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\DOI{10.5802/crgeos.305}
\datereceived{2024-11-03}
\daterevised{2025-06-19}
\dateaccepted{2025-07-28}
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\dateposted{2025-09-10}
\begin{document}

\begin{noXML}

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

\CDRsetmeta{articletype}{review}

\title{Permafrost and ice sheets: potential abrupt and/or irreversible
changes}

\alttitle{Perg\'elisol et calottes glaciaires : des changements
potentiellement abrupts et/ou irr\'eversibles}

\author{\firstname{Gerhard} \lastname{Krinner}\CDRorcid{0000-0002-2959-5920}}
\address{Univ. Grenoble Alpes, CNRS, INRAE, IRD, IGE, 38000 Grenoble, France}
\email{gerhard.krinner@cnrs.fr}
\urladdr{https://www.ige-grenoble.fr/-Gerhard-Krinner-}

\keywords{\kwd{Tipping elements}\kwd{Irreversibility}\kwd{Permafrost}\kwd{Ice sheets}}

\altkeywords{\kwd{\'El\'ements de bascule}\kwd{Irr\'eversibilit\'e}\kwd{Perg\'elisol}\kwd{Calottes glaciaires}}

\begin{abstract} 
This short article presents a summary of a public lecture held at the
Acad\'emie des Sciences in March, 2024 on continental tipping elements in
the climate system. It first discusses some critical aspects of the
tipping elements/tipping points concept as currently used in climate
science. It is argued that using more clearly defined concepts such as
abruptness and irreversibility instead of the ``tipping element'' framing,
as done in the IPCC AR6 WGI report, can avoid confusion. The permafrost
carbon reservoir and large continental ice sheets are then presented as
prime examples of climate system elements possibly subject to abrupt
and irreversible changes. 
\end{abstract}

\begin{altabstract} 
Cet article court pr\'esente un r\'esum\'e d'une conf\'erence publique 
donn\'ee \`a l'Acad\'emie des sciences en mars 2024 sur les 
\'el\'ements continentaux de bascule du syst\`eme climatique. Il aborde
d'abord certains aspects critiques du concept d'\og \'el\'ements de
bascule \fg{} ou de \og \'el\'ements de bascule \fg{} tel qu'il est
actuellement utilis\'e en science du climat. Il est avanc\'e que
l'utilisation de concepts mieux d\'efinis, tels que le caract\`ere
abrupte et l'irr\'eversibilit\'e, comme le propose le rapport du GIEC
AR6 --- Groupe I, permettrait d'\'eviter certaines confusions li\'ees 
au cadre conceptuel des \og \'el\'ements de bascule \fg{}. Le
r\'eservoir de carbone du perg\'elisol et les grandes calottes
glaciaires continentales sont ensuite pr\'esent\'es comme des exemples
majeurs d'\'el\'ements du syst\`eme climatique susceptibles de subir
des changements \`a la fois abrupts et irr\'eversibles.
\end{altabstract} 

\maketitle

\twocolumngrid

\end{noXML}

\defcitealias{schuur2022permafrost}{ibid.}
\defcitealias{deconto2016contribution}{ibid.}

\section{Introduction}\label{sec1}

The concept of abrupt and irreversible changes in the climate system as
a consequence of human interference, in particular emissions of
greenhouse gases, has been popularized under the expression ``tipping''
elements/points/events. In this context, the expression ``tipping
element'' refers to the part of the climate system under scrutiny
(e.g., the West Antarctic Ice Sheet), while the corresponding ``tipping
point'' is usually a point in time at which the tipping event 
occurs---this can also sometimes be quantified as a deviation of global
mean surface air temperature from a ``preindustrial reference'' value
(which, under continuous warming, implicitly corresponds to a given
point in time). The idea of possible tipping events in the climate
system is tightly linked to the concept of bifurcations in complex
system dynamics, but, in the context of the climate system, noise- and
rate-induced ``tipping'' has also been studied \citep{Ashwin:2012}.
Related concepts such as regime shifts and critical transitions have
been the focus of numerous studies in past decades. However, since the
2000s, the ``tipping element/point/event'' concept has been dominant,
both in scientific and public discourse, at least in relation to the
topic of climate change \citep{Milkoreit:2018}.

