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\DOI{10.5802/crgeos.323}
\datereceived{2025-04-04}
\daterevised{2025-11-04}
\dateaccepted{2026-01-05}
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\section*{Declaration of interests}
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\COI{The author does not work for, advise, own shares in, or receive
funds from any organization that could benefit from this article, and
has declared no affiliations other than their research organization.}

\dateposted{2026-06-01}
\begin{document}

%\dateposted{2026-02-16}

\begin{noXML}

\CDRsetmeta{articletype}{review}

\TopicFR{Sciences du climat}
\TopicEN{Climate Sciences}

\editornote{Article submitted by invitation}
\alteditornote{Article soumis sur invitation}

\title{Scenarios, projections, and climate-related risks}

\alttitle{Sc\'enarios, projections, et risques climatiques}

\author{\firstname{Val\'erie} \lastname{Masson-Delmotte}\CDRorcid{0000-0001-8296-381X}}
\address{Institut Pierre Simon Laplace, Laboratoire des Sciences du
Climat et de l'Environnement (UMR 8212 CEA-CNRS-UVSQ), Universit\'e
Paris Saclay, Gif-sur-Yvette, France}
\email{valerie.masson@lsce.ipsl.fr}

\keywords{\kwd{Socio-economic scenarios}
\kwd{Greenhouse gas emissions}
\kwd{Projections}
\kwd{Climate change}
\kwd{Climate science}
\kwd{Robust decision making under uncertainty}}

\altkeywords{\kwd{Sc\'enarios socio-\'economiques}\kwd{\'Emissions de
gaz \`{a} effet de serre}\kwd{Projections}\kwd{Changement
climatique}\kwd{Science du climat}\kwd{Prise de d\'ecision robuste
devant l'incertitude}}

\begin{abstract}
Socio-economic scenarios and resulting global warming levels structure
the understanding of causal chains of greenhouse gas emissions, air
pollution control, land-use changes, the response of the Earth system,
climatic impact-drivers, exposure, vulnerability, adaptation responses,
and climate-related risks. 

Following the French Academy of Sciences workshop on ``The climate
emergency, a turning point'' (2024), this manuscript provides an
overview of the concepts and methodologies underpinning the Sixth
Assessment Report of the Intergovernmental panel on Climate Change
(IPCC) regarding the development of scenarios and the assessment of
risks from the perspective of the physical science basis of climate
change, and selected recent updates and advances. 

It includes (i) a brief overview of the current state of the global and
regional climate and impacts in France, (ii) an overview of future
warming scenarios spanning socio-economic scenarios,\break (iii) constrained
climate projections for global warming, and (iv) low-likelihood,
high-impact outcomes which are relevant to inform robust
decision-making and risk management.
\end{abstract}

\begin{altabstract}
Les sc\'enarios socio-\'economiques et les niveaux de r\'echauffement
plan\'etaire qui en r\'esultent structurent la compr\'ehension des
cha\^{i}nes de causalit\'e entre les \'emissions de gaz \`{a} effet de
serre, le contr\^{o}le de la pollution atmosph\'erique, les changements
d'usage des terres, la r\'eponse du syst\`{e}me Terre, les facteurs
climatiques g\'en\'erateurs d'impacts, l'exposition, la
vuln\'erabilit\'e, les r\'eponses d'adaptation, et les risques li\'es
au climat.

Suite au colloque de l'Acad\'emie des Sciences fran\c{c}aises,
\og Urgence climatique, un tournant
d\'ecisif \fg, ce manuscrit fournit une vue d'ensemble des
concepts et m\'ethodes \'etayant le 6\textsuperscript{\`{e}me}
cycle d'\'evaluation du Groupe intergouvernemental d'experts sur
l'Evolution du Climat (GIEC), concernant le d\'eveloppement des
sc\'enarios, l'\'evaluation des risques, du point de vue des bases
physiques du changement climatique, et quelques avanc\'ees et mises
\`{a} jour r\'ecentes.

Le manuscrit contient (i) un point sur l'\'etat du climat au niveau
mondial et r\'egional et ses impacts en France ; un tour d'horizon des
sc\'enarios de r\'echauffement futur abordant les (ii) sc\'enarios
socio-\'economiques, (iii) les projections contraintes du
r\'echauffement plan\'etaire, et  (iv) les \'eventualit\'es de
probabilit\'e d'occurrence faible, de forts impacts, qui sont
pertinents pour \'eclairer une prise de d\'ecision robuste et la
gestion de risques.
\end{altabstract}

%\input{CR-pagedemetas}

\maketitle

\twocolumngrid

\end{noXML}

\defcitealias{IPCC2021}{ibid.}
\defcitealias{IPCC2023}{ibid.}

\section{Introduction: climate-related risks}\label{sec1}

France, in mainland and oversea territories, is particularly exposed to
the consequences of climate change, through changes in regional
climate, the mountain cryosphere, the ocean as well as sea-level  rise
\citep{Chiffres2024}, themselves directly related to human-caused global
climate change. 

{\advance\baselineskip.2pt

During the last decade, France mainland and oversea territories have
experienced the intensification of multiple climatic impact-drivers due
to human-caused climate change, providing evidence for limits to
current adaptation levels, and key vulnerabilities to land and marine
hot extremes, drought (with a large number of buildings exposed to clay
swelling), water scarcity, wildfires, extreme rainfall and flooding,
and intense tropical cyclones, leading to increasing insurance costs
\citep{HCC2024a,HautConseilpourleClimat2025} 
and losses, in particular for  food production \citep{HCC2024b}. 


In mainland France, managed forests have also been strongly affected by
reduced growth and increased tree mortality resulting from heat and
drought, leading to a 50\% reduction of the managed forest carbon sink
since around 2010 \citep{AcadSci2023}{, challenging the ability of
France to meet its pledged mitigation goals and constraining the
sustainable use of biomass in mitigation strategies \citep{HCC2024a}.

Worldwide, around 3.3 to 3.6 billion people live in contexts highly
vulnerable to climate change, and, by 2050, around 1 billion people
will be exposed to hazards resulting from sea-level rise, in low-lying
islands and coasts, coastal cities, and agricultural deltas}
\citep{IPCC2023}. Vulnerable communities, who have least contributed to
current global warming \citep{Jones2023}, are disproportionately
affected by the escalation of losses and damages, which affect water
and food security, well-being and health, critical infrastructures,
livelihoods and economic activity \citep{IPCC2023}. Today's young
generations will face unprecedented lifetime exposure to climate
extremes \citep{Grant2025}, with adverse effects of climate change
hindering the full enjoyment of human rights \citep{ICJ2025}.

