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\DOI{10.5802/crgeos.290}
\datereceived{2024-09-27}
\daterevised{2025-04-01}
\dateaccepted{2025-04-07}
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Project name and location} &amp; 
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Co-benefits of the project} &amp; 
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\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\shortrunauthors

\TopicFR{Sciences du climat}
\TopicEN{Climate Sciences}
\TopicFR{D\'{e}veloppement durable, am\'{e}nagement}
\TopicEN{Sustainable development, landscaping}

\title{Sea-level scenarios aligned with the 3rd adaptation plan in
France}

\alttitle{Sc\'{e}narios d'\'{e}l\'{e}vation du niveau de la mer
align\'{e}s sur le 3$^{\mathrm{e}}$ plan d'adaptation en France}

\author{\firstname{Gon\'{e}ri} \lastname{Le Cozannet}\CDRorcid{0000-0003-2421-3003}\IsCorresp}
\address{BRGM, 3 avenue Claude Guillemin, 45060 Orl\'{e}ans C\'{e}dex,
France}
\email[G. Le Cozannet]{g.lecozannet@brgm.fr}

\author{\firstname{R\'{e}mi} \lastname{Thi\'{e}blemont}\CDRorcid{0000-0003-0160-2276}}
\addressSameAs{1}{BRGM, 3 avenue Claude Guillemin, 45060 Orl\'{e}ans
C\'{e}dex, France}

\author{\firstname{Jeremy} \lastname{Rohmer}\CDRorcid{0000-0001-9083-5965}}
\addressSameAs{1}{BRGM, 3 avenue Claude Guillemin, 45060 Orl\'{e}ans
C\'{e}dex, France}

\author{\firstname{C\'{e}cile} \lastname{Capderrey}\CDRorcid{0000-0003-1723-4279}}
\addressSameAs{1}{BRGM, 3 avenue Claude Guillemin, 45060 Orl\'{e}ans
C\'{e}dex, France}

\keywords{\kwd{Sea-level rise}
\kwd{Coastal}
\kwd{Adaptation coastal}
\kwd{Climate resilient development}}

\altkeywords{\kwd{\'{E}l\'{e}vation du niveau de la mer}
\kwd{Littoral}
\kwd{Adaptation c\^{o}ti\`{e}re}
\kwd{D\'{e}veloppement r\'{e}silient au climat}}

\thanks{European Union's Horizon 2020 research and innovation program
under grants agreements No 869304, 101003598 and 101180125; PROTECT
contribution number 85, French National Research Agency under FUTURISKS
(ANR-22-POCE-0002) project.}

\begin{abstract} 
As part of its third national adaptation plan to climate change, France
is developing new scenarios approximately aligned with the current
commitments of states to reduce their greenhouse gas emissions and
anticipating 3 \textdegree C of global warming by 2100. Here, we
present sea-level projections aligned with this assumption. New
methodological developments include a statistical treatment to ensure
the continuity of trends and uncertainties before and after 2100. Our
results show that there is a probability of approximately 50\% that the
sea-level rise guidance of 60 cm---used for coastal risk prevention in 
France---will be exceeded by 2100 at 3 \textdegree C global warming
levels (GWL). This no longer provides safety margins expected in 2011
based on the 4th Assessment Report of the IPCC, suggesting an update is
now needed. The sea-level scenarios presented here might support the
3rd National Adaptation Plan, published in March 2025. While the
publication of the plan was generally welcomed, concerns were raised
regarding the lack of specific adaptation objectives in coastal areas
and uncertainties regarding the potential use and enforcement of the
new climate change scenario. We argue that an open debate on the
political choices involved in coastal adaptation would build an
opportunity to create more resilient coastal zones for the benefits of
human communities and for economic activities and coastal ecosystems.
\end{abstract}

\begin{altabstract} 
Dans le cadre de son troisi\`{e}me plan national d'adaptation au
changement climatique, la France a s\'{e}lectionn\'{e} un sc\'{e}nario
de changement climatique approximativement align\'{e} sur les
engagements actuels des \'{E}tats en mati\`{e}re de r\'{e}duction des
\'{e}missions de gaz \`{a} effet de serre, amenant \`{a} un
r\'{e}chauffement climatique global de 3 \textdegree C d'ici 2100. Nous
pr\'{e}sentons ici des projections du niveau de la mer coh\'{e}rentes
avec ce sc\'{e}nario. Sur le plan m\'{e}thodologique, nous proposons un
traitement statistique garantissant la continuit\'{e} des tendances et
des incertitudes avant et apr\`{e}s 2100. Nos r\'{e}sultats indiquent
que la probabilit\'{e} de d\'{e}passer le seuil de 60 cm
d'\'{e}l\'{e}vation du niveau de la mer en 2100 --- actuellement
utilis\'{e} en France pour la pr\'{e}vention des risques c\^{o}tiers
--- est d'approximativement 50 \% pour un niveau de r\'{e}chauffement
global de 3 \textdegree C. Nous notons que lorsque le sc\'{e}nario de
60 cm d'\'{e}l\'{e}vation du niveau de la mer a \'{e}t\'{e}
int\'{e}gr\'{e} \`{a} la r\'{e}glementation fran\c{c}aise en 2011, il
\'{e}tait consid\'{e}r\'{e} comme pessimiste. Ceci sugg\`{e}re qu'une
mise \`{a} jour est aujourd'hui n\'{e}cessaire afin de pr\'{e}server
les marges de s\'{e}curit\'{e}s qui avaient \'{e}t\'{e} anticip\'{e}es
\`{a} l'\'{e}poque. Les sc\'{e}narios que nous proposons pourraient
soutenir le troisi\`{e}me plan national d'adaptation, publi\'{e} en
mars 2025. Bien que la publication de ce plan ait \'{e}t\'{e}
globalement bien accueillie, des pr\'{e}occupations ont \'{e}t\'{e}
exprim\'{e}es quant \`{a} l'absence d'objectifs d'adaptation pr\'{e}cis
pour les zones c\^{o}ti\`{e}res et la mani\`{e}re dont seront
utilis\'{e}s les nouveaux sc\'{e}narios climatiques pour soutenir
l'adaptation. Nous d\'{e}fendons l'id\'{e}e qu'un d\'{e}bat ouvert sur
les choix politiques en mati\`{e}re d'adaptation c\^{o}ti\`{e}re
pr\'{e}senterait une opportunit\'{e} pour rendre les zones
c\^{o}ti\`{e}res plus r\'{e}silientes et b\'{e}n\'{e}fiques pour les
communaut\'{e}s humaines, des activit\'{e}s \'{e}conomiques et des
\'{e}cosyst\`{e}mes littoraux.
\end{altabstract} 

\maketitle

\vspace*{7pt}

\twocolumngrid

\end{noXML}

\defcitealias{Fox-Kemperetal2021}{ibid.}
\defcitealias{MTECT2023b}{ibid.}
\defcitealias{Edwardsetal2021}{ibid.}
\defcitealias{MEDDTL2011}{ibid.}
\defcitealias{OLearyetal2023}{ibid.}

\section{Introduction}\label{sec1}  
Sea-level rise is one of the most certain consequences of anthropogenic
climate change \citep{Fox-Kemperetal2021}. Due to global warming,
glaciers and ice-sheets are melting and adding mass to the ocean, and
the ocean is accumulating heat and expanding. Additionally, surface
water and groundwater management also contributes to sea-level changes,
albeit to a lesser magnitude. Globally, sea level is projected to rise
at rates between 5 to 30~mm/year by the end of the 21st century
depending on greenhouse gas emissions and the response of ice-sheets to
climate change \citepalias{Fox-Kemperetal2021}. In fact, sea levels are
projected to continue rising for decades and centuries. Hence,
mitigating climate change can only increase the probability of slower
rates of sea-level changes, but sea-level rise itself cannot be stopped
over the foreseeable 
\mbox{future.} 

Contemporary sea-level change is taking place in an unprecedented
context in human history because the ongoing urbanization of coastal
zones has resulted in higher than ever exposure to coastal \mbox{hazards} such
as coastal flooding, erosion and salinization. Globally, approximately
900 million people, or 11\% of the world's population, are currently
living below 10~m above sea levels 
{\citep{Haasnootetal2021}.} 
{In France,}
the focus of this article, the population density in coastal
municipalities is 2.5 times greater than the national average
\citep{MTECT2023a}. Exposure to sea-level rise is not only a concern
for coastal regions: it is also projected to affect human and economic
activities indirectly due to trade being highly dependent on ports and
harbors \citep{HansonNicholls2020,Verschuuretal2022}. 

