The cryosphere – composed of Earth’s ice,
snow and frozen ground – is a critical
component of the global system that is
undergoing rapid change with direct impacts
to society. Consequently, the Global Climate
Observing System Implementation Plan defines
several Essential Climate Variables focused
on or related to the cryosphere, as follows:
− Sea level:
globally, sea level is rising due to the
combined effects of the thermal expansion of
warming ocean water and loss of ice from
Greenland and Antarctica, as well as from
other smaller ice caps and glaciers
throughout the world. Studying the
contribution from land ice is critical
because these systems demonstrate the
potential for dynamic, rapid change that
could contribute enough water to the ocean
to reshape coastlines and displace millions
of people.
− Ice sheets:
combined, the Greenland and Antarctic ice
sheets contain approximately 33 million
km3 of ice, enough to raise sea
level by nearly 64 m. In general, snow and
ice accumulate in the interior of the
continental ice sheets, and mass is lost to
the ocean through surface melting,
subsurface melting and iceberg calving. The
processes are dynamic and complex. Surface
melting has been continuously moving to
higher elevations in Greenland. The ice
itself flows from the interior towards the
coast at rates of up to 20 m per day, and
may extend out into the ocean, forming
floating ice shelves, particularly in
Antarctica. These shelves are subject to
melting from beneath by warm ocean waters,
and their breakup has triggered surging of
their feeder glaciers.
− Glaciers and ice caps:
outside the major ice sheets, glaciers and
ice caps represent the vast majority of the
remaining ice on Earth. These smaller ice
masses have a relatively short response time
to climate perturbations, making them
early-warning indicators of global climate
change. Furthermore, although they represent
only 0.4% of Earth’s ice by area, glaciers
and ice caps have made the largest recent
(1993-2003) contribution to global sea-level
rise, after thermal expansion. Finally, ice
loss from such glaciers and ice caps has
serious impacts on the terrestrial water
cycle and on societies dependent on glacial
melt water.
− Albedo:
a certain percentage of the solar radiation
reaching Earth’s atmosphere and surface is
reflected back to space. This percentage
depends on the albedo, or reflectivity, of the
surface. Ice and snow have a very high albedo,
reflecting about 80% of incident sunlight. As
snow or ice cover decreases, however, less of
this incident radiation is reflected – as
albedo is reduced – and more is absorbed by
the atmosphere and the surface.
− Sea ice:
decreases in the extent and thickness of the
Arctic sea ice, particularly of the perennial
ice, have a significant impact on the exchange
of energy between the atmosphere and the
ocean. Open water areas exposed by melted sea
ice are subject to wind-driven circulation and
absorb more incident solar energy, increasing
ocean temperatures, which increases melting.
At the same time, thin ice – or its absence
entirely – allows more heat to escape from the
ocean to the colder atmosphere, raising
surface air temperatures and potentially
driving thawing of frozen ground on
surrounding lands. Furthermore, as sea ice
melts, fresh water is introduced to the
surrounding ocean, decreasing the salinity and
density, and increasing the buoyancy of the
surface water. All of these processes can
disturb the thermohaline ocean circulation
pattern and thereby alter global weather
patterns.
− Snow cover:
as with sea ice, snow cover plays an important
role in controlling the albedo of Earth’s
surface, with the seasonal component
dominating the annual and interannual
variations. On a local scale, snow cover is
critical to regional hydrology. Alterations in
the mass of snow and timing of melting greatly
affect water resources for consumption,
irrigation and even power generation.
− Permafrost:
ground that remains frozen throughout the year
is defined as permafrost. Like snow cover,
permafrost occurs principally in the northern
hemisphere, although it exists in mountainous
regions throughout the world. Its depth and
extent are sensitive indicators of temperature
change, but, more importantly, permafrost
thawing may lead to the release of large
volumes of greenhouse gases. Thawing
permafrost also alters ecosystems and causes
the collapse of infrastructure.
Role of Satellites
The cryosphere is found in the most
inaccessible areas of Earth: the Arctic, the
Antarctic and the high mountains of the
temperate zones. For four decades, satellites
have been indispensible tools in monitoring
them. Sea ice is primarily observed by passive
microwave instruments that provide wide-area,
all-weather, day-and-night observations that
enable nearly complete and routine monitoring.