}

Characterizing future climate impacts and risks is a critical element
to inform climate action, both in terms of mitigation efforts to avoid
unmanageable and intolerable risks, and in terms of adaptation
pathways, adaptation limits, residual risk and risk management. Risk is
defined in the IPCC AR6 as ``the potential for adverse consequences for
human or ecological systems, recognizing the diversity of values and
objectives associated with such systems. Risks can arise from potential
impacts of climate change as well as human responses to climate-related
risks''  (Figure~\ref{fig1}). 

\begin{figure*}
\includegraphics{fig01}
\bcaption{\label{fig1}}{0}{Schematic of the IPCC AR6 framework for
assessing future greenhouse gas emissions, climate change, risks,
impacts and mitigation. (a)~The integrated framework encompasses
socio-economic development and policy, emissions pathways and global
surface temperature responses to the five scenarios considered by WGI
(SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5) and eight global
mean temperature change categorisations (C1--C8) assessed by WGIII, and
the WGII risk assessment. The dashed arrow indicates that the influence
from impacts/risks to socio-economic changes is not yet considered in
the scenarios assessed in the AR6. Emissions include greenhouse gases
(GHGs), aerosols, and ozone precursors. CO$_2$ emissions are shown as
an example on the left. The assessed global surface temperature changes
across the 21st century relative to 1850--1900 for the five GHG
emissions scenarios are shown as an example in the centre. Very likely
ranges are shown for SSP1-2.6 and SSP3-7.0. Projected temperature
outcomes in 2100 relative to 1850--1900 are shown for C1 to C8
categories with median (line) and the combined very likely range across
scenarios (bar). On the right, future risks due to increasing warming
are represented by an example `burning ember' figure. 
(b)~Description and relationship of scenarios considered across AR6 Working
Group reports. (c)~Illustration of risk arising from the
interaction of hazard (driven by changes in climatic impact-drivers)
with vulnerability, exposure and response to climate change. Replicated
from \citep{IPCC2023}.}
\end{figure*}

The core dimensions of integration between mitigation and adaptation
efforts include global warming levels, as a result of changes in
emissions and land-use driving changes in atmospheric concentrations
and radiative forcing, and as a key element driving changes in climatic
impact-drivers (Figure~\ref{fig1}). The best estimate of future global
warming levels over time arising from current policies can also be
translated into regional warming levels, as reflected in the reference
trajectory for national adaptation for mainland France \citep{Corre2025}. 

As invited to the related keynote lecture presented at the French
Academy of Sciences workshop on ``The climate emergency, a turning
point'' in 2024, this manuscript provides an overview of the concepts
and methodologies underpinning the Sixth \mbox{Assessment} Report of the
Intergovernmental panel on Climate Change (IPCC) regarding the
assessment of risks from the perspective of the physical science basis,
and selected recent updates and discussions.

It includes a brief overview of the current state of heating of the
global and regional climate ({Section~\ref{sec2}}), and an overview of
future warming scenarios ({Section~\ref{sec3}}), spanning socio-economic
scenarios (Section~\ref{sec31}), and methodologies to provide
constrained climate projections (Section~\ref{sec32}). Within the
framework of robust decision-making under deep uncertainty and risk
management, I then place an emphasis on aspects related to deep
uncertainty and the importance of considering low-likelihood,
high-impact outcome (Section~\ref{sec33}), followed by final remarks on
advances in future climate scenarios\break (Section~\ref{sec4}).

\section{The current state of climate} \label{sec2}

\subsection{Observed and attributed changes in global indicators of the
state of climate}\label{sec21}

In 2024, for the first time in instrumental records, estimates of
global surface temperature exceeded 1.5~\textdegree C above 1850--1900 
(Figure~\ref{fig2}), a reference period often used to characterize
pre-industrial conditions \citep{SGC2025}. This new temperature record
in both land surface air temperature and sea surface temperature
reflects internal climate variability on top of the long-lasting
heating of the climate system resulting from human activities
\citep{SGC2025,Terhaar2025}. Indeed, the internal variability in any
single year is estimated to be ${\pm}$0.25~\textdegree C (5--95\%
range) based on CMIP6 climate models, and ${\pm}$0.17~\textdegree C
based on observations \citep{IPCC2021}.

\begin{figure*}
\includegraphics{fig02}
\vspace*{-2pt}
\caption{\label{fig2}Screenshot of the dashboard of key
indicators of the state of climate and human influence
\citep{Forster2025}. The left upper panel displays the current level
of human-caused global warming (1.36~\textdegree C by 2024) and the
years when specific levels of human-caused global warming are expected
to be reached assuming the continuation of the rate of human-caused
warming estimated during the last decade. The left middle panel
displays observed annual global surface temperature anomalies up to
2024 with respect to 1850--1900 (grey) and the outcome of the assessment
of human-caused global warming (orange). The left lower panel displays
the observed change in global mean sea level from 1993 to 2024. The
right upper panel shows annual emissions of greenhouse gases identified
with different colors as indicated in the legend. The right lower panel
indicates the estimated remaining carbon budget here associated with
limiting global warming to 1.5~\textdegree C with a 50\% likelihood, and
when it is expected to be exhausted, at current emissions levels. Note
that the interactive website allows us to select different global
warming levels and likelihoods. Replicated from
\url{https://climatechangetracker.org/igcc}.}
\vspace*{-3pt}
\end{figure*}

Advances in attribution methods provide a framework \citep{Hope2021}
{to understand observed changes (Table~\ref{tab1}), and to identify
climatic impact-drivers: attribution of changes in atmospheric
composition and resulting radiative forcing to human activities;
attribution of changes in a range of variables describing the global
and regional state of climate to human influence; attribution of
weather and climate events to human-caused climate change}
\citep{PerkinsKirkpatrick2024}; and attribution of observed impacts in
various sectors, regions, natural and human systems to climatic
drivers, themselves {attributable} to human influence. Here, we 
{focus} on
indicators of global climate, and attribution outcomes.\looseness=-1

\begin{table*}[t!]%tab1
\caption{\label{tab1}Synthesis of the assessment of changes in
large-scale indicators of the state of components of the climate system
for observed changes (described in the left column) for observed
changes (middle column) and their attribution to human influence up to
the year 2019 (right column). The color coding indicates the assessed
confidence/likelihood of the observed change, using the calibrated IPCC
methodology to report confidence in findings, and the human
contribution as a driver or main driver (more than 50\% of observed
change, as specified in the table box). Reproduced from the IPCC AR6
Synthesis Longer Report  \citep{IPCC2023}, based on
\citep{IPCC2021}.}
\begin{tabular}{c}
\tbody
\inlinefig{fx01}
\botline
\end{tabular}
\vspace*{3pt}
\end{table*}

Based on methods and datasets assessed in the Sixth Assessment Cycle
of the Intergovernmental panel on Climate Change \citep{IPCC2021}
(Table~\ref{tab1}), updated until the end of year 2024, the observed
level of global warming has reached 1.24 (1.11--1.34)~\textdegree C 
above 1850--1900 in 2015--2024, of which 1.22
(1.0--1.5)~\textdegree C is attributable to human influence.
Human-caused global warming continued at a record pace of 0.27
(0.2--0.4)~\textdegree C per decade in \mbox{2015--2024}, reaching 1.36
(1.1--1.7)~\textdegree C by 2024 \citep{Forster2024,Forster2025}. The
increased global warming rate results from increased radiative
forcing. It is primarily due to anthropogenic emissions of greenhouse
gases, to which is added the loss of the cooling effects resulting from
reduced emissions of aerosols, which track scenarios of strong
pollution control. The probability of seeing an observed temperature of
1.52~\textdegree C in 2024 considering a human-induced warming equal
to 1.36~\textdegree C is about one chance out of six; the same
probability but conditional on the fact that 2024 followed an El
Ni\~{n}o year and that the Atlantic multidecadal variability (AMV) was
in a positive phase rises to one chance out of two. 2024 can therefore
be treated as a ``normal'' year, i.e.\ very much expected at the actual
human-caused global warming level when the internal modes of
variability are taken into \mbox{account} and when assessed from a very large
number of simulations from large ensembles.