Over the years, many potential tipping elements in the climate system
have been proposed and discussed
\citep[e.g.,][]{Lenton:2008,Schellnhuber:2009,TippingPointsReport2023,
McKay:2022,Wang:2023}, such as loss of Amazon rainforest, loss of the
West Antarctic Ice Sheet, loss of summer Arctic sea ice, or a slowdown
of the Atlantic Meridional Overturning Circulation (AMOC). In is
notable that many of the proposed tipping elements are situated in the
high latitudes, and that the ensemble of tipping elements discussed in
the literature is quite variable, depending in part on whether one
considers only the climate system element per se (e.g., sea ice), or
potential ecological or social impacts of large changes in that
physical {element} (e.g., species extinctions linked to potentially
reversible, temporary sea-ice decline). In addition to the study of
individual tipping elements in isolation, several authors have drawn
attention to \mbox{potential} \mbox{interactions} between tipping elements, subject
to particularly large uncertainties \citep[e.g.,][]{Wunderling:2024}.

The purpose of this short article is in no way to provide a full
synthesis of past and ongoing research about continental tipping points
in the climate system. Rather, this modest article will first issue a
short caveat on the tipping element concept, based on the author's
personal thoughts and published critiques of the concept. Subsequent
sections will then discuss the potential for abrupt and irreversible
changes linked to the physics and biogeochemistry of the permafrost
carbon reservoir and large continental ice sheets, both of which can be
considered as prime examples of the multitude of proposed ``tipping
elements'' in the climate system. Tipping elements in the
socio-economic system, linked to climate change impacts, adaptation to
climate change, and its mitigation, are increasingly discussed in the
literature \citep[e.g.,][]{Otto:2020,TippingPointsReport2023}, but are
not treated here.

\section{A note on the concept of tipping points in the climate system}

Definitions of ``tipping'' vary somewhat in the scientific literature.
Following \citet{Milkoreit:2018} and \citet{McKay:2022}, the recent
``Global Tipping Points'' report \citep{TippingPointsReport2023}
defines a tipping point as occurring when change in part of a system
becomes self-perpetuating beyond a threshold, leading to substantial,
widespread, frequently abrupt and often irreversible impacts. It is
notable that in this definition, abruptness and irreversibility---the
latter referring to characteristics of the change itself, as well as
its impacts---are not strictly speaking necessary conditions for the
existence of a tipping point. In any case, the existence of positive
feedbacks, and their dominance during the phase of transition between
two states, is a common feature of hypothetical or real tipping
mechanisms in the climate system \citep{Lenton:2008}. The somewhat
unclear definitions of what a tipping element in the climate system
really is, such as the definition cited above, reflect the difficulties
the \mbox{scientific} community has in treating the concept. In such cases, it
is often useful to take one step back. In the case of tipping elements
in the physical climate system, taking this step back means
concentrating on more clearly defined characteristics of the relevant
elements of the climate system. This is why the IPCC AR6 Working Group
I report \citep{AR6WGI}, while acknowledging the concept and referring
to it, neither provides a detailed assessment of the existence of
tipping elements in the climate system, nor quantifies tipping points
as such. Rather, the report assesses the potential of abrupt changes
and the irreversibility of these changes (in case the initial forcing
leading to these changes is reversed), as can be seen in Table~\ref{T1}.

\begin{table*}
\caption{\label{T1}Continental tipping elements, potential abruptness
and irreversibility of future changes, projected change over the 21{st}
century as assessed in the IPCC AR6 Working Group I report (excerpt
from Table 4.10, Chapter 4 \citep{IPCC_2021_WGI_Ch_4}}
\begin{tabular}{@{}cccc@{}}
\thead
\parbox[t]{8pc}{\centering Earth system component/tipping element} & 
\parbox[t]{6pc}{\centering Potential abrupt climate change?} &
\parbox[t]{9pc}{\centering Irreversibility if forcing reversed (time scales indicated)} &
\parbox[t]{11pc}{\centering Projected 21st century change under continued warming} \\
\endthead
Tropical Forest & Yes, \textit{low confidence} &
\parbox[t]{9pc}{\centering Irreversible for multi-decades,
\textit{medium confidence}} & \parbox[t]{11pc}{\centering \textit{Medium
confidence} in increasing vegetation carbon storage depending on human
disturbance}\vspace*{3pt} \\

