Pathways to coastal retreat

Abstract

The shrinking solution space for adaptation calls for long-term dynamic planning starting now

Full text

SCIENCE sciencemag.org
only the first step. In the coming years, fur-
ther investment should build on the prom-
ises of longer-term risk modeling and cou-
ple its results with impact assessments so
that countries can build displacement es-
timates into their multiyear development
plans (15). Understanding needs and pri-
orities in the decision-making processes of
affected populations, institutional capaci-
ties, and socioeconomic dynamics, even if
less systematically assessed, will be at least
as important at indicating what the future
holds. Given the scope and complexity of
the problem, a pluralistic methodological
setup is required to contribute to a better
understanding of displacement risk and to
inform effective policy and response under
a broad range of circumstances. j
R EFERENCES AND NOTES
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Fifth Assessment Report of the Intergovernmental
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2. Internal Displacement Monitoring Centre (IDMC),
Global Report on Internal Displacement 2021 (2021);
www.internal-displacement.org/global-report/
grid2021.
3. IDMC, Global Internal Displacement Database; www.
internal-displacement.org/database.
4. M. Br zoska, C. Frö hlich , Migr. Dev. 5, 190 (2016).
5. Economics of Climate Adaptation (ECA), “Shaping
climate-resilient development: A framework for
decision-making. A report of the Economics of Climate
Adaptation Working Group” (ECA, 2009); www.ethz.
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6. C. Raymond et al., Nat. Clim. Chang. 10, 611 (2020).
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displacement. 2020 report,” S. Ambrus, Ed. (IDMC,
2020); www.internal-displacement.org/sites/default/
files/publications/documents/IDMC_CostEstimate_
final.pdf.
8. E. du Parc, L. Yasukawa, “The 2019–2020 Australian
bushfires: From temporary evacuation to longer-term
displacement,” J. Lennard, Ed. (IDMC, 2020);
www.internal-displacement.org/sites/default/files/
publications/documents/Australian%20bushfires_
Final.pdf.
9. K. K. R igaud et al., “Groundswell: Preparing for
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handle/10986/29461.
10. IDMC, “Global disaster displacement risk – A baseline
for future work” (2017); www.internal-displacement.
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baseline-for-future-work.
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42, 727 (2020).
13. IDMC, IIASA, “Points of no return: Estimating govern-
ments’ fiscal resilience to internal displacement” (IDMC,
2020); www.internal-displacement.org/sites/default/
files/publications/documents/201903-fiscal-risk-pa-
per.pdf.
14. J. Linnerooth-Bayer, S. Hochrainer-Stigler, Clim. Change
133, 85 (2015).
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24, 482 (2017).
10.1126/science.abh4283
POLICY FORUM
Pathways to coastal retreat
The shrinking solution space for adaptation calls for
long-term dynamic planning starting now
By Marjolijn Haasnoot1,2, Judy Lawre nce3,
Alexandre K. Magnan4,5
There is an urgent need to take coastal
retreat more seriously as an option
for adapting to sea level rise (SLR)
and as a strategy capable of provid-
ing positive outcomes, if planned
ahead. Early signs of such think-
ing are emerging. We demonstrate how
exploring pathways to managed retreat
adds value in the context of irreversible
long-term SLR. Retreat is typically framed
and understood as a single action, largely
used after events rather than preemptively,
and considered as a last resort. However,
implementing managed retreat constitutes
a multidecadal sequence of actions (i.e.,
across pathways) including community en-
gagement, vulnerability assessment, land
use planning, active retreat, compensa-
tion, and repurposing. This Policy Forum
advances practical knowledge on what
pathways to coastal retreat may look like
and how they can pave the way for flexible
and positive transformational adaptation,
if started now.
SHRINKING SOLUTION SPACE
SLR globally accelerated from 1.4 mm/year
(1901–1990) to 3.6 mm/year (2006–2015)
and will continue to do so during this cen-
tury (10 to 20 mm/year in 2100) ( 1). Sea
levels could rise between 0.43 and 0.84 m
globally by 2100, relative to 1986–2005,
as a median estimate under low and high
emission scenarios, respectively. However,
a rise of 2 m by 2100 cannot be ruled out
(1). There is also a clear commitment to
SLR centuries into the future due to iner-
tia in both the climate and ocean systems;
for every degree of warming, sea levels will
eventually rise ~2.3 m (2).
Inexorable SLR makes some degree of re-
location of coastal residents, buildings, in-
frastructure, and activities inevitable, even
if global warming is mitigated to 1.5° or 2°C.
