responsible for ice flow acceleration is hydro-fracturing
(DeConto and Pollard 2016). This mechanism is related to
water coming from ice melting at the surface of the ice shelf
that may percolate inside the ice-shelves. Crevasses can form
and widen when the water pressure is high enough, thereby
favoring iceberg calving. Ultimately, this process may lead
to ice-shelf collapse. It may also favor the marine ice sheet
instability. Indeed, once the ice shelves have collapsed, ice
cliffs become unstable and fall down if their height is greater
than * 90 m. However, this process remains poorly constrained and is still a matter of debate.
A special attention is given to the evolution of ice sheets.
Indeed, ice sheets strongly interact with the other components
of the climate system. These interactions can lead to a highly
non-linear climate response. This means that the effects of the
radiative disturbance (caused by variations in insolation or in
the amount of greenhouse gas in the atmosphere) can be
amplified or mitigated by these feedback processes, as
illustrated in Chap. 28 for abrupt events. Many studies on these
interactions have been published over the past two decades.
They would deserve a full chapter. Here, we only give some
quick examples for time scales ranging from a few decades to
a few centuries. Ice sheet melting is first of all accompanied
by possible changes in albedo and therefore in the surface
energy balance. This effect is almost instantaneous. In turn, a
change in the energy balance can lead to changes in the mass
balance of the ice sheets. Another consequence of the melting
and/or mechanical destabilization of the ice sheets, widely
discussed in the literature, concerns the freshwater flux
released in the ocean. Locally, this release leads to a decrease
in ocean surface temperatures, a change in sea ice cover and a
reduction of ocean density in the vicinity of the ice sheets.
Density changes also cause a disruption of large-scale ocean
circulation by altering deep-water convection. For example,
meltwater from Greenland has the potential to weaken the
Atlantic Meridional Overturning Circulation. These changes
can have effects in regions far from the polar zones. For
example, the recent study by Defrance (2017) showed that for
a substantial melting of Greenland under a climate change
driven by the RCP8.5 scenario, Greenland meltwater could
cause a massive alteration of the monsoon regime with a
drastic decrease in West African rainfall, and subsequently
with a significant reduction in cultivable areas. Therefore, in
addition to the direct and obvious consequences on sea level,
the future of polar ice sheets is of primary importance for the
future of human societies, all the more so if they are already
economically fragile.
Sea Level Changes
Sea level variations are the result of changes in both the
volume of the oceans and ocean basins, as well as changes in
the mass of water contained in the oceans. Depending on the
time scale being considered, these types of variations have
different origins. Over geological time scales, changes in the
shape of ocean basins and in the continent/ocean distribution
are the main factors affecting sea level. Over
glacial-interglacial cycles, sea-level variations are mainly
related to changes in continental ice volume and to the isostatic adjustment due to the vertical movements of the
Earth’s crust in response to changes in the mass of land ice.
Following the last deglaciation initiated about 21,000 years
ago, sea level rose by * 120 m and then stabilized around
6000 years ago. Geological data indicate that sea level has
not changed by more than 30 cm from that time until the end
of the nineteenth century.
On time scales ranging from a few years to a few decades,
variations in the mean sea level are the result of two factors,
mainly related to climate change:
(a) Variation of the ocean volume caused by changes in sea
temperature. As temperature increases, the volume of
water expands. This process is called thermal
expansion;
(b) Variations in the bodies of oceanic water resulting
mainly from exchanges with continental reservoirs,
such as rivers, lakes and inland seas, snowpack, ground
water, but also mountain glaciers and polar ice sheets.
Other factors such as ocean circulation or atmospheric
pressure can bring about local variations, without altering the
mean global sea level. In addition, some human interventions have the effect of modifying regional hydrology by
modifying the runoff of freshwater released to the ocean, and
thus the sea level. This occurs in the case of dams, irrigation,
urbanization, water extraction from aquifers and deforestation. Some of these processes have the effect of increasing
runoff (urbanization, deforestation); others, such as dams and
irrigation, contribute to the sequestration of freshwater on the
continents. Current estimates of the net land water storage
are based on observations and models, and vary between
−0.33 and 0.23 mm/yr over the period 2002–2014/15
(WCRP Global Sea Level Budget Group 2018).
Measurements of current sea-level variations are based on
two different techniques: tide gauges, which began to be
installed in the nineteenth century, and altimetry data from
satellite observations since 1992. The two main limitations
of tide gauges are their inhomogeneous spatial and temporal
coverage, and the inclusion of vertical land motion in their
record, which needs to be corrected for to obtain relative
sea-level changes. To limit the uncertainties related to these
movements, which are difficult to quantify in current models,
only a few tens of geologically stable sites, mainly located
along the coasts of North America and Europe, are taken into
account to inform us of the sea level evolution in the course
of the twentieth century. Based on a compilation of the most
recent estimates, the sea-level rise as indicated by tide gauge
31 From the Climates of the Past to the Climates …
457
(DeConto and Pollard 2016). This mechanism is related to
water coming from ice melting at the surface of the ice shelf
that may percolate inside the ice-shelves. Crevasses can form
and widen when the water pressure is high enough, thereby
favoring iceberg calving. Ultimately, this process may lead
to ice-shelf collapse. It may also favor the marine ice sheet
instability. Indeed, once the ice shelves have collapsed, ice
cliffs become unstable and fall down if their height is greater
than * 90 m. However, this process remains poorly constrained and is still a matter of debate.
