Vincent 2002 Influence of climate change over the 20th century...

Réf. Vincent 2002 - A

Référence bibliographique complète

VINCENT, C. Influence of climate change over the 20th century on four french glacier mass balances. Journal of Geophysical Research, 2002, vol 107.

Abstract: The sensitivity of summer ablation to temperature is maximum in low-elevation zones (1.4 m w.e. °C-1 at 1800 m asl) and decrease with altitude (0.5 m w.e. °C-1 at 2900 m asl). As a consequence, the sensitivity of equilibrium line altitude to temperate is 60-70 m °C-1, half that of the results found in the literature. Therefore, the alpine glacier retreat scenarios for the 21th century have been largely overestimated. Model sensitivity analysis shows that a 25-30% increase in precipitation would compensate 1°C temperature rise for the mass balances of glaciers. The 20th century may be divided into four periods: two steady state periods (1907-1941 and 1954-1981) during which the mass of glaciers remained almost constant, and two deficit periods (1942-1953 and 1982-1999) marked by a sharp reduction in glacier mass. Regarding mean ablation at 2800 m asl, a 22 W m-2 increase in energy balance is required to explain the ablation difference between the 1954-1981 and 1982-1999 periods. According to the energy balance analysis the increase in air temperature explains more than 60% of this ablation rise.

Mots-clés

Glaciers, climate, mass balance, accumulation, ablation

Organismes / Contact

Laboratoire de Glaciologie et de Géophysique de l’Environnement (LGGE), CNRS, St Martin d’Hères
vincent@lgge.obs.ujf-grenoble.fr

(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
Precipitation Temperature	Glaciers

Pays / Zone	Massif / Secteur	Site(s) d'étude	Exposition	Altitude	Période(s) d'observation
French Alps	Mt Blanc, Vanoise and Grandes Rousses	1.St Sorlin (45°10’N; 6°10’E) 2.Gébroulaz (45°19’N; 6°40’E) 3.Argentière (45°55’N; 6°57’E) 4.Mer de glace (45°55’N; 6°57’E)	1 : N to E 2 : N 3 : N to S 4 : N to S	1 : 2700-3400m 2 : 2600-3500m 3 : 1600-3600m 4 : 1600-4000m	1 : since 1957 2 : since 1993 3 : since 1975 4 : since 1983

(1) - Modifications des paramètres atmosphériques
Reconstitutions
Observations
Modélisations
Hypothèses

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

(2) - Impacts du changement climatique sur le milieu naturel
Reconstitutions	Reconstruction of mass balances over the 20th century The overall trend is constrained by glacier volume variations deduced from maps, the short-scale components are driven by the meteorological signal. Trends for accumulation and ablation respectively are therefore not derived from meteorological data. The cumulative mass balances of glaciers 2, 3 and 4 have declined only slightly over the 20th century (by ~ 13 m w.e.) and have been almost 0 since 1950. This overall trend contrasts with large local changes observed on the tongues of these glaciers. But the cumulative mass balance of glacier 1 has strongly declined (by ~ 30 m w.e.). The four glaciers have lost mass mainly over two periods : 1942-1953 and 1982-1999. Reconstructed mean accumulation and ablation on the glacier 1 at 2800 m asl show a very sharp mass balance decline between 1942 and 1953 (both increasing ablation and low precipitation). During this period the ablation was very high (even more than in the last period of regression). Glacier advances observed between 1954 and 1981 are clearly related to low ablation values. For the 1982-1999 period, ablation rise significantly and accumulation increase slightly. There is a very strong correlation between these results and results from a previous study on the Sarennes glacier (located 3 km from the glacier 1). The mean ablation rate rose by 44% (from 1.9 to 2.8 m w.e. yr-1) between 1954-1981 and 1982-1999, which corresponds to an energy difference of 22 W m-2. The air temperature increase of 0.8°C between the two periods is responsible for a large part of the ablation rise. A limited increase in incident solar radiation, probably due to decreased cloudiness, could be responsible for a part too.
Observations	Sensitivity of summer mass balance to temperature For the four glaciers, ablation increases linearly with CPDD (cumulated positive degree-day), concerning summer mass balance measurements since at least 1993. It means that globally, the trend is: the higher the CPDD is and the more important ablation is. The relationship between CPDD and ablation is an empirical one. It is not a causality relationship, since both terms are distinct surface-energy balance results. On the glacier 1, at the altitude of 2820 (close to the mean equilibrium line), the relation between ablation and temperature is not very good since temperature explains only 50% of the variance. Because of the influence of the albedo, ice ablation is greater than snow ablation for the same cumulated temperature. Surface albedo is directly linked to the ratio of snow ablation to total ablation. By including this ratio in the correlation, 85% of the variance can now be explained. The positive degree-day factor is close to 0.004 m w.e. °C-1 when ablation only concerns snow. Summer ablation is best correlated to temperature in the low-elevation regions of the glaciers free of rock debris (less influence of the albedo). For the four glaciers, ablation sensitivity to temperature variations globally decreases strongly with elevation. It seems that dispersion is greater over the region close to the equilibrium line, where the albedo pattern can be very different from one year to another. Winter mass balance and winter precipitation The general trend is that the winter mass balance increases considerably with the valley winter precipitation. A large difference between valley precipitation and winter accumulation is observed on glaciers especially in high-elevation regions. The ratios between winter mass balance and winter valley precipitation are relatively constant with time for each altitude range (above 2400 m asl) but very different from one site to another depending on the topography of the englacial basin.
Modélisations
Hypothèses

Sensibilité du milieu à des paramètres climatiques

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

Since 1993, systematic winter and summer mass balance measurements (May and September respectively) have been carried out on the glaciers. Some measurements available before 1993 have been included in the study. In the accumulation zone, cores are first drilled to measure winter mass balances from snow layering (stratigraphy) and density measurements. Then, stakes are inserted in the boreholes down to the previous summer surface. In the ablation area, winter mass balances are measured by drilling and measuring the snow thickness above the ice. Annual mass balances are determined from stakes inserted in ice. The summer mass balance is then the difference between these two balance terms.

Measured summer mass balances have been compared to the cumulated positive degree-day (CPDD) calculated from valley meteorological data. The CPDD is the cumulated temperature higher than 0°C obtained from valley measurements by applying a fixed lapse rate (temperature gradient with altitude) of 6°C km-1. The meteorological station is Lyon for glacier 1 and Chamonix for the other glaciers. In order to estimate ablation sensitivity to temperature variations over the whole summer period, each degree-day factor has been multiplied by the mean number of days for which temperature is higher than 0°C at the observation elevation. This calculation has been made using the average daily temperature (1923-1999) of the Lyon meteorological station.

Data on winter mass balance were plotted against valley winter precipitation observed over the same intervals. The meteorological stations used are Besse en Oisans for the glacier 1 and Chamonix for the other glaciers. Only precipitation at temperatures below freezing (at the observation elevation) is taken into account. The ratio between winter mass balance and winter valley precipitation, expressed as a function of elevation, for each glacier has been calculated. As Chamonix precipitation is ~1.3 times Besse precipitation, the ratio observed over glacier 1 has been divided by 1.3.

Total cumulative mass balances have been calculated using old maps with elevation contours and recent geodetic measurements (topographic measurements and aerial photographs). Surface area changes over the century have been taken into account using maps and parameters have been adjusted to match reconstructed mass balance with field measurements.

Then the magnitude of various climatic forcings capable of explaining the large ablation variations observed has been calculated. The total summer ablation has been converted into energy (assuming that the ablation is due only to melting and that heat conduction into the ice or snow is negligible, as sublimation).

(3) - Impacts 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