They have revealed a dramatic decline in the
average annual sea ice extent of 4.4% per
decade over the last 32 years. More recently,
other satellite instruments have augmented
these passive microwave records, including:
synthetic aperture radars (such as ERS-1/2,
RADARSAT-1/2, Envisat ASAR, and ALOS PALSAR),
scatterometers (such as ERS Windscat, QuikScat
and ASCAT) and spectroradiometers (such as
MERIS, MODIS and AVNIR). Together with buoy
data, these satellite observations are used to
monitor ice drift, to understand seasonal to
interannual variations in sea-ice growth and
melt processes, and to support operational
sea-ice services.
For the Greenland and Antarctic ice sheets,
satellite-based altimeters and SARs are
providing key insights into ice topography and
the velocities at which ice flows to the sea.
In particular, ESA’s ERS-1, ERS-2 and Envisat
satellites compiled a long-term radar
altimetry record, beginning in 1991 and
extending to 2011. NASA’s ICESat satellite,
which operated from 2003 to 2009, extended
coverage of the ice sheets, using a laser
altimeter to map the surface with much higher
resolution. ESA has operated the CryoSat-2
satellite since April 2010, with its
SAR/Interferometric Radar Altimeter (SIRAL),
which operates in three modes, to provide
detailed geodetic elevations across ice sheets
and sea ice. Data collected by CryoSat-2 has
already been used to produce preliminary maps
of Arctic sea-ice thickness, Arctic Ocean
circulation and a topographical relief map of
Antarctica.
Figure 1. Extent of Arctic sea ice coverage
at its seasonal minimum in September 2011,
which nearly equalled the record low set in
2007. This minimum is 2.38 million square
kilometers below the average minimum extent
observed between 1979 and 2000 (courtesy
NSIDC).
(Click image to view full size)
In addition to their radar altimeters, SARs
aboard the ERS and Envisat satellites have
provided unprecedented detail about the flow
of ice, highlighting its importance for
regional mass balance, as well as the critical
roles played by ice streams and ice shelves in
the overall stability of large ice sheets.
Further SAR-derived contributions have also
been made by the Japanese ALOS satellite,
launched in 2006 and operated until 2011;
the RADARSAT-1 and -2 satellites, launched
by Canada in 1995 and 2007, and -2 is still
operating; the TanDEM-X and TerraSAR-X
satellites, launched by Germany in 2007 and
2010 and still operating; and the
Cosmo-Skymed four-satellite constellation
launched between 2007 and 2010 and currently
in operation. Recently, scientists compiled
SAR data from these satellites to construct
the first comprehensive pole-to-coast map of
ice velocity in Antarctica.
Together, satellite altimeters and SARs have
characterised seasonal to inter-annual
changes in the ice sheets over moderately
sloping regions from basin to continental
scales. These data indicate that, although
the central parts of the ice sheets appear
stable, dramatic changes are taking place
along their margins, throughout Greenland
and particularly around the West Antarctic
ice sheet. Further insight is also being
provided by the gravity surveys of the
Gravity Recovery and Climate Experiment
(GRACE) satellite mission, which provides
independent estimates of ice mass. The
accuracy of overall ice sheet mass balance
has been improving significantly with
additional observations. The dramatic
disintegration of ice shelves, such as
Larsen-B in Antarctica, indicate that ice
sheet and ice shelf dynamics may be
considerably more sensitive to short-term
climate fluctuations than formerly
believed.
Airborne surveys such as NASA’s Operation
IceBridge and ESA’s CryoVEx campaigns have
also provided more detailed altimetry
surveys while collecting a variety of
complementary measurements, including ice
thickness and bedrock topography beneath
floating ice shelves to better characterise
ice flow dynamics, which are essential to
improving predictive models of ice sheet
behaviour.
As with other components of the cryosphere,
changes in snow cover significantly
influence both the energy and freshwater
balances for Earth. Long-term passive
microwave records, supplemented with
spectrophotometer, scatterometer and SAR
data, indicate that snow covers up to 30% of
the land surface seasonally. As global
warming continues, predictions suggest that
regions currently experiencing snowfall will
increasingly receive precipitation in the
form of rain, and for every 1oC
increase in temperature, the snowline will
rise by 150 m in altitude. This has
important implications for water storage,
particularly in communities that rely on the
melting of snow in the spring.
Finally, permafrost or perennially frozen
ground is estimated to underlie 24% of the
exposed land area in the northern
hemisphere. Fluxes of gases from northern
ecosystems represent a highly uncertain
contributor to future global climate change,
and in situ observations suggest that
further warming will strongly modify these
fluxes. The wet lowlands of the Arctic
permafrost landscapes, for example, are
poorly constrained natural sources of
greenhouse gases. Furthermore, satellite
observations have detected accelerated
melting of Siberian bogs, which may release
significant amounts of methane into the
atmosphere. Melting of permafrost, and the
collapsing of soil that follows, can also
lead to significant damage to
infrastructure, such as roads, houses and
pipelines. Scatterometers and SARs have been
used to observe the characteristics of
permafrost areas, but more systematic
observations at higher spatial resolution
are needed.