The necessity of paying more serious atten-
tion to pathways to managed retreat is be-
coming urgent (3). To begin with, observed
coastal flooding is already reaching unac-
ceptable levels for communities and infra-
structure in many low-lying coastal settle-
ments around the world (1), and unless
adaptation starts now, in a few generations,
more regions (e.g., small islands, parts of
the US coast, major deltas) will be at risk of
coastal flooding (1). Additionally, retreat re-
quires decadal lead time to plan and imple-
ment equitably (3, 4). Furthermore, many
decisions taken today have a long legacy ef-
fect and create path dependencies, closing
off some options in the future. For example,
coastal defenses last for many decades and
protected areas attract people and assets,
which lead to expectations of further pro-
tection. On the other hand, creating space
for wetlands to grow as sea levels rise pro-
vides a temporary buffer, keeping future op-
tions open for later development or a lower
barrier to retreat.
Ongoing and accelerating SLR, com-
pounded with other climate-related
changes (e.g., intensification of extreme
events such as storms, heavy rainfall, and
river flows) and increasing population at
the coast, is already progressively shrink-
ing the solution space of available adap-
tation options. Accommodation options
(e.g., elevated buildings, early warning,
and shelter) will not be enough to reduce
coastal risks to acceptable levels under
SLR-induced flooding and erosion. As sea
levels rise, groundwater salinization will
render water supplies unusable and limit
food production to saline-tolerant crops.
Nor will nature-based solutions, such as
offshore reefs or wetland restoration, be
likely to keep pace with combined climate
change impacts (1) and human pressures
that have reduced space and sediment sup-
ply to the coast. Such responses are there-
fore expected to be only temporary adapta-
tions in many places (5).
Hard protection, either through holding
the line (protect) or advancing seaward
(advance) using levees, barriers, or artifi-
cial islands, can be beneficial, for example,
in resource-rich megacities but also has
limitations, as sustained and rapid SLR
would make it increasingly difficult to ex-
tend infrastructure within available time
frames (6). Also, hard protection will not
be an affordable long-term solution for
1Deltares, Delft, Netherlands. 2Faculty of Geosciences,
Utrecht University, Utrecht, Netherlands . 3New Zealand
Climate Change Research Institute, Te Herenga Waka–
Victoria University, Wellington, New Zealand. 4IDDRI
(Sciences Po), Paris, France. 5LIENSs, La Rochelle University,
La Rochelle, France. Email: marjolijn.haasnoot@deltares.nl
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GRAPHIC: KELLIE HOLOSKI/SCIENCE BASED ON M. HAASNOOT ET AL.
every community, nor will it address the
impacts of rising groundwater and river
flows in every coast (6) or the existing and
increasing residual risks (e.g., when levees
fail). In low-lying coastal areas across dif-
ferent geomorphologies and levels of de-
velopment, retreat offers an alternative
option (see the first figure) that ultimately
removes vulnerability and risk in situ.
A DYNAMIC STRATEGY
Retreat is not easy, for various reasons,
including attachment to place, high costs,
lack of risk awareness, impacts on inland
settlements, and political resistance (3).
For example, retreat means sunk costs of
existing investments in public infrastruc-
ture and private property and does not ad-
dress the risk to cultural assets that cannot
be relocated. However, among the reasons
that make managed retreat beneficial is
that it enables long-term change at the
coast to be anticipated and planned for in
an orderly way, which can minimize both
stress on people and agencies and inequi-
table outcomes.
Exploring pathways can support stag-
ing retreat and help to break retreat into
manageable steps over time, align it with
maintenance or other social goals (e.g.,
economic development or environmental
conservation), and implement retreat de-
pending on how the future unfolds. This
could help to overcome the societal resis-
tance to retreat. Dynamic Adaptive Policy
Pathways (DAPP) (7) planning is a practi-
cal approach developed to do exactly this
and is increasingly used to support cli-
mate change adaptation decision-making.
To date, DAPP planning has been used to
address adaptation to SLR in several loca-
tions, including the Netherlands, the UK,
the US, and New Zealand, where measures
have included no-build zones and com-
munity and assets relocation (5, 6, 8). The
long-term perspective puts retreat on the
table next to protection and accommoda-
tion measures (see the first figure), avoid-
ing increasing investments that eventually
become higher sunk costs.
A first step in pathways planning is to
assess the hazard, vulnerabilities, and
uncertainties and to identify adaptation
options. An adaptation option may fail
to achieve objectives and/or may reach a
performance limit or threshold (also re-
ferred to as an adaptation tipping point) as
conditions change (e.g., SLR); a new or ad-
ditional measure is then needed. Similarly,
opportunities may arise (e.g., when infra-
structure needs replacing or when people
cannot tolerate SLR impacts and the need
for retreat becomes obvious). The first fig-
ure presents some thresholds and oppor-
tunities for adaptation to SLR that change
the solution space.
