. However, the approxi-
mation for tropical cyclone intensification as a function of warming
SST neglects key meteorological influences, which have been discussed
previously, including humidity, winds and atmospheric temperature.
Future projections
Coastal flooding probability associated with landfalling tropical
cyclones depends both on the probability of tropical cyclone occur-
rence and the behaviour of relative sea level. Accurate predictions of
future flood risk, therefore, must consider the two jointly. The specific
role of SLR and the potentially higher occurrence of intense storms
in future tropical cyclone flooding have been the focus of a number
of recent studies
34–39
. Many studies assume that tropical cyclone surge
and SLR are independent, thus the two may be linearly summed: flood
elevation equals surge plus SLR. This approach is a relatively simplistic
means of obtaining a global forecast of changes in extreme flood prob-
abilities and associated risk to coastal populations
34,35
. Although SLR
Evaluating changes related to tropical cyclone impacts requires a general understanding of the statistical metrics conventionally used for their
assessment. The likelihood of winds or flood levels exceeding a threshold is often presented either as the probability of occurrence in a particular
year, or with a return period equal to the inverse of this annual probability. For example, a 1% probability of winds or floodwaters exceeding a
certain level in any year is equivalent to the event having a 100-year return period. Another useful metric of hazard exposure is the chance that a
certain extreme event will be exceeded over a specified interval of time:
R = 1 − (1 − Q)T
where R is the chance of an event with an annual exceedance probability of Q occurring over the time period T99. This relationship reveals
that there is a 63% chance of a 100-year event (Q = 1.0%) occurring in the next 100years, and a 10% chance of a 1,000-year event (Q = 0.1%)
occurring in the next 100 years. This 10% is still fairly high and serves to highlight why coastal planners often consider events with return periods
well beyond the time frame of interest, particularly with respect to sensitive infrastructure. However, the probability of these low-frequency events
are the most difficult to constrain, particularly in the context of changes to tropical cyclone climatology.
BOX 1
Tropical cyclone probabilities
Uh
W
2(1)
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rates, storm intensification, and time periods differ among studies, the
general consensus is for an increase in future extreme flood elevations.
More sophisticated techniques that include a hydrodynamic mod-
elling component directly consider non-linearities between SLR and
storm surge
36,37,39,40
. Simulations in surge-prone Bangladesh were among
the first numerical studies to consider both SLR and a potential increase
in the tropical cyclone occurrence
36
. Results show that projected SLR by
the 2050s, along with the increased occurrence of intense storms, may
inundate up to 15% of the country and could result in a 12% rise in water
levels by extreme events. In a more recent study along the coastline
of Cairns, Australia, the 100-year return period of a flood event was
decreased to a 40-year event using statistically generated storms for
the 2050s, along with 0.2 m of SLR and a 10% increase in storm wind
speeds
37
. To assess the combined impact of SLR and changes in tropi-
cal cyclone activity for the Atlantic basin a modified joint probability
method has been proposed
38
. For the fourth Intergovernmental Panel
on Climate Change (IPCC) “middle-of-the-road” scenario (A1B) on
an idealized coast, this study projects the present-day 100-year return
period flood elevation becoming the 60-year event by the 2050s. All of
the above mentioned results are for relatively moderate rates of SLR by
the 2050s and do not account for the more rapid rates of SLR projected
for the latter half of the twenty-first century (Fig.2).
Enhanced rates of relative SLR in regions of rapid land subsidence
will further amplify tropical cyclone flooding. This enhanced subsid-
ence is common along populated deltaic and coastal plain systems owing
to groundwater, oil and gas extraction, and reductions in fluvial sedi-
ment supply. Megacities where past rates of human-induced subsidence
exceeded an average of 1 cm yr−1 include Osaka, Japan (2.8 m of subsid-
ence between 1935 and 1995); Manila, Philippines (>1 m of subsidence
between 1991 and 2003); Tainjin, China (3.1 m of subsidence between
1959 and 2003); and Tokyo, Japan (5 m of subsidence between 1930 and
1995)
41,42
. Shanghai, China, is one of the largest megacities that could
potentially be affected by elevated rates of relative SLR (2.8m of subsid-
ence between 1921 and 1995)42. Here a 4.3 m projected rise in sea level
due to additional land subsidence along the Yangtze River delta by 2100
would result in half of Shanghai being flooded by extreme storm-water
levels43. Similar increases in tropical cyclone impacts are projected at
other locations where SLR rates are expected to significantly exceed the
global average — for example the Red River Delta, Vietnam
44
, and the
Mississippi Delta45. All of these conclusions assume that no counter-
measures are taken to alleviate artificial causes of land subsidence.
