However, the role of the cryosphere is not limited to its
impact on the surface albedo. On the oceans, sea ice cuts off
the ocean from the atmosphere, and blocks the air-sea
exchanges of water, salt and other chemicals. When it is
forming, sea ice eliminates salt which makes the surrounding sea water denser causing it to sink to depths, thus
feeding the ocean thermohaline circulation. Conversely,
when it melts in summer, the salinity of surface water
decreases abruptly. On land, glaciers are not static. They
flow under their own weight at a speed of up to several
hundred meters per year. When they reach the coast, the ice
breaks into icebergs that are propelled by winds and currents, and melt when they arrive in warmer water, causing a
drop in salinity of surface waters of the ocean. Such changes
are likely to bring about important feedbacks, such as
stopping deep-water formation and the thermohaline circulation (see Chap. 29, Volume 2).
A climate change favoring the flow of glaciers could
promote the destabilization of ice sheets, such as the one
covering West Antarctica. Indeed, it rests on a base that
would normally be covered by the sea, and the ice sheet is in
contact with the ocean on all sides. Such a mass of ice is
unstable. Because of this instability, gigantic floating ice
platforms, several hundred meters thick, surround the entire
part of the ice sheet in contact with the ocean.
The sheets covering the Ross and Ronne Seas, each with
a surface area similar to that of France, calve huge icebergs,
tens of kilometers in length, into the Weddell Sea. Without
any rocky terrain, they are in direct contact with the ice cap
and their speed often reaches several meters per day. What
helps to safeguard the West Antarctic ice sheet is the presence of several islands around it. These anchor points slow
down the ice flow. Ice therefore progresses slowly until that
last barrier which, once crossed, leaves the field open to the
calving of icebergs resulting from the fragmentation of the
ice platform. The future of the West Antarctic ice cap in
response to ongoing climate change is a real concern for the
centuries to come.
The Cryosphere in the Past and Paleoclimates
The variability in the cryosphere is considerable, no matter
what time scale is considered. Over the span of a season,
snow cover and sea ice show the biggest variations. Measurements taken since the mid-twentieth century show that
the area of sea ice each year goes from 15 million km
2 in
early spring to just 6 million km
2 at the end of the summer.
However, since 1980, a clear decrease in this range has been
observed and it has reduced to nearly 3 million km
2 in 2012
summer. A similar trend in the retreat of mountain glaciers is
observed during the twentieth century, whether in the Alps,
or in the mountains of Africa or South America.
On a geological scale, the variability of the cryosphere is
even greater. It is seen in the geological traces left by glaciers recording their passage and their broadest expansion
(moraines, boulders streaked by friction marks as they were
transported by the glacier over the surface bedrock on which
it rested, vast continental shields like Canada eroded away).
Geological observations have thus led us to believe that
some 750 million years ago, all continents were covered
with glaciers, and that the oceans were probably covered
with perennial sea ice at the same time.
About 450 million years ago, during the Ordovician, a
gigantic ice cap covered the Sahara where even today one
can see striated rocks, glacial valleys, remains of moraines
and channels that collected the water from the melting ice in
summer and brought it to North Africa. Flying over western
Mauritania, one can recognize the sandy bed of a great river
that came into being beside an ice cap and traced out many
meanders before flowing further north, to the seas bordering
the glaciated African continent. These rivers were covered
by icebergs that melted slowly and released stones they
carried. They can be found today in exposed terrains, in
Morocco, Galicia and even in the Armorican massif, south
of Caen.
About two hundred million years ago, a long, globally
warm era commenced (Jurassic and Cretaceous), during
which time glaciers appear to have been rare, if they had not
completely disappeared. The wide variety of flora and fauna
reflects a variety of environmental conditions, from temperate in Japan, Siberia and Australia to very hot in America,
Africa and Eurasia. There are no known tracks of large
glaciers, even on the Antarctic continent, although, of
course, as the continent is now covered with a thick ice cap it
is only accessible to geological observations at its periphery.
The glaciations of the Quaternary are the culmination of a
long process of cooling of the Earth that began more than
thirty million years ago, firstly with the development of an
ice sheet on Antarctica, and then on Greenland. For the last
million years, glacial-interglacial oscillations have dominated the climate of our planet. While today it is the
Southern hemisphere which is the most glaciated with
Antarctica containing about 28 million km
3 of ice for only
1 million km
3 of ice in Greenland, it was the Northern
hemisphere that was the most glaciated at the height of the
last glaciation 20,000 years ago, with 50 million km
3 of ice
over Canada (Laurentide Ice Sheet, 4 km thick) and northern
Europe.