Boreal Forest & Yes, \textit{low confidence} &
\parbox[t]{9pc}{\centering Irreversible for multi-decades,
\textit{medium confidence}} & \parbox[t]{11pc}{\centering \textit{Medium
confidence} in offsetting lower latitude dieback and poleward extension
depending on human disturbance}\vspace*{3pt} \\

Permafrost Carbon & Yes, \textit{high confidence} &
\parbox[t]{9pc}{\centering Irreversible for centuries, \textit{high
confidence}} & \parbox[t]{11pc}{\centering \textit{Virtually certain}
decline in frozen carbon; \textit{low confidence} in net carbon change}
\vspace*{3pt}\\

Greenland Ice Sheet & No, \textit{high confidence} &
\parbox[t]{9pc}{\centering Irreversible for millennia, \textit{high
confidence}} & \parbox[t]{11pc}{\centering \textit{Virtually certain} 
mass loss under all scenarios}\vspace*{3pt}\\

\parbox[t]{8pc}{\centering West Antarctic Ice Sheet and shelves} & 
Yes, \textit{high confidence} & 
\parbox[t]{9pc}{\centering Irreversible for decades to millennia, \textit{high confidence}} & 
\parbox[t]{11pc}{\centering \textit{Likely} mass loss under all scenarios; deep uncertainty in
projections for above 3~\textdegree C}\vspace*{2pt}
\botline
\end{tabular}
\tabnote{The continental tipping elements mentioned in this table are
tropical forests, boreal forests, permafrost carbon, the Greenland Ice
Sheet, and the West Antarctic Ice Sheet with its large ice shelves.}
\vspace*{-.5pc}
\end{table*}

Moreover, the triggering of abrupt changes, such as a rapid ice sheet
decay, may in many cases require not only exceeding a given threshold
(in most cases, a certain temperature level), but also a certain amount
of time during which that exceedance takes place
\citep[e.g.,][]{schleussner2024overconfidence}. Therefore focusing on
``tipping points'' in terms of a single threshold in public discourse
can lead to misconceptions such as the imaginary existence of a single
global temperature level which would be a sharp divide between a safe
world and the global catastrophe. Therefore the IPCC, in its most
recent synthesis report \citep{AR6SYR}, took great care to reduce the
use of the word ``tipping point'' to a minimum. A more detailed
discussion of these aspects can be found in \citet{kopp2025tipping}.

In many cases, an assessment of the risk of tipping, and associated
tipping points, cannot be made based on evidence delivered by climate
models alone, for several reasons. First, as will be discussed for
specific cases in later sections of this text, critical processes are
in some case simply not represented in complex climate models. For
example, abrupt thaw processes, which can lean to strong local
acceleration of permafrost decay, are not represented in
current-generation comprehensive (CMIP-type) climate models. This is
sometimes because these processes take place at very small scales.
Another source of difficulty is that tipping of major elements of the
climate system has not been observed in the recent past. Therefore,
calibration of models in this respect would need to rely on information
from past climates, which is subject to multiple sources of uncertainty
(e.g., limited spatial coverage, limited temporal resolution, or
difficulties in interpreting the proxy record).