One of the most comprehensive projection studies of the combined
influence of recent SLR projections and future tropical cyclone climate
on storm surge assesses changes in flood probabilities in the New York
City region at the end of the twenty-first century
39
. In this study, a nested
modelling technique was used, combining output from global climate
model simulations with a physical tropical cyclone model to generate
synthetic tropical cyclone tracks for driving hydrodynamic storm surge
simulations. Results differ greatly depending on the climate model used,
with changes in the return frequency of tropical storms in the New
York region ranging from −15% to 290% by the end of the twenty-first
century. However, all simulations show increased flooding when a 1 m
rise in sea level is included, with the present-day 100-year return period
flood event reduced to the 3–20year event (Box2 discusses SLR and
flooding by Hurricane Sandy in 2012).
These studies highlight current uncertainties associated with future
changes in flood frequency that are linked with variability of landfall-
ing tropical cyclones. More importantly, however, they all point to the
clear increase in flood frequency associated with an accelerating SLR,
regardless of tropical cyclone climatology projections.
Shoreline dynamics
Recent results highlight the importance of relative SLR in increasing
coastal flood frequency34,35–39. However, the compounding effects of
future shoreline change are not accounted for in most of these assess-
ments. Potential changes in tidal regime may also be important
46
. Coast-
lines vary greatly in their morphology; however, broad low-lying regions
at the greatest risk of tropical cyclone flooding generally share the com-
monality of being fairly dynamic (Fig.3). These low-lying shores are
often built by mobile sediments (for example, barrier beaches and del-
taic coastlines) and/or by biogenic systems (for example, reefs, man-
grove wetlands and salt marshes) that are particularly susceptible to
climatic and anthropogenic stressors47–49. The frequency and intensity
of tropical cyclone flooding has been, and will continue to be, tightly
coupled to the morphological development of these coastal systems.
Geomorphic function of tropical cyclones
Tropical cyclones are natural phenomena that have greatly contributed
to the morphology of modern shorelines. In many cases, storms serve as
a construction mechanism. For instance, sands along the back of barrier
beaches are largely storm derived. Deposits from sediments overwash-
ing barrier islands might provide a key mechanism for determining
Figure 2 | Global sea-level trends. Local sea-level trends based on individual
tidal gauge records more than 50years old24,90. Green arrows indicate regions
where rates of SLR have been near the long-term global average, whereas red
and yellow indicate areas where SLR exceeds the global mean. For comparison,
arrows on the bottom right show (from left to right) the global instrumental
averages from 1900 to present, the projected average rate from present to 2100,
and the projected rate at 2100 (ref.23; see Fig.4b for SLR time series derived
from ref.23). Dashed lines outline regions of tropical cyclone activity defined
by ACE in Fig.1a. Spatial coverage is limited by the availability of long-term
tide gauge records. However, most of the key population centres affected by
tropical cyclones are focused in locations of rising sea level. For instance, by
2020, of the world’s top 30megacities 13 are projected to be along coasts affected
by tropical c yclones91 (see Fig. 3 for locations). With the exception of Chennai,
India, all of these population centres have experienced a rise in relative sea level
in recent decades, with rates at 10 of these 13 locations greater than the global
mean41,90,92,93. Figure adapted with permission from ref.94.
140º E 100º E 60º E 180º W 140º W 100º W 60º W 20º W
40º S
20º S
0
20º N
40º N
20º E
–3 to 0 0 to 3 3 to 6 6 to 10 >10
Sea-level trends (mm yr–1)
Global average
Projected
averages
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vertical accretion rates within back-barrier marshes50–52. Furthermore,
waves from distant tropical cyclones frequently mobilize offshore sedi-
ments that are normally unavailable for littoral transport, allowing this
material to be redistributed along the shore face and shallow shelf53.
