the data obtained on 169 glaciers since 1700, Oerlemans
(2005) shows that the retreat of glacier fronts began in the
nineteenth century, and accelerated strongly from 1850
onwards, with a continuation throughout the twentieth century and early decades of the twenty-first century.
Regional analyses have shown that, until around 2000,
the average mass balance cumulated over all European
glaciers was close to zero, with significant mass losses for
Alpine glaciers being compensated for by advances of glaciers in western Norway stemming from a sharp increase in
precipitation in response to a positive phase of the North
Atlantic Oscillation. From the year 2000 onwards, the
Norwegian glaciers began to retreat in response to a decrease
in precipitation. Over the period 2003–2009, the most negative mass balances occurred for glaciers in the northwestern
United States and southwestern Canada, Central Europe,
Southern Andes and low latitude areas. In the Alps, glaciers
have been retreating since the mid-nineteenth century. In
Switzerland, they currently cover only 60% of the area they
occupied in 1850. Due to the heat wave, an exceptionally
high loss of mass occurred in 2003, corresponding to a
reduction of 2500 kg m
−2 yr
−1 for the nine glaciers studied.
This value exceeds the previous record 1600 kg m
−2 yr
−1 in
1996 and is four times higher than the average measured
between 1980 and 2001 (600 kg m
−2 yr
−1 ). In Africa, the
glacier area at the top of Kilimanjaro is now only 20% of
what it was at the beginning of the twentieth century. In
Patagonia, the ‘icefields’ have lost between 3 and 13 km
3 /yr
of ice since the 1970s. The retreat of the glaciers in Nepal
and Himalayas seems to have accelerated over the past
twenty years, and in Tibet, the number of glaciers in retreat
has recently multiplied.
The Fifth IPCC Assessment Report estimated that the
contribution of glaciers and small ice caps to sea level rise
over the 1993–2010 period was about 0.76 mm/yr (IPCC
2013). Since then, new estimates based on new data indicate
a higher contribution, demonstrating that mass loss from
glaciers and small ice caps has accelerated significantly since
the early 1990s, and currently contributes between 1.05 and
1.12 mm/yr to the rise of the global sea level.
Polar Ice Sheets
Polar ice sheets are huge masses of ice formed, like glaciers,
by continual accumulation of snow in excess of ablation,
which gradually turns into ice under the effect of compaction. As for glaciers, this transformation occurs in a
transition zone about one hundred meters thick called firn.
Currently, the ice sheets are located at high latitudes, one
near the North Pole, Greenland, the other centered on the
South Pole, Antarctica. The area of the Greenland ice sheet
is about 1.8 Â 10
6 km
2 . In the center, the ice thickness is
greater than 3000 m. The Greenland ice volume
(* 3.0 Â 10
6 km
2 ) represents about 10% of the worldwide
freshwater supplies. Antarctica is composed of two effectively distinct ice sheets in the east and west, separated by
the Transantarctic Mountains. Its ice volume is close to
27 Â 10
6 km
2 and its surface, almost 98% covered by ice, is
about 14 Â 10
6 km
2 . A large part of the western ice sheet
lies below sea level. The West Antarctic ice sheet extends
locally over the sea to form floating ice shelves, mainly in
the embayments of the coast, as in the Weddell and Ross
Seas. In contrast, East Antarctica, which is larger, rests largely on bedrock. It forms a plateau with an area exceeding
10 Â 10
6 km
2 covered by a large ice layer of more than
4000 m thick in the center.
The evolution of the part of an ice sheet grounded on the
bedrock depends on its surface mass balance and its flow due
to the deformation of the ice itself. When the temperature is
high enough, the ice melts at the surface. As for glaciers, the
surface mass balance is determined by the difference
between accumulation and ablation. In addition, under the
effect of its own weight, the ice flows by plastic deformation
along the line of steeper slope, as well as by sliding on the
bedrock when the local temperature is close to the melting
point: this is called basal sliding. As ice is an insulating
material, a temperature gradient is established between the
colder surface and the warmer base. Furthermore, by
changing the ice viscosity, the temperature also affects the
flow velocities from the surface to the base of the ice sheet.
Thus, the processes involved in the ice deformation are not
the same at the surface and at the base of the ice sheet.
In the case of the Antarctic ice sheet, the surface temperature generally remains low enough so that ablation is
negligible. The ice then drains into the ocean or feeds the
floating ice shelves through ice streams which are characterized by a rapid outflow (i.e. low basal friction). The
sources of these ice streams are found far upstream, and their
contribution to the evacuation of grounded ice from the
center of the ice sheet towards the edges is estimated at
nearly 90%. The flow regime through the ice shelves is very
different from that of grounded ice. In fact, whereas grounded ice is characterized by a shear regime in the vertical
plane, the predominant constraints on the ice shelves are the
horizontal shear and the pressure forces exerted by the sea.
The destabilization of these glacial platforms is due to the
increase in sea level, but also to the basal melting beneath
the platforms. This melting is therefore related to the ocean
temperatures under the ice shelves and to the energy released
by ocean currents. The dislocation of these glacial plateaus
leads to the formation of icebergs (i.e. ice calving). This is
exactly what happened with the dislocation of the Larsen B
ice shelf in 2002 which resulted in a surface loss of about
3250 km
2 . This could also occur in the coming years with
the break-up of the Larsen C ice shelf in July 2017
(6000 km
2 of surface loss) and the continuous acceleration
of the Twaithes glacier reported by satellite observations
454
S. Charbit et al.
(2005) shows that the retreat of glacier fronts began in the
nineteenth century, and accelerated strongly from 1850
onwards, with a continuation throughout the twentieth century and early decades of the twenty-first century.