Figure 2. CryoSat-derived topography of the
Antarctic ice sheet plotted onto the
Antarctic bedrock (courtesy BEDMAP). The
plot indicates the limit of orbital coverage
at 88oS and advantages in
coverage in relation to the circle at
82o, which describes the former
southern limit of ERS and Envisat radar
altimetry coverage (courtesy UCL/Planetary
Visions).
CryoSat-2
The European Space Agency’s
CryoSat-2 ice satellite was launched
in April 2010 and is now realising
its potential en route towards
achieving the objectives of the
original planned three-year mission.
A significant setback had occurred
in 2005 with a launcher malfunction
and loss of the original satellite.
The rapid decision to approve a
replacement signalled the scientific
priority and urgency attached to the
mission’s goals to survey natural
and human-driven changes in Earth’s
cryosphere.
The primary SIRAL instrument on
CryoSat employs new technology by
comparison to the previous
generation of radar altimeters, and
is designed to provide more precise
all-weather, year-round elevation
measurements of the surface of
marine ice and the large Greenland
and Antarctic ice sheets.
Characterised by a near-polar orbit,
which provides dense polar coverage
up to a latitude of 88°, the mission
is designed for the purpose of
measuring the seasonal and
inter-annual changes in ice surface
elevation. Its precise measurements
of the rate of change in the
elevation of the ice over time are
now providing direct evidence of the
changes that the polar regions are
experiencing in relation to global
climate change.
The ice sheets that blanket
Antarctica and Greenland are several
kilometres thick and the growth and
shrinkage of these ice masses have a
direct influence on global sea
level. Figure 2 shows an example of
a compilation of CryoSat elevation
measurements in the form of an
Antarctic topographic map, and a
cross-section through the ice to the
bedrock beneath. The elevation
dataset in conjunction with the
bedrock beneath provides an
essential combination of data that
are required to develop realistic
models of ice sheet behaviour.
By comparison to the several
kilometre-thick Antarctic ice sheet,
floating marine ice or sea ice is
only up to a few metres-thick. In
spite of this, sea ice has a
significant regulating influence on
regional and global temperatures by
reflecting radiation back into
space, as well as insulating the
polar oceans and redistributing
freshwater in the form of ice. By
contrast to the kilometre-thick ice
sheets, measurement of sea-ice
surface elevation presents a unique
set of challenges. To cope with
this, CryoSat is equipped with a
radar that provides high along-track
resolution, in order to resolve and
provide accurate ranging to the
narrow cracks or leads within the
extensive Arctic sea-ice cover. This
provides the ability to measure the
elevation of the surface of sea-ice
floes in relation to the sea surface
accurately, a quantity known as
freeboard. With the density of the
ice, the freeboard is converted into
sea-ice thickness. With its dense
orbital coverage, CryoSat allows
monthly statistics of the elevation
and thickness to be derived, and
thus the seasonal to inter-annual
variations in thickness can be
detected.
Figure 3 shows CryoSat results
indicating the Arctic winter sea-ice
thickness distribution in
January–February 2011. An important
new feature of the CryoSat mission
is the need to obtain measurements
of the surface of the polar ocean.
Former altimeter missions were not
optimised to obtain robust
measurements of sea-surface
topography in the presence of sea
ice. Figure 4 indicates the absolute
Dynamic Topography, which is the
height variation of the ocean
surface relative to a gravitational
geoid (such as the latest GOCE
geoid). Monthly plots such as this
can be used in future to provide
significant new insight into
high-latitude ocean circulation and
freshwater storage in the large gyre
circulation beneath the Arctic sea
ice.
Figure 3. CryoSat-derived sea ice thickness
in the Arctic Ocean in January–February 2011
(courtesy CPOM/UCL/ESA).
International Cooperation
The Integrated Global Observing Strategy
Cryosphere Theme created a framework for
improved international coordination of
measurements to foster a more comprehensive
and integrated cryosphere observing system. It
is a combined initiative of the World Climate
Research Program, Climate and Cryosphere
(CliC) Project, and the Scientific Committee
on Antarctic Research (SCAR). This framework
facilitates the flow of data and information
in cryospheric research, long-term scientific
monitoring, and operational applications. The
Cryosphere Theme Report was presented to and
approved by the IGOS Partners in May 2007 and
is now in the implementation phase. Many of
its recommendations for improving the
cryosphere-observing system will be
implemented by the World Meteorological
Organization’s new Global Cryosphere Watch
(GCW), which will facilitate the use of
satellite data and products in studies of the
changing cryosphere.
