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\DOI{10.5802/crgeos.288}
\datereceived{2024-09-13}
\daterevised{2025-02-12}
\dateaccepted{2025-03-17}
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\dateposted{2025-04-29}
\begin{document}

\begin{noXML}


\TopicFR{G\'eophysique de surface}
\TopicEN{Surface geophysics}

\shortrunauthors

\CDRsetmeta{articletype}{review}

\title{The European Alps in a changing climate: physical trends and
impacts}

\alttitle{Les Alpes Europ\'eennes dans un climat en \'evolution :
tendances physiques et impacts}

\author{\firstname{Marie} \lastname{Dumont}\CDRorcid{0000-0002-4002-5873}\IsCorresp}
\address{Univ. Grenoble Alpes, Universit\'e de Toulouse, 
M\'et\'eo-France, CNRS, CNRM, CEN, 38000 Grenoble, France}
\email[M. Dumont]{marie.dumont@meteo.fr}

\author{\firstname{Diego}  \lastname{Monteiro}\CDRorcid{0000-0002-7702-3644}}
\addressSameAs{1}{Univ. Grenoble Alpes, 
Universit\'e de Toulouse, M\'et\'eo-France, 
CNRS, CNRM, CEN, 38000 Grenoble, France}
\email[D. Monteiro]{diego.monteiro@meteo.fr}

\author{\firstname{Simon}  \lastname{Filhol}\CDRorcid{0000-0003-1282-7307}}
\addressSameAs{1}{Univ. Grenoble Alpes, 
Universit\'e de Toulouse, M\'et\'eo-France, 
CNRS, CNRM, CEN, 38000 Grenoble, France}
\email[S. Filhol]{simon.filhol@meteo.fr}

\author{\firstname{Simon}  \lastname{Gascoin}\CDRorcid{0000-0002-4996-6768}}
\address{Centre d'\'Etudes Spatiales de la Biosph\`ere, 
CESBIO, Universit\'e de Toulouse, 
CNES/CNRS/INRAE/IRD/UT3-Paul Sabatier, Toulouse, 
France}
\email[S. Gascoin]{simon.gascoin@univ-tlse3.fr}

\author{\firstname{Christoph}\nobreakauthor\lastname{Marty}\CDRorcid{0000-0002-0398-6253}}
\address{WSL Institute for Snow and Avalanche Research SLF, 
Davos, Switzerland}
\email[C. Marty]{marty@slf.ch}

\author{\firstname{Pascal}  \lastname{Hagenmuller}\CDRorcid{0000-0002-6581-2048}}
\addressSameAs{1}{Univ. Grenoble Alpes, 
Universit\'e de Toulouse, M\'et\'eo-France, 
CNRS, CNRM, CEN, 38000 Grenoble, France}
\email[P. Hagenmuller]{pascal.hagenmuller@meteo.fr}

\author{\firstname{Samuel}  \lastname{Morin}\CDRorcid{0000-0002-1781-687X}}
\address{CNRM, M\'et\'eo-France, CNRS, 
Universit\'e de Toulouse, 31057 Toulouse, France}
\email[S. Morin]{samuel.morin@meteo.fr}

\author{\firstname{Philippe}  \lastname{Choler}\CDRorcid{0000-0002-9062-2721}}
\address{Univ. Grenoble Alpes, 
Univ. Savoie Mont Blanc, CNRS, LECA, Grenoble, France}
\email[P. Choler]{philippe.choler@univ-grenoble-alpes.fr}

\author{\firstname{Wilfried}  \lastname{Thuiller}\CDRorcid{0000-0002-5388-5274}}
\addressSameAs{4}{Univ. Grenoble Alpes, 
Univ. Savoie Mont Blanc, CNRS, LECA, Grenoble, France}
\email[W. Thuiller]{wilfried.thuiller@univ-grenoble-alpes.fr}

\keywords{\kwd{Mountain}
\kwd{Climate change}
\kwd{Snow}
\kwd{Glaciers}
\kwd{Permafrost}
\kwd{Ecosystems}}

\altkeywords{\kwd{Montagne}
\kwd{Changement climatique}
\kwd{Neige}
\kwd{Glaciers}
\kwd{Perg\'elisol}
\kwd{\'Ecosyst\`emes}}

\thanks{European Research Council (ERC) (IVORI; grant no. 949516)}

\begin{abstract} 
Mountain areas crown their surrounding landscapes and host a great
diversity of ecosystems and human activities. Due to their high
altitudes and low temperatures, mountains, in many regions of the
world, including Europe, allow the existence of the solid phase of
water, ice, seasonally or perennially, which is found in glaciers, snow
cover and permafrost, key components of the cryosphere. In the European
Alps, the air temperature has increased by about 2 \textdegree C
compared to the pre-industrial period (end of 19th century), alongside
with a slight change in the seasonality of precipitation. Both are
projected to intensify in the future. The increase in temperature
induces profound changes for the mountain cryosphere with in particular
the scarcity of snow cover, the retreat of glaciers and the thawing of
permafrost. These changes are causing a cascade of upheavals for the
water cycle, mountain ecosystems, the economic and touristic activity
of mountain societies. They also induce changes in the natural hazards
associated with the cryosphere such as avalanches, risks of glacial and
peri-glacial origins as well as floods and droughts. Thus, in a
changing climate, the fragile beauty and balance of the European Alps
is undergoing profound disruptions whose consequences extend to the
lowlands. These will continue to intensify as long as the temperature
continues to rise and this is why every increment of temperature
matters for the state of the European mountains.
\end{abstract}