The IPCC nevertheless provides an assessment of the global risk level
(``Reasons for Concern'') associated with ``large scale singular
events'' (see Figure~\ref{SPM4A}), based on structured expert
elicitation. The tipping of a major element of the climate system, for
example the irreversible destabilisation and, at least partial, loss of
the West Antarctic Ice Sheet, would be such a ``large scale singular
event''. The ensemble of these ``large scale singular events'' give
rise to a global risk level that AR6 assessed to be moderate for a
sustained warming of 1.5~\textdegree C with respect to the reference
period 1850--1900, transitioning to high at about 2~\textdegree C
warming.

\begin{figure*}
\vspace*{-4pt}
\includegraphics{fig01}
\caption{\label{SPM4A}``Reasons for Concern'' reproduced from the IPCC
AR6 Synthesis Report Figure SPM4a \citep{AR6SYRSPM}, containing, on the
right, a ``burning ember'' diagram that shows the assessed risk level
associated to ``large-scale singular events'' as a function of the
Global Warming Level (global mean surface air temperature increase
above the 1850--1900 level). The figure compares the assessed risk
levels for AR5 and AR6, showing that, for a given global mean
temperature change, the risk level linked to ``large-scale singular
events'' is estimated higher in AR6 than in AR5.}
\end{figure*}

In the following, the permafrost carbon reservoir and the large
continental ice sheets, as prime examples of potentially unstable major
elements of the global climate system, will be discussed in terms of
how abruptly they might change in the future and inhowfar the changes
might be irreversible.

\section{Permafrost}

Permafrost is perennially frozen soil. In high latitudes and
mountainous regions, permafrost is \mbox{ubiquitous:} the total area of the
northern circumpolar permafrost region is about 17.8 ${\times}$
10\tsup{6}~km\tsup{2} \citep{schuur2022permafrost}. Carbon has accumulated in
these soils for millennia, because the cold soil temperatures inhibit
microbial decomposition of organic matter. Estimates of the total
quantity of carbon in permafrost soil are variable but in the order of
about 1500 PgC \citepalias{schuur2022permafrost}. Under a warming climate,
it is very plausible that high-latitude soils at a given depth will
remain seasonally unfrozen for a longer time, leading to more efficient
microbial decomposition of the available organic matter; in areas close
to the southern limit of the permafrost area, permafrost might be
replaced by seasonally frozen soil. The decomposition of organic matter
that has been locked away in perennially or almost perennially frozen
soil for decades to millennia would thus lead to additional greenhouse
gas \mbox{emissions} (CO\tsub{2} or CH\tsub{4}, depending primarily on the
moisture of the soil). This positive feedback to climate
change---warming leading to additional greenhouse gas emissions,
\mbox{leading} to \mbox{enhanced} warming---has been recognized for decades, but its
quantification has been difficult, for many reasons. First, there have
been large uncertainties about the total quantity of organic carbon in
permafrost soils \citep[e.g.,][]{zimov2006permafrost}. Second, there is
a large spatial heterogeneity in northern permafrost soils
\citep[e.g.,][]{olefeldt2016circumpolar,nitzbon2020fast,mishra2021spatial}, 
strongly influencing estimates of future thaw. Third, and in part due
to the strong spatial heterogeneity, the quality of the organic matter
has not been well quantified until relatively recently
\citep[e.g.,][]{schadel2014circumpolar}. Fourth, many climate models do
not represent a substantial permafrost carbon reservoir in the
high-latitude soils, or simply do not well represent permafrost-related
soil processes \citep{schadel2024earth}. The consequence is that when
the climate warms, the models cannot simulate greenhouse gas emissions
from permafrost regions, yielding an unphysical total carbon reservoir
increase in northern high latitudes (see Figure~\ref{AR6WGI5.26}).

\begin{figure*}
\includegraphics{fig02}
\caption{\label{AR6WGI5.26}Net carbon changes under four Shared
Socio-economic Pathway (SSP) scenarios, as evaluated from nine CMIP6
Earth system models (IPCC AR6 Working Group I, Figure 5.26
\citep{IPCC_2021_WGI_Ch_5}). It is noteworthy that the total carbon
stored in northern high latitudes is projected to increase in these
models, due to increased storage of carbon in vegetation and soils,
while there is no or only little loss of permafrost carbon simulated
because relevant processes are not represented in the models.}
\end{figure*}

