The Long-Term Future of the Polar Ice Sheets:
Impact and Irreversibility
As seen earlier, the response times of the different components of the climate system are extremely variable, ranging
from a few minutes to a few days for the atmosphere and
from a few months to several hundreds of years for the
ocean. While the ice sheets have long been considered as a
slow component of the Earth system (with characteristic
time scales ranging from thousands to hundreds of thousands
of years), recent observations provide evidences that they
can react to climate change far more quickly than previously
thought. The last glacial-interglacial cycle demonstrates how
deeply the climate is influenced by the slow development
and rapid collapse of the ice sheets.
The ice core records retrieved from the Vostok and
Dome C sites in Antarctica show that, for 800,000 years, the
world has alternated between four ice sheets (during glacial
periods) and only two ice sheets (during interglacial periods). In other words, Greenland and Antarctica withstood
the warming that led to the disappearance of the
Fennoscandian and North American ice sheets. There is no
doubt that Greenland and West Antarctica have not always
emerged unscathed from glacial-interglacial cycles. Indeed,
during the last interglacial period (i.e. 130–115 ka ago), the
sea level was 6 to 9 m higher (Kopp et al. 2009). It is
therefore possible that anthropogenic activity could lead to
the partial or total melting of Greenland (Charbit et al. 2008).
Recent observations of Greenland, and, more surprisingly, of Antarctica mass balances (Velicogna 2009) show
that these ice sheets have become one of the main contributors to the increase in sea level (Cazenave et al. 2009).
Their complete disappearance would lead to a sea-level rise
of nearly 60 m: 6.6 m for Greenland and 52.8 m for
Antarctica, of which 3.3 m would come from West
Antarctica. These figures should be compared to the 120 m
sea-level rise corresponding to the disappearance of the
North American and Fennoscandian ice sheets in response to
very small changes in insolation compared to the additional
radiative forcing from anthropogenic activities. Also, the
disappearance of past ice sheets was spread over
14,000 years (from the Last Glacial Maximum to the
Mid-Holocene). The long-term effects (several hundred
years) of anthropogenic forcing on ice sheets are not easy to
model, partly because of uncertainties in the future
socio-economic pathways over the twenty-first century and
therefore in the evolution of the greenhouse gas concentrations in the atmosphere. It is therefore even more difficult to
establish scenarios over several centuries (Charbit et al.
2008). However, several assumptions can be made about the
long-term evolution of greenhouse gas emissions to explore
the sensitivity of the present-day ice sheets with numerous
scenarios. Performing simulations over several centuries or
several millennia cannot be achieved through the use of
general circulation models, which are too expensive in terms
of computational time, and simplified climate models coupled with efficient ice-sheet models are required.
In fact, there are three different time frames relevant in the
study of the evolution of ice sheets. The evolution over the
twenty-first century, the basis of all IPCC analyzes, will very
likely bring about a rise in sea level of several tens of centimeters (or even more), although there is a great deal of
uncertainty linked to the emission scenarios, to climate
model biases and to difficulties of ice-sheet models to capture the processes causing rapid dynamical change. The
second horizon, beyond the twenty-first and the following
centuries, is one where the CO 2 level may stabilize at three
or four times that of the pre-industrial level, likely resulting
in a massive retreat of Greenland and West Antarctica. These
changes are also likely to modify the ocean circulation and
the global climate. If this high level of CO 2 persists for a
long time in the atmosphere, the ice-sheet melting could be
irreversible, in the sense that there would no longer be any
perennial snow in Greenland (Charbit et al. 2008). Finally,
there is the much more distant third horizon, which raises the
following question: for the last million years, our climate has
oscillated between ice ages (long periods of about
100,000 years) and interglacial periods (short periods of
about 10,000 years). Is it possible that the anthropogenic
perturbation might cause a switch to another climate mode
with strongly reduced ice sheets or even no ice sheet at all,
similar to the hot climate mode of the pre-Quaternary era? In
other words, is it possible that anthropogenic disturbance
could induce modifications such that the next marked
decline in summer insolation due in about 100,000 years
(Loutre and Berger 2000) might not bring about an ice age?
Even if this question seems ‘futuristic’, we have good
reasons to believe that the glacial-interglacial cycles of the
last million years will no longer occur because the expected
decline of insolation will not be large enough to compensate
for the radiative forcing due to the high atmospheric CO 2
levels. This is what is suggested by the results of simulations
carried out for periods such as the Pliocene around 3 Ma
where the CO 2 level is estimated to have been around
405 ± 50 ppm.
