Environmental and Climate Records from High Elevation Alpine Glaciers (ALPCLIM)

EU-Project carried out at VAW/ETH Zürich in the EU (BBW) funded Environment and Climate program

Project duration: 1998-2001

People

Dr. Martin Hoelzle (GI/UNIZH)

Dr. Stephan Suter (MeteoSwiss)

Dr. Stephan Wagner

Associated partners to the project:

IAC/ETHZ Institut for Atmosphere and Climate ETH Zurich, A. Ohmura

IUP Institut für Umweltphysik University of Heidelberg, D. Wagenbach

LGGE Laboratoire de Glaciologie et Géophysique de l’Environnement (CNRS) St Martin d’Hères, M. Legrand

CFR Centre des Faibles Radioactivités CNRS-CEA Gif sur Yvette, H. Cachier

ZAMG Central Institute for Meteorology and Geodynamics Vienna, R. Böhm

PIUB Physikalisches Institut, Abteilung für Klima- und Umweltphysik University of Bern, T. Stocker

GIUZ Department of Geography University of Zurich, W. Haeberli

DISAT Dipartimento di Scienze dell’Ambiente e del Territorio Università degli Studi di Milano-Bicocca, V. Maggi

Display image of Energy Balance Tower installation on Seserjoch Monte Rosa Wallis Swiss Alps

Scientific objectives, approach and major activities

The overarching scientific objective of ALPCLIM was aimed at the exploitation of ALPCLIM glaciers for climate related records. These concerns the archived history of isotope temperature, atmospheric trace constituents as well as the englacial temperature distribution. Different to the vast polar ice sheets, there is a primary need for small scale Alpine glacier archives to evaluate and validate such proxy records in view of their hitherto unknown reliability, spatio-temporal significance and, most important, in view of their underlying net atmospheric signals. In this way, ALPCLIM findings are expected to eventually provide the basic prospect on the potential of Alpine glacier archives in current environmental and climate research foci. The conceptual framework had to narrow down crucial shortcomings of Alpine glaciers, which mainly comprised dating problems, large and even unknown glaciometeorological noise as well as the virtually unexplored (paleo-climate) potential of trace gases, englacial temperature profiles and cold ice bodies at lower altitudes.

The specific ALPCLIM approach was build on following innovations:

• to recover multiple ice core records from various drill sites clearly distinguished by their glacio-meteorological feature
• to tackle the dating problem by novel methods as e.g. methane matching
• to make use of well known atmospheric signals as provided by radionuclides and isotopic trace gas signatures
• to establish a detailed glaciological and climatological glacier site characterisation (also extended to low altitude cold ice bodies) supplemented by year round atmospheric observation
• to accomplish validation of the climatic glacier evidences via instrumentally based long term climatologies

ALPCLIM Final Report Executive summary

Major ALPCLIM activities essentially comprised:

• the evaluation of several ice cores drilled to bedrock in the Monte Rosa, Mont Blanc and Piz Corvatsch summit ranges for ice chemistry, trace gases and stable water isotope records
• extensive glaciological field programs at and in the vicinity of the drill sites devoted to additional new ice core drillings, englacial temperature mapping and automatic recordings of meteorological as well as aerosol input parameters
• modelling efforts focussing on the glacier energy balance, spatial cold firn distribution, flow dynamics of ice crests, inversion of borehole temperature profiles and ice core chronologies
• sophisticated evaluations of long term instrumental climate series aimed at establishing recent and long term climatologies related to ALPCLIM glacier sites

WP3: Cold firn and ice

Background

Cold glaciers are defined as firn- and ice bodies showing negative temperatures as a whole and over a minimum time period of one year. If the entire glacier is at pressure-melting point it is said to be temperate. Many existing glaciers are neither cold nor temperate throughout. Temperate parts of partially cold glaciers often occur at the glacier bed or in the accumulation area. Such ice bodies are called polythermal (e.g. Paterson, 1994).

In order to quantify firn- and ice temperatures, a mean annual firn temperature (MAFT) and a mean annual ice temperature (MAIT) can be defined, respectively. Seasonal surface-temperature fluctuations are normally reflected within the uppermost 10-20 m of the firn and ice. Englacial temperatures measured to this depth are called near-surface firn- and ice temperatures, adapting the concept by Hooke et al., 1983. The MAFT or MAIT is found at a depth where the seasonal temperature fluctuations vanish or - in practice - where they are within the accuracy range of the measurement.

