Sea-Level Rise and Its Impact on Coastal Zones

Abstract

Global sea levels have risen through the 20th century. These rises will almost certainly accelerate through the 21st century
and beyond because of global warming, but their magnitude remains uncertain. Key uncertainties include the possible role of
the Greenland and West Antarctic ice sheets and the amplitude of regional changes in sea level. In many areas, nonclimatic
components of relative sea-level change (mainly subsidence) can also be locally appreciable. Although the impacts of sea-level
rise are potentially large, the application and success of adaptation are large uncertainties that require more assessment
and consideration.

Full text

REVIEW
Sea-Level Rise and Its Impact on
Coastal Zones
Robert J. Nicholls
1
*and Anny Cazenave
2
*
Global sea levels have risen through the 20th century. These rises will almost certainly accelerate
through the 21st century and beyond because of global warming, but their magnitude remains uncertain.
KeyuncertaintiesincludethepossibleroleoftheGreenland and West Antarctic ice sheets and the
amplitude of regional changes in sea level. In many areas, nonclimatic components of relative sea-
level change (mainly subsidence) can also be locally appreciable. Although the impacts of sea-level rise
are potentially large, the application and success of adaptation are large uncertainties that require
more assessment and consideration.
The Fourth Assessment Report (AR4) of
the Intergovernmental Panel on Climate
Change (IPCC) projected that global sea
level will rise by up to ~60 cm by 2100 in re-
sponse to ocean warming and glaciers melting
(1). However, the recently identified accelerated
decline of polar ice sheet mass (2–5)raisesthe
possibility of future sea-level rise (SLR) of 1 m
or more by 2100 (6,7). Today, low-elevation
coastal zones below 10-m elevation contain ~10%
of the world population (8). Here, non–climate-
related anthropogenic processes (such as ground
subsidence due to oil and groundwater extrac-
tion, or reduced sediment supply to river deltas
caused by dam building) often amplify local
vulnerability associated with climate-related
SLR. The extent of future SLR, the resulting im-
pacts on low-elevation coastal zones, and the
ability of society to cope via adaptation remain
uncertain. Here, we review current knowledge on
the magnitude and causes of contemporary SLR,
examine future projections and their uncertain-
ties, and discuss SLR impacts. These impacts
are sensitive to how societies prepare for and
adapt to SLR.
What Are the Causes of Contemporary
Sea-Level Rise?
Although mean sea level remained nearly stable
since the end of the last deglaciation [~3000 years
ago; e.g., (9)], tide gauge measurements available
since the late 19th century indicate that sea level
has risen by an average of 1.7 ± 0.3 mm/year
since 1950 (10). Since the early 1990s, SLR has
been routinely measured by high-precision al-
timeter satellites. From 1993 to 2009, the mean
rate of SLR amounts to 3.3 ± 0.4 mm/year (Fig. 1)
(11), suggesting that SLR is accelerating.
Two main factors contribute to SLR: (i) ther-
mal expansion of sea water due to ocean warm-
ing and (ii) water mass input from land ice melt
and land water reservoirs (1). Ocean temperature
data collected during the past few decades in-
dicate that ocean thermal expansion has signifi-
cantly increased during the second half of the
20th century [e.g., (12)]. Thermal expansion ac-
counts for about 25% of the observed SLR since
1960 (13) and about 50% from 1993 to 2003 (1).
Since then, upper-ocean warming has been smaller
[e.g., (12,14)], and on average over the satellite
altimetry era (1993 to 2009), the contribution of
ocean temperature change to the global mean sea
level may be ~30% (15).
Numerous observations have reported world-
wide retreat of glaciers and small ice caps during
recent decades, with an appreciable acceleration
of this retreat during the 1990s (1,16). The gla-
cier contribution to SLR from 1993 to 2009 may
be ~30% (1,17). Change in land water storage,
due to natural climate variability and human ac-
tivities (e.g., underground water mining, irriga-
tion, urbanization, and deforestation), contributes
little (<10%) to current sea-level change (18).By
contrast, intensive dam building along rivers dur-
ing the second half of the 20th century lowered
sea level by ~ −0.5 mm/year (19).
