Evolution of the Cryosphere
The exact estimate of the influence of human activities on
climate is still limited because it is critically dependent on
our ability to distinguish the signal related to this additional
radiative forcing from the natural variability of the climate.
However, there is a growing number of tangible factors
indicating that man has had a perceptible influence on the
climate. In particular, the global warming observed over the
past century has been accompanied by a rise in sea level
(Clark et al. 2016), largely attributed to the thermal expansion of the oceans, but also to significant changes in the
cryospheric components of the climate system over the
whole planet. The cryosphere represents all the water in
solid form on Earth and contains more than 70% of the
Earth’s freshwater reservoir. It is an excellent indicator of
climate change. It includes ice sheets, floating ice-shelves,
mountain glaciers, snow and sea ice, but also the water in
rivers and lakes that freezes in winter, as well as the permafrost, that is to say the permanently frozen ground, covered by an ‘active’ soil layer which melts each summer and
whose thickness is variable.
Each of these components interacts in various ways with
the other components of the climate system over a wide
range of time scales, from seasonal (snow, permafrost, rivers, lakes, sea ice) to a hundred thousand years for the
glacial-interglacial cycles. While the cryosphere is particularly important in the polar regions, there are also many
glaciers in the low and mid-latitude regions, which provide
an overview of the relationship between climate change and
changes in the cryosphere.
Snow Cover
Estimating the extent of snow cover and the physical properties of snow is of paramount importance for both hydrological applications, such as modeling or predicting runoff
due to snowmelt, and for the understanding of local or
regional weather patterns. Typical snow parameters, derived
from radar data, include the extent of snow cover, the water
equivalent of the snow layer, and the state of the snow (wet
or dry).
The extent of snow cover has a direct influence on the
energy balance on the Earth’s surface, but also on the soil
water content. Fresh snow reflects between 80 and 90% of
incident solar radiation. The warming trend decreases the
snow cover, which in turn decreases the fraction of solar
energy reflected back to space, and increases the absorption
of incoming radiation, thereby increasing warming, which in
turn accelerates the snow melting. This amplifying mechanism is known as the ‘temperature-albedo’ effect. Thus, the
surface temperature is strongly dependent on the presence or
absence of snow. Another important aspect of snow cover is
the role it plays in thermal insulation. In winter, snow covered grounds cool much less quickly than bare grounds,
hence the importance of snow depth for plant and animal
life. Finally, melting snow in spring and summer requires a
high latent heat of fusion, so that snow cover represents a
significant heat loss for the atmosphere during the melting
season. As a result, seasonal snow produces thermal inertia
within the climate system, as it involves significant energy
exchanges, with little or no change in temperature.
In the northern hemisphere, snow cover varies seasonally
with a maximum in winter and a minimum in summer, but
with large inter-annual variations. Since the end of the
nineteenth century, daily records of snowfall and snow depth
have been kept by many countries. Nevertheless, these
measurements were only fully developed after 1950, and in
particular from 1966 onwards, with the arrival of satellites.
All of these data series reveal that snow cover has
decreased in spring and summer since the 1920s, with an
even more striking decrease since the end of the 1970s.
According to the Fifth IPCC Assessment report, this
decrease in March-April snow cover extent ranges
from −0.8% per decade over the 1922–2012 period
to −2.2% per decade between years 1979 and 2012. For the
fall and winter seasons, the signal is less clear: some data
sets suggest positive trends as a result of increased snowfall
in a warming climate, while others suggest negative trends
similar to what is observed in spring and summer. Nevertheless, there is a consensus that the mean annual snow
cover has decreased with a shift from February to January of
the maximum extent, an earlier onset of melting (*5.3 days
since winter 1972–1973) and thus a reduction of snow cover
duration. This drop in snow cover is mainly observed in the
northern hemisphere. In the southern hemisphere, very few
data exist outside of Antarctica, and these are often of much
lower quality than in the northern hemisphere.
Evolution of Sea Ice
Sea ice is frozen seawater. When freezing occurs, salt is
expelled from the ice crystals, thus raising the density of the
surface ocean waters. The formation of sea ice can therefore
have a direct impact on the intensity of the thermohaline
circulation. Sea ice is a highly reflective surface with a high
albedo of about 0.8. Conversely, when sea ice melts, the
ice-free ocean surface absorbs about 90% of the radiation
due to large albedo changes, causing the ocean to warm up,
followed by a further increase in surface temperature. This
phenomenon is a positive feedback between temperature and
albedo. Thus, sea ice regulates heat exchanges between the
atmosphere and the polar ocean. It isolates the relatively
‘warm’ ocean waters from the much colder atmosphere,
except when there are winter ‘leads’ occurring as a result of
sea-ice break-up. These leads allow the exchange of heat and
450
S. Charbit et al.
The exact estimate of the influence of human activities on
climate is still limited because it is critically dependent on
our ability to distinguish the signal related to this additional
radiative forcing from the natural variability of the climate.