To summarize, both the certainty of future sea-level rise and the
growing exposure to coastal hazards contribute to increase coastal
risks and adaptation needs. 

To support coastal adaptation planning, sea-level scenarios are
increasingly considered \citep{Nichollsetal2014}. In Europe, such
scenarios support a large panel of adaptation strategies, including the
production of maps to limit urban expansion in high-hazard areas or
virtual stress tests of critical coastal infrastructures such as
estuarine barriers \citep{Bednar-Friedletal2022,McEvoyetal2021}.
Designing and adopting such scenarios is not that simple. On the one
hand, sea-level rise displays regional and local variability due to
ocean density and circulation changes, atmospheric pressure variations
and due to the gravitational, rotational and deformational effects of
water and ice mass redistributions as well as vertical land motions
that can affect some areas locally \citep{Slangenetal2017}. On the
other hand, the utility of sea-level scenarios can be increased by
considering their intended application. For example, adaptation
planning of nuclear plants may consider scenarios involving an early
collapse of the west Antarctica ice-sheet, even if their likelihood is
very low, because damages to such infrastructures cannot be accepted.
Recognizing the variety of needs, previous national scenarios have
considered a spread of scenarios rather than a single value
\citep{McEvoyetal2021,Palmeretal2024,Weeksetal2023}.\looseness=1

In this paper, we present sea-level scenarios in support of the third
French adaptation plan to climate change. Since 2011, France is
considering a unique sea-level scenario of 60 cm with respect to the
end of the 21st century in order to support the implementation of its
coastal risk prevention plans \citep{LeCozannetCazenave2024}. To
support its third adaptation plan, the Ministry in charge of
Environment requested an update of this unique scenario considering
regional sea-level \mbox{variations.} \mbox{Interestingly,} the ministry requested a
unique scenario aligned with global warming levels (GWL) reaching
3~\textdegree C above the preindustrial era by 2100 \citep{MTECT2023b}.
One reason for selecting a specific scenario instead of a range of
values could have been to simplify the message for users, as suggested
in the document presenting the proposed national scenario
\citepalias{MTECT2023b}. Thus, this choice could aim to maximize stakeholder
engagement in adaptation planning, including those without advanced
knowledge of climate change.

A first research question is therefore to understand, contextualize and
refine this national scenario of 3~\textdegree C of global warming
\citep{Cabanaetal2023}, in order to design a method that responds to
the needs of this policy. This is addressed in  Section~\ref{sec2}. 

A second question is to develop the sea-level scenario itself, which
requires regionalizing sea-level rise scenarios dependent of global
temperatures, including beyond 2100. While such scenarios have been
delivered in the 6th Assessment report (AR6) of the IPCC
(Intergovernmental Panel on Climate Change), their projections are not
directly useable for two reasons: (1)~first, vertical land motions
other than those due to the Glacial Isostatic Adjustment are computed
using the non-climate signal in tide gauges; this approach misses local
ground motions not captured by tide gauges and reduces the confidence
in coastal projections, as acknowledged page 1301 of the Working Group
1 report \citep{Fox-Kemperetal2021};  (2)~the processing of
uncertainties in AR6 is not similar before and after 2100, leading to
breaks in rates and uncertainties of sea-level rise projections at this
time horizon. This difficulty needs to be addressed as the French
regulation explicitly requires to consider sea-level rise over the
coming century, that is, up to 2125. The methods used to produce
sea-level scenarios are presented in  Section~\ref{sec3} and the
results are shown in  Section~\ref{sec4}. 

A final question related to the production of these scenarios is to
understand to what extent they can support coastal adaptation
effectively. The utility of sea-level scenarios can be compromised due
to their intrinsic limitations or by ambiguities in their intended use.
In  Section~\ref{sec5}, we discuss the decision and policy context in
which they will be used and show that the lack of shared and explicit
objectives of coastal adaptation is a major issue that will require
policy decision. 

\section{Methods and data}\label{sec2} 

\subsection{Refining the French climate scenarios}\label{sec2.1}  
The third national adaptation plan of France is being prepared since
2022. One of the elements supporting this plan is a climate scenario
(TRACC, for ``trajectoire de r\'{e}chauffement de r\'{e}ference pour
l'adaptation au changement climatique'' or ``reference warming scenario
supporting climate change adaptation'', hereafter ``French climate
scenario'' or ``TRACC''). The TRACC is currently described in the
technical document supporting the consultation on the French climate
scenario that was openned in 2023 \citep{MTECT2023b}. In this document,
two scenarios were considered: 

\begin{itemize}
\item 
A first scenario in which global climate warming is stabilized at
1.5~\textdegree C GWL above preindustrial levels. This scenario is
aligned with the objectives of the Paris agreement. 
\item 
A second scenario in which global climate warming reaches 3~\textdegree
C GWL above preindustrial levels by 2100 and then is stabilized at this
level. This scenario is presented as an average between the commitments
made by the countries and the policies in place in 2020. It is referred
to as the ``3~\textdegree C scenario'' hereafter, though the
ministerial communication also refers to a ``4~\textdegree C
scenario'', which is approximately the level of warming in France for
3~\textdegree C of global warming. 
\end{itemize}
This 3~\textdegree C scenario was selected by the ministry to serve as
a reference for adaptation. 

It can be noted that as it is presented in the consultation document
\citepalias{MTECT2023b}, the 3~\textdegree C scenario is not fully aligned
with the current scientific knowledge. First, the Working Group III of
the  6th Assessment report of the IPCC and the 2022 emission gap report
shows that unconditional nationally determined contributions, and,
{a fortiori}, the policies in place, are not sufficient to reach
net-zero emissions by 2100 \citep{IPCC2022,UNEP2022}. Consequently, the
climate will continue to warm after 2100 if these policies and
contributions are implemented. This also poses practical issues to
design sea-level scenario because most climate scenarios available
today that reach 3~\textdegree C GWL in 2100 assume further warming
beyond 2100. Based on these arguments, we removed this constraint of a
stabilization of climate change at 3~\textdegree C GWL and developed
sea-level scenario that make no assumption regarding warming beyond
2100.

One assumption underlying these scenarios is that impacts of climate
change will be similar for a given level of warming whatever the
precise time horizon by which that level of warming is reached and as
long as climate change is stabilized at that particular global warming
level. This assumption is true for many impacts of climate change, as
illustrated by the burning embers in the IPCC reports
\citep{Zommersetal2020}, but it does not hold for sea-level rise due
to the latency of the response of the ocean and ice to global warming.
To resolve this issue, we tied each level of global warming to specific
time horizons. The refinements to these scenarios are presented in 
\mbox{Table~\ref{tab1}} below.