Mean annual firn and ice temperatures are mainly a result of the energy- and mass balance at the glacier surface. Important parameters affecting the mean annual firn and ice temperatures are MAAT, radiation flux, rate and seasonality of accumulation/ablation, volume of penetrating and re-freezing meltwater in firn areas, topographic location (depression or ridge) and wind effects. Some of these parameters are closely interconnected like, e.g. topography, wind effects and snow deposition. Other parameters which can influence the near-surface thermal regime are glacier flow, extension and characteristics of the catchment basin (e.g. deposition of cold snow by avalanche activity), intensity of friction at the glacier bed and ice deformation, ground heat flux and occurrence of crevassed zones (e.g. Paterson, 1994; Suter et al., 2001). Negative firn- and ice temperatures are of relevance in glacio-climatological studies for the following reasons:

• they are linked to the parameters of the ice flow law and to the presence/absence of basal sliding (e.g. Paterson, 1994);
• they can be considered a key parameter for detecting global warming or cooling trends (see e.g., Robin, 1983; Zotikov, 1986; Haeberli, 1990; Haeberli and Funk, 1991; MacAyeal et al., 1991; Fischer et al., 1995; Cihlar et al., 1997; Lüthi and Funk, 2001). In glacierized polar and high-mountain regions, negative firn- and ice temperatures can be interpreted in terms of natural thermographs with a pronounced long-term memory function as they reflect the surface-temperature conditions of the past. Observed temperature profiles from polar ice sheets, for instance, clearly indicate a warming trend for the 20th century (Robin, 1983);
• steep hanging glaciers entirely or partly frozen to their beds are stabilized (Alean, 1985; Haeberli et al., 1989; Wagner, 1996; Lüthi and Funk, 1997);
• they can significantly influence the mass-balance evolution of a glacier (e.g. Greuell and Oerlemans, 1986; Konzelmann and Braithwaite, 1995) and are of importance in mass-balance/sea-level scenarios (Meier, 1984);
• they are essential for glaciological trace-element investigations; cold firn and ice ensure an optimum embedding and conservation medium for natural trace elements like terrestrial and cosmic dusts, marine salts, isotopes and gases as well as anthropogenic contributions such as soot particles, (exhaust) gases, organic compositions and numerous radio-active isotopes (e.g., Wagenbach, 1989; Haeberli and Stauffer, 1994).

For a long time it was assumed that glaciers in the Alps were generally temperate, although Vallot (1893 and 1913) already observed cold firn in the Mont Blanc Massif at the end of the 19th and the beginning of the 20th century. Lliboutry et al. (1976) and Haeberli (1976) were among the first who systematically assessed the distribution of cold firn and ice in the Alps. In the past, extensive firn- and ice-temperature measurements took predominately place at locations of specific practical or scientific interest in connection with construction work (e.g. Haefeli and Brentani, 1955; Haeberli et al., 1979), glacier-risk prevention (Lüthi and Funk, 1997) and high-alpine core drillings (e.g. Oeschger et al., 1977; Alean et al., 1983; Haeberli and Funk, 1991; Vincent et al., 1997). A scheme for the spatial distribution of cold firn and ice was outlined by Hooke et al. (1983) including near-surface temperatures of alpine and polar regions and by Haeberli and Alean (1985) focusing on firn- and ice temperatures from the Alps. An altitude- and aspect-dependent statistical model on the distribution of cold firn and ice in accumulation areas of the Alps has recently been developed by Suter et al. (2001).

The potential of borehole-temperature records for paleo-temperature reconstructions (borehole thermometry) has recently been discovered and was successfully applied to temperature records from Greenland (e.g. Dahl-Jensen and Johnsen, 1986) and Antarctica (e.g. Ritz, 1989; MacAyeal et al., 1991; Salamatin et al., 1994; Dahl-Jensen et al., 1999) using forward approaches or sophisticated inverse modeling based on control or Monte-Carlo methods.

The near-surface thermal regime of cold firn and ice can only be fully understood by means of an energy-balance study. Modern energy-/mass-balance investigations (measurements and modeling) have mostly neglected the near-surface temperature regime, although the assumption of a steady 0°C surface temperature during the melt season may lead to substantial errors (Greuell and Oerlemans, 1986). Glacier melt in cold ablation areas is significantly reduced resulting in lower degree-day factors (melt index according to the sum of positive degree days of the air temperature) as the warming of cold ice represents a substantial heat sink (Konzelmann and Braithwaite, 1995).