Since the early 1990s, different remote-
sensing tools [airborne and satellite radar and
laser altimetry; synthetic aperture radar interfer-
ometry (InSAR); and, since 2002, space gravimetry
from the Gravity Recovery and Climate Exper-
iment (GRACE) mission] have provided good
data on the mass balance of the polar ice sheets.
These data indicate that Greenland and West
Antarctica mass loss is accelerating [e.g., (2)].
Between 1993 and 2003, <15% of the global SLR
was due to the ice sheets (1). However, since
about 2003, their contribution has nearly doubled
(3–5,20); increasing glacier and ice sheet mass
loss has compensated for reduced ocean thermal
1
School of Civil Engineering and the Environment and the
Tyndall Centre for Climate Change Research, University of South-
ampton, Southampton SO17 1BJ, UK.
2
Laboratoire d’Etudes
en Géophysique et Océanographie Spatiales LEGOS-CNES,
Observatoire Midi-Pyrénées, 18 Av. E. Belin, 31401 Toulouse
cedex 9, France.
*To whom correspondence should be addressed. E-mail:
r.j.nicholls@soton.ac.uk (R.J.N.); anny.cazenave@legos.obs-
mip.fr (A.C.)
1850
-20
20
40
60
80
100
120
140
160
180
200
0
1900 1950 2000 2050 2100
Time (year)
Projections
Sea level (cm)
Observations
20122010200820062004200220001998199619941992
20
Date (years)
10
30
40
50
60
70
80
Trend = +3.26 mm/year
Mean sea level (mm)
Fig. 1. Global mean sea level evolution over the 20th and 21stcenturies.Theredcurveisbasedontide
gauge measurements (10). The black curve is the altimetry record (zoomed over the 1993–2009 time
span) (15). Projections for the the 21st century are also shown. The shaded light blue zone represents IPCC
AR4 projections for the A1FI greenhouse gas emission scenario. Bars are semi-empirical projections [red
bar: (32); dark blue bar: (33); green bar: (34)].
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expansion, such that SLR continues at almost the
same rate (Fig. 1). Although not monotonic
through time, we estimate that on average over
the altimetry era (1993 to 2009), total land ice
mass loss explains ~60% of the rate of SLR (15).
Accelerated loss of ice sheet mass partly re-
sults from rapid outlet glacier flow along some
margins of Greenland and West Antarctica where
the grounding line is below sea level, and further
iceberg discharge into the surrounding ocean [e.g.,
(21–23)]. Recent observations suggest that warm-
ing of subsurface ocean waters triggers coastal ice
discharge (22,24,25). Although
surface mass processes (snow ac-
cumulation versus surface melt-
ing) also contribute to Greenland
mass loss (26), in West Antarctica
mass loss essentially results from
ice dynamics [e.g., (2,3)].
Satellite altimetry shows that
sea level is not rising uniformly
(Fig. 2). In some regions (e.g.,
western Pacific), sea level has ris-
en up to three times faster than the
global mean since 1993. Spatial
patterns in sea-level trends mainly
result from nonuniform ocean
warming and salinity variations
(1,27), although other factors al-
so contribute, including the solid
Earth response to the last degla-
ciation and gravitational effects
and changes in ocean circulation
due to ongoing land ice melting
and freshwater input (28,29). Spa-
tial patterns in ocean thermal ex-
pansion are not permanent features:
They fluctuate in space and time
in response to natural perturbations
of the climate system (1); as a
result, we expect that the sea-
level change patterns will oscil-
late on multidecadal time scales. IPCC AR4
projections suggest appreciable regional variabil-
ity around the future global mean rise by 2100 in
response to nonuniform future ocean warming
(1), but agreement between the models is poor.
However, accurate estimates of future regional
sea-level changes are required for coastal impact
and adaptation assessment.
How Much Will Global Sea-Level Rise
in the 21st Century?