Amongst greenhouse gases, the largest contribution to the observed
warming effect arises from emissions of $\mathrm{CO}_{2}$, followed by
emissions of CH$_4$ with a direct effect and an indirect effect
through its role as a precursor of tropospheric ozone formation
\citep{IPCC2021}. Combustion and fugitive emissions for fossil fuels
account from around 70\% of \mbox{greenhouse} gas---caused warming, with other
sources of \mbox{greenhouse} gas emissions arising from land-use, agriculture,
cement production, waste, and F-gas emissions \citep{Forster2025}.
Around 10\% of global CO$_2$ emissions are driven by change in land-use
and in particular deforestation, and 90\% are driven by fossil fuel
combustion and cement production \citep{Friedlingstein2025}.
\looseness=-1

In a fast changing climate, these annual updates of the key indicators
of global climate change and of the global carbon budget represent a
major effort in how the research community builds on the in-depth
assessment of datasets and methodologies within IPCC reports to inform
evidence-based decision-making with up-to-date and timely information.
Annual update approaches have recently been implemented for wildfires
\citep{Kelley2025}. A new methodological challenge will be to assess the
contributions of carbon cycle feedbacks such as wetland methane
emissions or wildfire CO$_2$ emissions to changes in
atmospheric greenhouse gas concentrations. There is also a need for
annual updates of climate forcing datasets for climate simulations
\citep{Naik2025}. Moreover, annual updates of forced warming can also be
used to update constrained projections \citep{Ribes2025}. \looseness=-1

Human-caused heating of the climate system drives widespread and rapid
changes in each component of the climate system, atmosphere, ocean,
biosphere and cryosphere (Table~\ref{tab1}) and exacerbates the
frequency and intensity of extreme events such as marine and
terrestrial heatwaves, agricultural (soil moisture) drought, and
extreme rainfall \citep{SGC2025,IPCC2021}, leading to negative
impacts and losses and damages for ecosystems and human activities
\citep{IPCC2022a}.
                                                                                                                                                                                                                                                                                                                                                                                                                 
Global mean sea-level rise is also a direct consequence of the Earth's
energy imbalance and heating of the climate system, with an
acceleration of its pace from 2 mm/year in the 1990s to more than 
4~mm/year in the recent decade, with the mass loss from Greenland and
Antarctica adding to glacier retreat and increased rates of ocean heat
uptake and resulting thermal expansion \citep{SGC2025}
\citep{Forster2025}. Due to the slow adjustment and delayed response of
glaciers, ice sheets and ocean heat at depth, past greenhouse gas
emissions and the current level of global warming already \mbox{imply}
multi-centennial committed sea level rise of more than 1~meter
\citep{Mengel2018,Nauels2019}. Annual updates of committed future
sea-level rise resulting from past emissions to date are not yet
available. However, the magnitude and rate of future sea-level rise and
the timing of when 1 m of sea level rise will be exceeded depend on
both future emissions and ice sheet processes associated with deep
uncertainty \citep{IPCC2021}.

The level of human-caused global warming is related to changes in
global (Table~\ref{tab1}) and regional climate characteristics,
resulting in changes in characteristics (intensity, frequency,
duration, timing, and spatial extent) of multiple climatic
impact-drivers, which are ``physical climate system conditions (e.g.,
means, events, and extremes) that affect an element of society or
ecosystems'' \citep{Ruane2022}. 

This climatic impact-driver framework is based on essential climate
variables and physical thresholds, accounting for instance for the
combination of extreme heat and humidity which challenges the ability
to regulate body temperature (heat health index), or combined hot, dry
and windy weather conditions favorable to wildfires (forest fire danger
index). 

Compound, sequential or simultaneous climatic impact-drivers can affect
systems in ways that extend beyond individual components. This is
typically the case for compound flooding, with overlapping extreme sea
level resulting from storm surge and fluvial flood resulting from
extreme rainfall \citep{Hermans2024}. In Europe, there is the emergence
of compound events such as the co-occurrence of marine and atmospheric
heatwaves, drought and fire activity \citep{Santos2024}, and the
pre-conditioning of compound heavy hourly rainfall events by heatwaves.
An example of a sequential ecoclimatic event consists of a warm early
season, leading to an early onset of vegetation growth, followed by
late frost, damaging grapevines and orchards \citep{Bastos2023}. In
France, winter wheat crop yields are more severely affected by
successive events, such as winter and spring drought, followed by
summer heat and drought, than by each individual driver
\citep{Shan2024}. Simultaneous events can be for instance concurrent
extreme events affecting large crop production regions.

Depending on system tolerance, climatic impact-drivers and their
changes can be detrimental, \mbox{beneficial,} neutral, or a mixture of each
across interacting system elements and regions. Current and future
impacts and climate-related risks thus depend on the interplay between
response measures affecting exposure, vulnerability, and hazards
resulting from climatic impact-drivers, themselves related to global
warming levels (Figure~\ref{fig1}).

\subsection{Amplified warming observed in Europe and mainland France}
\label{sec22}

Warming is amplified over land, compared to the ocean. Europe is the
continent outside the Arctic characterized by the largest rate of
regional warming \citep{Copernicus2024}. 
The observed increase in regional surface air
temperature averaged over the last decade has already exceeded
2~\textdegree C above 1850--1900 \citep{Heino2023}---with a similar
result in mainland France \citep{MeteoFrance2025a}. The enhanced
Europe warming rate results from feedbacks associated with the reduced
snow cover duration, in the north-east parts and in mountains, as well
as feedbacks associated with agricultural drought and surface heat,
with a rapid increase in heatwave frequencies. 

The European ``heatwave hotspot'' is increasing faster in observations
than in climate model simulations, due to regional trends in
atmospheric circulation \citep{Vautard2023} and aerosol emissions
\citep{Schumacher2024}. Attribution methodologies have been applied to
the increase in surface air temperature in mainland France
\citep{Ribes2022}. During the last decade, the observed level of
warming in mainland France has already reached the level expected to be
reached by 2030, for a level of human-caused global warming of
1.5~\textdegree C \citep{MeteoFrance2025b}. 

\subsection{Systematic comparisons between observed trends and climate
simulations} \label{sec23}

Systematic comparisons have been performed between observed changes in
the climate system, and the expected human-caused signals depicted in
simulations performed with climate or Earth system models, decades ago
\citep{Hausfather2020,IPCC2021,Stouffer2017}, including early
projections of sea-level rise \citep{Tornqvist2025}. 

A recent overview \citep{Simpson2025} highlights the success of climate
models to accurately \mbox{represent} historical increases in global mean
temperature and global column water vapour, the global \mbox{increase} in
extreme precipitation event intensity, the decline in wintertime cold
extremes in the northern hemisphere, the increased recurrence of marine
heat waves, and the increased amplitude of the seasonal cycle of
sea-surface temperature in the northern hemisphere. Models have also
successfully captured the expansion of the Hadley
cell\footnote{The Hadley circulation describes a
direct, thermally driven overturning cell in the atmosphere consisting
of poleward flow in the upper troposphere, subsiding air into the
subtropical anticyclones, return flow as part of the trade winds near
the surface, and with rising air near the equator in the so-called
Inter-tropical Convergence Zone (definition from the IPCC Glossary,
available at \url{https://apps.ipcc.ch/glossary/}).} edges, the weakening
of the northern hemisphere jet stream and summer storm track, and the
shift in the southern hemisphere mid-latitude jet in response to ozone
depletion and its pause driven by ozone recovery. 

Models have been successful in depicting the sign of the following
changes, but could underestimate the magnitude of changes for the
top-of-atmosphere radiative imbalance, for Arctic amplification and
sea-ice retreat, for the weakening of the tropical overturning
circulation, the increased contrasts of precipitation between tropical
dry and wet regions, the increased precipitation variability; they
could overestimate the magnitude of tropical troposphere \mbox{warming}. 