In spite of these difficulties, a consensus has nevertheless emerged
over the last decade or so, based on availability of new data, better
process \mbox{understanding} and improved modeling. This now allows to provide
more consolidated estimates of future greenhouse gas emissions from
thawing permafrost in a warming world, with admittedly still large
uncertainty ranges (see Figure~\ref{SROCC3.11}). The emerging consensus
suggests weaker future permafrost carbon losses than earlier estimates.
In the IPCC AR6 Working Group I report, the permafrost CO\tsub{2}
feedback per \textdegree C of global warming is thus estimated to be 18
(3.1--41, 5--95\% range) PgC${\cdot}$\textdegree C$^{-1}$
\citep{IPCC_2021_WGI_Ch_5}.\looseness=1

\begin{figure*}
\includegraphics{fig03}
\caption{\label{SROCC3.11}Evolution of the estimates of potential
future carbon loss from northern circumpolar permafrost soils.
Reproduced from Figure 3.11, IPCC Special Report on the Ocean and
Cryosphere in a Changing Climate \citep{meredith2019polar}.}
\end{figure*}

However, these assessments are partly based on models that neglect
abrupt thaw processes. Thawing of ice-rich permafrost leads to local
land subsidence, a process called thermokarst formation. These
distinctive landforms, with local depressions often filled with water
(``thermokarst lakes''), can lead to increased thaw rates along the
newly formed local slopes, often accelerated by increased erosion
rates. Model studies taking into account abrupt thaw processes
\citep{turetsky2020carbon,nitzbon2020fast} consistently suggest
higher future permafrost thaw rates for a given global warming
trajectory, and therefore a stronger permafrost carbon feedback. This
is recognized in the most recent IPCC assessment that only attributes
\textit{low confidence} to its estimate of future greenhouse gas
emissions from permafrost thaw \citep{IPCC_2021_WGI_Ch_5}.

If we define a perturbed state of a dynamical system as ``irreversible
on a given time scale if the recovery from this state due to natural
processes takes substantially longer than the time scale of \mbox{interest''}
(see~the~IPCC AR6 Working Group I glossary
\citep{IPCC_2021_WGI_Annex_VII}), then permafrost carbon release in a
warming world will clearly be irreversible. But is permafrost really a
tipping element in the global climate system? The definition of tipping
elements, as stated before, requires the change to become
self-perpetuating. It is clear that abrupt thaw, once locally
initiated, is a process that tends to reinforce itself---this is
precisely why it is called abrupt thaw. However, although permafrost
carbon release is a positive feedback to climate change, and would
therefore sensu stricto be self-amplifying, it is not obvious why
permafrost thaw should be a coherent global tipping element of the
Earth system with a single abrupt threshold, as sometimes suggested
\citep[e.g.,][]{drijfhout2015catalogue}. Rather, abrupt thaw is a local
process, although it is often initiated by a coherent large-scale
driver (global warming), and in particular by extreme events such as
fires \citep{gibson2018wildfire} or heavy precipitation events
\citep{magnusson2022extremely}, the frequency and amplitude of which is
linked to long-term warming. There is no known process by which
thermokarst formation at one place would spur thermokarst formation in
its surroundings. Moreover, the large spatial heterogeneity of
permafrost landscapes \citep{mishra2021spatial} can lead to a large
range of temperature thresholds for triggering abrupt thaw at rather
small spatial scales. Therefore, aggregated across the pan-Arctic
region, there is very probably no single ``tipping point'' (i.e.,
global warming level) beyond which the entire northern permafrost
reservoir would more or less suddenly start collapsing. There is no
known mechanism that would initiate a large-scale chain reaction of
abrupt permafrost thaw once a specific global warming level is
exceeded.

\section{Continental ice sheets}

The two existing large continental ice sheets in Greenland and
Antarctica would raise the global sea level by about 7~m
\citep{dowdeswell2006greenland} and 58~m \citep{morlighem2020deep},
respectively, if they melted completely. Although it is clear that
there is no risk of a complete decay of these two ice sheets in the
foreseeable future, concerns are real that large-scale instabilities of
both ice sheets could be triggered by human-caused global warming,
potentially leading to several meters of additional sea-level rise
within a few centuries \citep{IPCC_2021_WGI_TS}. Potential mechanisms
of self-amplifying mass loss are of two kinds: The melting can be
driven by oceanic or by atmospheric warming.