Next, by sequencing options, starting with
low-regret and preparatory actions that can
and/or need to be taken in the near term,
pathways are designed while also testing op-
tions for their sensitivity to a range of SLR in-
crements and to their path dependency. Path-
ways design is often done in a staged manner,
with increasing depth of analysis. For part of
the city of Miami, Florida, potential pathways
were first developed using narratives, by ask-
ing stakeholders: What could be short-term,
mid-term, and long-term adaptation options?
What is the next option? Promising options
and pathways were then further assessed us-
ing detailed models. In the Netherlands, a
study assessed the solution space for multi-
ple meters of SLR before exploring pathways.
The study concluded that spatial planning
that recognizes the consequences of long-
term SLR is needed, because of the uncertain,
potentially high SLR.
Monitoring is typically used to evalu-
ate success of implementation but is also
needed for detecting early warning sig-
nals on approaching thresholds and win-
dows of opportunities for preemptive ac-
tions (e.g., new insights on future risks or
new social values). This helps to identify
when a decision to shift to another ac-
tion is necessary. For adaptation to SLR,
signals can be derived from climate driv-
ers (e.g., mass loss from Antarctica, local
SLR), impact signposts (e.g., flooding or
freshwater availability) based on observa-
tions, and scientific studies and assess-
ment [e.g., the Intergovernmental Panel on
Climate Change (IPCC)] and, maybe more
critically, from social, economic, and cul-
tural signposts (e.g., insurance withdrawal,
increased costs, and others developed
with communities). Monitoring levels of
(in)tolerable risk, increasing exposure to
Lack of time
Coastal
population growth
and migration
Salination threatens
fresh water for food
and drinking
Gravity drainage
not possible,
pumpi ng required
Unaordable
Lack of
available land
Attachment
to place
No-build
zones Socioeconomic
developments
outside coastal zone
Culture of living
with the sea
End of
infrastructure
lifetime
Eective retreat
to other areas
Lack of
insurance
Other solutions
no longer viable
or aordable
Funding, e.g.,
buyout
Salination threatens
fresh water for food
and drinking
Insucient risk
reduction
Awareness Sunk costs
Histo ry of
protection
Flooding of stormwater and
wastewater infrastructure
Flooding of storm-
water and wastewater
infrastructure
Culture of living at the coast
Sea level rise
International
funding
Insucient risk reduction
Protect or advance
Retreat
Accommodate
Shrinking solution space
Widening the solution space
Urban atoll
islands Large agricultural
tropical deltas
Resource-rich
megacities
Stylized examples of coastal archetypes
Arctic
communities
The evolving and shrinking solution space to address sea level rise
The colored areas show how the solution space to protect or advance, accommodate, and retreat changes as sea level rises. Different drivers and soft or hard
limits shape this space. The figure highlights, first, a general narrowing of the solution space as a whole and, second, a change in the ratio between the three adaptation
strategies, with retreat becoming dominant. This applies differently across coastal archetypes [derived from (1); see inset] owing to local contexts.
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damage through population changes, and
infrastructure aging could warn about po-
tential lock-in or lock-out situations. Poten-
tial signals need to be evaluated for timeli-
ness and reliability, while considering the
required lead time for planning and imple-
mentation of next actions. This is problem-
atic in a context of increasing and acceler-
ating coastal risks, where physical and so-
cietal thresholds occur close together, with
limited time left for implementation, and
where communities are dependent on criti-
cal infrastructure, the functioning of which
is already threatened. For example, in Flor-
ida, several water infrastructure thresholds
are close or have been reached, where nui-
sance flooding is observed and the septic
systems are being compromised by rising
groundwater tables. New infrastructure
with pumps and drainage can only buy a
limited amount of time (8).
Beyond mapping the solution space that
includes retreat, pathways thinking is also
critical to supporting the design and imple-
mentation of the transition to retreat, as
presented with the nested path-
ways in the second figure.