The rapid changes observed in polar regions sug-
gest that the ice sheets respond to current warm-
ing on much shorter time scales than previously
anticipated [e.g., (1)]. However, it is unknown
whether these processes will continue into the
future, resulting in a partial collapse of the ice sheets
after a few centuries, or whether a new equilib-
rium will be reached (30,31). For the near term
(next decades), the largest unknown in future SLR
is the behavior of the ice sheets. Although IPCC
AR4 projections did not account for dynamical
changes of large ice sheets, simple kinematics
and observations of current velocities of marine-
terminated glaciers in Greenland and West Ant-
arctica suggest that future ice-dynamics discharge
could lead to SLR of about 80 cm by 2100 (6).
Several groups have developed semi-empirical ap-
proaches in which a simple relation between past
sea-level rate and temperature or radiative forcing
is determined, and then extrapolated through the
21st century using IPCC temperature or forcing
projections [e.g., (32–34)]. Depending on some
model variants, these studies yield SLR between
~30 and 180 cm by 2100 (Fig. 1). The upper limit
of these estimates is well above IPCC AR4 SLR
projections [of ~60 cm for the business-as-usual
A1FI greenhouse gas emissions scenario (1)].
What Are the Main Impacts of Sea-Level Rise?
The physical impacts of SLR are well known
(35). The immediate effect is submergence and
increased flooding of coastal land, as well as
saltwater intrusion of surface waters. Longer-term
effects also occur as the coast adjusts to the new
conditions, including increased erosion and salt-
water intrusion into groundwater. Coastal wet-
lands such as saltmarshes and mangroves will
also decline unless they have a sufficient sedi-
ment supply to keep pace with SLR. These phys-
ical impacts in turn have both direct and indirect
socioeconomic impacts, which appear to be
overwhelmingly negative (35). Although climate-
induced SLR is important, coastal impacts also
result from relative (or local) SLR (e.g., from geo-
logical processes such as subsidence). For exam-
ple, relative sea level is presently falling where
land is uplifting considerably, such as the north-
ern Baltic and Hudson Bay—the sites of large
(kilometer-thick) glaciers during the last glacial
maximum. In contrast, relative sea level is rising
more rapidly than climate-induced trends on subsid-
ing coasts. In many regions, human activities are
exacerbating subsidence on susceptible coasts,
including most river deltas [e.g., the Ganges-
Brahmaputra, Mekong, and Changjiang deltas
(36,37)]. The most dramatic subsidence effects
have been caused by drainage and groundwater
fluid withdrawal; over the 20th century, coasts
have subsided by up to 5 m in Tokyo, 3 m in
Shanghai, and 2 m in Bangkok (38). To avoid
submergence and/or frequent flooding, these
cities now all depend on a substantial flood de-
fense and water management infrastructure. South
of Bangkok, subsidence has led to substantial
shoreline retreat of more than 1 km, leaving tele-
graph poles standing in the sea.
These and other human-induced changes in
coastal areas (such as coastal defenses, destruction
of wetlands, port and harbor works, and reduced
sediment supply due to dams) obscure the im-
pacts of climate-induced SLR during the 20th
century (39,40). The nonclimate components of
SLR receive much less attention than climate com-
ponents, because they are considered a local issue.
However, they are so widespread that they amount
to a global problem warranting more systematic
90°
-20.0 -17.5 -15.0 -12.5 -10.0 -7.5 -5.0 -2.0 -1.0 0.0 1.0 2.0 5.0 7.5 10.0 12.5 15.0 17.5 20.0
-80°
-60°
-40°
-20°
20°
40°
60°
80° 180° 270°
270°
360° 90° 80°
60°
40°
20°
0°
0°
-80°
-60°
-40°
-20°
360°
mm/year
180°
Fig. 2. Regional sea-level trends from satellite altimetry (Topex/Poseidon, Jason-1&2, GFO, ERS-1&2, and Envisat
missions) for the period October 1992 to July 2009 (48).
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study, including appropriate
mitigation (of human influ-
ence) as well as adaptation
options.