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}Approach used in this study to regionalize
sea-level scenarios. ``O(10)'' models means ``roughly 10'' models.}
\end{figure*}

\begin{table*}
\caption{\label{tab1}Climate scenarios used in this study}
\begin{tabular}{ccccc}
\thead
\xmorerows{2}{Time horizon} &amp; \multicolumn{4}{c}{Global warming above the
pre-industrial period  (1850--1900)}\\\cline{2-5}
 &amp; \multicolumn{2}{c}{``1.5~\textdegree C'' scenario} &amp; 
\multicolumn{2}{c}{French ``3~\textdegree C'' adaptation scenario}\\
\cline{2-3}\cline{4-5}
 &amp; Global mean &amp; Mainland France &amp; Global mean &amp; Mainland France \\
\endthead
2030--2040 &amp; 1.5~\textdegree C &amp; 2~\textdegree C &amp; 
1.5~\textdegree C &amp; 2~\textdegree C \\ 
2050 &amp; 1.5~\textdegree C &amp; 2~\textdegree C &amp; 
2~\textdegree C &amp; 2.7~\textdegree C \\ 
2100 &amp; 1.5~\textdegree C &amp; 2~\textdegree C &amp; 
3~\textdegree C &amp; 4~\textdegree C \\ 
2150 &amp; 1.5~\textdegree C &amp; 2~\textdegree C &amp; 
Not specified &amp; Not specified
\botline
\end{tabular}
\tabnote{\fontsize{9}{11}\selectfont These scenarios are refined from
the French climate scenarios. Because the ministerial and media
communication has used levels of warming in mainland France instead of
global warming levels, we present both global warming levels and their
implications in France to help the French readers find their way.}
\vspace*{-4pt}
\end{table*}

\subsection{Methods to produce sea-level scenarios\newline
consistent with the French climate scenarios}\label{sec2.2}  
The approach to develop sea-level scenarios presented in this paper is
shown in  Figure~\ref{fig1}. The overall principle is to select model
outcomes that meet the assumptions of  Table~\ref{tab1} for each
contribution to sea-level rise, to regionalize these contributions and
finally sum them up \citep{JacksonJevrejeva2016}. This  subsection
presents the details of this approach.

\subsubsection{Selection of simulations compliant with the French
climate scenarios (TRACC)}\label{sec2.2.1} 
Climate scenarios currently do not deliver results for a given level of
global warming but follow assumptions on future greenhouse gas
concentrations. Depending on the climate sensitivity of each climate
model, the same concentration of greenhouse gas may result in different
levels of global warmings. Hence, we proceeded by selecting climate
model simulations or emulator realizations that have a global mean
surface temperature trajectory consistent with those provided in 
Table~\ref{tab1}, considering a margin of 0.3~\textdegree C around each
temperature threshold. For the sterodynamic component of sea-level rise
(i.e., ocean density and circulation changes corrected from the inverse
barometric effect, which therefore includes the thermal expansion)
\citep{Gregoryetal2019},  we selected temperature trajectories
compliant with the French climate scenarios. For climate-dependent mass
components, an additional level of complexity results from the fact
that glaciers and ice-sheet models must be forced with climate
scenarios to obtain the resulting melting, which is computationally
demanding. For example, the Greenland ice-sheet melting experiments of
the Ice Sheet Model Intercomparison project for CMIP6 (ISMIP6)
considers 14 ice-sheet models (with up to 2 or 3 different initial
conditions), 2 greenhouse gas emission scenarios and 6 climate models.
This is not sufficient to cover all potential climate scenarios of
interest \citep{Goelzeretal2020}. To overcome this difficulty,
emulators of ice melting models have been produced
\citep{Edwardsetal2021}. Emulators are statistical approximations of
the response of glaciers or ice-sheet models trained over a limited set
of computer\break experiments. 

Figure~\ref{fig2} presents a selection of glacier model projections
from the Emulandice emulator \citepalias{Edwardsetal2021}, based on the
French 3~\textdegree C climate scenario. Among the  ${\sim}$3000
emulated simulations, about 250 reach 2~\textdegree C GWL in 2050 and
3~\textdegree C GWL in 2100. This selection of 250 emulated simulations
gives a median glacier contribution to sea-level rise of 
$5.5\pm1.0$~cm in 2050~and  $14.0\pm2.5$~cm in 2100 with respect to the
reference period 1995--2014.

\begin{figure*}
\vspace*{-1pt}
\includegraphics{fig02}
\vspace*{-1pt}
\caption{\label{fig2}Subset of emulated glacier melting projections
meeting the French climate projection assumptions. The right panel
displays the projected temperature of emulated model projections
meeting these assumptions. The left panel shows the resulting
contribution of glaciers to sea-level rise to 2100. Note that
simulations meeting the constraints are not limited to RCP4.5 or
SSP2-4.5 scenarios, because some climate models have lower climate
sensitivity and only reach 3~\textdegree C by 2100 despite higher
greenhouse gas emissions.}
\vspace*{-1pt}
\end{figure*}

\subsubsection{Climate and emulated ice-melting projections and the
case of Antarctica}\label{sec2.2.2} 
The specific datasets used in this study are listed in  Supplementary
Material~1. Specifically, we use CMIP6 and the emulated sterodynamic
component \citep{Fox-Kemperetal2021}, GlacierMIP simulations for
glaciers \citep{Marzeionetal2020},  ISMIP6 emulated projections of
Greenland ice-sheet melting \citep{Edwardsetal2021,Goelzeretal2020},
projections of land water contributions from the AR6
\citep{Fox-Kemperetal2021}  and the GIA model results of 
\citet{Caronetal2018}. For Antarctica, two datasets are used: emulators
of ISMIP6 simulations \citep{Seroussietal2020} and LARMIP-2 simulations
\citep{Levermannetal2020}. In addition, we use separately simulations
considering the potential for an early onset of the Antarctica
ice-sheet  \citep{DeContoetal2021}. These simulations consider the
possibility of an onset of Marine Ice-Cliffs Instability (MICI), a
low-likelihood but high impact event that cannot be excluded according
to the  6th assessment report of the IPCC \citep{Ariasetal2021}. 
\looseness=1

\subsubsection{Projections beyond 2100}\label{sec2.2.3} 
Coastal risk prevention plans in France consider coastal impacts of
sea-level rise over the coming 100 years, that is, 25 years beyond
2100. Yet many datasets used in this study are only available until
2100. Illustrative model simulations beyond 2100 start to be available
in the literature, including for intermediate scenarios extending the
SSP2-4.5 \mbox{scenario} \citep{Turneretal2023}. However, given that the
policy need holds only until 2125, we use a quadratic extrapolation of
Emulandice and LARMIP-2 simulations to deliver projections beyond 2100.
This 
\mbox{approach} allows avoiding discontinuities in the trends around
2100. Note that the emulator uncertainties are also extrapolated to
avoid a spurious reduction of the uncertainty envelop beyond 2100 (see 
Supplementary Material~2 for details). Notwithstanding, the confidence
in the results decrease as the projections are extrapolated farther in
the future. Hence, we limited these projections to 2120 and
referred to SSP projections of the AR6 to deliver indicative values to
2150 (see  Supplementary Material~3). 