\begin{altabstract} 
Les r\'egions de montagne dominent les paysages environnants et
abritent une grande diversit\'e d'\'ecosyst\`emes et d'activit\'es
humaines. En raison de leur haute altitude et de leurs basses
temp\'eratures, les montagnes, dans de nombreuses r\'egions du monde, y
compris en Europe, permettent la pr\'esence de l'eau \`a l'\'etat
solide, sous forme de glace, de mani\`ere saisonni\`ere ou permanente.
On la retrouve dans les glaciers, le manteau neigeux et le
perg\'elisol, qui sont des composants cl\'es de la cryosph\`ere. Dans
les Alpes europ\'eennes, la temp\'erature de l'air a augment\'e
d'environ 2 \textdegree C par rapport \`a la p\'eriode pr\'eindustrielle (fin du
XIX\textsuperscript{e} si\`ecle), accompagn\'ee d'un l\'eger changement dans la
saisonnalit\'e des pr\'ecipitations. Ces tendances devraient
s'intensifier \`a l'avenir. L'\'el\'evation des temp\'eratures
entra\^\i ne des changements profonds dans la cryosph\`ere alpine,
notamment la rar\'efaction du manteau neigeux, le recul des glaciers et
le d\'egel du per\'elisol. Ces transformations provoquent une cascade
de bouleversements affectant le cycle de l'eau, les \'ecosyst\`emes de
montagne ainsi que les activit\'es \'economiques et touristiques des
soci\'et\'es alpines. Elles entra\^\i nent \'egalement des
modifications des al\'eas naturels li\'es \`a la cryosph\`ere, tels que
les avalanches, les risques d'origine glaciaire et p\'eri-glaciaire,
ainsi que les inondations et les s\'echeresses. Ainsi, dans un climat
en mutation, la beaut\'e fragile et l'\'equilibre des Alpes
europ\'eennes subissent des perturbations profondes dont les
cons\'equences s'\'etendent aux r\'egions de plaine. Ces
bouleversements continueront de s'accentuer tant que la temp\'erature
poursuivra son augmentation, soulignant ainsi l'importance de chaque
fraction de degr\'e suppl\'ementaire pour l'\'etat des montagnes
europ\'eennes.
\end{altabstract} 

\maketitle

\vspace*{2pc}

\twocolumngrid

\end{noXML}

\section{Introduction}
Models for painters, poets, and writers, scenario of many myths and
legends, often mysterious, mountains also provide a wealth of ecosystem
services. Mountains are characterised by a rugged topography and higher
elevation than the surrounding landscape, therefore exhibiting strong
elevation-dependent gradients in terms of temperature and
precipitation. Cryospheric components are part of many mountain ranges,
with seasonal snow lasting up to several months at intermediate and
high elevations, and the presence of permafrost (ground frozen for at
least two consecutive years) and glaciers at the highest elevations.
The European Alps cover 1.8\% of the continent (190,000~km$^{2}$), and
are located in the mid-latitudes  (43--47~\textdegree N), hosting a
diversity of ecosystems, landscapes, cultures and socio-economic
activities. The Europeans Alps spans a large range of elevations
culminating at the top of Mont Blanc, above 4800 m a.s.l.\ 
(Figure~\ref{fig:map}). Mountains of the European Alps provide water to
the largest European rivers, and contribute key natural resources such
as hydropower and timber. The European Alps are also the scenes of many
natural hazards such as rockfalls, floods, avalanches, landslides to
which human societies have long been exposed.  

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig01}
\vspace*{-3pt}
\caption{\label{fig:map}The European Alps as defined by the Alpine
convention perimeter in \citet{Piva}. The circles represent snow depth
measurements stations where the time serie of measurement is longer
than 10~years \citep{matiu2021,monteiro2023}.}
\vspace*{-5pt}
\end{figure*}

Climate change affects the entire planet. Anthropogenic greenhouse
gases emission has led to a global near-surface air temperature
increase that has reached  ${+}$1.1~\textdegree C in 2011--2020 above the
reference period  1850--1900 \citep{IPCC_2021_WGI}. As the rest of the
planet, mountains areas are affected by climate change with a broad
range of impacts for ecosystems, human societies and geomorphological
processes. The Copernicus Climate Change Service reports on the
European State of the Climate highlights that Europe is the fastest
warming continent in the world, with a warming occurring at a rate
twice that of the rest of the world  \citep{copernicus2023}. Due to
climate change, the European Alps landscapes and their surrounding are
being affected by changes in precipitation regime and higher
temperatures, raising the average rainfall-snowfall elevation limit,
melting snow and glaciers, and thawing permafrost. Implications to
changes of the cryosphere are far-reaching as well as the consequences
for mountain ecosystems and societies. 

Several studies have reviewed changes in one specific component of the
mountain environment, such as the cryosphere in
\citet{beniston_european_2018}. In 2019, the Special Report on the
Ocean and Cryosphere in a Changing Climate (SROCC)  \citep{srocc2019}
assessed the knowledge on climate change and its impacts in mountains
regions for all the different components of the cryosphere. The present
paper builds on SROCC and brings together more recent studies to review
the observed and projected changes in meteorological conditions in the
European Alps and the impacts of these changes on all the main
components of the mountain system, with a focus on the French and Swiss
Alps. Otherwise explicitly mentioned in the text, European Alps refer
to the area contains within the boundaries of the Alpine Convention 
\citep{Piva} depicted in  Figure~\ref{fig:map}. 

\section{Past and projected changes in\newline meteorological variables}
Climate change induces profound modifications of the Earth System. The
first, and most known manifestation of climate change, is near surface
air temperature rise. Changes in temperature triggers numerous
feedbacks that in turn modify other meteorological variables. Here we
review the changes in temperature and precipitation affecting the
European Alps, the two main drivers of the cryosphere changes. 

\subsection{Air temperature}
Quantifying accurately the magnitude of the temperature increase in
mountainous areas is a difficult task. Firstly, it varies in space due
to the complex topography. Secondly, mountains are remote places,
experiencing harsh meteorological conditions and exposed to natural
hazards. These factors make it difficult to access and maintain
monitoring stations over the long term, leading to geophysical records
including air temperature usually scarce and shorter than in
surrounding lowlands. Nevertheless, a number of studies assessed the
change in temperature in the European Alps  \citep{auer2007, srocc2019,
isotta2019, beaumet2021,pepin2022, philipona2023}. These studies used
available in situ measurements, statistical methods to reconstructed
and homogenized the time series of measurements as well as global and
regional reanalysis evaluated against the in situ measurements. These
studies estimate that, since 1970, the rate of temperature increase has
been  $0.3 \pm 0.2$~\textdegree C per decade.  The current increase in
temperature in the European Alps since 1850--1900 is therefore
approximately   $2.0\pm 0.3$~\textdegree C (see Figure~\ref{fig:tempe},
methods and references therein). This increase is higher than for the
rest of the planet since air temperature over land surfaces increases
faster than over oceans  \citep{IPCC_2021_WGI} but is in line with the
average warming in Europe  (2.3~${\pm}$~0.4~\textdegree C over the past 5
years,   \citet{copernicus2023}) and a little higher than the average
warming over metropolitan France  (1.7~\textdegree C,
\citet{ribes2022}). 

