VINCENT C., LE MEUR E., SIX D., POSSENTI P., LEFEBVRE E., FUNK M. Climate warming revealed by englacial temperature at Col du Dôme (4250 m, Mont Blanc area). Geophysical Research Letters, 2007, Vol. 34, 5 p.

Basal temperature is -11°C for both boreholes and the lower 50 meters of ice do not show any significant change in englacial temperature. On the other hand, a strong warming of firn or ice can be seen in the 90-meter upper part from 1994 to 2005. Moreover, note that the 1994 temperature profile was far from a steady state profile, which would exhibit a sustained cooling from the bottom to the top, apart from the top 15 meters influenced by seasonal variations.

Heat production coming from deformation is neglected. At Dôme du Goûter, the glacier is frozen to its bed, no sliding occurs and the bottom horizontal velocity is zero. As the 1994 and 2005 boreholes were drilled only 9 m apart, the basal heat flux can be considered unchanged. Again, given that the boreholes were drilled at the same location and that the surface horizontal velocity is only 8 m/yr [Vincent et al., 2007], it can be reasonably assumed that the snow layers in the two boreholes originate from the same area. Consequently, the observed temperature changes between 1994 and 2005 are mostly driven by vertical advection in firn/ice, heat conduction and latent heat resulting from surface meltwater refreezing at depth.

The numerical simulations show that reconstructed temperatures in 2005 cannot match the observed temperatures if the latent heat resulting from surface meltwater refreezing is ignored. A simple latent heat flux formulation has been included using a degree-day factor. This factor has been calibrated with observations to a value of 1 ± 0.3 mm/°C/day. Using the same latent heat formulation with the same degree-day factor, the 1994 temperature profile can also be satisfactorily reconstructed from 20th century meteorological data. Again, the numerical simulations show that reconstructed temperatures in 1994 cannot match the observed temperatures if latent heat resulting from surface meltwater refreezing is ignored. This leads to the conclusion that latent heat flux coming from meltwater plays a significant role.

Between the beginning of the 20th century and 1940, englacial temperatures were very close to a steady state profile. During the forties, englacial temperatures increased to reach values in 1950 similar to those obtained at the beginning of the nineties. This warm event is visible in the 1960 temperature profile, once again with a "bump'' at a depth of 50 m. 1980 shows a very near steady state profile preceding the strong warming of recent decades. This result is consistent with observations at Colle Gnifetti (Monte Rosa) in 1982 [Haeberli and Funk, 1991; Lüthi and Funk, 2001]. Over the 1982-2005 period, the summer mean temperature has increased by 1.1°C compared to the 20th century average. It leads to a temperature profile very far from steady state conditions. The exceptional hot summer in 2003, with summer temperature higher by 4.4°C compared to the 20th century average explains only a small part of this change.

For the air temperature warming scenarios, a moderate basal warming of 1°C is predicted for 2050. The englacial temperature increase is between 0 and 5°C below a depth of 30 m. For the warmest scenario, the upper 30 m of ice becomes temperate in 2100. Moreover, with the highest value of latent heat flux from refreezing surface meltwater, the glacier could be entirely temperate, apart from the bottom 20 meters.

In the Mont Blanc area, temperate firn has been observed between 3500 m and 3800 m depending on exposition and on ice advection from upstream [Suter, 2002; Lliboutry et al., 1976]. Thus, following an atmospheric warming of +4°C and a vertical temperature gradient of 0.0056°C/m, this limit should rise to 4210 m for northern expositions. The lower limit of the dry zone, close to 4200 m during the 20th century [Lliboutry et al., 1976], should climb to 4870 m or more. Between 3500 m and 4200 m, the ice temperature at the bed could reach the melting point, which would greatly modify the ice dynamics. As a result, the glacier could start to slide over its bed. Therefore this warming could strongly affect the stability of hanging glaciers.

Observations
Two deep ice borehole temperature profiles were measured in 1994 and in 2004-2005 at Col du Dôme (4250 m, Mont Blanc area). For easier analysis of temperature change, these two boreholes were drilled at the same location (9 meters apart). This allows to remove the effect of basal heat flux variability and reduce the effect of horizontal advection in ice.

Thermistors with 0.05°C accuracy were installed in both boreholes after drilling completion. Tests repeated in October 2004, January 2005 and April 2005 show that these deep borehole temperatures, measured with different thermistors, were consistent (±0.06°C).

Modelling
Analysis of temperature profiles requires heat flow and ice vertical/horizontal advection modelling. The heat-transfer equation within a cold glacier of Malvern [1969] and Hutter [1983] has been used. Englacial temperatures and their changes are computed at daily intervals using an explicit finite-difference scheme with a one meter horizontal layer thickness. Vertical advection in ice is derived from the analytical formulation used by Ritz [1987], Vincent et al. [1997], including horizontal flow.

Boundary surface temperatures have been obtained using valley meteorological data and a fixed vertical lapse-rate (5.6°C/km). Meteorological temperatures come from Lyon (Météo France). Surface accumulation has been inferred from precipitation at Besse using a multiplication factor of 3. Basal heat flux has been set to 15 mW/m2, which leads to a temperature gradient of 0.0067°C/m close to bedrock. This value is consistent with the temperature gradient observed at the borehole bases. Model studies were then carried out back to the beginning of the 20th century.

Further simulations were therefore conducted to reconstruct englacial temperatures for each decade of the 20th century. For these studies, homogenised temperature data [Böhm et al., 2001] since 1808 were used with the mean precipitation rate at Besse.

Numerical heat flow modelling was used to simulate englacial temperatures in the future for different air-temperature scenarios. Simulations were performed starting with the 2005 temperature profile using the last 20-year average temperature for Lyon and two linear surface temperature increases of 1°C and 2°C until 2050, i.e. 2°C and 4°C up to 2100. The degree-day factor value has been set to 0.7 and 1.3 mm/°C/day to cover the maximum range obtained from the previous results.