Among the major efforts recommended by the
Theme was the International Polar Year
2007-08, which actually extended from March
2007 to March 2009. Organised by the
International Council for Science and WMO,
this was actually the fourth IPY, following
those in 1882-83, 1932-33, and 1957-58. It
involved over 200 projects, with thousands
of scientists from over 60 nations examining
a wide range of physical, biological and
social research topics.
One of the flagship IPY projects was the
Global Interagency Polar Snapshot Year,
which was implemented by the IPY Space Task
Group. This group coordinated international
space agency planning, processing and
archiving of the IPY Earth Observation
legacy dataset, including: the first
high-resolution, comprehensive map of ice
velocity in Antarctica, derived from various
interferometric SAR satellites;
high-resolution digital elevation models of
the perimeters of the ice sheets and ice
caps, synthesised from stereo pair imagery
collected by France’s SPOT satellite; a
25-year record of wind, cloud and surface
properties, and radiation, using historical
AVHRR measurements; and studies of polar
atmosphere dynamics and chemistry performed
with NOAA and EUMETSAT operational data.
Figure 4. CryoSat-derived absolute Dynamic
Topography in the Arctic Ocean and north
Atlantic, with the red region indicating
high elevations in the Beaufort gyre region
(courtesy CPOM/UCL/ESA).
Mapping Antarctica
Scientists have assembled a
comprehensive, high-resolution,
digital mosaic of ice motion in
Antarctica, using 900 satellite
tracks and more than 3000 orbits of
radar data collected during the
International Polar Year 2007 to
2009. Published in a September 2011
issue of Science, this study,
by Rignot et al., and entitled, ‘Ice
Flow of the Antarctic Ice Sheet’ was
enabled by an extraordinary
collaboration between the space
agencies of the United States,
Europe, Canada and Japan.
Until now, there has been no clear
picture of ice-sheet motion at the
continental scale. The vast extent
of East Antarctica, representing
~77% of the continent, has been
devoid of quality data. Only a few
floating ice shelves have been
mapped, and comprehensive velocity
mapping has been limited to the
lower reaches of key outlet
glaciers. This lack of broad-scale
detailed observations of ice motion
has placed a fundamental limit on
the capability of numerical models
of ice-sheet evolution. This recent,
comprehensive survey of Antarctica
was obtained by combining data from
a variety of orbiting
interferometric synthetic aperture
radar (InSAR) instruments, including
RADARSAT-2 (Canada), Envisat ASAR
(Europe), ALOS PALSAR (Japan) and
ERS 1/2 (Europe). Each instrument
contributed unique coverage and
performance.
The resulting mosaic, shown in
Figure 1, confirms some well-known
behaviour, but also reveals a wealth
of new information. The ice velocity
ranges from a few cm/year near ice
divides to a few km/year on
fast-moving glaciers and floating
ice shelves. The distribution of
velocities has one peak at 4-5
m/year, for the slow-moving ice in
East Antarctica, and another peak at
250 m/year, for fast-flowing
glaciers and ice shelves. The
highest velocities are found at the
Pine Island and Thwaites glaciers of
West Antarctica, with rates several
times those of any other glacier.
This sector of the ice sheet is
undergoing the most rapid change at
present, over the widest area, and
with the greatest impact on the
total ice-sheet mass balance.
Importantly, the mosaic also
provides insight into preferred
channels of ice transport. It
reveals that every major glacier is
the merger of several tributaries
that extend hundreds of km inland.
Of particular note, in the Antarctic
peninsula, the tributaries of
Wilkins Ice Shelf, and of the
northern sector of George VI Ice
Shelf, abruptly transition to zero
velocity when they mix with the
floating ice shelves, which the
authors attribute to massive rates
of ice-shelf melt by the underlying
warm ocean.
The observation that ice flow in
Antarctica is driven by a complex
set of meandering, size-varying,
speed-varying, intertwined
tributaries, most likely dominated
by basal-slip motion, challenges the
traditional view of ice-sheet flow
constrained by internal deformation
and disconnected from coastal
regions. Since this latter view has
usually been adopted as the basis
for continental-scale ice-sheet
modelling, this new reference map
will help to improve reconstructions
of past and ongoing changes in
Antarctica, as well as predictions
of future ice-sheet evolution in a
warming climate.