Throughout the last ice age, the Laurentide Ice Sheet
extended beyond the American continent to reach the
Atlantic continental shelf and the Labrador Sea. This cap
could become unstable, suddenly releasing huge numbers of
icebergs which invaded the entire North Atlantic where they
melted. The sudden appearance of these armadas of icebergs,
designated as ‘Heinrich events’ caused a significant decrease
1 The Climate System: Its Functioning and History
19
impact on the surface albedo. On the oceans, sea ice cuts off
the ocean from the atmosphere, and blocks the air-sea
exchanges of water, salt and other chemicals. When it is
forming, sea ice eliminates salt which makes the surrounding sea water denser causing it to sink to depths, thus
feeding the ocean thermohaline circulation. Conversely,
when it melts in summer, the salinity of surface water
decreases abruptly. On land, glaciers are not static. They
flow under their own weight at a speed of up to several
hundred meters per year. When they reach the coast, the ice
breaks into icebergs that are propelled by winds and currents, and melt when they arrive in warmer water, causing a
drop in salinity of surface waters of the ocean. Such changes
are likely to bring about important feedbacks, such as
stopping deep-water formation and the thermohaline circulation (see Chap. 29, Volume 2).
A climate change favoring the flow of glaciers could
promote the destabilization of ice sheets, such as the one
covering West Antarctica. Indeed, it rests on a base that
would normally be covered by the sea, and the ice sheet is in
contact with the ocean on all sides. Such a mass of ice is
unstable. Because of this instability, gigantic floating ice
platforms, several hundred meters thick, surround the entire
part of the ice sheet in contact with the ocean.
The sheets covering the Ross and Ronne Seas, each with
a surface area similar to that of France, calve huge icebergs,
tens of kilometers in length, into the Weddell Sea. Without
any rocky terrain, they are in direct contact with the ice cap
and their speed often reaches several meters per day. What
helps to safeguard the West Antarctic ice sheet is the presence of several islands around it. These anchor points slow
down the ice flow. Ice therefore progresses slowly until that
last barrier which, once crossed, leaves the field open to the
calving of icebergs resulting from the fragmentation of the
ice platform. The future of the West Antarctic ice cap in
response to ongoing climate change is a real concern for the
centuries to come.
The Cryosphere in the Past and Paleoclimates
The variability in the cryosphere is considerable, no matter
what time scale is considered. Over the span of a season,
snow cover and sea ice show the biggest variations. Measurements taken since the mid-twentieth century show that
the area of sea ice each year goes from 15 million km
2 in
early spring to just 6 million km
2 at the end of the summer.
However, since 1980, a clear decrease in this range has been
observed and it has reduced to nearly 3 million km
2 in 2012
summer. A similar trend in the retreat of mountain glaciers is
observed during the twentieth century, whether in the Alps,
or in the mountains of Africa or South America.
On a geological scale, the variability of the cryosphere is
even greater. It is seen in the geological traces left by glaciers recording their passage and their broadest expansion
(moraines, boulders streaked by friction marks as they were
transported by the glacier over the surface bedrock on which
it rested, vast continental shields like Canada eroded away).
Geological observations have thus led us to believe that
some 750 million years ago, all continents were covered
with glaciers, and that the oceans were probably covered
with perennial sea ice at the same time.
About 450 million years ago, during the Ordovician, a
gigantic ice cap covered the Sahara where even today one
can see striated rocks, glacial valleys, remains of moraines
and channels that collected the water from the melting ice in
summer and brought it to North Africa. Flying over western
Mauritania, one can recognize the sandy bed of a great river
that came into being beside an ice cap and traced out many
meanders before flowing further north, to the seas bordering
the glaciated African continent. These rivers were covered
by icebergs that melted slowly and released stones they
carried. They can be found today in exposed terrains, in
Morocco, Galicia and even in the Armorican massif, south
of Caen.
About two hundred million years ago, a long, globally
warm era commenced (Jurassic and Cretaceous), during
which time glaciers appear to have been rare, if they had not
completely disappeared. The wide variety of flora and fauna
reflects a variety of environmental conditions, from temperate in Japan, Siberia and Australia to very hot in America,
Africa and Eurasia. There are no known tracks of large
glaciers, even on the Antarctic continent, although, of
course, as the continent is now covered with a thick ice cap it
is only accessible to geological observations at its periphery.
The glaciations of the Quaternary are the culmination of a
long process of cooling of the Earth that began more than
thirty million years ago, firstly with the development of an
ice sheet on Antarctica, and then on Greenland. For the last
million years, glacial-interglacial oscillations have dominated the climate of our planet. While today it is the
Southern hemisphere which is the most glaciated with
Antarctica containing about 28 million km
3 of ice for only
1 million km
3 of ice in Greenland, it was the Northern
hemisphere that was the most glaciated at the height of the
last glaciation 20,000 years ago, with 50 million km
3 of ice
over Canada (Laurentide Ice Sheet, 4 km thick) and northern
Europe.
Throughout the last ice age, the Laurentide Ice Sheet
extended beyond the American continent to reach the
Atlantic continental shelf and the Labrador Sea. This cap
could become unstable, suddenly releasing huge numbers of
icebergs which invaded the entire North Atlantic where they
melted. The sudden appearance of these armadas of icebergs,
designated as ‘Heinrich events’ caused a significant decrease
1 The Climate System: Its Functioning and History
19