Model-data discrepancies are identified for the pattern of tropical
Pacific sea-surface temperature (SST) gradient, the strengthening of
the wintertime North Atlantic jet, the increased in summer Greenland
blocking, the exacerbated summer warming and drying in Europe (as
described above), near-surface specific humidity above arid regions,
and Southern Ocean SST and sea ice variations. The situation remains
uncertain for the winter Eurasian temperature trend, Hadley circulation
strength, southern hemisphere storm tracks and the poleward shift in
the zonal mean jet streams. 

Model-data discrepancies can arise from model caveats, including model
representation of processes shaping climate feedbacks and/or model
resolution, but also from issues related to observations, forcings used
in simulations, and from internal (unforced) climate variability.
Robust procedures are developed to identify and understand model-data
agreement as well as sources of discrepancies. Such comparisons are
embedded in diagnoses of the physical processes controlling the forced
climate response to anthropogenic forcings, to advance the
understanding of the physical climate response, the fitness for the
purpose of models, and they inform the assessment of confidence in
projections. 


In the IPCC AR6 \citep{Hausfather2022}, the comparison between
simulations and historical global temperature observations was used to
apply different weights to combine projections from different climate
models, further constrained by the assessment of climate sensitivity
\citep{Sherwood2020}. Indeed, due to differences in observed and
simulated patterns of warming, recent warming alone provides little
information on climate sensitivity and future responses
\citep{Armour2024}. Novel approaches are being explored to further
constrain projections at global and regional scales
\citep{Kim2023,OReilly2024,Li2025}, combining observations,
simulations, machine learning \citep{Nowack2025} and theoretical
understanding \citep{Byrne2024}.

\section{Future global warming scenarios}\label{sec3}

The dominant driver of current and future warming arises from
anthropogenic CO$_2$ emissions, as the responses of the
climate system and carbon cycle lead to a close relationship between
cumulative CO$_2$ emissions and global warming levels \citep{Canadell2021}.
This so-called transient climate response to cumulative emissions
arises from counterbalancing non-linear effects, and has implications
for relating a global surface temperature goal to a corresponding
remaining carbon budget \citep{Lamboll2023}. If all human greenhouse gas
and aerosol emissions were set to zero, the IPCC AR6 assessed uncertain
but close to zero additional warming based on climate 
model simulations \citep{Jones2019,IPCC2023,PalazzoCorner2023}. 
\looseness=-1

As a result, future warming will primarily depend on pathways of future
emissions, primarily on global net CO$_2$ emissions, but also on the
net effect of {non-CO$_2$} anthropogenic drivers, and hence emissions of
other greenhouse gases, and short-lived \mbox{climate} \mbox{pollutants.}
The second
most important driver of current and future warming is indeed the
balance between emissions of methane and aerosols \citep{IPCC2021}.
Future changes in drivers of climate change resulting from human
activities are explored based on socio-economic emissions scenarios.

A broad range of modelled scenarios and pathways are used to explore
quantitatively future emissions, climate change, climate-related
impacts and risks, and mitigation and adaptation pathways. These
scenarios and pathways are built on sets of assumptions regarding
socio-economic aspects, to characterize socio-economic uncertainties.

\vspace*{-6pt}
\subsection{Socio-economic scenarios} \label{sec31}
\vspace*{-3pt}

Socio-economic scenarios are an ensemble of opportunity, incomplete in
essence, distinct from predictions. Five Shared socio-economic
pathways~(SSP1, sustainability; SSP2, current trends; SSP3, regional
rivalry; SSP4, inequality; SSP5, fossil-fueled development) have been
designed to encompass a broad range of challenges for climate change
adaptation and mitigation \citep{ONeill2017} (Figure~\ref{fig3}). For
climate-related impacts and risks, SSPs are used to evaluate exposure,
vulnerability, and adaptation challenges.\looseness=-1

\begin{figure*}
\vspace*{-3pt}
\includegraphics{fig03}
\vspace*{-3pt}
\caption{\label{fig3}A summary of SSP elements that contribute
to high or low challenges to (a) mitigation and (b) adaptation
\citep{ONeill2017}, and (c) indicative temperature evolution (left)
and radiative forcing categorization (right) of the core set of
scenarios used in the IPCC AR6 WGI report (SSP1-1.9, SSP1-2.6,
SSP2-4.5, SSP3-7.0 and SSP5-8.5) together with four additional
scenarios that are part of ScenarioMIP and previous RCP scenarios. The
black stripes on the left-hand panel indicate a larger set of IAM-based
SSP scenarios that span the scenario range more fully but are not used
in the IPCC AR6 WGI report. Note that the descriptive labels for the
five SSP narratives refer mainly to the reference scenario futures
without additional climate policies \citep{IPCC2021}.}
\vspace*{-3pt}
\end{figure*}

Depending on mitigation levels, modelled emissions pathways from SSPs
\citep{Riahi2017} can be consistent with different global warming
levels (Figure~\ref{fig3}). 


For instance, scenario SSP3 displays high challenges for adaptation,
and high challenges for mitigation---making limiting global warming
well below 2~\textdegree C unfeasible. As this scenario echoes some
recent geopolitical events, it deserves specific attention regarding
its implication for climate change: ``SSP3 explores a narrative of
resurgent nationalism, concerns about competitiveness and security, and
regional conflicts which push countries to increasingly focus on
domestic or, at most, regional issues. This trend is reinforced by the
limited number of comparatively weak global institutions, with uneven
coordination and cooperation for addressing environmental and other
global concerns. Policies shift over time becomes increasingly oriented
toward national and regional security issues, including barriers to
trade, particularly in energy resources and agricultural markets.
Countries focus on achieving energy and food security goals within
their own regions at the expense of broader-based development, and in
\mbox{several} regions move toward more \mbox{authoritarian} forms of government with
highly regulated economies. 
\mbox{Investments} in education and \mbox{technological}
development decline. Economic development is slow, consumption is
material-intensive, and inequalities persist or worsen over time,
especially in developing countries. ({\ldots}) Growing resource
intensity and fossil fuel dependency along with difficulty in achieving
international cooperation and slow technological change imply high
challenges to mitigation. The limited progress on human development,
slow income growth, and lack of effective institutions, especially
those that can act across regions, implies high challenges to
adaptation for many groups in all regions'' \citep{ONeill2017}. 

While the narrative of SSP can echo some aspects of the current world
context, it does not imply that model pathways based on SSP3 are closer
to current trends or should be considered of higher \mbox{relevance}, because
SSP storylines are scenarios for the medium and long term, and because
the implementation of the SSP3 storyline in models may not reflect
current trends, such as international trade, technological shifts
towards renewables, policy momentum in many nations, climate finance
and more cooperation than in the SSP3 worst case.


Different mitigation strategies can be consistent with different
warming levels by 2100. SSP-based scenarios are labelled SSPx-y, where
``x'' refers to the shared socio-economic pathway, describing
socio-economic trends, while ``y'' refers to the magnitude of resulting
radiative forcing resulting from that scenario, in 
W${\cdot}$m$^{-2}$ by 2100 (Figure~\ref{fig1}). The response of
the climate system was assessed \citep{IPCC2021} for five
illustrative pathways based on SSPs and which encompass the whole range
of future anthropic forcings available from the scientific \mbox{literature}.
These \mbox{scenarios} combine socio-economic assumptions, and different
levels of mitigation, land use, and atmospheric pollution controls for
emissions of aerosols and precursors of ozone formation. The comparison
between scenarios and emission inventories requires to accounting for
different accounting conventions for land-based carbon fluxes
\citep{Gidden2023}.

