Essential Climate Variables: Glaciers, Ice
Caps and Ice Sheets
The state of the polar ice sheets and their
volumes are both indicators and important
parts of climate change processes and
feedbacks. Consequently, it is important to
monitor and study them in order to
investigate the impact of global warming and
to forecast future trends. The IPCC expects
that, globally, ice sheets will continue to
react to climate warming and contribute to
sea-level rise for thousands of years after
the global climate has been stabilised. They
note that:
— Contraction of the Greenland ice sheet is
projected to continue to contribute to
sea-level rise after 2100. Current models
suggest virtually complete elimination of
the Greenland ice sheet and a resulting
contribution to sea level rise of about 7 m
if global average warming in excess of
1.9oC to 4.69oC
relative to pre-industrial values were
sustained for millennia;
— Ice dynamic models suggest that melting of
the West Antarctic ice sheet could
contribute up to 3 m of sea level rise over
the next 1000 years, but such results are
strongly dependent on model assumptions
regarding climate change scenarios, ice
dynamics and other factors.
Satellite remote sensing allows observations
of the changes in the shape of ice sheets,
and identification of the shape and size of
large icebergs that have detached from the
ice sheet. SAR instruments are one source of
data on the polar ice sheets. RADARSAT-2
provides routine surveillance of polar
regions and has created the first
high-resolution radar images of
Antarctica, enabling detection of changes in
the polar ice sheet and improved
understanding of the behaviour of the
Antarctic glaciers.
ASAR on the Envisat mission continued the
observations of polar ice topography started
by the ERS-1 and ERS-2 satellites.
Interferometric measurements by PALSAR,
together with observations by the AVNIR-2
instrument on JAXA’s ALOS mission (to be
continued with ALOS-2 from 2014) contributed
to understanding the ice-sheet mass balance
and glacier variation near the South Pole and
in Greenland.
Altimeters provide useful data on ice-sheet
topography. While many have high vertical
resolution, their limited horizontal
resolution means that their observations over
smoother, near-horizontal portions of ice
sheets are of greatest value. The RA-2
instrument on Envisat has provided improved
mapping of ice caps.
Given the significance of information on
changes in the continental ice sheets, two
missions dedicated to their study have been
developed: NASA’s ICESat (launched January
2003 and concluded in August 2010) and ESA’s
CryoSat-2 (launched 2010, following the loss
on launch of CryoSat in 2005). CryoSat-2
provides an instrument for the ice sheet
interiors and margins, for sea ice and other
topography, with three-mode operation:
— Conventional pulse-limited operation for the
ice-sheet interiors (and oceans if
desired);
— Synthetic aperture operation for sea ice;
— Dual-channel synthetic
aperture/interferometric
operation for ice-sheet margins.
Regular measurements of terrestrial snow
cover are important because snow
dramatically influences surface albedo,
thereby a significant impact on the global
climate, as well as influencing hydrological
properties and the regulation of ecosystem
biological activity. The IPCC has found that
– on the evidence of satellite data – there
is likely to have been a decrease of about
10% in the extent of snow cover since the
late 1960’s.
Snow forms a vital component of the water
cycle. In order to make efficient use of
meltwater runoff, resource agencies must be
able to make early predictions of the amount
of water stored in the form of snow.
Coverage area, snow water equivalent and
snow pack wetness are the key parameters to
be determined in this process.
Snow cover information has a range of
additional applications such as detecting
areas of winterkill in agriculture that
result from lack of snow cover to insulate
plants from freezing temperatures. Locally,
monitoring of snow parameters is important
for meteorology and for enabling warnings of
when melting is about to occur – which is
crucial for hydrological research and for
forecasting the risk of flooding.
A range of different instrument types can
contribute to measurements of snow.
Visible/near-infrared satellite imagery
provides information of good horizontal and
temporal resolution and accuracy on snow
cover in the daytime in cloud-free areas.
AVHRR provides snow cover information and
this will be continued in the future by
VIIRS on Suomi NPP and the planned JPSS
series. MODIS data are being used to monitor
the dynamics of snow and ice cover over
large areas (greater than 10 km2)
and, on a weekly basis, to report the
maximum area covered by both. The resulting
snow maps should be available within 48
hours of MODIS data collection.
Passive microwave instruments such as SSM/I
and AMSR/E (on Aqua until October 2011, and
on GCOM-W, launched 2012) have all-weather
and day/night monitoring capability, and are
able to estimate the thickness of dry snow
up to about 80 cm deep.
Data from RADARSAT and ERS-2 have shown the
usefulness of SAR remote sensing techniques
to determine snow area extent and to monitor
the physical conditions of snow. Envisat and
RADARSAT-2 have provided continuity of such
snow information.