PRACTICAL PATHWAYS
INSIGHTS
Although the relevance, extent,
rate, and modalities of managed
retreat will vary depending on
SLR and local context, three ge-
neric steps can be highlighted
across coastal settlements:
preparation, active retreat, and
cleanup (5). Enabling decision-
makers to progressively prepare
includes engagement to gain
community understanding of
the risks and to understand
social values and vulnerabili-
ties; planning to identify op-
tions, exploring pathways, and
establishing monitoring plans
to detect signals of opportu-
nities (e.g., early moves, end
of lifetime of infrastructure);
funding for property acquisi-
tion and infrastructure provi-
sion in alternative areas; and
adjustment of land use plans
and regulations. These prepa-
ratory actions support active
retreat, which comprises the
acquisition of property, buyout,
and removal of structures or re-
location of houses, people, and
economic activities. The last
step, cleanup, comprises land
rehabilitation and repurposing
(e.g., for coastal amenity and
recreational uses that can relo-
cate readily) until that land is permanently
flooded by the sea.
Because implementing managed retreat
can take decades, it needs to be considered
well ahead of any climate-induced soci-
etal and physical thresholds (9). The time
needed depends on each society’s willing-
ness and ability to anticipate the climate
risks and to act on them before observed
impacts. Time is also needed to plan and
engage with those affected about the ur-
gency to start the retreat process now, so
that individuals can make relocation deci-
sions as opportunities arise. For example, in
the Netherlands and New Zealand, retreat
to enable river floodplain restoration was
signaled well ahead of project implementa-
tion in anticipation of the effects of climate
change (5, 10), which gave time (25 and 10
years, respectively) for eventual removal of
houses and purchase of at-risk properties
on a voluntary basis. This contrasts with
instances where retreat has been triggered
after damaging climate events (e.g., after
hurricanes Sandy in New York and Katrina
in New Orleans (4, 10); where protection
proved ineffective and retreat was forced,
creating additional community stress and
costs [e.g., after a storm and mudslide in
New Zealand (11)]; or where forced retreat
to a flood-safe area was unsustainable be-
cause work was unavailable in the new lo-
cation [e.g., in the Philippines (12)]. In the
Carteret Islands, Papua New Guinea, reset-
tlement of island populations created nega-
tive outcomes owing to a lack of economic
opportunities in the relocation areas, land
tenure conflicts with established popula-
tions, and disruptions to local communities
that were not planned for (13). These exam-
ples illustrate the social consequences of re-
treat if it does not take a planned and staged
pathways approach.
To determine when to start active retreat,
one can assess under what conditions retreat
is required because of limitations of other
strategies, indicating the latest moment at
which active retreat should be realized. An-
other way is to assess the conditions under
which retreat becomes more beneficial than
SCIENCE sciencemag.org
GRAPHIC: KELLIE HOLOSKI/SCIENCE BASED ON M. HAASNOOT ET AL.
Transfer to new portfolio or action
Adaptation threshold
Portfolio or action eective
Uncertainty in eectiveness
Adaptation signal
Decision node
Design plan
No-build zone and temporary
protection or accommodation
Land rights
negotiation,
property acquisition
Engagement
Monitoring
Buyout, relocation of
public infrastructure
Development of
alternative land
Displacement and
relocation of people
Removal
Repurposing
Historic
pathway
Enabling
investments
and regulations
Frequent or
high ooding
Unaordability; pumping;
lack of time, support,
knowledge, material
Unaordability, salinization,
pumping, lack of support
Long lead time
Preparation Active retreat Cleanup
Advance
Protect
Accommodate
Retreat
Retreat pathway in more detail
No-build zone
Indicative adaptation pathways of retreat
Retreat is presented as a nested pathway within a broader pathways map, including advance, protect, and accommodate.
Retreat comprises three stages: preparation, active retreat, and cleanup. Engagement and monitoring support planning and
implementation (gray lines). After designing a plan, land use regulations and temporary measures can be implemented,
followed by buyout. Enabling investments and regulations are precursor actions.
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other strategies accounting for flood risk,
alignment with social goals, and costs. For
example, Kool et al. (14) worked backward
from an infrastructure threshold for SLR of
30 cm, at which point a gravity-based storm-
water and wastewater system would need to
be replaced by a pumped system. Before that
point, the costs for a new system, its lifetime,
and the opportunity costs to the community
would need to be assessed against the costs
and benefits of a retreat option that helps re-
move the ongoing impacts from SLR. Using
pathways for adjacent locations, they iden-
tified opportunities for drainage system re-
design to buy time for engagement with the
community before eventual retreat. Such a
strategy consisting of progressive steps can
result in a beneficial transition that is sup-
ported by the community.
An increasing number of studies (3, 5, 10,
15) provide lessons for developing robust
pathways to coastal retreat: (i) engaging
early with affected communities to build
understanding of their risk tolerance, vul-
nerabilities, and values; (ii) enhancing the
policy and public understanding of higher
risk levels than in the past; (iii) early design
of and contributions to design of funding
mechanisms and regulations that can en-
able implementation of retreat; (iv) avoid-
ing developments in places recognized
as risky and where existing urbanization
trends can be reversed through no-build
zones and prohibited land uses; (v) con-
sidering locations for new developments
or designing them to be movable; and (vi)
considering whether buying time through
temporary accommodation, protection, or
nature-based measures will trigger greater
risk exposure and therefore worsen the
problem over time, or whether these ap-
proaches facilitate a transition to retreat.
