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Réf. Six & Vincent 2014 - A

Référence bibliographique
SIX D., VINCENT C., Sensitivity of mass balance and equilibrium line altitude to climate change in the French Alps - Journal Of Glaciology, Vol. 60 No. 223, 2014 doi : 10.3189/2014JoG14J014

Abstract : Assessment of the sensitivity of surface mass balance and equilibrium-line altitude (ELA) to climate change is crucial for simulating the future evolution of glaciers. Such an assessment has been carried out using an extensive dataset comprising numerous measurements of snow accumulation and snow and ice ablation made on four French glaciers over the past 16 years. Winter mass balance shows a complicated pattern with respect to altitude, with no clear linear relationship. Although the ratios of winter mass balance to valley precipitation differ considerably from site to site, they are relatively constant over time. Relationships between snow/ice ablation and temperature are stable, with no link with altitude. The mean snow and ice positive degree-day (PDD) factors are 0.003 and 0.0061mw.e. °C–1 d–1. This analysis shows that, at a given site, ablation depends mainly on the amount of snow precipitation and on cumulative PDDs. The sensitivity of annual ablation to temperature change increases almost linearly from 0.25mw.e. °C–1 at 3500m to 1.55mw.e. °C–1 at 1650 m. ELA sensitivity to temperature change was found to range from 50 to 85m°C–1.

Mots-clés
 glacier mass balance, moutain glaciers, surface melt

Organismes / Contact

Auteurs/Authors :

  • Six D., Laboratoire de Glaciologie et Géophysique de l'Environnement (LGGE), Université Grenoble Alpes and Centre National de la Recherche Scientifique, Grenoble, France
  • Vincent C., Laboratoire de Glaciologie et Géophysique de l'Environnement (LGGE), Université Grenoble Alpes and Centre National de la Recherche Scientifique, Grenoble, France

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa

Temperature

climate change

mass balance
   

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
France Alpes

Glacier de Saint-Sorlin
Glacier de Debroulaz
Glacier d'Argentière
Mer de Glace

    1995-2011

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations

 

Modélisations
 
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)



(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations

 Temporal fluctuation in annual mass balance since 1995

The correlation between annual and summer mass balance was strong. For example, it reached R2 = 0.89 and R2 = 0.92 for all the stakes in the ablation zone of Glacier de Saint-Sorlin for the periods 1995–2002 and 2003–11 respectively. From this analysis, we conclude that the annual summer mass balance is largely responsible for changes in the annual mass balance. In addition, we conclude that the decrease in the annual mass balance after 2002 is mainly due to the increase in summer ablation.

Winter accumulation and precipitation

We compared the winter accumulation measured at each stake in the accumulation zone of the four glaciers with the cumulative valley precipitation calculated over the same period (Fig. 5). [...]
Precipitation at temperatures below 2°C (at the elevation of the observation) was taken into account. This temperature threshold was selected based on the best correlation found between winter mass balance and precipitation. Above this temperature, precipitation is rain and is assumed to run off the glacier and hence not contribute to the mass balance. No observations are reported below 2100ma.s.l. because melting in winter is by no means negligible and can influence the winter mass balance significantly. Figure 5 shows the marked differences, which cannot be explained by orographic effects alone, in the ratio between winter mass balance and valley precipitation. For each glacier, the pattern is highly variable, very far from a linear function of altitude.

Summer ablation and temperature

Summer mass balances were compared to the cumulative PDDs calculated from valley meteorological stations. Over the past 16 years, the number of PDDs between 1 May and 1 October has increased by 30 days at 2800m on Glacier de Saint-Sorlin (near the equilibrium line) based on data measured at Lyon, 19 days on Gebroulaz based on data measured at Bourg Saint-Maurice and 26 days on Glacier d'Argentière and Mer de Glace based on data measured at Chamonix. The CPDD increased by 92°C on Saint-Sorlin, 121°C on Gebroulaz and 184°C on Argentière and Mer de Glace (again at 2800 m).
We compared the periodically observed snow and ice ablation separately with the CPDD, in order to obtain snow and ice PDD factors. The snow PDD factor ranged between 0.0025 and 0.004mw.e. °C–1 d–1, and the ice PDD factor between 0.0045 and 0.007mw.e. °C–1 d–1. The mean ice PDD factor was 0.0061mw.e. °C–1 d–1, and the mean snow PDD factor was 0.003mw.e. °C–1 d–1. [...]
In our study, we found no relationship between the PDD factors and elevation. The ice PDD factors appear to depend on exposure since glaciers exposed to the south showed the largest ice PDD factors. However, they could also depend on local surface snow or ice albedo. [...] According to the results of the present study, the relationships are stable irrespective of whether the wide glacier mass balances are positive or negative. [...] Given that this analysis covered 16 years with highly contrasted mass balances, we conclude that these snow and ice PDD factors can be used during the coming decades. This means that, for a given site, ablation depends mainly on the amount of snow precipitation and the CPDDs.