Storms are also largely responsible for sediment redistribution across
barrier reef systems54, as well as the building of successive beach ridges
along seaward advancing or prograding coastlines (commonly referred
to as beach ridge plains)55.
Naturally, tropical cyclones also erode shorelines, and the building
of back-barrier environments often occurs at the expense of an eroding
foreshore
49,56
. Ultimately, this net transport of sediment from the fore-
shore to the backshore results in the landward retreat of the entire barrier
beach system through a barrier rollover mechanism49. Mechanisms of
shoreline retreat can be complex, with rates governed not only by SLR,
but also by sediment supply and the coastline’s pre-existing configuration
and lithology (geological or glacial inheritance). The opening of new
inlets by storms can also be particularly destructive to barriers because
this is often where the greatest loss of beach sediment is observed
57–59
.
Newly formed tidal inlet deltas act as a significant sink for beach sedi-
ments. The opening of new inlets can also measurably change tidal
exchange and allow ocean surges to more effectively propagate inland.
Thus, the surge hazard will be significantly greater if a tropical cyclone
occurs while a new inlet remains open. Similarly, inland areas become
more vulnerable to tropical cyclone surges through barrier island degra-
dation and inlet formation. Although wide and high barrier islands serve
as a natural surge impediment, degraded narrow, low barrier islands
readily allow overwash and breaching during tropical cyclones, leading
to increased surge levels behind these coastal barriers60.
140º E 100º E 60º E 180º W 140º W 100º W 60º W 20º W
40º S
20º S
0
20º N
40º N
20º E
10
4
6
0
–60
8
Elevation (m)
–30
30
2
Jakarta
Dhaka
Tokyo
Shanghai
Mumbai
(Bombay)
Karachi
Kolkata
(Calcutta)
Manila
Shenzhen
Guangzhou
Chennai
(Madras)
Osaka
Tianjin New York City
Madagascar
Australia
45º N
5º N
60º W
65º E
100º W
150º E
45º N
5º N
30º E 65º E 100º E 170º W
5º S
30º S
5° S
30° S
a
b c
d e
Figure 3 | Coastlines with broad low-lying elevations and shallow abutting
bathymetry. a, Regions where storm surge is enhanced by shallow depths
offshore are shown in pale blue, and low-lying regions generally at a greater
risk of coastal flooding are shown in red. Regions of tropical cyclone activity
defined by ACE (Fig.1a) are outlined by grey dashed lines in a. Broad regions
of low-lying topography and shallow near-shore bathymetry are a fairly good
proxy for dynamic and evolving low-gradient shorelines. b, The expansive
low-lying regions in the Western North Pacific and North Indian Ocean are
mainly along deltaic systems that are composed of unconsolidated subsiding
sediments. c, Similarly, most of the low-lying coasts affected by tropical
cyclones in the Gulf of Mexico and the Western North Atlantic are composed
of soft sediments often fronted by dynamic barrier beach systems. Finally,
small-island nations affected by tropical cyclones, often identified in b–e as
isolated light blue regions, are typically fronted by living reef and mangrove
systems, which are particularly sensitive to changing environmental
conditions. Topographic and bathymetric data are from ref.95. Coastal cities
indicated with circles are ranked among the top 30 of the world’s largest urban
centres by 2025 (ref. 91).
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Potentially, many of these coastal systems have tipping points, at
which coupled changes in SLR, vegetation coverage and sediment sup-
ply result in rapid conversion from one equilibrium state to another,
for example gradual barrier island migration compared with com-
plete break up of the barrier island system
49
, or salt marshes compared
with open-water tidal flats
61,62
. Furthermore, the landward retreat of
inhabited barrier beaches is inhibited by artificial structures, result-
ing in shoreline degradation and a loss of the natural buffer that pro-
tects infrastructure and homes from large wave forces during tropical
cyclone events.
Although initial damage to coastal landforms by tropical cyclones
often seems catastrophic, given enough time, these coastal systems gen-
erally have the means to recover. The entire barrier beach profile can
rebuild if there is sufficient sediment supply57,63, storm-produced inlets
can close, and vegetative cover and reef systems can regrow
64
. Shoreline
resilience to severe tropical cyclone disturbance requires that enough
time lapses between extreme events to allow for recovery; barrier
57,65
and reef systems
66
are particularly vulnerable to subsequent flood events
during this recovery period.