In the case of melting driven by atmospheric warming
(Figure~\ref{FAQ91}, right-hand side), the melt-induced lowering of the
ice-sheet surface leads to warmer surface conditions, amplifying the
mass loss. Surface melt rates on glaciers, driven by exceptionally warm
summer temperatures, can be of the order of several meters per year
\citep{cremona2023european}. This positive feedback can thus, in
extreme cases of an ice sheet very far from equilibrium, lead to a
rapid decay of an ice sheet. However, this feedback will not
immediately be in full swing once an ice sheet starts to melt, as the
additional disequilibrium induced by surface lowering is
cumulative---it is, necessarily, zero at the beginning of the melting
process. If the initial warming is modest and only leads to a weakly
negative ice sheet mass balance, this ice-sheet altitude feedback will
thus not be perceptible at the beginning, and the ice-sheet decay
driven by atmospheric warming can in such cases, theoretically, go on
for thousands of years before the amplifying ice-sheet altitude really
``kicks in'' \citep{robinson2012multistability}. Melting driven by air
temperature is responsible for about 50\% of the mass loss of the
Greenland Ice Sheet between 1992 and 2018 \citep{imbie2020GL}, while
the surface mass balance of the Antarctic Ice Sheet over the period
1992--2017 was close to equilibrium \citep{imbie2018AA}. Therefore, at
least currently, the ice sheet altitude feedback, which involves
substantial surface melting, is more relevant for Greenland than for
Antarctica. 

\begin{figure*}
\includegraphics{fig04}
\caption{\label{FAQ91}Mechanisms of self-amplifying ice sheet mass
loss. Left: melting driven by ocean temperature; right: melting driven
by air temperature. In both cases, initial ice sheet retreat can, under
certain conditions, be amplified by positive feedbacks. Reproduced from
FAQ9.1, IPCC AR6 Working Group I report \citep{IPCC_2021_WGI_Ch_9}.}
\end{figure*}

If height loss of the ice sheet is large enough to maintain further
mass loss even in case the initial cause of the mass loss---an
atmospheric warming---is reversed, then this is an irreversible
process. However, if the cumulative mass loss is weak, the ice sheet
can recover its initial surface height if the warming is reversed.
Therefore, ice-sheet melting driven by air temperature becomes an
irreversible process only at a certain stage, and it is usually not
abrupt except in its final phase, when the remainders of the
almost-gone ice sheet are far from equilibrium with the new surface
climate. Nevertheless, the conceptual framework of a ``tipping
element'' still remains applicable to the Greenland Ice Sheet: Once it
shrinks sufficiently after a certain global warming threshold has been
exceeded, it will not come back even if temperature falls back below
that threshold; at that stage, the process is clearly irreversible.
However, this tipping point is difficult to quantify, and temperature
needs to exceed the threshold for a sufficient amount of time, which is
also difficult to quantify. It is essentially for these reasons that
the IPCC AR6 Working Group I makes the somewhat fuzzy statement that
``there is \textit{limited evidence} that\,{\ldots}\ the Greenland ice sheet
will be lost almost completely and irreversibly over multiple
millennia'' at a ``sustained'' global warming level between
2~\textdegree C and 3~\textdegree C.

\begin{figure*}
\vspace*{-3pt}
\includegraphics{fig05}
\vspace*{-3pt}
\caption{\label{MICI}Schematic representation of Marine Ice Sheet
Instability (MISI, a) and Marine Ice Cliff Instability (MICI, b).
Reproduced from Figure CB8.1, IPCC Special Report on Ocean and
Cryophere in a Changing Climate \citep{meredith2019polar}.}
\vspace*{-3pt}
\end{figure*}