Regional analyses have shown that, until around 2000,
the average mass balance cumulated over all European
glaciers was close to zero, with significant mass losses for
Alpine glaciers being compensated for by advances of glaciers in western Norway stemming from a sharp increase in
precipitation in response to a positive phase of the North
Atlantic Oscillation. From the year 2000 onwards, the
Norwegian glaciers began to retreat in response to a decrease
in precipitation. Over the period 2003–2009, the most negative mass balances occurred for glaciers in the northwestern
United States and southwestern Canada, Central Europe,
Southern Andes and low latitude areas. In the Alps, glaciers
have been retreating since the mid-nineteenth century. In
Switzerland, they currently cover only 60% of the area they
occupied in 1850. Due to the heat wave, an exceptionally
high loss of mass occurred in 2003, corresponding to a
reduction of 2500 kg m
−2 yr
−1 for the nine glaciers studied.
This value exceeds the previous record 1600 kg m
−2 yr
−1 in
1996 and is four times higher than the average measured
between 1980 and 2001 (600 kg m
−2 yr
−1 ). In Africa, the
glacier area at the top of Kilimanjaro is now only 20% of
what it was at the beginning of the twentieth century. In
Patagonia, the ‘icefields’ have lost between 3 and 13 km
3 /yr
of ice since the 1970s. The retreat of the glaciers in Nepal
and Himalayas seems to have accelerated over the past
twenty years, and in Tibet, the number of glaciers in retreat
has recently multiplied.
The Fifth IPCC Assessment Report estimated that the
contribution of glaciers and small ice caps to sea level rise
over the 1993–2010 period was about 0.76 mm/yr (IPCC
2013). Since then, new estimates based on new data indicate
a higher contribution, demonstrating that mass loss from
glaciers and small ice caps has accelerated significantly since
the early 1990s, and currently contributes between 1.05 and
1.12 mm/yr to the rise of the global sea level.
Polar Ice Sheets
Polar ice sheets are huge masses of ice formed, like glaciers,
by continual accumulation of snow in excess of ablation,
which gradually turns into ice under the effect of compaction. As for glaciers, this transformation occurs in a
transition zone about one hundred meters thick called firn.
Currently, the ice sheets are located at high latitudes, one
near the North Pole, Greenland, the other centered on the
South Pole, Antarctica. The area of the Greenland ice sheet
is about 1.8 Â 10
6 km
2 . In the center, the ice thickness is
greater than 3000 m. The Greenland ice volume
(* 3.0 Â 10
6 km
2 ) represents about 10% of the worldwide
freshwater supplies. Antarctica is composed of two effectively distinct ice sheets in the east and west, separated by
the Transantarctic Mountains. Its ice volume is close to
27 Â 10
6 km
2 and its surface, almost 98% covered by ice, is
about 14 Â 10
6 km
2 . A large part of the western ice sheet
lies below sea level. The West Antarctic ice sheet extends
locally over the sea to form floating ice shelves, mainly in
the embayments of the coast, as in the Weddell and Ross
Seas. In contrast, East Antarctica, which is larger, rests largely on bedrock. It forms a plateau with an area exceeding
10 Â 10
6 km
2 covered by a large ice layer of more than
4000 m thick in the center.
The evolution of the part of an ice sheet grounded on the
bedrock depends on its surface mass balance and its flow due
to the deformation of the ice itself. When the temperature is
high enough, the ice melts at the surface. As for glaciers, the
surface mass balance is determined by the difference
between accumulation and ablation. In addition, under the
effect of its own weight, the ice flows by plastic deformation
along the line of steeper slope, as well as by sliding on the
bedrock when the local temperature is close to the melting
point: this is called basal sliding. As ice is an insulating
material, a temperature gradient is established between the
colder surface and the warmer base. Furthermore, by
changing the ice viscosity, the temperature also affects the
flow velocities from the surface to the base of the ice sheet.
Thus, the processes involved in the ice deformation are not
the same at the surface and at the base of the ice sheet.
In the case of the Antarctic ice sheet, the surface temperature generally remains low enough so that ablation is
negligible. The ice then drains into the ocean or feeds the
floating ice shelves through ice streams which are characterized by a rapid outflow (i.e. low basal friction). The
sources of these ice streams are found far upstream, and their
contribution to the evacuation of grounded ice from the
center of the ice sheet towards the edges is estimated at
nearly 90%. The flow regime through the ice shelves is very
different from that of grounded ice. In fact, whereas grounded ice is characterized by a shear regime in the vertical
plane, the predominant constraints on the ice shelves are the
horizontal shear and the pressure forces exerted by the sea.
The destabilization of these glacial platforms is due to the
increase in sea level, but also to the basal melting beneath
the platforms. This melting is therefore related to the ocean
temperatures under the ice shelves and to the energy released
by ocean currents. The dislocation of these glacial plateaus
leads to the formation of icebergs (i.e. ice calving). This is
exactly what happened with the dislocation of the Larsen B
ice shelf in 2002 which resulted in a surface loss of about
3250 km
2 . This could also occur in the coming years with
the break-up of the Larsen C ice shelf in July 2017
(6000 km
2 of surface loss) and the continuous acceleration
of the Twaithes glacier reported by satellite observations
454
S. Charbit et al.