Tropical cyclone climatology partly drives the length of recovery
time that coastal systems have between storm disruptions. However,
extreme-value flood statistics consistently point towards SLR as a com-
peting, if not more important, factor in driving the frequency of extreme
coastal flooding by tropical cyclones. Thus, although storms provide the
dominant mechanism for erosion, it is often an increase in SLR and/or a
drop in sediment supply that is the true underlying cause of long-term
rates of shoreline retreat63.
Insight from Holocene shoreline development.
Global SLR rates during the early Holocene (roughly 11,500 to 7,000
years before present), are of the same order as many current projections
of global SLR by the end of the twenty-first century, about 1cmyr−1
(Fig.4). The form and behaviour of shorelines during this earlier period
of rapid SLR therefore serves as an important analogue of future shore-
line change (although differences exist, including the location of the
coast and sediment availability). Often, SLR during this time period was
too fast for landforms such as barrier beaches to remain stable, resulting
in submergence or rapid landward retreat of these systems67. Remnants
of relic back-barrier salt marsh and estuarine material are observed kilo-
metres offshore and are evidence of substantial shoreline retreat during
the early to mid-Holocene
68,69
. This period of wide-spread shoreline
instability is commonly referred to as the Holocene transgression, a
period of rapid landward retreat of many low-lying sedimentary coast-
lines in response to high rates of SLR.
In general, the configuration and current function of most modern
low-gradient shorelines only established themselves after a significant
decline in global SLR rates, beginning around 9,000 to 6,000years ago
(Fig.4). Rates of sea-level change for the next 6,000years or so vary
regionally70, from areas of little change to areas of both net SLR and
net sea-level fall. However, with the exception of regions of significant
tectonic activity or rapid isostatic adjustment, most coastlines affected
by tropical cyclones have experienced moderate rates of sea-level change
over the past few millennia relative to the rapid SLR rates of the early
Holocene. Examples of current coastal settings, for which the existing
forms and behaviours commonly established themselves under these
fairly modest rates of sea-level change, include most of the world’s del-
taic systems71, barrier beaches67,72,73, contemporary beach ridge and
chenier plains74,75, wetland marshes76,77 and mangrove wetlands78,79.
Although rates of sea-level change remained relatively low over the
later Holocene, tropical cyclone activity did not (Fig.4c). Statistically
significant intervals of both quiescence and increased tropical cyclone
activity are evident in the timing of coarse-grained, tropical-cyclone-
induced event deposits from back-barrier salt marshes and coastal
ponds80,81. Overwash deposits can be delineated within these back-
barrier environments because they are later covered by finer-grained
organic substrate once sheltered conditions resume. These palaeo-storm
records, therefore, not only provide evidence of changes in storm activ-
ity over the past few millennia, but they also point to the resilience
of barrier beach systems to storms during times of modest sea-level
change. By contrast, there is a lack of early Holocene storm deposits pre-
served behind the modern coast, which points to the seaward location
of past shorelines and the frequent reworking of back-barrier sediments
by rapid shoreline retreat when past global rates of SLR were elevated
to the levels projected for the end of the twenty-first century (Fig.4).
Storm-induced beach ridges in the South Pacific and South Indian
Oceans also serve as a reliable marker of tropical cyclone activity, sup-
plementing overwash deposit information from the North Atlantic and
North Pacific
82
(Fig.4c). These beach ridge tropical-cyclone-proxies
are preserved along shorelines that have been prograding, partly due
to moderate rates of sea-level fall over the past 6,000–7,000years83.
Similar to back-barrier overwash reconstructions, the onset for the
formation of these beach-ridge shorelines begins only after the Holo-
cene transgression. These shorelines were either stationary or retreating
landward before this interval, because of rapid rates of relative SLR.
On October 29, 2012 Hurricane Sandy inundated New York City at
high tide, raising water levels to 3.5 m above mean sea level at the
Battery (located at the south end of Manhattan Island). Historical
records indicate that this event may have exceeded the maximum
water levels of the previous highest recorded flood, during a hurricane
in 1821 when the water rose roughly 3.2 m above mean sea level at
the time100. However, the 1821 event occurred closer to low tide and
when mean sea level at the Battery was roughly 0.5 m lower than
present94. If the 1821 event were to occur at today’s higher sea level
and at high tide the resulting flood level for the event would probably
have exceeded that observed during Hurricane Sandy. Thus, although
Sandy was potentially record-breaking in terms of the overall water
elevation reached, it was certainly not unique in terms of its overall
surge, with sea-level rise and tides two of the primary causes of
Sandy’s very high water levels relative the 1821 hurricane event.
