recent years, this trend seems to have been reversed. Alpine
glaciers are in an intermediate situation. We will not review
exhaustively the melting of glaciers, but we chose to illustrate this behavior through the study of two alpine glaciers.
At the Environment Geosciences Institute of Grenoble,
two studies were carried out on alpine glaciers, one on the
Saint-Sorlin, the other at the dome of Goûter, in the
Mont-Blanc massif.
The first study simulated the future evolution of the mass
balance of the Saint-Sorlin, located in the Grandes Rousses
massif whose highest point is at 3400 m altitude. The results
show that even with an optimistic greenhouse gas emission
scenario (+1.8 °C in 2100), the equilibrium line (i.e. the
lowest limit of eternal snow) is at a higher altitude than the
highest point of the glacier. This means that over a full year,
the glacier does not accumulate snow (or ice) and is therefore
in a state of chronic ablation. In order to simulate the dynamic
response of the glacier, these results were then used as inputs
for a two-dimensional ice flow model. Despite a moderate
global warming, the model suggests a complete disappearance of the glacier around 2070. The dynamic response of the
glaciers is complex because it depends not only on their
specific morphology, which is different from one glacier to
another, but also on many physical processes, which are
sometimes difficult to simulate, and which determine their
temporal response, i.e. on what timescale the glacier will
grow or, on the contrary, disappear. Nevertheless, this study
suggests that alpine glaciers similar in size and located at an
altitude close to that of the Saint-Sorlin, could undergo the
same type of evolution in the twenty-first century under the
influence of moderate global warming.
The second study is based on borehole temperature
measurements (140 m deep) located at the Dome du Goûter
at 4250 m altitude. These measurements showed a warming
of 1 to 1.5 °C over the first 60 m of ice, between 1994 and
2005. A physical modeling of the process of heat diffusion in
the ice made it possible to show that the observed warming
results not only from atmospheric warming, but also from
the heat produced by the refreezing of surface melt water
that percolates at depth.
Moreover, simulations carried out for different global
warming scenarios confirm the twenty-first century trend and
show that, regardless of the scenario, the Alpine glaciers,
currently classified as ‘cold’ and located between 3500 and
4250 m altitude with an internal temperature ranging from 0
to −11 °C, could become ‘temperate’, with an internal
temperature close to the melting point.
Other modeling studies using different greenhouse gas
scenarios lead to similar and equally disturbing conclusions.
A study conducted in the Montana region in the Glacier
National Park shows that, with a doubling of CO 2 , some
glaciers would disappear by 2030, despite an increase of 5–
10% in average precipitation in the mid and high latitudes.
The future of ice sheets
There are different approaches to determine how polar ice
sheets will evolve in the future. The first one is to determine
their surface mass balance which is directly dependent on the
climate. To simulate as precisely as possible the mass balance of the ice sheets, high-resolution climate models are
necessary to properly represent the topography of the ice
sheets, especially the steep slopes at the margins. These
areas correspond to the ablation zones and are also the
locations where most of the precipitation falls. However,
AOGCMs usually tend to overestimate precipitation and
underestimate ablation. In addition, snowpack models
implemented in general circulation models are often too
simplistic and do not account for key processes taking place
in the snowpack. Among the missing processes are the
refreezing of surface melt water that percolates at depth, the
transport of snow by winds, the snow metamorphism and the
transformation of snow into ice. These processes are crucial
for the estimates of surface mass balance. For example,
water refreezing modulates surface runoff and snow metamorphism strongly modifies the albedo and thus the surface
energy balance.
Regional atmospheric models generally offer a better
representation of the surface energy balance than AOGCMs
because of their finer resolution and the inclusion of more
sophisticated snow models. However, as they are forced at
their lateral boundaries by outputs from global climate
models, they may suffer from the global model deficiencies.
As a result, the uncertainties associated with the GCM-based
forcing represent about half of the uncertainty associated
with changes in surface mass balance inferred from regional
climate models (RCM). All RCM-based studies project an
increase in precipitation over large parts of Greenland and
Antarctic ice sheets in response to global warming, but there
is great uncertainty as to the magnitude of this increase.
Projections suggest that over Greenland, ablation will largely
exceed the increase in snowfall. Conversely, in East
Antarctica, precipitation is expected to exceed ablation
throughout the twenty-first century, but the key question is
whether the mass gain will be offset by dynamical ice losses
from the West Antarctic ice sheet.
The surface mass balance is not the only important
parameter in the determination of the evolution of polar ice
sheets. As explained earlier, their dynamic response must
also be considered. This can be achieved through the use of
three-dimensional ice-sheet models which include most of
the processes responsible for the ice-sheet evolution as a
function of climate forcing (temperatures and precipitation
or surface mass balance). For a more thorough description of
ice-sheet models, we ask the reader to refer to Chap. 24 of
this volume. However, several problems may arise when
using such models. As previously mentioned, the surface
31 From the Climates of the Past to the Climates …
471
glaciers are in an intermediate situation. We will not review
exhaustively the melting of glaciers, but we chose to illustrate this behavior through the study of two alpine glaciers.