High and very high emissions scenarios (SSP3-7.0 et SSP5-8.5) have
respectively a doubling of CO$_2$ {emissions by 2100 and 2050,
respectively (Figure~\ref{fig1}). Such high emissions scenarios are
less plausible owing to technological progress and mitigation policies
already implemented. Indeed, more than 24 countries, including France,
have passed peak CO$_2$ emissions and started sustained emissions
reductions \citep{Lamb2022}. On the global scale, CO$_2$ emissions
continue to increase, albeit at a smaller rate than one decade ago.
Avoided emissions arise from mitigation policies
\citep{Stechemesser2024}, such as reduced deforestation, low-carbon
electricity production, gains in energy efficiency and technological
innovation policies allowing for reduced costs and increased capacities
for PV electricity production and batteries \citep{IPCC2022b}. 

The intermediate emissions scenario (SSP2-4.5) is characterized by
relatively stable global CO$_2$ {emissions until mid-century, followed
by a slow decline (Figure~\ref{fig1}). This is the scenario closest to
the extrapolation of current mitigation policies and nationally
determined contributions pledged within the Paris Agreement
\citep{UNEP2024}. 

Low of very low emissions scenarios are marked by sharp declines in
global CO$_2$ emissions reaching net zero CO$_2$ emissions by 2050 and
2070, respectively, followed by various levels of net negative CO$_2$
emissions (Figure~\ref{fig1}). Both are also characterized by a rapid
reduction in methane emissions.

This core set of 5 scenarios, with differences in air pollution control
and mitigation stringency, cover a broader range of greenhouse gas and
air pollutant futures than assessed in earlier IPCC WGI reports. 

\subsection{Constrained global surface temperature projections and
implications for France}\label{sec32}

The uncertainty on future climate projections depends on the
socio-economic uncertainties affecting future emissions (scenario
uncertainty), but also on the relationship between emissions and
concentrations (carbon feedback uncertainty, affecting the natural
response of the land and ocean carbon sinks), and on the uncertainty of
the climate response, which is related to the understanding of the
physical response of the climate system to changes in atmospheric
composition resulting from climate feedbacks and climate sensitivity as
well as the structural uncertainties in the climate model. To that is
added the irreducible and intrinsic uncertainties related to external
natural forcings (solar and volcanic activity), and internal climate
variability.

One first approach to explore the uncertainty of the future climate
response is based on climate model results from the Sixth phase of the
Climate Model Intercomparaison Project CMIP6 \citep{Eyring2016}, and
the range of simulated responses to a given atmospheric concentration
or emissions scenario, which includes both large ensembles to span the
range of simulated internal variability, and multi-model ensembles, to
span the uncertainty of climate feedbacks and climate sensitivity
\citep{IPCC2021}. 


Earth system models which include the explicit modelling of some
processes of the carbon cycle interplay with climate can be driven by
emissions, or by prescribed atmospheric concentrations, based on a
central estimate of the carbon cycle feedbacks. For a few Earth system
models for which both emission-driven and concentration-driven
simulations were available for the same scenario, the differences were
small, and do not affect the assessment of global surface temperature
projections by more than 0.1~\textdegree C by 2100, with a
0.1~\textdegree C larger spread \citepalias{IPCC2021}. However, this is
only true for high emission scenarios, but not for overshoot scenarios,
due to non-linearities and higher uncertainties of the simulated carbon
cycle in response to a change from positive to negative emissions
\citep{Asaadi2024}. 

When comparing scenario-based, multi-model projections in CMIP5 to
CMIP6 \citep{Durack2025}, about half of the increase in simulated
warming in CMIP6 compared to CMIP5 arises from a higher radiative
forcing in nominally-corresponding scenarios: the radiative forcing is
larger in SSP5-8.5 than in the earlier RCP8.5 concentration pathway.
The plausibility of such high emissions scenarios is challenged due to
developments in climate policies and advances in clean technologies.
However, the high atmospheric greenhouse gas concentration levels
corresponding to these high emissions scenarios cannot be ruled out,
due to uncertain carbon cycle feedbacks, which, in lower emissions
\mbox{trajectories,} could result in concentrations above the central levels
used to drive climate model projections. The other half of the
increased simulated warming arises from a larger prevalence of high
climate sensitivity within CMIP6 compared to CMIP5 climate models
\citep{IPCC2021}.

Methodological advances \citep{Sherwood2020} now allow to combine new
evidence of the Earth's climate sensitivity, based on the understanding
and quantification of the Earth's energy imbalance, the instrumental
record of global surface temperature change, paleoclimate
reconstructions, and climate feedbacks, in a holistic view
\citep{Jeevanjee2025}. In particular, feedback processes are now
understood to become more amplifying on multi-decadal time scales,
depending on the pattern of warming \citep{Armour2024}. This has to be
accounted for when using insights from historical records or past cold
periods \citep{Cooper2024}. Accounting for the pattern effect provides
a framework to combine lines of evidence based on paleoclimate and
recent trends, and supports an updated assessment of the best estimate
of equilibrium climate sensitivity of 3~\textdegree C, with a
\textit{very likely} range of 2 to 5~\textdegree C, and a
\textit{likely} range of 2.5~\textdegree C to 4~\textdegree C which is
narrower than previously assessed. Clouds remain the main cause of
uncertainty in climate feedback, but advances in the understanding of
cloud processes have led to a narrowing of the associated uncertainty
range by about 50\% in the AR6 compared to the AR5 \citep{IPCC2021}.
Uncertainties resulting from cloud processes are further evidenced by
recent studies \citep{Goessling2025}. 

On average, CMIP6 models have higher mean values and wider spreads than
these assessed best estimates and ranges, resulting from larger cloud
feedback, and some of the high-sensitivity and low-sensitivity CMIP6
models are less consistent with the observed historical warming trend
and paleoclimate data \citep{Lunt2024}. The models with very high
responses provide insights into low-likelihood, high-impact futures
which cannot be excluded based on the current state of knowledge
\citep{IPCC2021}. 

In order to combine all available lines of evidence, and not just raw
climate model results, the IPCC AR6, for the first time, explicitly
combined new \mbox{projections} for the selected SSP-RCP scenarios with
observational constraints, based on past simulated warming, as well as
the updated assessment of equilibrium climate sensitivity and transient
climate response. Moreover, climate predictions initialized from the
observed climate state have also been used for the period 2019--2028}
\citepalias{IPCC2021}. More recently, methodologies have been developed to
allow for annual updates of forced warming and constrained projections}
\citep{Ribes2025}, and remain to be implemented on a routine basis. 

The outcome of these approaches is reflected in Figure~\ref{fig1},
showing the best estimate and likely range of the assessed climate
response to the five illustrative SSP-RCP scenarios. They show that the
effect of emissions reductions on the slowing down of the rates of
global warming would be detectable from natural climate variability
within around 20 years. As a result, a level of 1.5~\textdegree C of
global warming is expected to be reached (averaged over 20 years) in
the coming decade across all scenarios, with the crossing time expected
in the early 2030s. On other time horizons, future warming depends on
the scenario. For instance, a 2~\textdegree C level of global warming is
crossed under intermediate to high emissions scenarios, and is
\textit{unlikely} to be crossed for low and very low emissions
scenarios. 

The IPCC assessment does not provide guidance nor a likelihood
assessment of the plausibility of climate future scenarios, but
highlights that recent and future emissions trends expected from
current \mbox{policies} are closest to an intermediate scenario. Future
warming implied by current policies has also been updated annually
\citep{UNEP2024}. However, current policies might be reversed, and a
large uncertainty range is attached to the best estimate of the climate
response to a given scenario: under SSP2-4.5, the \textit{very
likely} range (90\% interval) of assessed global warming by 2081--2100
spans from 2.1 to 3.5~\textdegree C of global warming, for a best
estimate\break of 2.7~\textdegree C. 