\subsubsection{Regionalization and probabilistic
aggregation}\label{sec2.2.4}
The methods to regionalize sea-level rise have been established about
10 years ago
\citep{Slangenetal2012,Slangenetal2014,Koppetal2014} 
and taken over in the  5th and 
6th assessment reports of the IPCC. The principle is to regionalize the
impacts of each component first, then sum these components to deliver
regional sea-level projections. The physical mechanisms to be
considered are different for sterodynamic sea-level changes, for
sea-level changes involving a mass component, and for relative
sea-level changes due to 
\mbox{vertical} ground motions
\citep{Gregoryetal2019}: 
\begin{itemize} 
\item 
Sterodynamic sea-level changes representing the thermal expansion plus
the ocean dynamic corrected from the inverse barometer effects are
obtained from climate model simulations (i.e., CMIP6) with adjusted
climate sensitivities as in the  6th Assessment Report of the IPCC.  
\item 
For mass components, that is, those related to glaciers and ice-sheet
melting as well as land water, the principle is to consider the
gravitational, rotational and deformational effects induced by the mass
redistributions.
\item 
Vertical ground motions include the glacial isostatic adjustment (GIA),
the response of the solid Earth to the last deglaciation, for which
models exist already \citep{Caronetal2018}, as well as other subsidence
and uplift processes due to, e.g., groundwater pumping, seismic processes
or variations of the water content in recent sedimentary  layers
\citep{Nichollsetal2021}.
\end{itemize}

In this work we follow this standard approach, using codes developed in
R whose details have been presented in previous articles
\citep{Dayanetal2021,LeCozannetetal2019a,Thieblemontetal2019}.  The
main change is that we use the static fingerprints used in AR6 to
regionalize the mass components
\citep{Fox-Kemperetal2021,Koppetal2023},  instead of those used in
previous report \citep{Churchetal2013,Oppenheimeretal2019}.  As in
previous work, we exclude local vertical ground motions (besides GIA), 
because they can be assessed in various ways, e.g., using tide gauge
records as in the  6th Assessment Report of the IPCC, using Global
navigation satellite system GNSS data, levelling data or satellite
Interferometric synthetic aperture  radar
\citep{WoppelmannMarcos2016,Thieblemontetal2024}.  These processes are
typically best understood and quantified locally where geologists can
interpret the processes taking place in their region of interest, and
more research is needed to be able to include them in regional
projections without decreasing the confidence in these projections
\citep{Slangenetal2023}.

We apply a Monte-Carlo approach to provide probabilistic projections.
Specifically, we randomly select (sampling with replacement)
simulations out of those available, for example one out of the 250
simulations available from Emulandice for the 3~\textdegree C scenario.
Dependencies between components can have an impact to uncertainties
\citep{LeBars2018}, especially at the tail of the distribution
\citep{LeCozannetetal2019a}.  Here we assume perfect dependence between
the different glaciers regions and independence with other
contributions. Since we select models reaching the same levels of
global warming, this choice has limited impacts to the uncertainties of
projections, especially within the  5--95th range of percentiles. For
the Antarctic component of the TRACC scenarios without MICI, two
datasets are available: LARMIP-2 and Emulandice. We extract the
simulation that has the greatest deviation from the mean of the medians
of the two datasets. This procedure leads to maximizing the
uncertainties of the Antarctica ice-sheet component, which is in line
with the high uncertainties surrounding Antarctica's response to global
warming. A total of 5000 random samples was carried out for each
global warming level and time horizon.

\section{Results}\label{sec3}
The sea-level scenarios corresponding to a stabilization of climate
change at 1.5~\textdegree C GWL, and a warming leading to 2~\textdegree
C GWL by 2050 and 3~\textdegree C GWL by 2100 are presented in 
Figures~\ref{fig3} and~\ref{fig4} bellow, respectively, as well as in 
Table~\ref{tab2}. These scenarios are consistent with the assumptions
of the French climate scenarios presented in  Section~\ref{sec2.1}
above. 

\begin{figure*}[t!]
{\vspace*{-3pt}}
\includegraphics{fig03}
{\vspace*{-3pt}}
\caption{\label{fig3}Median regional sea-level projections for a
stabilization of climate at 1.5~\textdegree C GWL. Projections are
given in meters with respect to the reference period 1995--2014. Pink
triangles indicate the location of sites given in Supplementary
Material~3. These projections follow the French climate scenario that
is indicatively given to remind the climate mitigation objective of the
Paris agreement (see Section~\ref{sec2.1}).}
{\vspace*{-3pt}}
\end{figure*}

\begin{figure*}[t!]
{\vspace*{-2pt}}
\includegraphics{fig04}
{\vspace*{-2pt}}
\caption{\label{fig4}Median regional sea-level projections for climate
change scenario reaching 3~\textdegree C GWL in 2100 as assumed in the
French Climate Scenario (TRACC). Projections are given in meters with
respect to the reference period 1995--2014. Pink triangles indicate the
location of sites given in  Supplementary Material~3. }
{\vspace*{-2pt}}
\end{figure*}

\begin{table*}
\caption{\label{tab2}Global mean sea-level rise projections, in cm with
respect to the reference period 1994--2015  (50th [17th--83rd]
percentiles)}
\begin{tabular}{cccc} 
\thead
Scenario and time horizon &amp; 2050 &amp; 2100 &amp; 2120 \\ 
\endthead
1.5~\textdegree C &amp; 18 [15--22] &amp; 41 [29--54] &amp; 49 [33--68] \\ 
3.0~\textdegree C &amp; 21 [17--25] &amp; 60 [48--77] &amp; 80 [61--104] \\
3.0\textdegree\ (with models including MICI) &amp; 20 [17--22] &amp; 
63 [53--76] &amp; 86 [71--107] 
\botline
\end{tabular}
\vspace*{-4pt}
\end{table*}

\subsection{Regional projections by 2050, 2100 and 2120}\label{sec3.1} 
By 2050, sea-level at 1.5~\textdegree C or 2~\textdegree C GWL are
projected to rise by approximately 20 to 30~cm above 1995--2014 levels
in mainland France and inhabited overseas territories. Yet, in some
other regions, sea-level projections lie outside this range. For
example, off the eastern coast of the United States, sea levels are
\mbox{projected} to rise above 30~cm due to the combination of sterodynamic
effects \citep{Couldreyetal2021}  and background subsidence due to the
glacial isostatic adjustment \citep{Loveetal2016}. Sea levels are still
projected to decrease in Scandinavia due to the GIA effects. The
gravitational, rotational and deformational effects caused by the
melting of ice in Antarctica, Greenland and in the arctic glaciers are
already visible by 2050: our projections display a drop in sea-level
changes visible close to the region of mass loss by that time horizon
already. By 2050, the differences between the 1.5~\textdegree C and
2~\textdegree C GWL scenarios are small, because sea-level changes for
the \mbox{coming} three decades are very much constrained by past greenhouse
gas emissions \citep{Hinkeletal2019}. 

In 2100, the disparity between the 1.5~\textdegree C and 3~\textdegree
C GWL scenarios is more evident. Under the 1.5~\textdegree C GWL
scenario, the median sea-level rise for metropolitan France and
inhabited overseas territories is projected to lie in the range of
40--50~cm. In contrast, the median sea-level rise exceeds 60~cm for
3~\textdegree C GWL. Even by 2120, the 1.5~\textdegree C scenario
indicates that the median sea-level rise would remain below 60~cm along
most of the French coastlines.