Energy-balance studies over snow go back to the 1930s and 1940s, already (Sverdrup, 1936; Wilson, 1941). A series of modern energy-balance (component-) measurements and modelings was carried out for alpine glaciers (e.g. Föhn, 1973; La Casinière, 1974; Martin, 1975; Wagner, 1979 and 1980; Escher-Vetter, 1985a and 1985b; Funk, 1985; Oerlemans, 1992, van de Waal et al., 1992; Arnold et al., 1996; Hock and Noetzli, 1997; Hock, 1999; Oerlemans, 2001) most of them in connection with mass-balance and glacier-melt/discharge investigations on temperate valley glaciers.

Physically-based energy-balance models, including all energy-balance components, were successfully applied to complex alpine topography for simulating spring snow melt (Plüss, 1997; Fierz et al., 1997), glacier melt (Brock et al., 2000), permafrost occurrence (Hoelzle et al., 2001; Mittaz et al., 2000) or were used to model the snow cover evolution for avalanche forecasting (Brun et al., 1989; Lehning et al., 1999).

Results of ALPCLIM studies

The study on cold firn at high altitudes was divided into four main parts dealing with the atmosphere-ground interactions (surface energy-balance), the distribution of cold firn in space (spatial occurrence of cold firn), the thermometric evidence of observed firn- and ice-temperature profiles in terms of a climate signal (borehole thermometry) and the energy balance and the (future) englacial thermal conditions in space (coupled spatial energy-balance/firn-temperature model).

The atmospheric impact was investigated with the help of an energy-balance study at the cold 4300 m high Seserjoch firn saddle, Monte Rosa area (Italy and Switzerland). Measurements of short- and longwave radiation, wind speed and wind direction, air temperature, air humidity, snow height and snow- and firn temperatures were carried out between September, 1998 and August, 2000. A one-year time series of energy-balance measurements covering the period from May, 1999 to April, 2000 shows that the net radiation and turbulent heat fluxes are the major contribution to the energy balance. The heat fluxes due to surface melt in summer and re-freezing events (re-freezing of meltwater at the surface or rime accretion) have to be taken into account. Their precise magnitude is difficult to interpret as these fluxes also comprise the instrumental and methodological errors of the energy-balance calculation. Single surface melt events and the prevailing meteorological conditions favouring or preventing surface melt could be identified by precise high-resolution surface-temperature measurements (Figure 1).

Figure 1: Surface and air temperatures, net radiation and residual heat flux (melt energy and re-freezing energy) from August 4 (upper panel) and August 16, 1999 (lower panel). On August 4, surface melt occurred with a positive melt-energy flux during the day, whereas no surface melt was observed on August 16 (residual heat flux approaching zero during the day).

Although air temperature is below freezing point in both cases, melt does occur in the first case due to a high net radiation and relatively low wind speeds. In the latter case, net radiation is much smaller and wind speed is high which inhibits melt. The quite large residual heat flux during night may be due to re-freezing processes (either re-freezing meltwater or rime accretion due to fog). Monthly mean values of the energy balance using the bulk method are presented in Figure 2. The residual heat flux comprises the melt flux and the instrumental and methodological errors of the net radiation, sensible and latent heat flux and ground heat flux. A positive net radiation can only be observed from May to August. A quite large negative radiation balance occurs in December and January due to shading from the adjacent Parrotspitze and a very low longwave incoming radiation. The monthly means of the ground heat flux are very small and do not contribute to the energy balance in an important way. The turbulent fluxes calculated with the bulk method show steady positive values for the sensible heat flux and negative values for the latent heat flux. Thus, the positive net radiation during the summer months is mainly compensated by the latent heat flux; the negative net radiation in the winter months by the sensible heat flux. The negative residual heat flux during summer may be due to melt events. The residual heat flux from November to January is very high and must be explained by an underestimation of the sensible heat flux, possibly due to an underestimation of the wind speeds caused by hoar frost at the instruments. Another part of the residual heat flux must be attributed to rime accretion at the surface. The generally rather high latent heat fluxes could be due to the assumption of water-vapour saturation at the surface.

Figure 2: Monthly mean energy balance of May, 1999 to April, 2000 at Seserjoch using the bulk method.

As a summary, it can be stated that the cold firn saddle of Seserjoch shows predominantly negative fluxes of net radiation and latent heat and positive fluxes of sensible heat, whereas the direction of the sensible heat flux is not so clear during the summer months depending on the presence or absence of melt.

