satellite tracking. In the framework of the International Polar
Year (2007–2009), a large permafrost- monitoring network
was developed and ground temperatures (which control the
thermal state of the permafrost) have been measured in 575
sites located in arctic regions (North America, Nordic Eurasian regions and Russia). In most sites belonging to the
network, permafrost temperatures have increased in recent
decades. The observed rate of change of mean annual
ground temperatures from mid-1970s to 2010 ranges from
0.15 ± 0.03 to 0.82 ± 0.07 °C per decade, depending on
the site (Fig. 31.5). In cold permafrost regions (mostly in
continuous permafrost zones), the mean annual ground
temperatures have increased by up to 2 °C, compared to less
than 1 °C in warm, forested permafrost areas (discontinuous
permafrost). This can be explained by the fact that the
amount of ice in warm permafrost is usually larger than for
cold permafrost. Indeed, in case of ice melting, the overall
warming trend is counteracted by the latent heat effect.
Moreover, in forested areas, the snow insulating effect is
limited due to the interception of snow by vegetation.
One of the main consequences of permafrost warming is
increased thickness of the active layer, although some permafrost areas exhibit only modest thickening or even a
thinning. Indeed, a study based on the analysis of 169 circumpolar and mid-latitude sites revealed that only 43.2% of
them have experienced an increase of the active layer
thickness since the 1990s (Luo et al. 2016). However, there
is great spatio-temporal variability from one site to the other
ranging from a few tenths of cm/yr to more than 10 cm/yr.
Thickening of the active layer is a matter of great concern
since it may have large consequences on the stability of the
surface due to the melting of shallow ground ice. Potential
impacts include thaw settlement, soil creeps, slope failures
and ponding of surface water. All these features can cause
severe damages to infrastructures, such as roads, dams or
structural building foundations but also to vegetation. In
forested areas, thaw modifies the hydrological conditions
and can lead, for example, to the destruction of tree roots,
causing drastic changes in the ecosystems. When permafrost
thaws and the active layer thickens, more organic matter is
likely to be decomposed by bacteria that produce either
methane or carbon. In both cases, the bacterial action
enhances greenhouse gas emissions and thus promotes global warming. However, the magnitude of this thaw-related
feedback is a great unknown. The total amount of carbon
stored in the permafrost has been estimated at 1672 Gt, of
which 277 Gt is found in peat bogs. This is about twice the
amount of carbon in the atmosphere.
We still do not know for sure if the increase in atmospheric methane concentration observed in recent years, after
about ten years of relative stability, is due to the warming of
the high northern latitudes. Another amplification reaction
observed at high northern latitudes involves the microbial
transformation of nitrogen trapped in soils into nitrous oxide,
which could increase with increasing temperatures and thus,
in turn, amplify global warming.
In any case, even if there are still many uncertainties, the
Fifth IPCC assessment report emphasized on the positive
feedback of permafrost melting on climate warming: “the
release of CO 2 or CH 4 to the atmosphere from thawing
permafrost carbon stocks over the twenty-first century is
assessed to be in the range of 50 to 250 GtC for RCP8.5”
(IPCC 2013). The RCP scenarios are defined in the section
“Projecting the Future of the Climate System” of this chapter.
Glaciers
A glacier is a mass of ice formed by the successive accumulation of layers of snow year after year. Over the years,
under the pressure of its own weight, the snow hardens and
becomes granular (firn), then turns into ice and expels the air
it contained. Under the action of gravity, the ice flows along
the slope, thus supplying the lower parts of the glacier.
A glacier is in constant movement and carries mass from
high altitudes to lower altitudes. In winter, the glacier grows
due to snow accumulation on its surface. During the following summer, the glacier loses all or part of the mass it
gained during winter. The disappearance of ice through
surface processes is called ablation. The difference between
accumulation and ablation determines the surface mass
balance of the glacier. This brings about a change in ice
volume. A positive balance overall causes the glacier to
grow while a negative balance leads to a loss of ice volume
which can be accompanied by a retreat of the glacial front.
The mass balance of temperate glaciers in the mid-latitudes
is mainly dependent on winter precipitation, summer temperature and summer snowfalls (temporally reducing the melt
due to the increased albedo). In contrast, the glaciers in low
latitudes, where ablation occurs throughout the year and
multiple accumulation seasons exist, are strongly influenced
by variations in the atmospheric moisture content which
affects incoming solar radiation, atmospheric long-wave
emission, albedo, precipitation and sublimation. In monsoon areas, such as the Himalayas, accumulation and ablation
occur mainly in summer. Glaciers at high altitudes and in
polar regions can experience accumulation in any season.