\subsection{Impacts of an early onset of ice-sheet collapse in
Antarctica}\label{sec3.2}  
As explained in  Section~\ref{sec2.2.2}, we explore the potential
contribution of an early collapse of the Antarctica ice-sheet by
replacing the contribution of Antarctica from Emulandice and LARMIP-2
\citep{Edwardsetal2021,Levermannetal2020}  by a dataset implementing
the  MICI \citep{DeContoetal2021}, that is, following the same approach
as in AR6 \citep{Fox-Kemperetal2021}. The 83rd percentiles of these
simulations are presented and compared in  Figure~\ref{fig5}. We
present the  83rd percentile of simulations to be consistent with the
narrative of the ``low-likelihood high impact'' event presented in AR6.

\begin{figure*}
{\vspace*{4pt}}
\includegraphics{fig05}
{\vspace*{4pt}}
\caption{\label{fig5}Regional sea-level projections in 2050, 2100 and
2120 (in meters) corresponding to the 83rd percentile of the
3~\textdegree C GWL level scenario with respect to  1995--2014 with
MICI (left) and without MICI (right).}
{\vspace*{2pt}}
\end{figure*}

In 2100, sea-levels are projected to lie between 80 and 90~cm along
most coastal areas of mainland France and inhabited overseas
territories and regions (83rd percentile of projections). By 2120 the
projections are 10 to 15cm higher in tropical overseas regions and
territories than in mainland France due to the gravitational and
rotational effects of ice-melting. They exceed 1.20~m in La R\'{e}union
island, where both the gravitational and rotational effects of ice melt
are maximized. Importantly, the simulations with and without MICI are
similar, reflecting that 3~\textdegree C GWL is too low to
significantly activate the MICI in the simulations of 
\citet{DeContoetal2021} and that different statistical processing of
uncertainties have been 
\mbox{performed} for the simulations with and without
MICI (see Section~\ref{sec2.2.2}). 

\subsection{Relation with the adaptation policy}\label{sec3.3} 
In France, the primary adaptation policy that explicitly incorporates
sea-level projections is the implementation of coastal risk prevention
plans \citep{LeCozannetCazenave2024}.  These plans use numerical flood
simulations to prescribe accommodation measures in urbanized areas and
to limit further urbanization in the most exposed areas that have not
been developed yet \citep{DGPR2014}. Since 2011, a sea-level scenario
of 60~cm sea-level rise with respect to the end of the  20th century is
considered and added in flood simulations representing potential
flooding in 2100 \citep{MEDDTL2011}. 

For national authorities, a first difficulty arises due to
inconsistencies in the time horizons considered in the various articles
of law, decree and technical guidance. The regulation on the coastal
prevention risks (Article R562-11-5 of the Environmental Code,
available
\url{https://www.legifrance.gouv.fr/codes/article\_lc/LEGIARTI000038733755}) 
requires considering coastal hazard changes in 100 years. However, the
decree of 2011 gives a scenario of sea-level rise to 2100 
\citepalias{MEDDTL2011}. Yet the two time horizons---2100 and in  100
years---have been confused in the technical guidelines of 2014
delivered to practitioners and coastal consultants in charge of
implementing the regulation  \citep{DGPR2014}. While this did not make
a big difference in the early 2010s, considering a scenario in 2120 or
2125 now adds immediately about 20~cm to the national sea-level rise
guidance. For national authorities, resolving these inconsistencies
might be seen as a sensitive challenge due to the potential
implications for land use development in the low lying coastal plains.

A second challenge for national authorities will be to decide how the
new climate change scenario will be enforced in the area of coastal risk
prevention. A most immediate outcome of the adoption of the TRACC as a
reference scenario for coastal adaptation in France would be to update
the uniform guidance of 60~cm of sea-level rise used within the coastal
risk prevention plan policy. Importantly, we note that the decree that
established this guidance in 2011 considered this 60~cm sea-level rise
scenario as \mbox{``pessimistic''} \citep{MEDDTL2011}. Furthermore, the second
national adaptation plan explicitly mentioned that its aim was to adapt
to 2~\textdegree C GWL 
(\xcitealp{MTES2018}{2018}---see page~1). At that time, the
60~cm sea-level rise scenarios could still be justified by the fact
that these 60~cm corresponded to the upper limit of the likely range of
sea-level projections for the RCP2.6 scenario shown in the  5th
Assessment report of the IPCC. This would now correspond to a 83rd
percentile of probabilistic projections.

As sea-levels are projected to rise well above 60~cm sooner or later,
a key question raised by the French coastal risk prevention plans is
to determine when the sea-level guidance will be exceeded and the
benefits of this particular policy would be lost. In fact, once the
sea-level scenario will be exceeded, areas unaffected by development
restrictions will start to be exposed to flooding. According to the
sea-level scenarios shown above, there is approximately a 
\mbox{50--50} chance
that the 60~cm sea-level guidance is exceeded between 2120 and 2150 in
mainland France and overseas regions and territories at 1.5~\textdegree
C GWL (see  Supplementary Material~3). For the 3~\textdegree C GWL
scenario, the probability to exceed the 60~cm sea-level guidance is
above 50\% in 2100. This gives little
security margin, especially if we
consider that global warming levels above 3~\textdegree C GWL cannot be
excluded. 

Based on this analysis above, we note that selecting the 83rd
percentile of the 3~\textdegree C GWL scenario (without MICI) as a new
guidance would preserve the same security margins that were originally
anticipated when the national scenario was adopted in 2011 and when the
second national adaptation plan was published in 2018. As shown in 
Section~\ref{sec3.2}, the 83rd percentiles of the simulations with and
without MICI are very similar at 3~\textdegree C GWL. If
sea levels rise according to the median in the real world, having
chosen a higher percentile as the reference scenario will provide one
or two extra decades to prepare for sea levels that will be exceeded in
any case. This decision belongs to the ministry in charge of
Environment and has not been announced yet at the date of submission of
this article. 

One advantage of choosing a unique climate change scenario of
3~\textdegree C GWL could be to give a simple signal to the numerous
actors who are still in the early stages of engaging into adaptation.
However, for advanced users, higher scenarios (e.g., 5~\textdegree C
GWL) can be more relevant if their time horizon and aversion to
uncertainties justifies it \citep{Hinkeletal2019}. 
\looseness=-1

\section{Discussion}\label{sec4} 

\subsection{Limitations of this study}\label{sec4.1}
The sea-level projections developed within this study rely on
state-of-the-art modeling of each component of sea-level rise and used,
e.g., in AR6. These projections could be challenged in the future. Yet,
it can be noticed that the likely range of sea-level projections has
been remarkably consistent across assessments since 40 years
\citep{Garneretal2018}.  In contrast, high-end, or
low-likelihood/high-impact sea-level projections have shown
considerable variations \citep{PiecuchDas2022}. Hence it cannot be
ruled out that future projections of polar ice sheet melting are
revised upward in the future. 

The main limitations related to the regionalization are threefold:
first, coastal processes at higher resolutions than 
$1\text{\textdegree}\times1\text{\textdegree}$ are not represented in our
projections, although they can alter sterodynamic sea-level projections
by about 20\% on the continental shelf or in the Mediterranean
\citep{Chaigneauetal2024,Jevrejevaetal2024,Sanninoetal2022}. Second,
the regionalization of ice mass losses could be improved a bit with a
better characterization of the specific locations of melting areas in 
Greenland \citep{Meyssignacetal2017}. Lastly, vertical land motions
other than GIA can also add a sizeable contribution to relative
sea-level change. In mainland France, these processes are much less
critical than in Asia, but they still exceed 1~mm/year over more than
half of the low-lying coastal flood plains \citep{Thieblemontetal2024}.
It can be noted that the national guidance of coastal risk prevention
plans leaves the possibility to consider local knowledge on vertical
land motions and revise the national sea level rise scenario upward. To
our knowledge, this has never been done in a regulatory prevention
plan. Based on the literature above, we estimate that the final impacts
to the total mean relative sea-level projections in mainland France
would be limited to 20 or 30~cm in the worst case. In overseas regions
and territories, vertical land motions are less well characterized.
Thus, we are not able to bound uncertainties of relative sea-level
projections in these overseas regions and territories. 