These global constrained projections can also be used to develop
consistent regional warming levels. For instance, the French reference
trajectory for adaptation is based on the best estimate of the climate
response to this intermediate emissions scenario, assuming a level of
global warming of 2~\textdegree C by 2050 and 3~\textdegree C by 2100,
and then applying similar constrained projections methodologies to
translate a time horizon and level of global warming to the
corresponding level of warming in mainland France
\citep{Corre2025,Ribes2022}. The reference trajectory also assumes climate
\mbox{stabilization} at 3~\textdegree C by 2100, while net zero CO$_2$
emissions are not expected to be reached by that time horizon from
intermediate or current policies scenarios \citep{UNEP2024}. Such
differences have implications for long-term sea-level projections
consistent with different future climate scenarios.

Going back to the global scale, the assessment of the Earth system
response has also been used to develop simplified climate models, or
emulators \citep{Nicholls2022}. These emulators have been used to
classify more than 1200 modelled emissions pathways into 8 categories
\citep{Kikstra2022} as a function of the resulting level of global
warming, and to distinguish the trajectories allowing to limit global
warming to 1.5~\textdegree C (with 50\% probability), without overshoot
(as in SSP1-1.9) or with a limited (${<}$0.1~\textdegree C) or high
(0.1~\textdegree C to 0.3~\textdegree C) overshoot, the trajectories
allowing to limit global warming well below 2~\textdegree C (with 67\%
probability, as in SSP1-2.6 or 50\% probability), below 2.5~\textdegree
C or 3~\textdegree C (as in SSP2-4.5) or 4~\textdegree C with ${>}$50\%
probability (as in SSP3-7.0), or to exceed 4~\textdegree C by 2100 with
${>}$50\% probability (as in SSP5-8.5) (Figure~\ref{fig1}). These
approaches allow to describe what are the characteristics of mitigation
pathways consistent with specific warming levels, for instance in terms
of phasing out fossil fuels consistent with limiting global warming to
different levels \citep{Achakulwisut2023}. 

For consistency, emulators consistent with constrained global surface
temperature projections have been used for future projections of ocean
heat content, glaciers and ice sheet contributions to sea-level rise,
based on ice sheet processes associated with at least \textit{medium
confidence} (Figure~\ref{fig3}, colored lines in the sea-level
projection panel). As a result, global mean sea-level projections are
consistent with global surface temperature projections, also allowing
to relate global warming levels to future sea-level rise over the time
scales of centuries to millennia. The consistency between insights from
modelling and from paleoclimate evidence from past warm periods and
high stands of global mean sea level provide confidence in the outcome
\citep{IPCC2021}. 


Recent studies have identified deficiencies in state-of-the-art simple
climate models for the replication of physical mechanisms linking ocean
heat and carbon uptake. Future work is thus needed to improve the
geophysical consistency between models used in the scenario generation
process and assessment \citep{Sanderson2023,Seferian2024}. 

For many climatic impact-drivers, their characteristics and spatial
patterns directly depend on the level of global warming, independently
of the timing when this level of warming is reached, and of the
emissions and land use pathway leading to that level of warming. As a
result, the level of global warming is used as a dimension of
integration between mitigation pathways and climate-related risks. The
interactions between resulting hazards, exposure and vulnerabilities of
human activities and societies, species and ecosystems, lead to
climate-related risks. The assessment of regional and sectoral risks is
conducted within five broad risk typologies, assessed as a function of
global warming levels, and, where relevant, socio-economic scenarios
(Figure~\ref{fig3}). Mitigation and maladaptive responses can indeed
also drive negative effects. For instance, land-based mitigation can
exacerbate risks to land rights, water and food security, ecosystems
and biodiversity \citep{IPCC2023,IPCC2019}.

Risks and projected adverse impacts and related losses and damages
escalate with every increment of global warming (Figure~\ref{fig3}). For
many climate-related risks, they are assessed in the IPCC AR6 to be
higher for any future global warming level than assessed in the IPCC
AR5. While future changes in slow \mbox{components} in the climate system are
unavoidable, due to their adjustment times, they can be limited by
limiting future warming and thus by deep, rapid and sustained
reductions in greenhouse gas emissions \citep{IPCC2023}.

Current mitigation efforts are insufficient to respect the Paris
Agreement long-term temperature goal, limiting global warming well
below 2~\textdegree C with the aspiration to limit global warming to
1.5~\textdegree C. As a result, the risk assessment also encompasses the
possibility of exceeding a level of global warming, and then returning
below that level of global warming through assuming the ability to
reach and maintain net negative CO$_2$ emissions (so-called
``overshoot'' scenarios) (Figure~\ref{fig1}). 


The plausibility of overshoot scenarios depends on the plausibility of
upscaling carbon dioxide removal, with uncertainties on technologies,
political and public support, business models, sustainability and
equity considerations, and uncertainties associated with permanence and
feedbacks \citepalias{IPCC2023}. Any mitigation delay and associated
emissions leads to a higher level of peak warming, and every increment
of additional peak warming leads to increased impacts with no
expectation of immediate reversibility after overshoot
\citep{Kim2022}, such as continuous warming of the Southern Ocean,
changes in the deep ocean, ocean and land ecosystems and biodiversity,
and leads to an additional long-term sea-level commitment. A few
studies indicate distinct regional climate characteristics following
overshoot compared to stabilization scenarios. Long-term reversibility
of climatic impact-drivers after overshoot furthermore has limited
relevance for adaptation decision-making spanning the next 50 years.
Finally, uncertainties resulting from uncertain climate and carbon
cycle feedbacks triggered by peak warming may challenge the ability to
stabilize future warming, calling for sustainable carbon dioxide
removal capacity to hedge against high warming outcomes
\citep{Schleussner2024}.

\vspace*{-5pt}
\subsection{Deep uncertainty, storylines and low-likelihood,
high-impact eventualities}\label{sec33}

The current rise in atmospheric CO$_2$ level, mostly resulting from
the combustion of fossil fuels, is unprecedented in up to 16 million
years \citep{CenCO2PIP2023}. The resulting climate heating is driving the
emergence of novel climate conditions, unprecedented in terms of pace
and magnitude over centuries to millennia \citep{IPCC2021,Osman2021},
and, combined with human pressures on ecosystems and biodiversity
\citep{IPBES2019}, the degradation of ecosystems and loss of biodiversity
and nature's contributions to people \citep{IPCC2022a}. As described
above, future climate scenarios are associated with various types of
uncertainties, both in terms of socio-economic drivers and changes, in
terms of Earth system responses, and in terms of complex risks, and
systems transformations, with the potential for surprises. IPCC
assessment reports use risk frameworks to explore uncertainties, and
have developed a calibrated language to report confidence in future
projections, quantitatively (ranges of outcomes), and qualitatively. 

Information on uncertainty is critical to inform risk management, and
robust decision-making \mbox{under} uncertainty. The latter framework embraces
the ideas that science-based analysis should ``seek to facilitate human
creativity, deliberation and judgement in solving complex problems
rather than aspire to proscribe the best decisions'', and ``science can
help decision-makers manage deep uncertainty, not just reduce it''
\citep{Lempert2024}.

Regarding future climate scenarios, the IPCC AR6 has explicitly
expanded its guidance for the degrees of certainty to encompass deep
uncertainty, with the following definition: ``A situation of deep
uncertainty exists when experts or stakeholders do not know or cannot
agree on: (1) appropriate conceptual models that describe relationships
among key driving forces in a system; (2) the probability distributions
used to represent uncertainty about key variables and parameters;
and/or (3) how to weigh and value desirable alternative outcomes''. 