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig02}
\vspace*{-2pt}
\caption{\label{fig:tempe}Change in temperature with respect to 
1850--1900. The black lines and the global projected temperature change
are taken from \citet{IPCC_2021_WGI}. The  temperature change for the
European Alps (green box) was calculated from 1970 until today based on
the values from \citet{monteiro2023}, assuming that prior changes
followed the average change for mainland France with data from
\citet{ribes2022}. Projections for the European Alps are calculated
from Table~S2 in \citet{kotlarski2023}. The blue projections are for a
low emmission scenario (RCP2.6 for the European Alps, SSP1-2.6 for
global). The red projections are for very high emission scenario
(RCP8.5 for the European Alps, SSP5-8.5 for global) (more in
\citet{IPCC_2021_WGI}).}
\vspace*{-2pt}
\end{figure*}

The rate of temperature increase depends on season and elevation. It
exhibits a seasonal variations with stronger warming in spring and
summer than in autumn and winter, similarly to the situation prevailing
in central and southeastern Europe and around the Mediterranean 
\citep{simmons2022}. Warming in the Alps also depends on elevation, but
the magnitude and sign of this influence varies with the the season. In
the Swiss and the French Alps, winters experience a pronounced warming
at lowest elevations, while summers exhibit a pronounced warming at
elevations higher than 2000~m a.s.l.\ 
\citep{pepin2022,Scherrer_2020,isotta2019,rottler2019,beaumet2021,
monteiro2023}. These conclusions were drawn thanks to available in situ
measurements, statistical methods and regional reanalysis over the
French and the Swiss Alps. Several physical processes can explain this
elevation dependant warming: decrease in the aerosols content of the
atmosphere, increase and decrease in cloud cover as well as a darkening
of the surface in spring due to earlier snow retreat (a.k.a.\ the snow
albedo feedback, see  Section~\ref{sec:snow}) \citep{pepin2015}.
Locally, the contribution of these processes may vary, which makes it
difficult to derive common simple rules.

Climate projections indicate that warming in the European Alps will
continue at almost the same rate  (0.3 ${\pm}$ 0.2~\textdegree C) per
decade until about 2050 for as long as net greenhouse gases emission,
i.e.\ releases of greenhouse gases from anthropogenic sources minus
removals by anthropogenic sinks, is above zero  \citep{srocc2019}
(Figure~\ref{fig:tempe}). By the end of the 21st century, the increase
in temperature is highly dependant on the emission scenario 
\mbox{(Figure~\ref{fig:tempe})} 
\citep{kotlarski2023}. The projected increase in
\mbox{temperature} in 2070--2100 compared to the pre-industrial period
ranges between 3 to 6~\textdegree C in the European Alps (against 1.8
to 5~\textdegree C for the global temperature increase  in 
\citet{IPCC_2021_WGI}). In most cases, the scenarios indicate a more
pronounced increase in temperature during the summer and autumn months
compared to the winter and spring seasons  \citep{kotlarski2023}. As
highlighted in the later reference, projected temperature are subject
to considerable model uncertainty, the largest uncertainties being
found for the highest projected warming (see  Figure~\ref{fig:tempe}).

\subsection{Precipitations}
Precipitations, i.e.\ snowfall and rainfall, are affected by climate
change. Precipitation measurements in mountain regions are even scarcer
than temperature measurements and subject to greater uncertainties 
\citep{srocc2019,kochendorfer2018}. Several conclusions can however be
obtained using available in situ measurements, statistical methods for
homogenization of measurements, as well as 
\mbox{regional} reanalysis 
\citep{masson2016,menegoz2020, MeteoS, bozzoli2024}.  On an annual
scale, no clear trends are found on the total annual amount of
precipitation over the European Alps, but seasonal changes are
detected, with an increase in winter precipitation over the
northwestern Alps for the period 1903--2010, with changes typically
reaching up to 20\%  per century  \citep{masson2016,menegoz2020,
MeteoS, bozzoli2024}. In summer, a decrease in total precipitation is
found in the Southern Alps, remaining small (${&lt;}$10\% per century) at
elevation higher than 1500~m a.s.l., and is due to a decrease in the
number of rainfall days. An increase in extreme precipitations at the
annual timescale and also during the four seasons is observed 
\citep{scherrer2016, menegoz2020}. Based on regional reanalysis over
the French Alps, \citet{leroux2021} have shown that the magnitude of
extreme snowfall is increasing at high elevation and decreasing at low
elevation in favour of extreme rainfall due to the shift from snowfall
to rainfall in a warmer
climate. 

Projections of precipitation are subject to large uncertainties
concerning the magnitude of changes by the end of the 21st century.
They show an \mbox{intensification} of the seasonal changes already observed
over the last century, with wetter winters and drier summers. These
contrasting future seasonal changes are explained by large-scale
atmospheric circulation  \citep{rajczak2017, blanc2022}. Indeed, the
European Alps are in a transition zone between the Mediterranean basin,
on a drying trend, and the central and northern Europe, likely on a
wetting trend  \citep{rajczak2017}. The location of this transition
zone varies depending on the season and its future evolution will
likely determine the regional sign of precipitation changes. While the
respective trends do not vary with the scenario, the intensity of the
changes scales with the temperature rise and is more pronounced after
2050  \citep{kotlarski2023}. Despite these seasonal variations
concerning the amount of precipitation, extreme precipitation events
are expected to intensify in all seasons 
\citep{rajczak2017,gobiet2020}. 

Other meteorological variables, such as cloud cover, aerosols content
and deposition, longwave and shortwave radiations and winds may also be
affected by climate change, although their changes both from a model
and observation standpoint is more difficult to detect and less
documented  \citep{srocc2019, philipona2023}.