A final difficulty of these projections is their use for the coming
decades for problems such as the adaptation to high-tide flooding
events that have started to emerge due to sea-level rise
\citep{SweetPark2014,Thieblemontetal2023}.  Making sure that the
projections can make sense locally and are calibrated accurately with
observations will require further observations, reanalysis of
historical sea levels and research.

\subsection{Bridging the gap from sea-level scenarios to coastal
adaptation: the need for a clear\newline 
strategy}\label{sec4.2}
While the limitations listed above are important, they are clearly not
the main barrier preventing coastal adaptation to advance in France.
Understanding these barriers requires explaining the policy and
regulatory context in which these scenarios are developed
\citep{Cabanaetal2023}. 

Since June 2022, the Ministry in charge of environment repeatedly
postponed the publication of the national adaptation plan (See 
Supplementary Material~4). During the preparatory work and
consultations, from 2022 to 2024, the governmental communication
generally focused on the levels of warming required for the national
scenario supporting the plan rather than on the specific measures of
the plan themselves. This is evidenced in the selection of articles
shown in  Supplementary Material~4 illustrating this statement. Very
few objectives, political choices and financing mechanism supporting
adaptation can be identified in these articles. Yet, the climate
scenario itself was extensively discussed. When the national adaptation
plan was finally published in March 2025, its publication was welcomed
by the French Climate Council, which also raised concerns about its
legal fragility and how the new national climate change scenario would
be used and enforced  \citep{HautConseilpourleClimat2025}.

We argue that the discussion around the choice of global warming
scenarios is a technical one that is intended to support adaptation
professional and requires accurate knowledge of their decision context.
In the short term, a discussion around scenarios could stimulate
technical measures such as infrastructure reinforcement or
accommodation or adjustments in the regulation. Yet, we found no
evidence that this discussion facilitated adaptation stakeholders in
engaging in an informed process clearly highlighting the benefits and
trade-offs involved in upcoming coastal adaptation decisions. We argues
that scenarios support coastal adaptation in a useful way, but that
they cannot replace a strategy with clear and realistic objectives and
shared by all\break stakeholders.

There is a risk of locking-in coastal areas in current development
patterns if no clear strategy toward sustainability is set out. This
also increases the risks of social conflicts around coastal adaptation.
For \mbox{example} the newspaper article dated  9th April 2023 in 
Supplementary Material~4 suggests that the stage is set for conflicts
to emerge between social groups in favor of relocation and those
supporting engineered protection. Such a conflict could emerge after
the next major storm or while discussing any coastal adaptation
decision in favor of any of these options. An additional risk is that
by focusing on sea-level scenarios rather than on the policy choices
involved in coastal adaptation \citep{PerherinMeur-Ferec2022}, we miss
the opportunity to take advantage of the inevitable land use changes
induced by rising sea levels to create more resilient coastal systems,
which would be ultimately beneficial for human communities and for
economic activities and coastal ecosystems.

\subsection{The national adaptation plan: an\newline 
opportunity to progress toward coastal\newline 
resilient development?}\label{sec4.3}
At timescales of centuries, sea levels are projected to rise by several
meters \citep{Fox-Kemperetal2021}. In low-lying areas, responses to
such large sea-level changes will either involve relocation or
engineered protection \citep{LeCozannetetal2023,Muccioneetal2024}. 
This is because in the majority of cases, accommodation and nature
based solutions will lose effectiveness with larger and quicker
sea-level changes \citep{Bednar-Friedletal2022}. Both options change
will involve large costs and stringent changes in land use, but they
can be anticipated to minimize the negative impacts of current
decisions for future  generations \citep{LeCozannetetal2023}.

One obvious opportunity is to take advantage of the unavoidable
investments in coastal adaptation to protect and restore coastal and
estuarine ecosystems and improve their management, in order to enhance
their structure and functioning and then their ability to cope with
sea-level rise and other anthropogenic pressures.  Figure~\ref{fig6}
provides a selection of such 
{actions,} widely grouped under the 
{umbrella}
term {Nature} Based Solution (NBS), and their {co-benefits} 
relevant to
coastal environments that can be found in mainland France and overseas
regions and territories.  Table~\ref{tab3} 
shows that such management
actions are already implemented in many coastal locations in Europe. A
challenge in this area is to upscale local experiments in order to
progress toward resilient coastal zones management
\citep{Bednar-Friedletal2022}. \looseness=-1

\begin{figure*}
\vspace*{2pt}
\includegraphics{fig06}
\caption{\label{fig6}Examples of coastal Nature-Based Solutions (NBS)
and their co-benefits for different types of coastal environments
relevant to mainland France and overseas regions and territories.
NBS refer to a range of established approaches (restoration, protection,
ecosystem management) simultaneously providing societal and
environmental co-benefits (Figure adapted from ``Geochronique'',
N\textdegree164, Soci\'{e}t\'{e} G\'{e}ologique 
de France).}
\vspace*{3pt}
\end{figure*}