As the magnitude of climate-
induced SLR increases, the
impacts will become more ap-
parent (35), especially in certain
low-elevation coastal zones
(Fig. 3). Most countries in South,
Southeast, and East Asia ap-
pear to be highly threatened
because of the widespread oc-
currence of densely populated
deltas, often associated with large
growing cities (Fig. 3). Africa
also appears highly threatened
owing to the low levels of de-
velopment combined with ex-
pectations of rapid population
growth in coastal area: Egypt
and Mozambique are two “hot-
spots”for potential impacts.
However, the small island states
experience the largest relative
increase in impacts, including
regions of high islands like the Caribbean. Low
islands such as the Maldives or Tuvalu face the
real prospect of submergence and complete
abandonment during the 21st century (41).
Can Adaptation Help?
Many impact studies do not consider adaptation,
and hence determine worst-case impacts [e.g.,
(42)]. Yet, the history of the human relationship
with the coast is one of an increasing capacity to
adapt to adverse change [e.g., (43)]. In addition,
the world’s populated coasts became increasingly
managed and engineered over the 20th century
(35). The subsiding cities discussed above all
remain protected to date, despite large relative
SLR. Analysis based on benefit-cost methods
show that protection would be widespread as
well-populated coastal areas have a high value
and actual impacts would only be a small fraction
of the potential impacts [e.g., (44)], even assum-
ing high-SLR (>1 m/century) scenarios (45).
This suggests that the common assumption of a
widespread forced retreat from the shore in the
face of SLR is not inevitable. In many densely
populated coastal areas, communities advanced
the coast seaward via land claim owing to the
high value of land (e.g., Singapore). Yet, pro-
tection often attracts new development in low-
lying areas, which may not be desirable, and
coastal defense failures have occurred, such as New
Orleans in 2005. Hence, we must choose between
protection, accommodation, and planned retreat
adaptation options (35). This choice is both tech-
nical and sociopolitical, addressing which mea-
sures are desirable, affordable, and sustainable in
the long term. Adaptation remains a major un-
certainty concerning the actual impacts of SLR.
In one of the few strategic plans to respond to
SLR, the Netherlands is planning to upgrade
protection both for SLR and to provide higher
levels of safety (to a nominal chance of failure of
1 in 100,000) by building their North Sea coast
seaward using beach nourishment (46). The plan
stresses how adaptation to SLR must be integrated
into wider coastal management and development
plans. It also explicitly recognizes that adaptation
will continue beyond 2100 (35). In most develop-
ing countries, the issues are more challenging,
and the limits to adaptive capacity will be a key
constraint. National development plans will need
to address the growing risks of coastal occupancy
and identify the most appropriate approaches to
coastal management.
Outlook
The extent of future SLR remains highly
uncertain—more so than in 2007, when the IPCC
AR4 was published. A two-track solution is
required to advance the scientific understanding
of observed and future climate-induced SLR and
develop pragmatic impact and adaptation scenar-
ios that capture the uncertainties of future SLR.
The former analysis should focus on understand-
ing the processes that control SLR (e.g., ice sheet
instabilities), whereas the latter analysis requires a
range of plausible scenarios, including the low
probability–high consequence part of the possible
SLR range where our understanding is weaker
(7). More attention must also focus on the non-
climate components of SLR, especially for coasts
more susceptible to subsidence, such as deltas. Non-
climate processes tend to be larger where there
are high concentrations of people and economic
activity, and hence have a high impact potential.
The impacts of SLR can also be divided into
two distinct issues: impacts for climate policy, which
usually focus on the effects of climate-induced SLR
and the incremental benefits of different climate
mitigation policies, and impacts for coastal manage-
ment policy, which must consider all relevant cli-
mate and nonclimate coastal drivers. An improved
understanding of adaptation is fundamental,
because it is one of the biggest determinants of
actual rather than potential impacts. Studies such
as the World Bank assessment of adaptation costs
in developing countries (47) are useful starting
points to address these problems.
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10.1126/science.1185782
REVIEW
How Do Polar Marine Ecosystems
Respond to Rapid Climate Change?