\section{Past and future changes in the mountain cryosphere}
Consistent and recurrent below-freezing temperatures in the higher
elevations enable the presence of the solid phase of water, ice, and
therefore the existence of many cryospheric processes. The mountain
cryosphere includes several components, in particular snow cover,
glacier and permafrost, that are extremely sensitive to the rise in
temperatures  \citep{beniston_european_2018,srocc2019}. This  section
reviews past and future changes of the mountain cryosphere in the
European Alps. 

\subsection{Snow cover}\label{sec:snow}
The snow cover is a major component of the mountain environment
especially in high and mid elevation mountain ranges such as the
European Alps. The snow cover results from the accumulation of
snowfalls on the ground during winter, and gradually melts in spring or
transforms to firn, and ultimately to glacier ice, at high elevation.
In the Alps, snow covers the ground for at least 6.5 months per year
above 2000~m elevation  \citep{husler2014, marty2017}. 

The presence of snow on the ground is very important for many aspects
due to its unique properties. First, snow has an albedo,  i.e.\ fraction
of reflected solar radiation, close to 1 and thus limits the absorption
of solar radiation  \citep{warren1982}. Second, snow is a reservoir for
water in mountain regions and the surrounding lowland, buffering river
discharge from winter to spring and summer, providing high flow at the
time of melt  \citep{fayad2017}. Finally, as the snow cover has a high
air content  (e.g.\ 50--87\% of its volume), snow acts as an insulator,
protecting the ground from air temperature variations, buffering cold
temperatures during winter months to reach the ground, and preventing
warm temperatures during springtime  \citep{bender2020} to heat the
ground. Changes in the snow cover due to climate change thus have
important consequences for several components of the mountain system
that are detailed in  Section~\ref{sec:impacts}.

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig:snow}Evolution of air temperature (upper panel),
mean snow depth (middle panel) and total precipitation (rain+snow,
lower panel) since 1960 at two observations sites. All data are
averaged over the same period (December 1st of year $n-1$ to April 30th
of year $n$). The Col de Porte site is located in the French Alps at
1325~m elevation (deep blue marker, \citet{lejeune2019}). Precipitation
data are corrected from undercatch and other artifacts
\citep{lejeune2019}. The Weissfluhjoch observation site is located in
the Eastern Swiss Alps at 2536~m elevation (light blue markers).
Precipitation data for Weissfluhjoch measurements are taken from snow
board only since the rain contribution is negligible over the
considered months. For data and site description, see
\citet{marty2012}. The horizontal lines corresponds to the 30-years
mean (solid line 1961--1990 and dashed line 1991--2020). The absolute
and relative differences between the two 30-year means are indicated on
each panel.}
\end{figure*}

As a consequence of sustained warming, the snow cover is shrinking
worldwide  \citep{pulliainen2020}. Overall since 1950, the duration of
snow cover in the European Alps has shortened by roughly one month
below 2000~m elevation  \citep{matiu2021}.  Several snow observation
sites have been maintained in the Alps for a few decades, providing a
unique insight on the signal of climate change in mountain regions 
\citep{lejeune2019, guyomarch2019, marty2012}. For instance, at the Col
de Porte site, located at medium elevation (1325 m a.s.l.) in the
French Alps, the mean winter air temperature (December 1st to
April 30th) has increased by  1.05~\textdegree C between the two
30-year periods of 1960--1990 and 1990--2020, while the total amount of
winter precipitation has not changed significantly 
(Figure~\ref{fig:snow} upper and lower panel). As a result, the mean
snow depth decreased by 40\% between the two periods due to  (i)~a
temperature-induced change to more rainfall on the cost of snowfall at
this mid-elevation site where the winter temperature is close to 
0~\textdegree C  (0.55~\textdegree C from December to April) 
\citep{serquet2011}, and (ii)~stronger melting in a warmer climate, 
i.e.\ a  \mbox{melting}
that starts \mbox{earlier} in the \mbox{season.} The~\mbox{increase} of the rain 
vs.\ snow precipitation elevation limit is the first cause of an
increasing snow scarcity at low and mid-elevation  \citep{srocc2019}.
At higher elevation, as exemplified by the Weissfluhjoch site, 2536 m
a.s.l.\ in the Swiss Alps  (Figure~\ref{fig:snow} in light blue),
temperatures have increased roughly the same for the same periods than
at Col de Porte  \citep{marty2012}, i.e.\ 1.16~\textdegree C between
the two 30-year periods of 1960--1990 and 1990--2020. However, the
average snow depth from December to April did not change significantly
and no major change in total precipitation is detected 
(Figure~\ref{fig:snow}). The reasons for this are that (i)~the average
winter temperature at this 
\mbox{elevation} is still well below freezing 
(${-}$7.35~\textdegree C at Weissfluhjoch) and (ii)~the increase in
absolute temperature due to climate change is not sufficient to convert
snowfall to rainfall for the period December to April. Nevertheless,
the snow cover duration even at 2536~m a.s.l.\ has shortened in late
Spring. Our analysis in  Figure~\ref{fig:snow} shows how the snow
melt-out date occurs 15 days earlier on average between the two 30-year
mean. The increase in spring temperature leads to faster melt
explaining this earlier melt-out date. In fall, the snow onset date has
however not significantly changed for higher elevations with continuous
snow cover during winter  \citep{klein2016, essery2020,matiu2021,
vorkauf2021}. Thus, the shorter snow cover duration is the second cause
of snow disappearance in the Alps due to climate change. As illustrated
in  Figure~\ref{fig:snow}, the snow depth exhibits a very strong
interannual variability. The impact of climate change can thus not be
evaluated on a limited number of years but require average over long
time series (typically 30 years). This also implies that temperature
rise causes a decrease in the frequency of snowy winters in favour of
the frequency of winters with very shallow snowpacks. To summarise, the
snow cover is retreating on average as a response to air temperature
increase, occurring faster at low and mid-elevation due to the change
in the precipitation phase  (i.e.\ rain instead of snow) but also at
higher elevation due to earlier melt. 