The retreat of mountain glaciers is one of the most visible
examples of climate change. A compilation of observations
made by the World Glacier Monitoring Service on 19
regions around the world, gathering more than 40,000
observations, shows that the retreat is an almost global
phenomenon, despite some intermittent readvances related to
dynamical instabilities (e.g. Island, Svalbard) or to specific
climatic conditions (i.e. increased winter snowfall) observed
on a few individual glaciers in Scandinavia or New Zealand
in the 1990s. However, the periods of glacier front advances
are short compared to the overall ice retreat. By compiling
31 From the Climates of the Past to the Climates …
453
Year (2007–2009), a large permafrost- monitoring network
was developed and ground temperatures (which control the
thermal state of the permafrost) have been measured in 575
sites located in arctic regions (North America, Nordic Eurasian regions and Russia). In most sites belonging to the
network, permafrost temperatures have increased in recent
decades. The observed rate of change of mean annual
ground temperatures from mid-1970s to 2010 ranges from
0.15 ± 0.03 to 0.82 ± 0.07 °C per decade, depending on
the site (Fig. 31.5). In cold permafrost regions (mostly in
continuous permafrost zones), the mean annual ground
temperatures have increased by up to 2 °C, compared to less
than 1 °C in warm, forested permafrost areas (discontinuous
permafrost). This can be explained by the fact that the
amount of ice in warm permafrost is usually larger than for
cold permafrost. Indeed, in case of ice melting, the overall
warming trend is counteracted by the latent heat effect.
Moreover, in forested areas, the snow insulating effect is
limited due to the interception of snow by vegetation.
One of the main consequences of permafrost warming is
increased thickness of the active layer, although some permafrost areas exhibit only modest thickening or even a
thinning. Indeed, a study based on the analysis of 169 circumpolar and mid-latitude sites revealed that only 43.2% of
them have experienced an increase of the active layer
thickness since the 1990s (Luo et al. 2016). However, there
is great spatio-temporal variability from one site to the other
ranging from a few tenths of cm/yr to more than 10 cm/yr.
Thickening of the active layer is a matter of great concern
since it may have large consequences on the stability of the
surface due to the melting of shallow ground ice. Potential
impacts include thaw settlement, soil creeps, slope failures
and ponding of surface water. All these features can cause
severe damages to infrastructures, such as roads, dams or
structural building foundations but also to vegetation. In
forested areas, thaw modifies the hydrological conditions
and can lead, for example, to the destruction of tree roots,
causing drastic changes in the ecosystems. When permafrost
thaws and the active layer thickens, more organic matter is
likely to be decomposed by bacteria that produce either
methane or carbon. In both cases, the bacterial action
enhances greenhouse gas emissions and thus promotes global warming. However, the magnitude of this thaw-related
feedback is a great unknown. The total amount of carbon
stored in the permafrost has been estimated at 1672 Gt, of
which 277 Gt is found in peat bogs. This is about twice the
amount of carbon in the atmosphere.
We still do not know for sure if the increase in atmospheric methane concentration observed in recent years, after
about ten years of relative stability, is due to the warming of
the high northern latitudes. Another amplification reaction
observed at high northern latitudes involves the microbial
transformation of nitrogen trapped in soils into nitrous oxide,
which could increase with increasing temperatures and thus,
in turn, amplify global warming.
In any case, even if there are still many uncertainties, the
Fifth IPCC assessment report emphasized on the positive
feedback of permafrost melting on climate warming: “the
release of CO 2 or CH 4 to the atmosphere from thawing
permafrost carbon stocks over the twenty-first century is
assessed to be in the range of 50 to 250 GtC for RCP8.5”
(IPCC 2013). The RCP scenarios are defined in the section
“Projecting the Future of the Climate System” of this chapter.
Glaciers
A glacier is a mass of ice formed by the successive accumulation of layers of snow year after year. Over the years,
under the pressure of its own weight, the snow hardens and
becomes granular (firn), then turns into ice and expels the air
it contained. Under the action of gravity, the ice flows along
the slope, thus supplying the lower parts of the glacier.
A glacier is in constant movement and carries mass from
high altitudes to lower altitudes. In winter, the glacier grows
due to snow accumulation on its surface. During the following summer, the glacier loses all or part of the mass it
gained during winter. The disappearance of ice through
surface processes is called ablation. The difference between
accumulation and ablation determines the surface mass
balance of the glacier. This brings about a change in ice
volume. A positive balance overall causes the glacier to
grow while a negative balance leads to a loss of ice volume
which can be accompanied by a retreat of the glacial front.
The mass balance of temperate glaciers in the mid-latitudes
is mainly dependent on winter precipitation, summer temperature and summer snowfalls (temporally reducing the melt
due to the increased albedo). In contrast, the glaciers in low
latitudes, where ablation occurs throughout the year and
multiple accumulation seasons exist, are strongly influenced
by variations in the atmospheric moisture content which
affects incoming solar radiation, atmospheric long-wave
emission, albedo, precipitation and sublimation. In monsoon areas, such as the Himalayas, accumulation and ablation
occur mainly in summer. Glaciers at high altitudes and in
polar regions can experience accumulation in any season.
The retreat of mountain glaciers is one of the most visible
examples of climate change. A compilation of observations
made by the World Glacier Monitoring Service on 19
regions around the world, gathering more than 40,000
observations, shows that the retreat is an almost global
phenomenon, despite some intermittent readvances related to
dynamical instabilities (e.g. Island, Svalbard) or to specific
climatic conditions (i.e. increased winter snowfall) observed
on a few individual glaciers in Scandinavia or New Zealand
in the 1990s. However, the periods of glacier front advances
are short compared to the overall ice retreat. By compiling
31 From the Climates of the Past to the Climates …
453