Oscar Schofield,
1
*Hugh W. Ducklow,
2
Douglas G. Martinson,
3
Michael P. Meredith,
4
Mark A. Moline,
5
William R. Fraser
6
Climate change will alter marine ecosystems; however, the complexity of the food webs,
combined with chronic undersampling, constrains efforts to predict their future and to optimally
manage and protect marine resources. Sustained observations at the West Antarctic Peninsula
show that in this region, rapid environmental change has coincided with shifts in the food web,
from its base up to apex predators. New strategies will be required to gain further insight into
how the marine climate system has influenced such changes and how it will do so in the future.
Robotic networks, satellites, ships, and instruments mounted on animals and ice will collect
data needed to improve numerical models that can then be used to study the future of polar
ecosystems as climate change progresses.
How does a changing physical ocean en-
vironment affect regional and local ma-
rine food webs? Many regions, especially
polar seas (1,2), are experiencing changes in
atmospheric/ocean circulation (3), ocean prop-
erties (4,5), sea ice cover (6,7), and ice sheets
(8,9). These rapid climatic changes are trigger-
ing pronounced shifts and reorganizations in
regional ecosystems and biogeochemical cycles
(10,11). However, it remains difficult to link
these ecosystem changes to shifts in the physical
system. Overcoming this gap is a critical step in
establishing any level of predictive skill.
The West Antarctic Peninsula (WAP), north-
western North America, and the Siberian Pla-
teau are exhibiting rapid regional warming (1),
but only the WAP has a maritime climate.
Thus, the WAP is an ideal location to monitor
and understand the impacts of rapid climate
change on marine ecosystems. Other regions
of Antarctica are exhibiting much smaller rates
of warming—and some, such as the Ross Sea
(12), are even experiencing trends in the oppo-
site direction—but climate models predict strong
warming and circumpolar sea ice retreat around
Antarctica over the next century (13). Under-
standing the response of the WAP ecosystems to
climate change will thus help to predict further
changes in the polar ecosystem as a whole and
will provide insight into the planetary-scale
changes that are likely as greenhouse gas–driven
warming continues.
Physical Changes in the WAP
Changes in the WAP are profound (Fig. 1). Mid-
winter surface atmospheric temperatures have
increased by 6°C (more than five times the global
average) in the past 50 years (14,15). Eighty-
seven percent of the WAP glaciers are in retreat
(16), the ice season has shortened by nearly 90
days, and perennial sea ice is no longer a feature
of this environment (17,18). These changes are
accelerating (19,20).
Ocean warming has been implicated as a
major driver for this deglaciation (21). The ocean
has become warmer in the WAP (17). Most of this
heat comes from the warm, saline Upper Circum-
polar Deep Water (UCDW) that penetrates onto
the WAP shelf from the Antarctic Circumpolar
Current (ACC) in the adjacent deep ocean. The
increased supply of heat from the UCDW is
believed to be associated with the strengthening
of winds over the Southern Ocean (22,23). En-
hanced upwelling of heat to the WAP is comple-
mented by rising summertime surface-ocean
heating (24), which is associated with the strong
retreats in the seasonal sea ice cover (7,18).
This atmosphere–ocean-ice interplay at the
WAP results in a positive feedback that amplifies
and sustains atmospheric warming. Understand-
ing these feedbacks will require better knowledge
of the processes at the shelf edge and in the adja-
cent deep ocean to determine where and when the
UCDW intrudes from the ACC onto the WAP
shelf. Although the ACC is a major current in the
1
Coastal Ocean Observation Laboratory, Institute of Marine
and Coastal Sciences, School of Environmental and Biolog-
ical Sciences, Rutgers University, New Brunswick, NJ 08901,
USA.
2
The Ecosystems Center, Marine Biological Laboratory,
Woods Hole, MA 02543, USA.
3
Department of Earth and
Environmental Sciences, Columbia University, NY 10964,
USA.
4
British Antarctic Survey, Madingley Road, Cambridge
CB3 0ET, UK.
5
Biological Sciences Department and Center
for Coastal Marine Sciences, California Polytechnic State
University, San Luis Obispo, CA 93407, USA.
6
Polar Oceans
Research Group, Post Office Box 368, Sheridan, MT 59749,
USA.
*To whom correspondence should be addressed. E-mail:
oscar@marine.rutgers.edu
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