A special attention is given to the evolution of ice sheets.
Indeed, ice sheets strongly interact with the other components
of the climate system. These interactions can lead to a highly
non-linear climate response. This means that the effects of the
radiative disturbance (caused by variations in insolation or in
the amount of greenhouse gas in the atmosphere) can be
amplified or mitigated by these feedback processes, as
illustrated in Chap. 28 for abrupt events. Many studies on these
interactions have been published over the past two decades.
They would deserve a full chapter. Here, we only give some
quick examples for time scales ranging from a few decades to
a few centuries. Ice sheet melting is first of all accompanied
by possible changes in albedo and therefore in the surface
energy balance. This effect is almost instantaneous. In turn, a
change in the energy balance can lead to changes in the mass
balance of the ice sheets. Another consequence of the melting
and/or mechanical destabilization of the ice sheets, widely
discussed in the literature, concerns the freshwater flux
released in the ocean. Locally, this release leads to a decrease
in ocean surface temperatures, a change in sea ice cover and a
reduction of ocean density in the vicinity of the ice sheets.
Density changes also cause a disruption of large-scale ocean
circulation by altering deep-water convection. For example,
meltwater from Greenland has the potential to weaken the
Atlantic Meridional Overturning Circulation. These changes
can have effects in regions far from the polar zones. For
example, the recent study by Defrance (2017) showed that for
a substantial melting of Greenland under a climate change
driven by the RCP8.5 scenario, Greenland meltwater could
cause a massive alteration of the monsoon regime with a
drastic decrease in West African rainfall, and subsequently
with a significant reduction in cultivable areas. Therefore, in
addition to the direct and obvious consequences on sea level,
the future of polar ice sheets is of primary importance for the
future of human societies, all the more so if they are already
economically fragile.
Sea Level Changes
Sea level variations are the result of changes in both the
volume of the oceans and ocean basins, as well as changes in
the mass of water contained in the oceans. Depending on the
time scale being considered, these types of variations have
different origins. Over geological time scales, changes in the
shape of ocean basins and in the continent/ocean distribution
are the main factors affecting sea level. Over
glacial-interglacial cycles, sea-level variations are mainly
related to changes in continental ice volume and to the isostatic adjustment due to the vertical movements of the
Earth’s crust in response to changes in the mass of land ice.
Following the last deglaciation initiated about 21,000 years
ago, sea level rose by * 120 m and then stabilized around
6000 years ago. Geological data indicate that sea level has
not changed by more than 30 cm from that time until the end
of the nineteenth century.
On time scales ranging from a few years to a few decades,
variations in the mean sea level are the result of two factors,
mainly related to climate change:
(a) Variation of the ocean volume caused by changes in sea
temperature. As temperature increases, the volume of
water expands. This process is called thermal
expansion;
(b) Variations in the bodies of oceanic water resulting
mainly from exchanges with continental reservoirs,
such as rivers, lakes and inland seas, snowpack, ground
water, but also mountain glaciers and polar ice sheets.
Other factors such as ocean circulation or atmospheric
pressure can bring about local variations, without altering the
mean global sea level. In addition, some human interventions have the effect of modifying regional hydrology by
modifying the runoff of freshwater released to the ocean, and
thus the sea level. This occurs in the case of dams, irrigation,
urbanization, water extraction from aquifers and deforestation. Some of these processes have the effect of increasing
runoff (urbanization, deforestation); others, such as dams and
irrigation, contribute to the sequestration of freshwater on the
continents. Current estimates of the net land water storage
are based on observations and models, and vary between
−0.33 and 0.23 mm/yr over the period 2002–2014/15
(WCRP Global Sea Level Budget Group 2018).
Measurements of current sea-level variations are based on
two different techniques: tide gauges, which began to be
installed in the nineteenth century, and altimetry data from
satellite observations since 1992. The two main limitations
of tide gauges are their inhomogeneous spatial and temporal
coverage, and the inclusion of vertical land motion in their
record, which needs to be corrected for to obtain relative
sea-level changes. To limit the uncertainties related to these
movements, which are difficult to quantify in current models,
only a few tens of geologically stable sites, mainly located
along the coasts of North America and Europe, are taken into
account to inform us of the sea level evolution in the course
of the twentieth century. Based on a compilation of the most
recent estimates, the sea-level rise as indicated by tide gauge
31 From the Climates of the Past to the Climates …
457