Two framing concepts related to deep uncertainty have been developed in
climate science literature, and are thus reflected in
\citet{IPCC2021}, physical climate storylines, and low-likelihood,
high-impact outcomes. 

Physical climate storylines provide self-consistent narratives, without
a quantified probability of occurrence, of possible unfolding of a
physical trajectory of the climate system. In \citetalias{IPCC2021}, this
concept was used for the exploration of low-likelihood, high impact
events, or cross-scale interactions for informing adaptation; to put
historical events in the context of a changing climate and as an
alternative approach to event attribution studies; and within climate
services, as providing climate information integrated with
socio-economic information. An overview of recent developments
highlights the variety of approaches associated with this concept, and
the causal chain that they explore, the type of evidence used, and how
they incorporate value judgements \citep{Baldissera2024}. These
storylines play a key role in the links between climate science and
decision-making \citep{Sillmann2024}, but have not yet been explicitly
incorporated within the French national adaptation strategy.

Low-likelihood, high impact outcomes are defined as ``Outcomes/events
whose probability of occurrence is low or not well known (as in the
context of deep uncertainty) but whose potential impacts on society and
ecosystems could be high. To better inform risk assessment and
decision-making, such low-likelihood outcomes are considered if they
are associated with very large consequences and may therefore
constitute material risks, even though those consequences do not
necessarily represent the most likely outcome'' \citep{IPCC2021}.

Compound and current extremes contribute to the increasing probability
of low-likelihood, high-impact outcomes, and will be more frequent with
increasing global warming levels, which also increase the likelihood of
events unprecedented in the observational record of a given region,
giving a misleading perception of ``surprise'' as assessed in Box 11.2,
in \citetalias{IPCC2021}. For instance, the occurrence of extreme heat
reaching 50~\textdegree C in Paris is estimated to be implausible under
the current climate, but is possible, as a very rare event, in a
2~\textdegree C warmer world \citep{Yiou2024}. 

Low-likelihood, high-warming storylines describes future global warming
exceeding the assessed \textit{very likely} range, for a given
emissions scenarios, for instance based on models with large climate
sensitivities (not shown in Figure~\ref{fig1} for scenarios and
projections). This has for instance implications for the timing when
specific levels of global warming are reached and is described in Box
TS.3 in \citet{IPCC2021}.

Another example of a low-likelihood, high-impact storyline refers to
the possibility, consistent with paleoclimate evidence, of experience
several large volcanic eruptions, which would alter the multi-decadal
climate trajectory compared to scenario-based projections, as assessed
in Cross-Chapter Box~4.1 in \citetalias{IPCC2021}. There is a lack of
inclusion of such storylines in risk management and adaptation
strategies worldwide, which only focus on human drivers of future
climate change; such eventualities would have major implications for
e.g.\ food security.

Low-likelihood, high-impact storylines have also been used to describe
what would be the consequences of abrupt changes in components of the
climate system. There is \textit{medium confidence} that the projected
decline in the Atlantic Meridional Overturning Circulation will not
lead to an abrupt collapse before 2100, but such a collapse might be
triggered by unexpected meltwater inflow from the Greenland ice sheet.
The storyline approach allows to describe, if such a collapse were to
occur, the state of knowledge on its consequences for regional changes
in hydroclimate, as described in Box TS.3 in \citetalias{IPCC2021}.\looseness=1

\textit{Likely} range sea-level rise projections (Figure~\ref{fig4}) do
not include ice-sheet processes characterized by \textit{low
confidence} and deep uncertainty. Higher magnitudes of sea-level rise
could be driven by earlier-than-projected disintegration of marine ice
shelves, the onset of uncertain ice sheet instability processes in
Antarctic ice sheet sectors, or faster-than-projected melt and ice flow
from the Greenland ice sheet. In the IPCC AR6, a low-likelihood,
high-impact storyline was developed for the very high CO$_2$ emissions
scenario, and this worse case outcome was explicitly communicated
together with sea-level projections (Figure~\ref{fig3}, dashed line)
\citep{Kopp2023}. Further research has focused on high-end sea-level
rise as a function of the scenarios and time frame \citep{Stammer2019}, for
the purpose of informing practitioners and for stress tests in
risk-intolerant contexts \citep{vandeWal2022}. Such high-end storylines,
which are relevant for coastal adaptation pathways and their limits,
have been incorporated in national sea-level information, for instance
in the USA \citep{Kopp2023} and the UK \citep{Palmer2024}, but are not
included in the current approach to sea-level information in the French
context, which is raised as a concern for potential maladaptation
\citep{LeCozannet2024}. 

\begin{figure*}
\includegraphics{fig04}
\bcaption{\label{fig4}}{3}{Subset of assessed climate outcomes and
associated global and regional climate risks. The burning embers result
from a literature-based expert elicitation. (a)~Left---Global
surface temperature changes in \textdegree C relative to 1850--1900.
These changes were obtained by combining CMIP6 model simulations with
observational constraints based on past simulated warming, as well as
an updated assessment of equilibrium climate sensitivity. Very likely
ranges are shown for the low and high GHG emissions scenarios (SSP1-2.6
and SSP3-7.0) as in Figure~\ref{fig1}. Right---Global Reasons for
Concern (RFC), comparing AR6 (thick embers) and AR5 (thin embers)
assessments. Risk transitions have generally shifted towards lower
temperatures with updated scientific understanding. Diagrams are shown
for each RFC, assuming low to no adaptation. Lines connect the
midpoints of the transitions from moderate to high risk across AR5 and
AR6. (b) Selected global risks for land and ocean ecosystems,
illustrating the general increase in risk with global warming levels
with low to no adaptation. (c) Left---Global mean sea level
change in centimetres, relative to 1900. The historical changes (black)
are observed by tide gauges before 1992 and altimeters afterwards. The
future changes to 2100 (coloured lines and shading) are assessed
consistently with observational constraints based on the emulation of
CMIP, ice sheet, and glacier models, and likely ranges are shown for
SSP1-2.6 and SSP3-7.0.  Right---Assessment of the combined risk of
coastal flooding, erosion and salinization for four illustrative
coastal geographies in 2100, due to changing mean and extreme sea
levels, under two response scenarios, with respect to the baseline
period 1986--2005. The assessment does not account for changes in
extreme sea level beyond those directly induced by mean sea level rise;
risk levels could increase if other changes in extreme sea levels were
considered (e.g., due to changes in cyclone intensity).
``No-to-moderate response'' describes efforts as of today (i.e., no
further significant action or new types of actions). ``Maximum
potential response'' represent a combination of responses implemented
to their full extent and thus significant additional efforts compared
to today, assuming minimal financial, social and political barriers.
(In this context, ``today'' refers to 2019.) The assessment criteria
include exposure and vulnerability, coastal hazards, in situ responses
and planned relocation. Planned relocation refers to managed retreat or
resettlements. The term response is used here instead of adaptation
because some responses, such as retreat, may or may not be considered
to be adaptation. (d)~Selected risks under different
socio-economic pathways, illustrating how development strategies and
challenges to adaptation influence risk.  Left---Heat-sensitive human
health outcomes under three scenarios of adaptation effectiveness. The
diagrams are truncated at the nearest whole~\textdegree C within the
range of temperature change in 2100 under three SSP scenarios.
Right---Risks associated with food security due to climate change and
patterns of socio-economic development. Risks to food security include
availability and access to food, including population at risk of
hunger, food price increases and increases in disability-adjusted life
years attributable to childhood underweight. Risks are assessed for two
contrasted socio-economic pathways (SSP1 and SSP3) excluding the
effects of targeted mitigation and adaptation policies. Replicated from
the Summary for Policy Makers of \citep{IPCC2023}.}
\vspace*{-3pt}
\end{figure*}

Other areas of deep uncertainty are associated with the Antarctic
sea-ice, and with carbon cycle feedbacks. No specific storyline was
developed within the IPCC AR6, but carbon cycle feedback processes
where explicitly mentioned as a qualitative source of uncertainty for
the ability to return \mbox{following} a period of overshoot: ``Adverse
impacts that occur during this period of overshoot and cause additional
warming via feedback mechanisms, such as increased wildfires, mass
mortality of trees, drying of peatlands, and permafrost thawing,
weakening natural land carbon sinks and increasing releases of
greenhouse gases would make the return more challenging''. Recently, a
storyline approach combining narratives and scenarios was developed to
explore future CO$_2$ and methane emissions resulting from permafrost
thaw \citep{Schuur2022}, showing that a sole focus on country-level
emissions without accounting for Arctic carbon feedbacks is not likely
to be enough to limit future warming.