The projected changes in snow cover show a decline for all scenarios
considered, with a response of the snow cover directly modulated by the
temperature rise  \citep{srocc2019, marty2017}
(Figure~\ref{fig:snowproj}). \citet{srocc2019} and
\citet{kotlarski2023} estimate that snow depth at low and mid-elevation
(i.e.\ below 2000~m a.s.l.) will decrease by 25\% (${\pm}$15\%) in 2050 for
every emission scenario, associated with a decrease in snow cover
duration. Under high emission scenario (RCP 8.5) the decrease in snow
depth could reach  80--90\% in 2100 for these elevations. This decrease
is explained, as for the observed decrease, by a change in
precipitation phase, especially pronounced early and late in the snow
season  \citep{piazza2014}, and a more intense melt in a warmer
climate. The projected increase in winter precipitation will not
compensate for the loss of the snow season duration 
\citep{kotlarski2023}. At higher elevations, i.e.\ above 2000~m a.s.l.,
the decrease will be less pronounced. Though, the snow duration will
decrease due to an earlier disappearance in springtime. At higher
elevations (typically above 3000~m a.s.l.), where this not many
observation available  (Figure~\ref{fig:map}), uncertainties in
projections are still quite high with models having difficulties to
apprehend the sharp and rugged topography due to too coarse resolution 
\citep{terzago2017, monteiro2022, matiu2024}. All in all, the decrease
in snow cover will continue as long as global temperature will continue
to rise, and the seasonal snow cover will only be able to stabilise
once global temperatures will do so. 

\begin{figure*}
\includegraphics{fig04}
\vspace*{2pt}
\caption{\label{fig:snowproj}Past and projected evolution of mean snow
depth during the period November to April in the Mont-Blanc massif at
1500~m elevation. The colours indicates the snow depth. The horizontal
black lines corresponds to the average snow depth from the periods
1961--1990 and 1991--2020 (respectively 45.6 and 38.6~cm). Projected
snow depth for 2050 and 2100 are reported with the quantile. Q05, Q50
and Q95 respectively represent quantile 5\%, 50\% and 95\% of the
ensemble. Data from \citet{samacoits2022}. The emission scenario
corresponds to the French national pathway for adaptation to climate
change, ${+}$3~\textdegree C in 2100 for global temperature with respect
to 1850--1900 (SSP2-4.5).}
\vspace*{4pt}
\end{figure*}

Deposition of light absorbing impurities such as black carbon,
originating from fuel combustion, mineral dust (mostly originating from
the Saharan desert in the European Alps) or also algae accelerate snow
melt by decreasing the snow albedo 
\citep{dumont2020,reveillet2022,dumont2023, roussel2024}. The
deposition of such particles modulates the response of mountain snow
cover under climate change. While the amount of black carbon deposited
on snow has decreased since 1990, there is no trend on the amount of
mineral dust deposition in the last 40 years over the French Alps 
\citep{reveillet2022}. In the future, black carbon deposition are
projected to decrease while there is no consensus yet on the sign of
the trend for mineral dust  \citep{IPCC_2021_WGI, IPCC2022}. The future
evolution of these depositions will thus participate in the future
evolution of the snow cover in the European Alps. 

The reduction in snow cover has many consequences.  In the United
States, snow was estimated to contribute significantly to the western
US economy  \citep{sturm2017}. A lot of parallels could be drawn in
between the Alps and the western US in terms of socio-economic
activities (agriculture, hydropower and winter tourism), the European
Alps being even more densely populated. The reduction of snow cover
will thus profoundly change society and the economy. The main impacts
of a retreating snow cover are discussed in  Section~\ref{sec:impacts}.

\subsection{Glaciers}
Glaciers are often seen as sentinels of climate change. They are one of
the important symbols of mountain regions, imprinting their presence on
the shape of mountains. Glaciers cover 2092~km$^2$ in central Europe,
most of them found in the European Alps   \citep{srocc2019}. If their
entire volume of ice was to melt, glaciers of the European Alps would
contribute to $0.3\pm 0.1$~mm of global sea level rise, a small
contribution compared to, for example, the melting of the Greenland ice
sheet corresponding to more than 7~m in equivalent sea level rise 
\citep{srocc2019}. In the European Alps, glaciers have been retreating
since the little ice Age (1850) in response to changes in temperature
and precipitation. For the past 50 years, alpine glaciers have lost on
average 40\% of their  \mbox{surface} area   \mbox{\citep{sommer2020},}
as exemplified by La Mer de Glace glacier on  Figure~\ref{fig:mdg}. On
average, glaciers of  \mbox{central}  Europe have lost 1.02 ${\pm}$
0.21~m water  \mbox{equivalent} per year for the period  2000--2019 
\citep{hugonnet2021}.

\begin{figure*}
\includegraphics{fig05}
\caption{\label{fig:mdg}The Mer de Glace glacier (Massif 
du Mont Blanc
in 1919 (left panel) and 2019 (right panel). Source: 
\protect\url{http://ige-vis.univ-grenoble-alpes.fr/glacioclim/glacioclim.html}.}
\end{figure*}

Projections are showing that, regardless of the emission scenario,
glaciers of the European Alps below 3500 m a.s.l.\ will very likely
disappear before the end of the century  \citep{compagno2021}. Small
glaciers with accumulation areas below 3400 m a.s.l.\ such as Saint
Sorlin glacier, in the French Alps, should even disappear by 2050,
regardless of the climatic scenario. Such small glaciers account for
75\% of the glaciers in the French Alps  \citep{vincent2023}. For
larger glaciers such as the Mer de Glace  (Figure~\ref{fig:mdg}), their
state at the end of the century depends on the emission scenario. The
Mer de glace would loose 80\% of its surface area in 2100 under the
intermediate emission scenario (RCP 4.5), while it would disappear
before 2100 under the high emission scenario (RCP 8.5) 
\citep{vincent2019}. In contrast to the seasonal snow cover that mostly
evolves in response to the meteorological conditions of the current
year, the time evolution of glaciers results from a cumulative process
by which glaciers gain or lose mass on an annual scale---most often
lose, in the past decades, because of their negative annual mass
balance under current climate conditions. This explains why the future
of most low- and medium-elevation glaciers is sealed, regardless of the
emission scenario, towards total disappearance by 2050. In contrast,
the number of glaciers that will disappear and the evolution of larger
glaciers with high accumulation areas largely depends on the emission
scenario. This is why every increase in temperature matters 
\citep{rounce2023}. 