Flooding as a result of Hurricane Sandy is shown here along the New
Jersey coast.
BOX 2
Sea-level rise and Hurricane Sandy
MASTER SGT. MARK OLSEN, US AIR FORCE
5 DECEMBER 2013 | VOL 504 | NATURE | 49
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© 2013 Macmillan Publishers Limited. All rights reserved

Tropical-cyclone-derived beach-ridge deposits, therefore, highlight the
ability of some coastlines to generally advance seaward over a period of
varying tropical cyclone activity, with significant changes in the long-
term behaviour of this coastal system driven not by changes in storm
activity, but rather by the mid-Holocene transition from rapid rates of
SLR during the Holocene transgression to stable or moderate rates of
sea-level fall over the past few millennia.
Regional landscapes that were flooded during the Holocene trans-
gression often vary in composition and geometry compared with today’s
coasts. Thus, the future response of these shorelines to rapid SLR will
probably differ somewhat to responses during the early Holocene.
However, the marked difference in form and behaviour of most of the
world’s low-lying sedimentary coastlines during past rapid SLR over the
Holocene transgression is a clear example of the importance of sea-level
variability in initiating significant changes in shoreline behaviour and,
thus, should not be overlooked.
Managing future risk
By the end of this century there will probably be a higher occurrence of
more intense tropical cyclones globally
2
. However, considerable uncer-
tainty is associated with how the smaller subset of landfalling tropical
cyclones will change in the future. Efforts are ongoing to provide more
robust projections of the occurrence and intensity of these events. None-
theless, current uncertainties around the effect of future climate change
on tropical cyclone activity should not distract from the two additional
forces that will drive higher flood probabilities. First, increasing rates of
SLR will increase extreme flooding by tropical cyclones. Second, future
storm damage will be greatest not where tropical cyclone activity is the
highest, but rather where geomorphic changes along dynamic, popu-
lated shorelines greatly enhance storm impacts.
Most coastal populations are not prepared for an increase in extreme
flood frequency. Coastal planners and policy makers are challenged
by large uncertainties in flood projections related to changing tropi-
cal cyclone climatology, SLR and shoreline change. However, despite
these uncertainties, the high likelihood of increased catastrophic
coastal flooding in the future warrants preparation. Projected increases
in coastal development and population will only increase damages
from tropical cyclones
5
. Coastal populations need to develop adaptive
strategies, which in many cases must include plans and incentives for
landward or vertical retreat from the sea. Equally important is the devel-
opment of proactive policies for planning and engineering in communi-
ties that must remain in these vulnerable areas, because of, for example,
economic importance, national security or political boundaries. When
coastal defences are necessary to protect crucial infrastructure, it is
important that they are designed in a way that allows for future modi-
fication — because flooding risks will continue to increase over time as
SLR accelerates through the twenty-first century (Fig.4b). Crucial for
increasing resilience to the effects of future tropical cyclones are holistic
strategies that include consideration of the issues related to changes
in sediment supply and subsidence induced by groundwater, oil and
gas extraction. Such strategies will be particularly important along and
behind barrier beaches as well as for the major deltaic systems on which
many coastal megacities exist (Fig. 3).
Coastal communities in developing countries are possibly the most
susceptible populations to the adverse effects of increased tropical
cyclone flooding35,84,85. Here, urban centres and their projected growth
Figure 4 | Mean global sea level along with patterns and extent of preserved
sedimentary records of tropical cyclone activity following the most recent
glacial maximum. a, Four separate estimates of global sea-level elevation
since 10,000years before present96–98, with b, associated SLR observed over the
twentieth century23. The twenty-first century projections between intermediate
high (IH) and intermediate low (IL) ranges presented in ref.23 are shaded grey,
with the mid-point (dashed line). c, Tropical cyclone activities (adapted from
ref.82). Each rectangular line represents a tropical cyclone reconstruction (see
ref.82 for references for each individual reconstruction) with location grouped
by North West Atlantic, red; North West Pacific, blue; South West Pacific, green;
and South Indian, orange. Black represents active tropical cyclone periods
and light shading less active periods. Sedimentary reconstructions of tropical
cyclones exist only for the past few millennia, partly because coastlines were
generally more unstable before this period due to increased rates of SLR.