Another type of self-sustained ice-sheet decay is caused by the Marine
Ice Sheet Instability (MISI) mechanism, depicted on the left-hand side
of Figure~\ref{FAQ91}. This mechanism requires the base of an ice sheet
to be situated below the sea level, and the bedrock to be retrograde
(i.e.\ sloping towards the interior of the continent). This is the case
in particular for large parts of the West Antarctic Ice Sheet. The
vulnerability of this part of the Antarctic Ice Sheet to ocean-driven
melting has been recognized almost 50 years ago \citep{mercer1978west}.
When, during ocean-driven melting of the immersed part of an ice sheet,
the grounding line recedes behind a topographic maximum of the bedrock,
the ice-sheet retreat becomes self-sustained
\citep[e.g.,][]{ritz2015potential,pattyn2020uncertain}. The dominant
process that can trigger these large and abrupt changes is the loss of
the stabilizing effect of floating ice shelves on the inland ice sheet,
called ice shelf buttressing
\citep[e.g.,][]{furst2016safety,pattyn2020uncertain}.

MISI can become irreversible within a few decades and unfold over a few
centuries until the almost complete decay of the affected part of the
ice sheet \citep[e.g.,][]{feldmann2015collapse,rosier2021tipping}.
Therefore MISI is probably as close as it gets to a tipping mechanism
as it is commonly understood: a self-sustained, irreversible ice-sheet
retreat that can take place sufficiently rapidly to be \mbox{qualified} as
``abrupt'', with obviously large consequences. Similarly to abrupt
permafrost thaw, extreme events can, in certain cases, be the final
trigger of the tipping event. Loss of buttressing ice shelves can be
accelerated by massive surface melt events linked to heatwaves or even
short events such as atmospheric rivers which advect large amounts of
warm and moist air from lower latitudes
\citep{wille2019west,wille2022intense}. However, these extreme events
would only be the ``final kick'' leading to local and from there to
potentially larger-scale instability, with long-term oceanic and
atmospheric preconditioning as a necessary condition
\citep{bassis2024stability}. Therefore, such extreme events would
determine the timing of the tipping---in other words, the tipping 
point---, but would they not be a critical element of the instability
process as such.

A second potential ocean-driven ice sheet instability process is Marine
Ice Cliff Instability \citep{deconto2016contribution}. This process has
never been observed at scale and has been proposed on theoretical
grounds, linked to mechanical properties of ice which cannot support
high, partially immersed cliff faces in excess of about ${\approx}$800~m
thickness. Such a cliff face, initially generated by bottom melting
and/or destabilization of ice shelves by strong surface melt and
refreeze (hydro-fracturing), would theoretically be subject to repeated
structural failure, leading to abrupt and irreversible ice sheet
retreat \citepalias{deconto2016contribution}, as shown on the right-hand
side of Figure~\ref{MICI} (the left part of the figure displays marine
ice sheet instability, for comparison). However, a recent study
\citep{morlighem2024west} has shown that rapid thinning and ice flow
speed-up would rapidly stabilize such a cliff if it existed, suggesting
reduced vulnerability of the West Antarctic Ice Sheet to this
destabilizing process, at least during initial phases of ice sheet
\mbox{retreat} (that is, at least during the 21{st} century). Moreover, the
Marine Ice Cliff Instability process is not needed to explain
reconstructed rapid mass losses of the Antarctic Ice Sheet in the past,
neither for the mid-Pliocene epoch, nor for the last interglacial
period, and certainly not for the recent period of mass loss since 1992
where MICI has not been observed \citep{edwards2019revisiting}.
Therefore, the principle of parsimony, also known as ``Occam's razor'',
would lead us to discard this eventuality based on the available
evidence. 

\begin{figure*}
\vspace*{2pt}
\includegraphics{fig06}
\vspace*{3pt}
\caption{\label{SLRTiming}IPCC WR6 WGI assessment of the timing of when
global mean sea level (GMSL) thresholds of 0.5, 1.0, 1.5 and 2.0~m are
exceeded, based on four different ice-sheet projection methods
informing post-2100 projections. Methods are labelled based on their
treatment of ice sheets. ``No acceleration'' assumes constant rates of
mass change after 2100. ``Assessed ice sheet'' models post-2100 ice-sheet
losses using a parametric fit extending to 2300 based on a multi-model
assessment of contributions under RCP2.6 and RCP8.5 at 2300. Structured
expert judgement (SEJ) employs ice-sheet projections from
\citet{bamber2019ice}. Marine ice-cliff instability (MICI) combines the
parametric fit for Greenland with Antarctic projections based on
\citet{deconto2021paris}. Circles, thick bars and thin bars represent
the 50{th}, 17th--83{rd} and 5th--95{th} percentiles of the exceedance
timing for the indicated projection method. Figure and caption
(adapted) from IPCC AR6 Working Group I Figure 9.29
\citep{IPCC_2021_WGI_Ch_9}.}
\end{figure*}