Concerns about the loss of efficiency of natural carbon sinks have
recently been enhanced by increased tree mortality, ecosystem
degradation and large-scale wildfires \citep{Friedlingstein2025}, as
well as the recent intensification of wetland methane  feedback
\citep{Zhang2023}, processes which are not fully incorporated into
Earth system models. The ocean and land carbon responses to both CO$_2$
and climate change are a major source of uncertainty for understanding
the full Earth system long-term response to zero CO$_2$ emissions, and
could result in additional amounts of warming, challenging the
plausibility of overshoot scenarios \citep{PalazzoCorner2023}, and the
ability to limit future global warming without a carbon removal
capacity suitable for buffering potential carbon cycle feedbacks
\citep{Schleussner2024}. It has thus been argued that carbon-climate
interactions and feedbacks are central to how the Earth system will
evolve in the future, and should be central to future projections. This
calls for carbon emissions and land-activity-driven simulations to be
central to future climate scenarios, instead of concentration-driven
simulations \citep{Sanderson2023}. 

This brief overview highlights the importance of physical climate
storylines to high-impact eventualities and tail risks, in addition to
the best estimates and most likely climate futures and in complement of
probabilistic estimates of occurrences of events and to inform robust
decision-making \citep{Lempert2024}. Such storylines are important for
instance for climate proofing of risk-averse infrastructure, for
climate stress tests of critical infrastructure, and for cascading risk
management \mbox{strategies}. \mbox{Storyline}\unskip\break
\mbox{information} is not (yet) included in French adaptation
strategies, possibly for fear of over-adaptation (over-investments in
adaptation), a point explicitly mentioned in the National Adaptation
strategy (PNACC3). However, there is a growing interest in such
information to inform long-term strategic planning for energy, health
and telecommunication infrastructure, calling for credible and reliable
information on potential high-impact scenarios and storylines, as
developed for the UK \citep{Palmer2024,Arnell2025}. 

\section{Final remarks on advances on future climate scenarios}\label{sec4}

As summarized in this overview, scenarios support scientific
assessments of climate-related risks and the synthesis of climate
change information. \mbox{Scenarios}, referring to the combination of climate,
socio-economic and policy pathways within a coherent and internally
consistent plausible future, advance the understanding of how different
socio-economic choices lead to different outcomes and risks, and inform
society in general, and climate policies. They need to be complemented
by information on low-likelihood, high-impact outcomes to inform robust
decision-making under uncertainty.

Given the fast pace of human-induced climate change, and the escalation
of climate-driven impacts, the new frontier for the development of
socio-economic narratives requires to explicitly account for
human-caused climate change impacts and risks \citep{Pirani2024},
equity \citep{Kanitkar2024}, with new challenges for a climate research
agenda grounded in ethics \citep{Masson-Delmotte2024}. Other challenges
include better representation of economic and social dynamics such as
market frictions, financial flows, as well as the inclusion of new
technologies, notably information technologies.

Supporting in situ and remove observational networks is a cornerstone
for all aspects of advancing climate science and for the provision of
reliable, actionable information for society 
\citep{Stammer2025,Thierry2025}. Within a vision of climate science for
2050 \citep{Brasseur2025}, advances in the understanding of Earth
system responses at global and regional scales, based on advances in
model developments \citep{Jones2024}, very high resolution (kilometer
scale, convection permitting) modelling \citep{Fosser2024}, theory
\citep{Byrne2024}, as well as new methodologies \citep{Eyring2024} to
build reliable climate information and constrain uncertainties
\citep{Douville2025} are needed, in particular for dynamical and
hydroclimatic climate phenomena. 

The narrow set of five SSP narrative, which is a compromise between a
limited number of narratives for communication purposes, and exploring
a range of possible futures, does not incorporate scenarios with a
strong emphasis on sufficiency, post-growth perspectives
\citep{Hickel2021}, or social equity and justice \citep{Zimm2024}.
Specific developments in all these aspects are currently underway.

Scenarios assessed in the IPCC AR6 encompass a very high emissions
scenario, challenged for its plausibility. However, exploring Earth
system modelled responses to very high forcings and providing high
signal to noise experiments is useful, and can be achieved from
sensitivity studies or idealized simulations.

New scenario designs \citep{Meinshausen2024} are needed to explore the
potential climate outcome of current climate policy targets; the
worse-case outcomes for current policy scenarios in terms of high-end
climate sensitivity and carbon cycle feedbacks; what climate impacts
can still be avoided; what are the different feasible mitigation
strategies implied by the Paris Agreement temperature goals and the
interlinkages of related climate action with a broader sustainability
agenda; what are the consequences of delayed mitigation; what are the
climate effects of different regional emissions and land use changes;
what are the effects of non-CO$_2$ mitigation; what are the risks and
effectiveness of land-based carbon dioxide removal; and to what extent
are climate change and the impacts of climate change time-dependent and
reversible. Very high and very low emissions scenarios remain useful as
benchmarks for ``the world that could have been'' if mitigation efforts
had not existed or had been very stringent. The extension of scenarios
over time is also important to explore the long-term implications, well
beyond 2100. These approaches support the design of ScenarioMIP-CMIP7
\citep{vanVuuren2025}.

The biodiversity perspective is particularly important to strengthen,
as no SSP scenario, including sustainability-oriented SSP1, allows to
meet \mbox{biodiversity} targets, as highlighted in  Table~18.1 of
\citet{IPCC2022a}. Given the close interplays between climate change,
food and energy systems, water, health and well-being, and the erosion
of biodiversity and loss of ecosystem functions and nature's
contributions to people, new scenario approaches are also needed to
inform response options which account for nexus interactions and
support transformative changes \citep{McElwee2025,MedECC2024}. 

In the context of escalating climate change impacts, it is increasingly
relevant to reverse the causal chain, and focus on the end-user
perspective. Starting with adaptation limits and thresholds of
manageable or tolerable risks, for instance related to tolerable heat
for health, fire weather for forestry, or glacier mass loss for water
resources, in specific regions, can then be the starting point to infer
\mbox{compatible} \mbox{emissions} and land use pathways,
\mbox{strengthening} support for ambitious mitigation
\citep{Pfleiderer2025}.

\printCOI 

\vspace*{-3pt}

\section*{Financements}
\vspace*{-3pt}
For the IPCC Sixth Assessment Cycle, the technical support unit of the
IPCC Working Group I (of which the author was co-chair) was funded by
the French government through Ministries of Research, Foreign Affairs
and Ecological Transtion (through ADEME).

\vspace*{-3pt}

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\refinput{crgeos20250297-reference.tex}

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