Near-surface firn temperatures were measured in 22 steam-drilled boreholes in the summit region of Mont Blanc (France and Italy) between 3800 and 4800 m a.s.l. in June, 1998, and in 31 boreholes in the Monte Rosa area (Italy and Switzerland) between 3900 and 4500 m a.s.l. in May/July 1999. Borehole temperatures were sampled with removable thermistor chains to a depth of 22 m. The temperatures at 18 m depth ranged between temperate conditions and approximately -15°C. The thermal distribution pattern of cold firn suggests a strong influence of solar radiation and turbulent heat exchange (Figure 3).

Figure 3: Measured 18-m temperatures in the Mont Blanc area from 1998 (upper panel) and the Monte Rosa area from 1996 and 1999 (lower panel) showing grid-interpolated values using a spline interpolation.

During the melt season in summer, these energy fluxes mainly determine the melt-energy input into the snow and firn and, thereby, the observed near-surface firn temperatures. Mean annual air temperature is of secondary importance for the spatial distribution pattern, although the observed mean annual firn temperatures generally increase with decreasing elevation. A statistical analysis of the measured firn temperatures revealed that the parameters elevation, potential direct solar radiation, slope and accumulation are able to explain more than 80 % of the variation of the mean annual firn temperatures. The aspect-dependent lower boundaries for cold firn in the Mont Blanc and Monte Rosa areas range between 3500 and 3700 m a.s.l. on north-facing and between 3800 and 4100 m a.s.l. on south-facing slopes (Table 1; Suter and Hoelzle, 2002).

Table 1: Lower boundaries [m a.s.l.] of cold firn occurrence as a function of aspect for the Mont Blanc and Monte Rosa areas.

Theoretical calculations, using a one-dimensional time-dependent thermo-mechanical firn-temperature model including the effect of latent heat originating from surface melt, show that the englacial thermal regime is extremely sensitive to the magnitude and duration of surface melt and that melt events perturb the pure surface-temperature signal, considerably. A typical surface-temperature perturbation penetrates a 100 m thick glacier within 18 to 30 years, only (Figure 4).

Figure 4: Arrival time at depth of 1 % of a 1 °C surface perturbation exerted for 30 years (b) or for 1 year (c) for different surface accumulations and a linear and exponential vertical velocity model. Thick lines represent the exponential and thin lines the linear vertical velocity model. The input signal is shown in (a), where the dashed line represents the input signal for (b) and the solid line the input signal for (c), respectively.

Therefore, the possible time horizon for surface-temperature reconstructions using englacial temperature profiles is limited to a few centuries at best. The latent heat release within a cold firn- and ice body is of major importance for its thermal structure. The effect of the latent heat release on a steady-state temperature profile is shown in Figure 5 using realistic values for cold firn areas in the Alps.

Figure 5: Effect of latent heat release on a steady-state temperature profile for different durations of melt D (a) and melt-energy inputs M (b). Thick lines represent the exponential, thin lines the linear velocity-depth model. The deviations from the steady-state temperature profile after 10 years are shown.

In case of Figure 5a, only the duration of the surface melt was varied and the amount of melt energy kept constant (10 W/m2). In Figure 5b, the duration of the melt event was set constant to 30 days and the amount of melt energy varied. The effects on the temperature profiles (deviation from steady-state profile) from varying both the duration and melt-energy input are on the same order of magnitude and drastically change the near-surface thermal regime.

Englacial temperature profiles were measured with an absolute accuracy of ±0.01-0.03°C in a 29 m deep borehole at Seserjoch (4300 m a.s.l., Monte Rosa area), in a 25 m deep borehole at the saddle point of Colle Gnifetti (4450 m a.s.l., Monte Rosa area) and in a 40 m deep borehole on top of Dôme du Goûter (4300 m a.s.l., Mont Blanc area). The observed temperature profile and calculated steady-state and time-dependent situations for Dôme du Goûter are presented in Figure 6.

Figure 6: Measured and extrapolated temperature profile, calculated steady-state situation and surface-temperature change (a) and transient temperature profiles (b) at Dôme du Goûter, Mont Blanc area. Three different transient situations starting from a (seasonal) steady-state profile were calculated: dTs = +3.4 °C in 8 years with no melt; dTs = 0.0 °C and a mean daily melt-energy input of 5.5 W/m2 during August in 4 years and dTs = +1.0 °C and a mean daily melt-energy input of 4.5 W/m2 during August in 4 years.

Firn- and ice-temperature observations from Colle Gnifetti since the early 1980s are presented in Figure 7. The 20-year time series of englacial temperatures at Colle Gnifetti suggests a surface-temperature increase of approximately +0.6 °C since about 1990, under the assumption of a stable or negligible surface melt-energy input.

Figure 7: Measured englacial temperatures at Colle Gnifetti, Monte Rosa area from 1983 to 2000 showing the situations for 1983, 1991, 1999 and 2000.