The problem with climate change is that in addition to large
uncertainties, values are in dispute, stakes are high and decisions are
urgent---which is exactly the definition of ``post-normal science''
following \citet{funtowicz1991new}. In the specific case of MICI, which
could cause very high sea-level rise rates over the next centuries and
which cannot be excluded, this means that scientists, when they provide
scientific expertise at the science-policy interface (for example, as
IPCC authors), cannot neglect this eventuality. Therefore, MICI has
been explicitly taken into account in the most recent IPCC assessment,
as a specific ``low-likelihood, high-impact'' storyline building on
processes that ``[could] not be excluded'', a wording that was used
consistently starting from the underlying chapter 9 in the Working
Group I report \citep{IPCC_2021_WGI_Ch_9} up to the Summary for
Policymakers of the Synthesis Report \citep{AR6SYRSPM}. Given the new,
available \mbox{evidence} published since the literature cutoff date of the
last IPCC Working Group I report in 2021, one can wonder whether MICI
would still be taken into account if the report was written now.
Speculation is moot, but it is worth noting that the fundamental
\mbox{underlying} reasons for taking MICI into account in the assessment of
possible future sea-level rise still remain valid: large-scale
occurrence of MICI cannot fully be excluded (yet), and the impacts
would be very large.%\looseness=1

In any case, it is important to keep in mind that concerning sea-level
rise, the fundamental \mbox{question} humanity faces is less ``how much?'',
but rather ``when?'', as can be seen in Figure~\ref{SLRTiming}. Under a
strong emissions scenario (SPS5-8.5), for example, large-scale
occurrence of marine ice cliff instability would make sea-level rise
(relative to the 1995--2014 level) reach the 2~m milestone earlier than
otherwise (that is, during the first half of the 22{nd} century in the
corresponding ``low-likelihood, high-impact'' storyline), but even
without MICI, that milestone would eventually be reached, and it would
probably eventually even be reached in low-emissions pathways, only
much later.

\section{Conclusion}

In the oral presentation this article is based on, and in this article
itself, I have discussed in a little bit of detail two possible
``tipping elements'' in the climate system: permafrost carbon and large
continental ice sheets. The purpose was not to provide a comprehensive
review of continental tipping elements, but rather to help the public
and the non-specialist reader better understand the underlying concepts
and the current state of the science concerning the general concept of
tipping element/points/events and these two specific possible tipping
elements. However, instead of using the concept of ``tipping''
throughout, I preferred to analyse potential future permafrost carbon
and ice sheet changes in terms of irreversibility and abruptness. Both
permafrost and continental ice sheets can change irreversibly, and they
will undergo irreversible changes if global warming continues. These
changes can be abrupt, at least at the ``local'' scale (for example,
for a specific outlet glacier of a large ice sheet, including parts of
its drainage basin), but it is not obvious that they will be abrupt at
the large scale (that is, the scale of an entire ice sheet or the
entire northern circumpolar permafrost region). This article has been
largely based on the IPCC's 6{th} assessment report, which remains
relevant in its treatment of ``tipping elements'' in the light of new
publications, even several years after its literature cutoff.

\section*{Acknowledgments}

The author thanks the Acad\'emie des Sciences, and in particular its
member Jean Jouzel, for giving him the opportunity to give a lecture at
the colloqium ``L'urgence climatique: un tournant d\'ecisif ?'' in
March, 2024, where he presented the material summarized here.

\section*{Declaration of interests}

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.

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