\begin{sidewaystable*}
\vspthree
\caption{\label{tab3}Examples of NBS already implemented across Europe
and an insight of their associated level of transformative change}
\fontsize{7.75}{9.75}\selectfont\tabcolsep=3pt
\begin{tabular}{cccccccc}  
\thead
\parbox[t]{2.3cm}{\centering Project name and location} &amp; 
Ecosystem &amp; 
\parbox[t]{2.3cm}{\centering Ecosystem services targeted} &amp; 
\parbox[t]{2.3cm}{\centering Co-benefits of the project} &amp; 
\parbox[t]{2.3cm}{\centering Drivers of decision} &amp; 
\parbox[t]{2.3cm}{\centering Type of actions} &amp; 
\parbox[t]{2.3cm}{\centering Current and projected effectiveness} &amp; 
References
\vspace*{2pt} \\ 
\endthead
\parbox[t]{2.3cm}{\centering
Scheldt estuary: Hedwig-Prosper polder dike-retreat\Lbreak
\raisebox{1pc}{}
(Belgium)} &amp; 
\parbox[t]{2.3cm}{\centering
Saltmarhes and freshwater marshes} &amp; 
\parbox[t]{2.3cm}{\centering
Flood risk reduction} &amp; 
\parbox[t]{2.3cm}{\centering
Biodiversity\Lbreak
\raisebox{1pc}{}
Cultural (educational and recreational)} &amp; 
\parbox[t]{2.3cm}{\centering
Catastrophic floods and deaths} &amp; 
\parbox[t]{2.3cm}{\centering
Flood control areas\Lbreak 
\raisebox{1pc}{}
Managed realignment} &amp; 
\parbox[t]{2.3cm}{\centering
Consideration of~ecosystem adaptation capacity until 2100} &amp; 
\parbox[t]{3cm}{\centering
\url{https://climate-adapt.eea.europa.eu/fr/metadata/case-studies/a-transboundary-depoldered-area-for-flood-protection-and-nature-hedwige-and-prosper-polders}\Lbreak
\raisebox{1pc}{}
\url{https://www.vliz.be/imisdocs/publications/366209.pdf}} 
\vspace*{2pt}\\ 
\parbox[t]{2.3cm}{\centering
Scheldt estuary: Lippenbroek polder, Kruibeke polder\Lbreak 
\raisebox{1pc}{}
(Belgium)} &amp; 
\parbox[t]{2.3cm}{\centering
Saltmarhes and freshwater marshes} &amp; 
\parbox[t]{2.3cm}{\centering
Flood risk reduction} &amp; 
\parbox[t]{2.3cm}{\centering
Biodiversity\Lbreak
\raisebox{1pc}{}
Cultural (educational and recreational)} &amp;
\parbox[t]{2.3cm}{\centering
Catastrophic floods and deaths} &amp;
\parbox[t]{2.3cm}{\centering
Controlled reduced tide (temporary storages areas for overflow water)} &amp;
\parbox[t]{2.3cm}{\centering
Consideration of ecosystem adaptation capacity until 2100} &amp;
\parbox[t]{3cm}{\centering
\url{https://www.sigmaplan.be/en/projects/polders-kruibeke}}
\vspace*{2pt}\\
\parbox[t]{2.3cm}{\centering
Humber estuary: Alkborough Flats Managed Realignment\Lbreak
\raisebox{1pc}{}
(United Kingdom)} &amp;
\parbox[t]{2.3cm}{\centering
Saltmarhes and freshwater marshes} &amp;
\parbox[t]{2.3cm}{\centering
Flood risk reduction} &amp;
\parbox[t]{2.3cm}{\centering
Biodiversity: compensation site for habitat loss elsewhere in the
estuary} &amp;
\parbox[t]{2.3cm}{\centering
Exposed human activities: major part of the Environment Agency's flood
risk management strategy on the estuary} &amp;
\parbox[t]{2.3cm}{\centering
Managed realignment} &amp;
\parbox[t]{2.3cm}{\centering
Reduction of effectiveness expected due to accretion at the site} &amp;
\parbox[t]{3cm}{\centering
\url{https://www.therrc.co.uk/sites/default/files/projects/54\_alkborough.pdf}}
\vspace*{2pt}\\
\parbox[t]{2.3cm}{\centering
Hondsbossche and Pettemer Sea Dike\Lbreak
\raisebox{1pc}{}
(The Netherlands)} &amp;
\parbox[t]{2.3cm}{\centering
Beach and dune systems} &amp;
\parbox[t]{2.3cm}{\centering
Flood risk reduction} &amp;
\parbox[t]{2.3cm}{\centering
Biodiversity\Lbreak
\raisebox{1pc}{}
Educational and recreational} &amp;
\parbox[t]{2.3cm}{\centering
Decrease of the safety levels provided by the traditional engineering
structure} &amp;
\parbox[t]{2.3cm}{\centering
Removal of dikes and creation of a ``soft barrier'' as a new beach dune
system} &amp;
\parbox[t]{2.3cm}{\centering
Currently effective, the natural dune growth is predicted to keep up
with sea level rise up to 2050} &amp;
\parbox[t]{3cm}{\centering
\url{https://www.ecoshape.org/en/cases/sand-nourishment-hondsbossche-dunes-nl/}}
\vspace*{2pt}\\
\parbox[t]{2.3cm}{\centering
Urban hybrid dunes in Barcelona\Lbreak
\raisebox{1pc}{}
(Spain)} &amp;
\parbox[t]{2.3cm}{\centering
Beach and dune systems} &amp;
\parbox[t]{2.3cm}{\centering
Erosion risk reduction} &amp;
\parbox[t]{2.3cm}{\centering
Biodiversity\Lbreak
\raisebox{1pc}{}
Awareness} &amp;
\parbox[t]{2.3cm}{\centering
Destruction caused by urbanization} &amp;
\parbox[t]{2.3cm}{\centering
Beach nourishment and planting dunes with pioneer grass
(\textit{Ammophila} sp.)} &amp;
Not measured &amp;
\parbox[t]{3cm}{\centering
\url{https://oppla.eu/casestudy/17283}}
\vspace*{2pt}\\
\tabthreesplittabular
\parbox[t]{2.3cm}{\centering
Mud motor Port of Harlingen--Koehoal\Lbreak
\raisebox{1pc}{}
(The Netherlands)} &amp;
\parbox[t]{2.3cm}{\centering
Saltmarshes} &amp;
\parbox[t]{2.3cm}{\centering
Biodiversity: restoration, conservation} &amp;
\parbox[t]{2.3cm}{\centering
Erosion reduction} &amp;
\parbox[t]{2.3cm}{\centering
Loss of saltmarshes in the Wadden Sea} &amp;
\parbox[t]{2.3cm}{\centering
Dredging and disposing fine sediments to stimulate the growth of salt
marshes} &amp;
\parbox[t]{2.3cm}{\centering
Currently effective: high sediment transport rates} &amp;
\parbox[t]{3cm}{\centering
\url{https://www.ecoshape.org/en/cases/re-use-of-dredged-sediment-mud-motor-in-port-of-harlingen-koehoal-nl/}}
\vspace*{2pt}\\
\parbox[t]{2.3cm}{\centering
Coastal erosion control in Paphos\Lbreak
\raisebox{1pc}{}
(Cyprus)} &amp;
\parbox[t]{2.3cm}{\centering
Beach and dune systems} &amp;
\parbox[t]{2.3cm}{\centering
Regulating \Lbreak
\raisebox{1pc}{}
Erosion risk reduction} &amp;
\parbox[t]{2.3cm}{\centering
Water quality\Lbreak
\raisebox{1pc}{}
Recreation} &amp;
\parbox[t]{2.3cm}{\centering
Local threat (resort)} &amp;
\parbox[t]{2.3cm}{\centering
Removal of groynes} &amp;
\parbox[t]{2.3cm}{\centering
Currently effective; observed accretion and a newly formed beach} &amp;
\parbox[t]{3cm}{\centering
\url{https://discomap.eea.europa.eu/map/Data/Milieu/OURCOAST\_172\_CY/OURCOAST\_172\_CY\_Case\_CoastalErosion\_OverstructuredBeach.pdf}}
\vspace*{2pt}\\
\parbox[t]{2.3cm}{\centering
Mondego estuary\Lbreak
\raisebox{1pc}{}
(Portugal)} &amp;
Saltmarshes &amp;
\parbox[t]{2.3cm}{\centering
Water quality} &amp;
\parbox[t]{2.3cm}{\centering
Erosion reduction} &amp;
\parbox[t]{2.3cm}{\centering
Increased eutrophication and ecological losses} &amp;
\parbox[t]{2.3cm}{\centering
Reduction nutrient inputs from the catchment\Lbreak
\raisebox{1pc}{}
Improving water circulation through connexion of formerly embanked
channels} &amp;
\parbox[t]{2.3cm}{\centering
Currently effective; no harmful bloom recorded, healthy seagrass beds} &amp;
\parbox[t]{2.8cm}{\centering
\citep{Dolbethetal2010}\Lbreak
\raisebox{1pc}{}
\citep{Lepageetal2022}}
\vspace*{2pt}\\
\parbox[t]{2.3cm}{\centering
Mortagne polder in the Gironde estuary\Lbreak
\raisebox{1pc}{}
(France)} &amp;
Saltmarshes &amp;
Biodiversity &amp;
\parbox[t]{2.3cm}{\centering
Water quality and nutrient cycling} &amp;
\parbox[t]{2.3cm}{\centering
Accidental depoldering after Martin storm (1999)} &amp;
\parbox[t]{2.3cm}{\centering
Willingness to let the system evolve with no other intervention} &amp;
\parbox[t]{2.3cm}{\centering
Currently habitats with important ecological functions} &amp;
\parbox[t]{2.8cm}{\centering
\citep{Lepageetal2022}\Lbreak
\raisebox{1pc}{}
Life adapto project} 
\vspace*{2pt}\\
\parbox[t]{2.3cm}{\centering
Adaptive restoration of the former saltworks in Camargue (Rh\^{o}ne
delta)\Lbreak
\raisebox{1pc}{}
(France)} &amp;
Saltmarshes &amp;
Biodiversity &amp;
\parbox[t]{2.3cm}{\centering
Flood protection\Lbreak 
\raisebox{1pc}{}
Erosion reduction\Lbreak 
\raisebox{1pc}{}
Tourism} &amp; 
\parbox[t]{2.3cm}{\centering
Delta vulnerable to flooding and ecological loss (intense salt
production and polders)} &amp; 
\parbox[t]{2.3cm}{\centering
Dyke removal and restoration of gravitational flows} &amp; 
\parbox[t]{2.3cm}{\centering
Currently development of beaches but no quantitative data on
erosion effectiveness} &amp; 
\parbox[t]{2.8cm}{\centering
\url{https://climate-adapt.eea.europa.eu/metadata/case-studies/adaptive-restoration-of-the-former-saltworks-in-camargue-southern-france}} 
\vspace*{2pt}
\botline 
\end{tabular}
\end{sidewaystable*}