Sensitivity of ELA to temperature change

For a 1°C increase, we found an ELA increase of +75m on Saint-Sorlin, +50m on Argentière and +85m on Gebroulaz. In the present study, the altitudinal dependence of mass balance comes from direct surface mass-balance observations, but the weight given to the selected stakes (all the stakes on the glacier or stakes located around the ELA) influences the sensitivity. The major differences in ELA sensitivity to a temperature increase seem to be mainly driven by the massbalance gradient with altitude. [see table 3 p875].

Fluctuations temporelles du bilan de masse annuel depuis 1995

La corrélation entre le bilan de masse annuel et estival est importante. Ainsi, on observe respectivement un coefficient de corrélation R2= 0.89 et R2= 0.92 pour les périodes 1995-2002 et 2003-11, pour toutes les mesures sur la zone du glacier de Saint-Sorlin. A partir de cette analyse, nous pouvons conclure que le bilan de masse estival est responsable, de manière majoritaire, des changements du bilan de masse annuel. De plus, nous pouvons également conclure que la diminution du bilan de masse annuel après 2002 est principalement due à l’augmentation des taux d’ablation estivale.


Accumulation et précipitations hivernales

Nous avons comparé l’accumulation hivernale mesurée en chaque point (de mesure), des zones d’accumulation des 4 glaciers, avec le cumul des précipitations (valley precipitation) calculé sur la même période.
Seules les précipitations à une température inférieure à 2°C (à l’altitude d’observation) ont été prises en compte. Ce seuil de température a été choisi à partir de la corrélation la plus significative trouvée entre le bilan de masse hivernal et les précipitations. Au-
dessus de cette température, les précipitations tombent sous forme liquide (pluie) et sont considérées comme s’écoulant sur le glacier sans contribuer à l’évolution du bilan de masse. Aucune observation n’a été rapportée en dessous de 2100m car la fonte en hiver n’est pas négligeable et peut influencer le bilan de masse de manière significative. La figure 5 montre les différences marquées, qui ne peuvent pas être expliquées par les seuls effets orographiques, dans le ration entre le bilan de masse hivernal et les précipitations. Pour chaque glacier le schéma est très variable et est très eloigné d’une fonction linéaire d’altitude.


Ablation estivale et températures

Les bilans de masse estivaux ont été comparés aux cumuls des PDD calculés dans les stations météorologiques de vallée. Au cours des 16 dernières années, le nombre de PDDs entre le 1 mai et le 1er octobre a augmenté, selon les données obtenues à Lyon, de 30 jours à 2800 sur le glacier de Saint-Sorlin (à proximité de la LEA), de 19 jours à Gebroulaz (selon les données de Bourg Saint Maurice) et de 26 jours sur les glaciers d’Argentière et de la Mer de Glace (selon les données de Chamonix). Le cumul des PDD a augmenté de 82°C à Saint Sorlin, de 121°C à Gebroulaz et de 184°C à Argentière et à la Mer de Glace.
Nous avons observé l’ablation de la neige et de la glace séparement avec les CPDD afin d’obtenir les facteurs PDD associés. Le facteur PDD pour la neige varie entre 0.0025 et 0.004 m.e.e. °C-1 j-1. Le facteur PDD pour la glace varie, quant à lui, entre 0.0045 et 0.007 m.e.e °C-1 j-1. La moyenne des facteurs PDD pour la neige est 0.0061 m.e.e °C-1 j-1 et la moyenne des facteurs PDD pour la neige est 0.003 m.e.e °C C-1 d-1. […]
Dans notre étude, nous n’avons pu trouver aucune relation entre les facteurs PDD et l’altitude. Les facteurs PDD de la glace semblent dépendre de l’exposition. En effet, on a pu observer des facteurs PDD plus importants pour les glaciers exposés sud. Cependant, ils pourraient également dépendre de la couverture neigeuse locale ou de l’albédo de la glace. […] D’après les résultats de cette étude, les relations mises en évidence sont stables et ce, que le bilan de masse du glacier soit positif ou négatif. […] Etant donné que cette étude couvre une période de 16 avec des bilans de masse fortement contrastés, nous avons conclu que l’utilisation des facteurs PDD pour la glace et la neige mis ici en évidence, peut se poursuivre dans les prochaines décades. Cela signifie que, pour un site donné, l’ablation dépend principalement de la quantité de précipitations neigeuses et des CPDDs.

Sensibilité de la LEA aux variations de température

Pour une augmentation de 1°C, nous avons trouvé une augmentation de la LEA de 75m pour le glacier de Saint-Sorlin, de 50m pour Argentière et de 85m pour Gebroulaz. Dans cette étude, nous avons utilisé les observations du bilan de masse de surface pour mettre en évidence la dépendance altitudinale du bilan de masse. Cependant, le poids donné aux différents points de mesure (tous les points de mesure sur le glacier se situent aux alentours de la LEA) influence la sensibilité. Les différences les plus importante dans la sensibilité de la LEA aux variations de températures semble donc être induites par le gradient altitudinal du bilan de masse.

Modélisations
 
Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

 

 

(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 
 
 

(4) - Remarques générales
 

(5) - Syntèses et préconisations
 

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