Beyond global sea level rise, the shrinkage of glaciers has important
local and regional impacts on human societies, including hydrology,
geohazards, and mountain recreational activities and tourism 
\citep{MOUREY2020, SALIM2023,WEISSBRODT2024} (see 
Section~\ref{sec:impacts}). In glacierized catchments, glaciers act as
seasonal and long-term reservoirs, releasing water in rivers from the
melt of snow accumulated during winters, and the melt of ice
accumulated in past decades and centuries. Therefore, the retreat and
disappearance of alpine glaciers will lead to important changes in the
water cycle (see  Section~\ref{sec:hydro}) especially important to
downstream communities and ecosystems close to the glaciers. Moreover,
when glaciers retreat in a valley there are a number of
geomorphological cascading and compound effects:  (i)~the buttressing
effect glaciers can have on the surrounding slopes is removed, which
increases the instability of steep slopes  \citep{MCCOLL2012},
(ii)~unconsolidated debris generated by glacial erosion are exposed to
the surface, leading potentially to debris flow during high snow melt
or intense rain events  \citep{Ballantyne1994},  (iii)~the basal
topography of glaciers may reveal local depressions leading to the
formation of lakes below unstable slopes  \citep{vincent2023,
haeberli2017} and (iv)~the increasing chance of the formation of
glacial lakes and therefore of glacial lake outburst and debris flow 
(e.g.\ GLOF, see e.g.\ \citet{emmer2017}) (see 
Section~\ref{sec:hazards}). 

\subsection{Permafrost}
Permafrost, or ``permanently'' frozen ground, is a less visible
component of the mountain cryosphere than snow and glaciers nonetheless
important. Permafrost is defined as ground continuously remaining below
0~\textdegree C for at least two consecutive years. It can be found in
soft materials (e.g.\ soil, talus slope, rock glaciers) or in bedrock.
Permafrost is a subsurface phenomenon, less accessible to observational
means than snow and glaciers and thus generally less monitored and
understood  \citep{srocc2019}. Permafrost covers roughly 6220~km$^2$ in
the whole European Alps, almost 3 times the glacier area 
\citep{boeckli_permafrost_2012, marcer_permafrost_2017}.

Permafrost temperature is driven by (i)~the surface energy balance and
therefore by air temperature, solar and longwave net radiation, snow
cover, vegetation,  and  (ii)~by the ground thermal properties (i.e.\ 
soil moisture, soil stratigraphy). Because of the soil thermal inertia,
permafrost may build up over centuries and is often a remnant of
previous climatic conditions at depths. As a consequence, mountain
permafrost distribution is heterogeneous and responds to local
conditions in energy fluxes  \citep{magnin2023}. It is found, for the
most part, at the upper elevations of alpine mountains 
\citep{boeckli_permafrost_2012} and anecdotally in places at lower
elevation where local effects allow for sustained freezing conditions 
\citep{lambiel2008}. Snow plays a critical role in permafrost dynamic 
\citep{Luetschg01062005, gisnaas2016, noetzli2024}. We know that snow
acts as an insulator for permafrost and prevail warmer conditions than
areas bare of snow. The timing of arrival of snow in fall is also
crucial, as for instance a late snowpack arrival will cool the ground.
Snow, in steep permafrost areas was identified to be a significant
contributor to liquid water infiltration during snowmelt, a mechanism
known to trigger rockfalls and advect energy in the ground 
\citep{benasher2023}. Yet these feedback mechanisms are still poorly
characterised for steep slope permafrost, most common across the Alps
(e.g.\ rock cliffs).

Different than boreal and Arctic permafrost which holds large amount of
carbon  \citep{schuur2015}, mountain permafrost in the Alps is mainly
affecting slope stability of steep rock slopes emblematic of the
highest alpine peaks. Thawing permafrost has important consequences for
infrastructures located in the vicinity of unstable slopes or
downstream of alpine valleys affected by permafrost, and therefore for
the safety of human society in mountains regions (see 
Section~\ref{sec:hazards}). Permafrost in the European Alps has warmed
up and thawed for the last decades  \citep{srocc2019,haberkorn2021,
magnin2023}. The rate of warming exhibits high local variability.
Ice-poor bedrock warmed by up to 3 times faster than the average values
\citep{Etzelmuller2020, magnin2023}. Latest estimates based on borehole
observations located across the Alps shows  0.36~\textdegree C per
decade warming rate at 10~m depth for the last decade 
\citep{noetzli2024}. The active layer thickness, corresponding to the
depth of seasonal thawing, has also been increasing  \citep{srocc2019,
magnin2023}.  Subsurface liquid water content is increasing too,
indicating ice loss  \citep{srocc2019, bodin2009}. 

Rock glaciers, a permafrost landform made of a mixture of rock and ice,
experience more stable temperature benefiting from an insulating effect
of the upper layer of debris but exhibit a variety of responses to
rising temperature.  \citet{marcer2021} showed that some rock glaciers
can enter a phase of destabilisation during which their speed
increases. 5\% of the rock glaciers in the French Alps were found to be
in this phase, for which the mean annual temperature explains 55\% of
the displacement rate variance. Similar behaviour were found across the
Alps for a subset of rock glaciers  \citep{KellererPirklbauer2024}.
Most rock glaciers, however show no acceleration (potentially inactive)
or even decelerating flow speeds  \citep{KellererPirklbauer2024,
manchado2024}. The longest time series of observed permafrost
temperature from the rock glacier of Murt\`el-Corvastch in Switzerland
shows that since 1987, temperatures between 15 and 25~m depth as
increased by  1~\textdegree C in the last 30 years 
\citep{haeberli2023}.  

Projections indicates that mountain permafrost will likely undergo
increasing thaw and degradation during the 21st century, with
consequences scaling with the increase in temperatures 
\citep{srocc2019}. Peaks below 3850~m a.s.l.\ may lose permafrost
entirely under high emission scenario (RCP 8.5)  \citep{magnin2017}.
The local response of permafrost as well as a detailed process
understanding is however required to refine these projections. 