02,0004,0006,0008,00010,000
–25
–20
–15
–10
–5
0
Years before 1950 AD
Global mean sea level (m relative to present)
SLR
0.4–1.2 cm yr–1
SLR
0.2–0.05 cm yr–1
SLR
0.05–0.0 cm yr–1
~End of
Holocene
transgression
~Onset of
TC sedimentary
records (see c)
1950 2000 2050 2100
–0.2
0
0.2
0.4
0.6
0.8
1
1.2
Year (AD)
Global mean sea level (m above 1992
AD
)
Average SLR
tide gage
0.17 cm yr–1
Average SLR
satellite era
0.32 cm yr–1
Average projected SLR
twenty-rst century
0.8 cm yr–1
Globally
averaged
sea-level
curves
Average projected SLR
for 2100 AD
1.4 cm yr–1
ab
02,0004,0006,0008,000
c
Years before 1950 AD
50 | NATURE | VOL 504 | 5 DECEMBER 2013
REVIEW
INSIGHT
© 2013 Macmillan Publishers Limited. All rights reserved

are generally focused on coastal areas where existing infrastructure
and current management strategies are ill equipped for extreme tropi-
cal cyclone flooding. In terms of the number of people affected, the
impact of future tropical cyclone flooding will probably be focused on
key population centres built on broad, low-lying sedimentary coasts
86
(Fig.3). Essential strategies for mitigating risk at these locations include
improving flood forecasts and developing emergency shelter and effec-
tive evacuation procedures85.
Humans have adapted to environmental changes in the past. When
reacting to a growing hazard, however, it is important to understand its
root cause. It is possible that changes in future tropical cyclone activ-
ity could be an important component of flood risk, and management
strategies will need to be updated as the science advances on this impor-
tant topic. The evidence is now clear, however, that sea levels are rising
and at a rate that will continue to accelerate into the next century. The
era of relatively moderate SLR that most coastlines have experienced
during the past few millennia is over, and shorelines are now begin-
ning to adjust to a new boundary condition that in most cases serves
to accelerate rates of shoreline retreat. The potential for future tropical
cyclones to increase in their intensity has served as a prominent exam-
ple of increased risk that is associated with climate change. This has
placed a disproportionate emphasis on still uncertain changes to tropical
cyclone characteristics at the expense of factors with a potentially larger
and more certain impact, including accelerating SLR, rapidly evolving
coastlines and growing coastal populations. The combined considera-
tion of all of these elements is a much more accurate presentation of the
compounding factors that society must consider to successfully adapt
to future increases in tropical cyclone flooding. ■
Received 7 April; accepted 23 July 2013.
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Acknowledgements We wish to thank our colleagues for the many comments and
suggestions that improved this manuscript, as well as thoughtful discussions at
the 2013 Joint AGU/GSA Conference on ‘Coastal Processes and Environments
Under Sea-Level Rise and Changing Climate: Science to Inform Management’.
J.D.W. is funded through the US National Science Foundation (NSF, grant number
EAR-1158780 and EAR-1148244), the Risk Prediction Initiative at the Bermuda
Institute of Ocean Sciences (grant number RPI11-1-001/11-5110), and the
Hudson River Foundation. S.J.C. acknowledges funding from the National Oceanic
and Atmospheric Administration (NOAA, grant number NA11OAR4310093
and NA10OAR4310124) and NSF (grant number AGS-1143959 and AGS-
1064081). J.L.I. received funding for this work through NOAA’s National Sea Grant
College Program (grant number 24036078) and the South Atlantic Landscape
Conservation Cooperative (grant number 24036078). The views expressed herein
do not necessarily reflect the views of any of these organizations.
Author Information Reprints and permissions information is available
at www.nature.com/reprint. The authors declare no competing financial
interests. Readers are welcome to comment on the online version of this
article at go.nature.com/f6rg4i. Correspondence should be addressed to J.W.
(woodruff@geo.umass.edu).
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