In these scenarios, a key factor is the long time required
to reach atmospheric CO 2 equilibrium which can be several
tens to hundreds of thousands years (Archer et al. 1997). The
anthropogenic disturbance is almost instantaneous, but its
consequences will last for a very long time, and the
31 From the Climates of the Past to the Climates …
475
Impact and Irreversibility
As seen earlier, the response times of the different components of the climate system are extremely variable, ranging
from a few minutes to a few days for the atmosphere and
from a few months to several hundreds of years for the
ocean. While the ice sheets have long been considered as a
slow component of the Earth system (with characteristic
time scales ranging from thousands to hundreds of thousands
of years), recent observations provide evidences that they
can react to climate change far more quickly than previously
thought. The last glacial-interglacial cycle demonstrates how
deeply the climate is influenced by the slow development
and rapid collapse of the ice sheets.
The ice core records retrieved from the Vostok and
Dome C sites in Antarctica show that, for 800,000 years, the
world has alternated between four ice sheets (during glacial
periods) and only two ice sheets (during interglacial periods). In other words, Greenland and Antarctica withstood
the warming that led to the disappearance of the
Fennoscandian and North American ice sheets. There is no
doubt that Greenland and West Antarctica have not always
emerged unscathed from glacial-interglacial cycles. Indeed,
during the last interglacial period (i.e. 130–115 ka ago), the
sea level was 6 to 9 m higher (Kopp et al. 2009). It is
therefore possible that anthropogenic activity could lead to
the partial or total melting of Greenland (Charbit et al. 2008).
Recent observations of Greenland, and, more surprisingly, of Antarctica mass balances (Velicogna 2009) show
that these ice sheets have become one of the main contributors to the increase in sea level (Cazenave et al. 2009).
Their complete disappearance would lead to a sea-level rise
of nearly 60 m: 6.6 m for Greenland and 52.8 m for
Antarctica, of which 3.3 m would come from West
Antarctica. These figures should be compared to the 120 m
sea-level rise corresponding to the disappearance of the
North American and Fennoscandian ice sheets in response to
very small changes in insolation compared to the additional
radiative forcing from anthropogenic activities. Also, the
disappearance of past ice sheets was spread over
14,000 years (from the Last Glacial Maximum to the
Mid-Holocene). The long-term effects (several hundred
years) of anthropogenic forcing on ice sheets are not easy to
model, partly because of uncertainties in the future
socio-economic pathways over the twenty-first century and
therefore in the evolution of the greenhouse gas concentrations in the atmosphere. It is therefore even more difficult to
establish scenarios over several centuries (Charbit et al.
2008). However, several assumptions can be made about the
long-term evolution of greenhouse gas emissions to explore
the sensitivity of the present-day ice sheets with numerous
scenarios. Performing simulations over several centuries or
several millennia cannot be achieved through the use of
general circulation models, which are too expensive in terms
of computational time, and simplified climate models coupled with efficient ice-sheet models are required.
In fact, there are three different time frames relevant in the
study of the evolution of ice sheets. The evolution over the
twenty-first century, the basis of all IPCC analyzes, will very
likely bring about a rise in sea level of several tens of centimeters (or even more), although there is a great deal of
uncertainty linked to the emission scenarios, to climate
model biases and to difficulties of ice-sheet models to capture the processes causing rapid dynamical change. The
second horizon, beyond the twenty-first and the following
centuries, is one where the CO 2 level may stabilize at three
or four times that of the pre-industrial level, likely resulting
in a massive retreat of Greenland and West Antarctica. These
changes are also likely to modify the ocean circulation and
the global climate. If this high level of CO 2 persists for a
long time in the atmosphere, the ice-sheet melting could be
irreversible, in the sense that there would no longer be any
perennial snow in Greenland (Charbit et al. 2008). Finally,
there is the much more distant third horizon, which raises the
following question: for the last million years, our climate has
oscillated between ice ages (long periods of about
100,000 years) and interglacial periods (short periods of
about 10,000 years). Is it possible that the anthropogenic
perturbation might cause a switch to another climate mode
with strongly reduced ice sheets or even no ice sheet at all,
similar to the hot climate mode of the pre-Quaternary era? In
other words, is it possible that anthropogenic disturbance
could induce modifications such that the next marked
decline in summer insolation due in about 100,000 years
(Loutre and Berger 2000) might not bring about an ice age?
Even if this question seems ‘futuristic’, we have good
reasons to believe that the glacial-interglacial cycles of the
last million years will no longer occur because the expected
decline of insolation will not be large enough to compensate
for the radiative forcing due to the high atmospheric CO 2
levels. This is what is suggested by the results of simulations
carried out for periods such as the Pliocene around 3 Ma
where the CO 2 level is estimated to have been around
405 ± 50 ppm.
In these scenarios, a key factor is the long time required
to reach atmospheric CO 2 equilibrium which can be several
tens to hundreds of thousands years (Archer et al. 1997). The
anthropogenic disturbance is almost instantaneous, but its
consequences will last for a very long time, and the
31 From the Climates of the Past to the Climates …
475