However, there is a growing number of tangible factors
indicating that man has had a perceptible influence on the
climate. In particular, the global warming observed over the
past century has been accompanied by a rise in sea level
(Clark et al. 2016), largely attributed to the thermal expansion of the oceans, but also to significant changes in the
cryospheric components of the climate system over the
whole planet. The cryosphere represents all the water in
solid form on Earth and contains more than 70% of the
Earth’s freshwater reservoir. It is an excellent indicator of
climate change. It includes ice sheets, floating ice-shelves,
mountain glaciers, snow and sea ice, but also the water in
rivers and lakes that freezes in winter, as well as the permafrost, that is to say the permanently frozen ground, covered by an ‘active’ soil layer which melts each summer and
whose thickness is variable.
Each of these components interacts in various ways with
the other components of the climate system over a wide
range of time scales, from seasonal (snow, permafrost, rivers, lakes, sea ice) to a hundred thousand years for the
glacial-interglacial cycles. While the cryosphere is particularly important in the polar regions, there are also many
glaciers in the low and mid-latitude regions, which provide
an overview of the relationship between climate change and
changes in the cryosphere.
Snow Cover
Estimating the extent of snow cover and the physical properties of snow is of paramount importance for both hydrological applications, such as modeling or predicting runoff
due to snowmelt, and for the understanding of local or
regional weather patterns. Typical snow parameters, derived
from radar data, include the extent of snow cover, the water
equivalent of the snow layer, and the state of the snow (wet
or dry).
The extent of snow cover has a direct influence on the
energy balance on the Earth’s surface, but also on the soil
water content. Fresh snow reflects between 80 and 90% of
incident solar radiation. The warming trend decreases the
snow cover, which in turn decreases the fraction of solar
energy reflected back to space, and increases the absorption
of incoming radiation, thereby increasing warming, which in
turn accelerates the snow melting. This amplifying mechanism is known as the ‘temperature-albedo’ effect. Thus, the
surface temperature is strongly dependent on the presence or
absence of snow. Another important aspect of snow cover is
the role it plays in thermal insulation. In winter, snow covered grounds cool much less quickly than bare grounds,
hence the importance of snow depth for plant and animal
life. Finally, melting snow in spring and summer requires a
high latent heat of fusion, so that snow cover represents a
significant heat loss for the atmosphere during the melting
season. As a result, seasonal snow produces thermal inertia
within the climate system, as it involves significant energy
exchanges, with little or no change in temperature.
In the northern hemisphere, snow cover varies seasonally
with a maximum in winter and a minimum in summer, but
with large inter-annual variations. Since the end of the
nineteenth century, daily records of snowfall and snow depth
have been kept by many countries. Nevertheless, these
measurements were only fully developed after 1950, and in
particular from 1966 onwards, with the arrival of satellites.
All of these data series reveal that snow cover has
decreased in spring and summer since the 1920s, with an
even more striking decrease since the end of the 1970s.
According to the Fifth IPCC Assessment report, this
decrease in March-April snow cover extent ranges
from −0.8% per decade over the 1922–2012 period
to −2.2% per decade between years 1979 and 2012. For the
fall and winter seasons, the signal is less clear: some data
sets suggest positive trends as a result of increased snowfall
in a warming climate, while others suggest negative trends
similar to what is observed in spring and summer. Nevertheless, there is a consensus that the mean annual snow
cover has decreased with a shift from February to January of
the maximum extent, an earlier onset of melting (*5.3 days
since winter 1972–1973) and thus a reduction of snow cover
duration. This drop in snow cover is mainly observed in the
northern hemisphere. In the southern hemisphere, very few
data exist outside of Antarctica, and these are often of much
lower quality than in the northern hemisphere.
Evolution of Sea Ice
Sea ice is frozen seawater. When freezing occurs, salt is
expelled from the ice crystals, thus raising the density of the
surface ocean waters. The formation of sea ice can therefore
have a direct impact on the intensity of the thermohaline
circulation. Sea ice is a highly reflective surface with a high
albedo of about 0.8. Conversely, when sea ice melts, the
ice-free ocean surface absorbs about 90% of the radiation
due to large albedo changes, causing the ocean to warm up,
followed by a further increase in surface temperature. This
phenomenon is a positive feedback between temperature and
albedo. Thus, sea ice regulates heat exchanges between the
atmosphere and the polar ocean. It isolates the relatively
‘warm’ ocean waters from the much colder atmosphere,
except when there are winter ‘leads’ occurring as a result of
sea-ice break-up. These leads allow the exchange of heat and
450
S. Charbit et al.