\section{Impacts}\label{sec:impacts}

\subsection{Mountain hydrology}\label{sec:hydro}
Changes in snow cover due to climate change induce changes in mountain
hydrology, as conceptually illustrated in Figure~\ref{fig:hydro}
\citep{bloschl2017}. In a warmer climate, the liquid fraction of
precipitation increases, causing an increase in winter runoff  with an
associated larger risk of floods \citep{bocchiola2014,srocc2019,
marty2023}. The lower accumulated snow mass at the end of winter leads
to a lower and earlier peak of meltwater runoff in spring
\citep{bocchiola2014,srocc2019}. Another consequence is the increased
severity of low discharge in summer leading to an increase in the risk
of drought. The summer drought of 2022 in the European Alps due to the
compound effect of the snow droughts and heatwaves  provided a glimpse
into what could happen more regularly in the coming decades
\citep{gascoin2024,soubeyroux2023, montanari2023}.  More intense
snowfalls are also expected at high \mbox{elevations} with climate change,
increasing the risk of rain-on-snow events in the European Alps at high
elevation \citep{srocc2019}. Changes in meltwater from snow often
dominate the runoff response at high elevations, while at lower
elevations, changes in precipitation and evapotranspiration due to
climate change become increasingly important 
\citep{bocchiola2014,srocc2019}. The shift from snow- to rain-dominated
hydrological regime in alpine catchments will increasingly disrupt our
ability to predict the availability of water resources for various
usage including hydropower generation, irrigation and fluvial
navigation, consequently causing profound impacts on the operational
management of reservoirs \citep{srocc2019}. It also triggers changes
for natural hazards and ecosystems (see following  sections).
Projections show that these hydrological changes will continue and
intensify as long as the air temperature will continue to increase
\citep{srocc2019}.

\begin{figure*}
\includegraphics{fig06}
\vspace*{-3pt}
\caption{\label{fig:hydro}Simulated snow mass (upper panel, 
$\mathrm{kg}{\cdot}\mathrm{m}^{-2}$) and daily run-off (lower panel, 
$\mathrm{mm}{\cdot}\mathrm{d}^{-1}$) for an idealised snow dominated
mountain basin (light blue). In the dark blue simulation, the mean
annual temperature was increased by  ${+}$5~\textdegree C while the
precipitation remained unchanged. Details about the hydrological model
and its forcing are given in \citet{gascoin2021-hdr}.}
\vspace*{-5pt}
\end{figure*}

In glacierized catchments, glacier retreat also contributes to changes
in water availability \citep{srocc2019}. When a glacier is in
equilibrium with climate, its contribution to the water cycle is not
very different from seasonal snow, except that it is shifted later in
the melt season \citep{radic2014}. However, when the glacier is in a
warming climate, the amount of melt water provided in summer increases,
reaching a maximum, which is named ``peak water'' state of the glacier.
Beyond that point, the glacier surface becomes too low to provide
substantial melt water from ice melt. The glacial contribution to
runoff decreases to eventually stop once the glacier has fully
disappeared, leading to water shortage in summer. The FAQ 2.1 in
\citet{srocc2019} provides a schematic of this evolution. In the
European Alps, the peak water date passed for more than half of the
glaciers and will be reached before 2100 for all glaciers
\citep{huss2018global}. 
Summer runoff contribution from glaciers will
therefore be strongly reduced, intensifying issues of  water shortages
in glacierized catchments such as the Rhone, Rhine, Po and Danube
rivers \citep{huss2011}. Indeed during periods of droughts, glaciers
can represent a non negligible source of water \citep{huss2011}. 

\subsection{Winter tourism}
Changes in snow cover affect human societies in mountain areas and
beyond. One example is the impact of snow cover changes on winter
tourism  recently studied at the scale of the European continent 
\citep{franccois2023}. Winter tourism in the European mountain ranges
is an important economic and societal component (total turnover on the
order of 30 billion euros). If the global temperature change reaches 
${+}$2~\textdegree C compared to the pre-industrial era (see 
Figure~\ref{fig:tempe}), 53\% of the European ski resorts would be
affected by a high level risk of snow scarcity without snowmaking
capabilities. A high level risk of snow scarce winters is defined as
snow scarce conditions encountered at least once every two years on
average. The level of risk varies from 33\% of the ski resorts in the
French Alps being at high level risk to more than 80\% of the ski
resorts in the mid-elevation ranges of France and Switzerland
(including Jura, Vosges, Massif Central) reaching high level risks.
Under a global temperature change of  ${+}$4~\textdegree C, 98\% of the
European ski resorts would reach high level risk. Snowmaking, when
applied to 50\% of the surface of ski slopes, could reduce by 7\% to
9\% the amount of ski resorts at high level risk for  ${+}$2~\textdegree
C scenario in the European Alps, while for mid-elevation mountain range
the amount of ski resorts at high level risk reduces from 80\% to by
56\%. However, snowmaking is also impacted by temperature rise as it
requires sufficiently low temperature. Climate change impacts ski
tourism, but ski tourism contributes to greenhouse gas emissions.
Transport and accommodation are the main contributors to the carbon
footprint of winter tourism and ski tourism is a good example where
adaptation and mitigation needs to be jointly analysed for planning the
future of this industry and its relationship with the mountain
environment \citep{franccois2023}.

\subsection{Ecosystems}
Climate change is significantly impacting European mountain ecosystems,
leading to observable changes such as  ``greening'', species
redistribution, community structure alterations, and shifts in
ecosystem services  \citep[e.g.][]{brambilla2024}. The European Alps
have shown clear signs of  ``greening'', where vegetation growth
increases due to rising temperatures and longer growing seasons.
\citet{choler2021} discussed how these changes are altering the spatial
distribution of vegetation, affecting ecosystem services such as water
regulation. Additionally, \citet{vitasse2021} note that phenological
shifts in plant species are closely tied to the timing of snowmelt and
the subsequent rapid greening of the landscape  \citep[also
in][]{kosolapova2024}. The redistribution of species, driven by climate
change, is another significant impact on mountain ecosystems. Species
compositions are shifting upwards in elevation, which may lead to
changes in community dynamics and the potential loss of species unable
to migrate  \citep{hufnagel2013}. This shifts in species distributions,
particularly in the Alps, can further exacerbate the loss of
specialized alpine species not able to sustain competition from 
lower-land species  \citep{rogora2018}. Both greening and species
redistribution in response to climate change induce strong structural
changes within communities, with shifts in species composition leading
to new community dynamics  \citep{bektas2021}. These community shifts
are not only altering biodiversity but also the ecosystem functions
these communities provide and the associated ecosystem services 
\citep{reed2013}.   In this context, an outstanding question is whether
the current protected area network will mitigate and safeguard the last
pieces of mountain ecosystems. Using semi-mechanistic species
distribution models together with scenarios of climate and land use
changes, a recent study showed that significant modification of the
network will be needed, as it does not adequately represent
biodiversity patterns  \citep{chauvier2024transnational}. To be
efficient, a 
\mbox{substantial} shift in 
\mbox{conservation} 
\mbox{priorities} over time
will be needed, moving from lower to higher elevations and across
latitudes, as plants migrate upslope and their distributions 
\mbox{contract.} 