At the Environment Geosciences Institute of Grenoble,
two studies were carried out on alpine glaciers, one on the
Saint-Sorlin, the other at the dome of Goûter, in the
Mont-Blanc massif.
The first study simulated the future evolution of the mass
balance of the Saint-Sorlin, located in the Grandes Rousses
massif whose highest point is at 3400 m altitude. The results
show that even with an optimistic greenhouse gas emission
scenario (+1.8 °C in 2100), the equilibrium line (i.e. the
lowest limit of eternal snow) is at a higher altitude than the
highest point of the glacier. This means that over a full year,
the glacier does not accumulate snow (or ice) and is therefore
in a state of chronic ablation. In order to simulate the dynamic
response of the glacier, these results were then used as inputs
for a two-dimensional ice flow model. Despite a moderate
global warming, the model suggests a complete disappearance of the glacier around 2070. The dynamic response of the
glaciers is complex because it depends not only on their
specific morphology, which is different from one glacier to
another, but also on many physical processes, which are
sometimes difficult to simulate, and which determine their
temporal response, i.e. on what timescale the glacier will
grow or, on the contrary, disappear. Nevertheless, this study
suggests that alpine glaciers similar in size and located at an
altitude close to that of the Saint-Sorlin, could undergo the
same type of evolution in the twenty-first century under the
influence of moderate global warming.
The second study is based on borehole temperature
measurements (140 m deep) located at the Dome du Goûter
at 4250 m altitude. These measurements showed a warming
of 1 to 1.5 °C over the first 60 m of ice, between 1994 and
2005. A physical modeling of the process of heat diffusion in
the ice made it possible to show that the observed warming
results not only from atmospheric warming, but also from
the heat produced by the refreezing of surface melt water
that percolates at depth.
Moreover, simulations carried out for different global
warming scenarios confirm the twenty-first century trend and
show that, regardless of the scenario, the Alpine glaciers,
currently classified as ‘cold’ and located between 3500 and
4250 m altitude with an internal temperature ranging from 0
to −11 °C, could become ‘temperate’, with an internal
temperature close to the melting point.
Other modeling studies using different greenhouse gas
scenarios lead to similar and equally disturbing conclusions.
A study conducted in the Montana region in the Glacier
National Park shows that, with a doubling of CO 2 , some
glaciers would disappear by 2030, despite an increase of 5–
10% in average precipitation in the mid and high latitudes.
The future of ice sheets
There are different approaches to determine how polar ice
sheets will evolve in the future. The first one is to determine
their surface mass balance which is directly dependent on the
climate. To simulate as precisely as possible the mass balance of the ice sheets, high-resolution climate models are
necessary to properly represent the topography of the ice
sheets, especially the steep slopes at the margins. These
areas correspond to the ablation zones and are also the
locations where most of the precipitation falls. However,
AOGCMs usually tend to overestimate precipitation and
underestimate ablation. In addition, snowpack models
implemented in general circulation models are often too
simplistic and do not account for key processes taking place
in the snowpack. Among the missing processes are the
refreezing of surface melt water that percolates at depth, the
transport of snow by winds, the snow metamorphism and the
transformation of snow into ice. These processes are crucial
for the estimates of surface mass balance. For example,
water refreezing modulates surface runoff and snow metamorphism strongly modifies the albedo and thus the surface
energy balance.
Regional atmospheric models generally offer a better
representation of the surface energy balance than AOGCMs
because of their finer resolution and the inclusion of more
sophisticated snow models. However, as they are forced at
their lateral boundaries by outputs from global climate
models, they may suffer from the global model deficiencies.
As a result, the uncertainties associated with the GCM-based
forcing represent about half of the uncertainty associated
with changes in surface mass balance inferred from regional
climate models (RCM). All RCM-based studies project an
increase in precipitation over large parts of Greenland and
Antarctic ice sheets in response to global warming, but there
is great uncertainty as to the magnitude of this increase.
Projections suggest that over Greenland, ablation will largely
exceed the increase in snowfall. Conversely, in East
Antarctica, precipitation is expected to exceed ablation
throughout the twenty-first century, but the key question is
whether the mass gain will be offset by dynamical ice losses
from the West Antarctic ice sheet.
The surface mass balance is not the only important
parameter in the determination of the evolution of polar ice
sheets. As explained earlier, their dynamic response must
also be considered. This can be achieved through the use of
three-dimensional ice-sheet models which include most of
the processes responsible for the ice-sheet evolution as a
function of climate forcing (temperatures and precipitation
or surface mass balance). For a more thorough description of
ice-sheet models, we ask the reader to refer to Chap. 24 of
this volume. However, several problems may arise when
using such models. As previously mentioned, the surface
31 From the Climates of the Past to the Climates …
471