NECESSARY ENABLERS
Inexorable SLR that will continue for centu-
ries means that for many low-lying coastal
areas worldwide, retreat is an inevitable
adaptation action. If planned now and in-
tegrated with social, economic, and cultural
goals, the anticipatory and dynamic path-
ways to retreat can be a positive approach
to reduce coastal risks and minimize regret
of investments and social inequities.
To allow retreat to be considered a se-
rious option and implemented where ap-
propriate, there are a number of necessary
enablers that require further attention by
the research and policy communities. These
include: (i) improved understanding of how
SLR is a changing risk over time that re-
quires a shift from static to dynamic path-
ways decision-making and how this affects
communities differently now than in the
past; (ii) improved understanding of what
managed retreat comprises and how it can
be staged over time through monitoring and
sharing experiences; (iii) development of
policies and regulations that are grounded
in anticipatory planning supported by sus-
tainable funding arrangements; (iv) further
development of analytical methods relevant
to changing risk, such as for mapping the
shrinking solution space and identifying if
and when retreat will be needed; (v) further
assessment of the effectiveness of the range
of adaptation responses under alternative
futures and how retreat can be integrated
with wider societal goals; and (vi) enhance-
ment of the role of political leadership in
building community trust in preparation
for managed retreat, and embedding com-
mitment devices to maintain the long-term
dynamic approaches for reducing SLR risks.
Notably, the development and the imple-
mentation of any retreat pathway funda-
mentally depends on the past trajectory of
coastal risks; the present situation (gover-
nance, coastal strategy, observed impacts,
individual and institutional values and at-
titudes toward climate-related risks); the
envisioned future; and when and under
what conditions adaptation opportunities
and limits appear. Whatever the context
considered, it is increasingly evident that
the shrinking solution space for adaptation
in low-lying coastal areas calls for long-
term dynamic pathways planning now. j
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ACKNOWLEDGMENTS
We thank C. Kraan, S. McEvoy, and A. Reisinger for feedback
and I. van den Broek for the figures. J.L. thanks the NZ
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Adaptation Programme (GNS-RNC040) and NZ SeaRise
Endeavour Programme (RTUV1705). A.K.M. thanks the
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10.1126/science.abi6594
POLICY FORUM
High-density
population and
displacement
in Bangladesh
The strategy promotes
“migrant friendly” towns and
selective relocation abroad
By Mizan R. Khan, Saleemul Huq,
Adeeba N. Risha, Sarder S. Alam
Among the many adverse impacts of
climate change in the most vulner-
able countries, climate change–in-
duced displacement increasingly
caused by extreme weather events
is a serious concern, particularly in
densely populated Asian countries. Reports
by the Intergovernmental Panel on Climate
Change (IPCC) project a grim picture for
South Asia, the most populous region on
Earth, home to about one-quarter of global
population, with the highest poverty in-
cidence. A combination of poor socioeco-
nomic indicators and increased frequency
and intensity of cyclones and floods renders
the region extremely vulnerable. Mean-
while, slow-onset climate hazards, such
as sea level rise, salinity intrusion, water
stress, and crop failures gradually turn into
larger disasters. Within South Asia, Ban-
gladesh stands as the most vulnerable: 4.1
million people were displaced as a result
of climate disasters in 2019 (2.5% of the
population), 13.3 million people could be
displaced by climate change by 2050, and
18% of its coastland will remain inundated
by 2080 (1). We describe how, faced with
such natural and human-made adversities,
Bangladesh can stand as a model of disaster
management, adaptation, and resilience.
The Paris Agreement goal of keeping the
temperature rise at 1.5°C or well below 2°C
compared to pre-industrial times may not
be achieved, given the lack of ambitious
mitigation. As a result, the number of peo-
ple estimated to be displaced by slow-onset
events will stand at ~22.5 million by 2030
and ~34.4 million by 2050 (2). A combi-
International Centre for Climate Change and Development
(ICCCAD), Independent University, Dhaka, Bangladesh.
Email: adeeba.nuraina@icccad.or
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Pathways to coastal retreat
Marjolijn Haasnoot, Judy Lawrence and Alexandre K. Magnan
DOI: 10.1126/science.abi6594
(6548), 1287-1290.372Science
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