Future Challenges
Although satellite observations reveal
significant changes occurring in the
cryosphere, the causes of and processes that
drive change are only partly understood. Some
key questions cannot be answered with current
technologies, limiting knowledge of the
couplings amongst the cryosphere, oceans and
atmosphere, understanding of which is key to
improved predictions. Particular observational
challenges include:
— Evaluating feedbacks to the ocean and
atmosphere induced by changes in sea-ice
cover;
— Determining the distribution, thickness, and
mass balance of sea ice, and also addressing
the impact of surface melt, albedo change and
snow cover;
— Isolating the various contributions to
ice-sheet mass balance, such as accumulation
rates, melt rates and ice–ocean interactions,
and evaluating their sensitivities to
forcing;
— Mapping bedrock topography beneath ice
sheets and ice shelves, and improving
knowledge of the physical processes
controlling fast glaciers and ice streams;
— Quantifying changes in snow cover, including
snow water equivalent, and connections in the
global hydrological cycle and regional water
resources;
— Developing remote sensing techniques for
characterising and monitoring permafrost
regions.
Operation IceBridge
Operation IceBridge is a NASA airborne
mission that began in 2009, making
altimetry, radar and other geophysical
measurements to monitor and
characterise Earth’s cryosphere. Its
primary goal is to extend the record
of ice altimetry begun by ICESat. The
IceBridge mission will continue until
the launch of ICESat-2, scheduled for
2016.
The IceBridge mission seeks to
understand the factors controlling the
retreat and growth of the world’s
major sea- and land-based ice sheets
and their interactions with the ocean,
atmosphere, solid Earth and solar
radiation. Linking the altimetry
measurements of ICESat and ICESat-2,
as well as Europe’s Envisat and
CryoSat-2, facilitates accurate
intercomparisons, and the production
of a long-term ice altimetry
record.
IceBridge aircraft also carry radars
to map and to characterise: snow and
firn on land and sea-ice, to improve
the interpretation of altimetry data
and estimates of surface mass balance;
the bedrock beneath land-based ice in
Greenland and Antarctica to improve
ice-sheet models; and ice layers
within the ice sheets. Furthermore,
the mission collects airborne gravity
measurements to infer the bathymetry
beneath ice shelves and sub-ice sheet
bed topography beneath outlet glaciers
that cannot be mapped by radar.
Taken together, these data will help
to improve understanding of the
mechanisms governing mass balance and
the dynamics of the Greenland and
Antarctic ice sheets, and those that
drive sea-ice cover, particularly in
the Arctic. They will also be used to
validate and to improve predictive
models of land-based ice contributions
to sea level, and of sea ice cover,
during this century.
Continuous, uniform, long-term monitoring
observations are essential in assessing and
understanding the response of the global
cryosphere to climatic variations. Owing to
large year-to-year variations in seasonal
signals, the duration of monitoring records
has a critical impact on the certainty with
which trends can be assessed. Data gaps
could also significantly impair our ability
to track down rapid events, such major
calving events, ice shelf collapse, glacier
draw down or exceptional melt seasons. The
recent conclusions of the ICESat, ALOS and
Envisat missions, combined with the limited
remaining lifetimes of the RADARSAT-2 and
GRACE satellites, represent a significant
challenge to the various space agencies
tasked with maintaining these long-term
times series of measurements. This effort
would be strengthened by the continued
operation of RADARSAT-2 and CryoSat-2. From
2014, ESA will launch two Sentinel
satellites to complement the radar altimetry
and SAR data records begun by ERS-1, and
Japan will launch its ALOS-2 satellite.
Furthermore, NASA has committed to extending
the unique datasets established by ICESat
and GRACE, scheduling launches of the
ICESat-2 and GRACE follow-on satellites in
2016 and 2017, and has initiated studies for
the development and eventual launch of the
DesDynI SAR satellite.
Finally, the vast amount of data being
collected by cryosphere-observing satellites
poses its own challenges in stewardship,
especially ensuring usability by a broad
user community to understand the couplings
of the cryosphere to the global climate
system. But most importantly, concerted
effort is needed to make these data freely
available to scientists who can interpret
the state of the cryosphere, and help to
improve predictions of the state of the
future Earth.
Figure 5. Antarctic ice velocity derived
from ALOS PALSAR, Envisat ASAR, Radarsat-2
and ERS 1/2 satellite interferometry,
overlaid on a MODIS mosaic of Antarctica.
Thick black lines delineate ice divides.
Thin black lines outline a selection of
sub-glacial lakes (courtesy Science).