\subsection{Natural hazards}\label{sec:hazards}
The mountain cryosphere is directly involved in several natural
hazards. As described above, the increase in temperature causes
profound changes in the mountain cryosphere, modifying consequently the
magnitude, frequency, and types of these cryosphere related natural
hazards  \citep[e.g.][]{JACQUEMART2024}. 

The disruption of the water cycle due to an increase in snow cover
scarcity and glacier retreat (see Section~\ref{sec:hydro}) induces changes
in rain-on-snow related floods that may become more frequent in winter
at high elevation (and scarcer at low elevation)  \citep{stoffel2018}.
Spring floods are happening earlier due to earlier snow melts 
\citep{bloschl2017}. Climate change also induces an increased risk of
droughts and water shortages in summer due to the increased lack of
water stored as snow in upper elevation  \citep{montanari2023}. 

Snow cover is also responsible for snow avalanches, responsible of
approximately 100 death per year on average (mainly mountain
practitioners in uncontrolled terrain) over the Alps 
\citep{techel2016} as well as disturbances to 
{transportation} and
threats to mountain infrastructures. Snow avalanches number, types and
the related risks are also impacted by climate change 
\citep{eckert2024}. In the French Alps, the number of large natural
avalanches as well as their run-out distance decreased where snow depth
decreased  \citep{Eckert2013}. Wet snow avalanches also tend to happen
earlier  \citep{srocc2019}. In general, avalanche natural activity
decreased at low elevation  \citep{giacona2021}. At higher elevations
more mixed changes are observed with an increase in wet snow activity
and these changes will be exacerbated in the future 
\citep{srocc2019,castebrunet2014}. \looseness=-1

The retreat of glaciers leads to several glacier related hazards. For
cold glaciers, where the ice is below 0~\textdegree C, the observed
increase in ice temperature can be higher than the increase in air
temperature  \citep{vincent2020} and could lead to ice gliding and ice
avalanches that can endanger population  \citep{faillettaz2015,
Francese2024}. For temperate glaciers (ice at  0~\textdegree C), an
increase in glacier lake formation due to accelerated melt is observed
and can be related to climate change  \citep{srocc2019}. However, it is
not clear if the frequency of glacier lake outburst floods (GLOF) has
evolved and how it will evolve in the future  \citep{srocc2019}. For
polythermal glaciers, accelerated melt may favour the formation of
aquifers inside the glacier that can lead to the collapse of part of
the glacier  \citep{gilbert2012, BONDESAN2023}.

Permafrost thawing and degradation induce an increase in the observed
frequency and magnitude of rockfalls and rock avalanches 
\citep{ravanel2017, walter2020}. In combination with heavy
precipitation and increase soft soil exposure, it can increase the risk
for debris flow  \citep{Ballantyne1994}. Other damages such as sudden
drainage of  lakes and destabilisation of infrastructure are also
observed  \citep{haeberli2016new, duvillard2021,magnin2023,bodin2015}. 

For all these risks, cascading and combined hazards can occur. While
there is a limited number of observed direct chain of events  to
project future trends, it seems reasonable to assume that such chain of
events combining some of the natural 
\mbox{hazards} 
\mbox{described} in this  section
could increase in frequency and magnitude. Adaptation strategies are
thus required for the safety of mountain population as well as for
mountain practices  \citep{mourey2019, MOUREY2020, SALIM2023}.

\section{Concluding remarks}
Climate change already strongly influences the meteorological
conditions in the European Alps, with an air temperature increase of
2~\textdegree C compared to the pre-industrial period (end of  19st
century). While the annual amount of precipitation is remaining stable,
there is a slight seasonal shift towards wetter winters and drier
summers in some regions of the European Alps. All components of the
mountain cryosphere, glaciers, snow cover and permafrost, are declining
and retreating as a consequence of this temperature increase.  Mountain
landscapes will be permanently altered, for instance due to the loss of
glaciers. These changes trigger profound disruptions for mountain
ecosystems and human societies. They will continue to intensify as long
as the temperature continues to rise.\looseness=-1

This paper reviews recent studies on the trends and impact of climate
change in the European Alps, with a focus on the French and the Swiss
Alps, based on existing knowledge and highlighting most recent advances
in this field. It however does not claim to be exhaustive, calling for
a wider report genuinely spanning the entire European Alps domain in a
changing climate, involving a larger and more diverse team of authors
and including further advances from the research and practitioners
communities, and updated climate projections from latest international
coordinated climate modelling experiments. 

\section*{Dedication}
The manuscript was written by M. Dumont through contributions of all
authors. All authors have given approval to the final version of the
manuscript.

\section*{Declaration of interests}
The authors do not work for, advise, own shares in, or receive funds
from any organization that could \mbox{benefit} from this article, and have
declared no affiliations other than their research organizations.

\section*{Funding}
MD has received funding from the  European Research Council (ERC) 
under the European Union's Horizon 2020 research and innovation program
(IVORI; grant no. 949516). 

\CDRGrant[IVORI]{949516}

\section*{Acknowledgements}
The authors are grateful to
Aur\'elien Ribes (CNRM) and Martin Menegoz (IGE) for useful discussions.
The authors are grateful to national services (MeteoSwiss, WSL/SLF,
M\'et\'eo-France, GLACIOCLIM and GLAMOS) for maintaining their
long-term mountain observatories. IGE, LECA and CNRM/CEN are part of
Labex OSUG@2020.

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