Nature based solutions are often associated with projects involving
some planned relocation. In fact, relocations provide space that is
presently lacking for coastal sediments and ecosystems in many
anthropogenized coastal zones. However, nature based solutions can also
be combined with coastal engineering projects, a practice referred to
as ``hybrid nature-based solutions''
\citep{Sowinska-SwierkoszGarcia2022}.\looseness=-1

Typical barriers to the implementation of coastal nature based
solutions include a perceived lack of effectiveness, lack of ecological
knowledge, lack of comparative economic scenarios of benefits,
insufficient funding and the inability of current governance to manage
such complex projects and address the related social challenges.
Research projects supporting ongoing experimentations and
implementations can help better assessing the effectiveness of
different options and move from a reactive adaptation to a strategic
planning approach anticipating the meters of sea-level rise that are
projected at multi-centennial timescales while also addressing other
challenges of resilient development
\citep{LeCozannetetal2023,Lepageetal2022,
Moraesetal2022,PhilippenkoLeCozannet2023,Schibalskietal2022}. Examples
shown in  Table~\ref{tab3}
can inspire new projects. For example, the Sigma plan
conducted in Belgium is known already by many French practitioners, but
it could also inspire new adaptation actions at least in the Seine,
Loire, Gironde estuaries and Rh\^{o}ne delta. The Sigma plan
implemented a number of different adaptation options along the Scheldt
estuary, including protection works, managed realignment, flood control
area, temporary storage area for overflow areas (aka control reduced
tide areas) while allowing for large areas of tidal wetlands and their
associated ecosystem services. Such an implementation ultimately creates
more space for hydro-sedimentary processes and ecosystems in the
estuary while offering sufficient protection at least for a few decades
\citep{BoeremaMeire2017,Jacobsetal2018,Lepageetal2022}. This may avoid
either developing costly estuarine barriers with potentially large
adverse impacts to coastal ecosystem \citep{Ortonetal2023}  or large
human and economic losses due to storm surges if sea-level rise
adaptation is not sufficiently 
\mbox{anticipated.}

While there is an increasing number of such actions, including in
France with the adapto project of the Coastal Conservation Agency
(Conservatoire du Littoral) and the nationally-defined coastal
adaptation strategy \citep{LeCozannetCazenave2024}, the implementation
of nature based solutions has been slower in marine and coastal and
estuarine ecosystems than in other ecosystems so far
\citep{OLearyetal2023}. The forthcoming regulatory context in Europe
offers opportunities to accelerate in this area and implement
transformative actions that are required for scaling up coastal nature
based solutions. The EU Nature Restoration Law that entered in force on
August 18th 2024 has a long term objective to restore  ${&gt;}$90\% of
degraded ecosystems by 2050, with an immediate target in 2030 of
restoring at least 30\% of them. A success in this area will
require specific attention to loosely defined objectives, such as those
referring only to surface areas and not specific levels of quality
ecosystem functioning. It will also require research to learn as these
solutions are implemented \citep{OLearyetal2023}. 

Overall, we argue that an essential aspect of a successful coastal
adaptation should be on enabling system-wide actions to reduce direct
and indirect drivers of biodiversity loss and environmental degradation
and restore healthy ecosystems for the benefits of all. Such a success
can obviously neither be guaranteed by the publication of a sea-level
scenarios such as those provided within this study, nor by any
adaptation plan avoiding any substantial discussion about the strategic
objectives of adaptation. On the contrary, the success of coastal
adaptation will require clear objectives, a transparent debate on the
opportunities of coastal adaptation and political will.

\section{Conclusions}\label{sec5}
The decision on the sea-level guidance to be considered after the 3rd
adaptation plan belongs to the Ministry in charge of Environment.
However, the future of the 3rd adaptation plan is uncertain due to
political instability in France marked by the absence of a societal
consensus on climate change responses, despite their being assessed in
scientific reports such as those of the IPCC. More problematically, the
public debate from 2022 to December 2024 has often focused on the
climate change scenarios rather than the adaptation measures
themselves. The consultation of the national adaptation plan in
December 2024 created an opportunity to discuss coastal adaptation
measures and strategies, but then the issue of the legal fragility of
the climate scenario emerged. Obviously, defining a sea-level scenario
is not sufficient to decide whether, e.g., relocation or protection is
more appropriate. Previous adaptation plans suggest that such political
decisions could be left to regional authorities in France, but it is
unsure that their current governance and finance is sufficiently robust
to plan efficiently for relocation plans or infrastructures that cost
billions of Euros. 

As stated by the national court of auditors in its annual report 2024
\citep{CourdesComptes2024}, there is a danger that a narrow vision of
adaptation limited to technical choices and regulation application
eludes any strategic ambition for coastal development. In the worst
case, this can result in a lock-in in unsustainable practices and large
damage and human losses during coastal disasters. By avoiding a clear
debate on coastal adaptation, France may be missing the opportunity to
take advantage of the upcoming investments in adaptation to create more
resilient coastal and estuarine ecosystems and generally more resilient
coasts. 

\section*{Declaration of interests}
The authors declare no specific conflict of interest that would result
in any benefit from the publication of this research. The first author
initially contributed to a study funded by the ministry in charge of
Environment linked to the topic of this manuscript, withdrew from this
project to resolve conflicts of interests due to him also being member
of the French Climate Change Committee in charge of evaluating climate
policies in France. Hence, this work is published independently of any
political or administrative influence, under the provisions of the
European contracts that funded this research and in accordance with the
Article L411-3 of the French Research Code.

\section*{Acknowledgements} 
GLC, JR, and RT are
supported by the CoCliCo, PROTECT and SeaClim projects. These projects
received funding from the European Union's Horizon 2020 research and
innovation program under grants agreements No 869304, 101003598 and
101180125; PROTECT contribution number 85. GLC and JR are also
supported by the French National Research Agency under FUTURISKS
(ANR-22-POCE-0002) project.
CC was supported by the Adapto project. We thank Jean
Jouzel and Etienne Ghys for organizing the workshop at the Academy of
Sciences in March 2024 and this special issue. We used artificial
intelligence to correct grammatical errors in the manuscript.

\CDRGrant[EU]{869304} 
\CDRGrant[EU]{101003598} 
\CDRGrant[EU]{101180125} 
\CDRGrant[ANR]{ANR-22-POCE-0002} 

\section*{Supplementary data}
Supporting information for this article is available on the journals
website under \printDOI\ or from the author.

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crgeos-290-suppl.pdf}}

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