Sea-ice variability is a key indicator of
climate variability and change which is
characterised by a number of parameters.
Systematic global observation of sea-ice
extent and concentration, inferred from
passive microwave radiometry, has produced a
30-year record. The length and consistency
of this record has made it the most often
cited data source for sea-ice climate
research. Sea-ice observations from newer
instruments have relatively short records,
but offer complementary characteristics such
as greater accuracy for determining ice
concentration and improved resolution.
In addition to monitoring ice extent (the
total area covered by ice at any
concentration) and concentration (the area
covered by ice per unit area of ocean), it
is necessary to know ice thickness in order
to estimate sea ice volume or mass balance.
In the past, only scarce in situ data from
boreholes or upward-looking sonar from
moored instruments or submarines, were
available for this purpose. Now,
satellite-borne altimeters are emerging as
an important new data source. Early work
with radar altimeters demonstrated the
utility of altimetry for ice thickness. The
Geoscience Laser Altimeter System (GLAS) on
ICESat, launched in 2003 and completed in
2010, has provided high-resolution ice
thickness maps. CryoSat-2, launched in April
2010, has a radar altimeter that provides
precise ice-thickness maps. A precise first
map of Arctic sea-ice thickness was produced
by CryoSat-2 in June 2011, complementing
observations of the dramatic decrease of
sea-ice extent at the end of summer from
around 8 million sq km in the early 1980s to
2007’s historic minimum (nearly equalled in
2011) of just over 4 million sq km.
ICESat-II (Ice, Cloud and land Elevation
Satellite-2) is the second-generation of the
laser satellite ICESat, scheduled for launch
in 2016.
All-weather, day and night active radar,
including the low-resolution QuikSCAT
(complete 2010) scatterometer and
high-resolution RADARSAT synthetic aperture
radar, is sensitive to the unique
electromagnetic signature of multiyear ice.
This ice has survived a summer’s melt and is
generally thicker than younger ice. Active
radar and other new sensors played an
important part in attributing the
surprisingly low Arctic ice extent of
September 2007 to various causes. Summer ice
extent has had a downwards trend since the
1990s, as determined by the passive
microwave record. The active microwave
sensors provided data that showed that the
Arctic Ocean had lost a considerable amount
of multiyear sea ice over the past few years
as a result of the prevailing circulation
pattern, suggesting that the ice cover was
unusually thin as summer began and
predisposed to melting back further.
Wide-area sea-ice motion and deformation
products from visible band sensors, as well
as higher resolution AMSR data, provided
corroborating evidence. Finally,
investigators using ICESat confirmed that
the ice thickness at the beginning of summer
was well below its typical average value,
with thin seasonal ice replacing thick older
ice as the dominant type between the winters
of 2004 and 2008 for the first time on
record.
Operational ice services place a higher
priority on timeliness and accuracy than on
consistency over a long data record, and
accordingly use a wide variety of near-real
time remote sensing data to construct ice
charts. These charts are used by shipping to
avoid damage and delay, and to reduce fuel
costs; offshore drilling companies; maritime
insurance companies; and government
environmental regulatory bodies.
High-resolution synthetic aperture radars,
such as those on Envisat and RADARSAT-2,
offer the best source of data for
operational services. Data from these
instruments provide information on the
nature, extent and drift of ice cover and
are used not only for status reports, but
also for ice forecasting and as an input for
meteorological and ice drift models. JAXA’s
PALSAR radar provided polarimetric data,
which improved the accuracy of sea-ice
classification. Low-resolution scatterometer
observations, such as those from ASCAT on
MetOp, can also be used to retrieve
information on sea-ice extent and
concentration in all weather conditions, day
or night. Looking to the future,
continuation of RADARSAT/Envisat class
radar-equipped missions, such as ESA’s
Sentinel-1 (first launch 2014, so there will
be a service gap for European missions given
the recent conclusion of Envisat), is
important in providing complementary
high-resolution data to further elucidate
sea-ice processes.
JAXA’s AMSR-E radiometer on Aqua (failed
October 2011) and GCOM-W (launched 2012) and
operational sensors such as the DMSP SSM/I
will ensure continuity of the passive
microwave global sea-ice concentration data
source in the near term.
In 2006, CEOS defined a series of actions to
better meet the GCOS-defined needs for the
sea-ice Essential Climate Variable:
— CEOS agencies will examine their
respective plans to maintain provision of
microwave brightness temperatures and
visible/infrared radiances for the sea-ice
ECV;
— CEOS space agencies will consult with the
science community on appropriate retrieval
algorithms of passive microwave observation
for reprocessing sea-ice products;
— New space-based measurements and products,
including ice thickness and ice drift, will
be considered by CEOS agencies as part of
their future research missions.