Observed and calculated thermal records for Seserjoch are shown in Figure 8. A deviation between calculated steady-state (surface) temperature and observed 18-m temperature of ?T=0.48+0.27/-0.11°C results (Figure 8a). A time-dependent calculation is needed to quantify the time horizon of the observed warming. The calculated scenario starts from a (seasonal) steady-state situation for January, 25. The scenario was prescribed with dTs=+1.0 ?C and a mean daily melt-energy input of 2.1 W/m2 during 48 days in 5 years (8.71 MJ/m2 total yearly melt-energy input) and, subsequently, dTs=-0.5 °C and a mean daily melt input of 1.0 W/m2 during 7 days in 2 years (0.60 MJ/m2 total yearly melt-energy input) and is shown in Figure 8b. As in case of the Dôme du Goûter situation, the observed warming can hardly be explained by a surface-temperature increase nor an increase in surface melt alone. The combined scenario gives a very good representation of the observed profile, which can be explained by a surface cooling and corresponding decrease of surface melt during 2 years, after a strong surface warming over 5 years starting from the (assumed) steady-state profile. The mean surface-temperature increase of +0.5 °C over the calculation period of 7 years is in good
agreement with the warming derived from the comparison between the observed profile and the numerically calculated steady-state profile (see above).

Figure 8: Measured and extrapolated temperature profile, calculated steady-state situation and surface-temperature change (a) and transient temperature profile (b) at Seserjoch, Monte Rosa area. Starting from a (seasonal) steady-state profile, the transient situation in (b) was calculated with dTs = +1.0 °C and a mean daily melt-energy input of 2.1 W/m2 during 48 days in 5 years and, subsequently, dTs = -0.5 °C and a mean daily melt input of 1.0 W/m2 during 7 days in 2 years.

Thus, the borehole-temperature records from the three sites suggest a surface-temperature increase on the order of 0.5-1°C for the last decade (Figures 6-8). A possible future temperature evolution is modelled in Figure 10.

In addition, the potential of ground surface-temperature (GST) reconstruction based on inverse theory was assessed using a least-squares inverse theory by Tarontola and Valette (1982) and a Fourier frequency-domain discrete method by Wang (1992). This theory was applied to the 3 sites with deeper borehole-temperature records (Dôme du Goûter, Colle Gnifetti and Seserjoch). Whereas the inferred GST's were difficult to interpret for Colle Gnifetti and Seserjoch, a reasonable coincidence was found for the 40-m deep borehole at Dôme du Goûter (Figure 9). The higher magnitude of the warming (about 4 °C) found in the GST-reconstruction may be due to an increased release of latent heat within the firn as a consequence of a higher melt-water input. Thus, the warming found in the inverse model may represent both an increase in the GST and the surface melt regime.

Figure 9: Calculated ground surface-temperature changes for the Dôme du Goûter borehole. It reflects quite well the shape of the air temperature changes observed in the Western Alps.

A spatial energy-balance model was coupled with a one-dimensional thermal firn-temperature model and applied to the Monte Rosa study area. The spatial energy-balance model used was developed for complex alpine topography and is based on former works by Funk and Hoelzle (1992); Konzelmann et al. (1994); Plüss (1997); Fierz et al. (1997); Hock (1999) and Hoelzle et al. (2001). The firn-temperature model is based on the one-dimensional time-dependent thermo-mechanical firn-temperature model from above.

Figure 10: Air temperature at Colle del Lys from December, 1996 to December, 2000 showing monthly mean values and extremes.

Although a non-ventilated air-temperature sensor was used at Colle del Lys, natural ventilation was strong enough to avoid sensor heating by solar radiation and a good coincidence results between observed air temperatures at Colle del Lys and Seserjoch (Figure 11).

Figure 11: Comparison between observed air temperature (daily mean values) at Colle del Lys and at Seserjoch. A very good linear relation is found.

Although the spatial energy-balance model yielded some encouraging results, the errors in the calculated surface temperature turned out to be too large for a direct application in a coupled energy-balance/firn-temperature model. A simplified formulation of the upper boundary condition in terms of surface temperature and melt-energy input was made and coupled with the firn-temperature model. The model is considered robust enough to give a statement on the future thermal evolution of the cold firn saddles of Seserjoch (Figure 12) and Colle Gnifetti based on IPCC warming scenarios.

Figure 12: Observed (2000) and modeled future firn- and ice-temperatures profiles at Seserjoch showing the state in 2020, 2050 and 2100.

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