CryoSat-2 (Earth Explorer Opportunity Mission-2)
CryoSat-2 is the follow-on Earth Explorer Opportunity Mission in ESA's Living Planet Program. It replaces CryoSat, which was selected for development in 1999 and lost as a result of launch failure on October 8, 2005. CryoSat-2 will have the same mission objectives as the original CryoSat mission; it will monitor the thickness of land ice and sea ice and help explain the connection between the melting of the polar ice and the rise in sea levels and how this is contributing to climate change.
The original CryoSat mission was proposed by Duncan Wingham of the University College London (UCL) and an international science team. Duncan Wingham is also the mission PI. A nominal mission duration of three years is planned (excluding the commissioning and validation phases, which may last up to six months).
In Feb. 2006, ESA received the green light from its Member States to build and launch a CryoSat recovery mission, CryoSat-2, based on the same objectives as the original CryoSat mission. However, the design of the CryoSat-2 spacecrasft is being updated. The changes required to the design of CryoSat-2 were scrutinized from December 2006 to January 2007. The Δ-CDR (Critical Design Review) was completed on Feb. 1, 2007. 1) 2) 3) 4) 5)
A total of 85 improvements/modifications have been approved in the design of CryoSat 2 (of which 30-40% have been small software changes that make the satellite much easier to operate). The new key features of CryoSat-2 include the following items:
• The SIRAL-2 (SAR/Interferometric Radar Altimeter-2) design includes a full backup SIRAL system, in case the primary payload malfunctions. Once in orbit, a special algorithm will be used to convert data collected by the CryoSat-2 satellite to create more accurate ice maps. - As a result of the dual SIRAL payload and associated interfaces, and other improvements to reliability, there has been a knock-on effect to the design of the satellite. For example, the backup SIRAL system has to be kept warm while it is switched off - the additional heater power is provided by increasing the size of the satellite's battery.
• A heat-radiating panel is being added. The path of CryoSat-2's orbit means it will face extremes of temperature. The panel ensures the electronics are protected
• Solar panels on the satellite's back are being added to account for the additional power requirements. Unlike many spacecraft, CryoSat 2 does not have solar wings.
In addition to building the new satellite, a number of field experiments to support the CryoSat-2 mission, were conducted or are getting underway in the Arctic. First is the Arctic Arc Expedition, part of the IPY (International Polar Year) 2007-2008. 6) 7)
- Antarctic 2008/9 CroVEx campaign in the blue ice region (see Figure 1) in December 2008: German scientists from the Technical University of Dresden and the AWI (Alfred Wegener Institute) are spending up to four months venturing out onto the vast frozen reaches of what is known as the 'blue ice' region near the Russian Novo airbase in Dronning Maud Land in Antarctica. The aim is to take very accurate measurements of the surface topography, both from the air and on the ground to contribute to the validation program for CryoSat-2. 8)
In parallel to the efforts on the ground, the Alfred Wegner Institute (AWI) will be flying their POLAR5 aircraft across the blue ice site – starting just before Christmas and finishing before the New Year. From the plane, the AWI team will collect laser and radar height measurements along the very same tracks as the ground team. To do this they are using ESA's ASIRAS (Airborne Synthetic Aperture and Interferometric Radar Altimeter System), which simulates the measurements CryoSat.
- CryoVEx (CryoSat Validation Experiment) 2008 (3-week campaign in May 2008 in the far north of Greenland and Canada). CryoVEx 2008 is a continuation of a number of earlier campaigns that focus on collecting data on the properties snow and ice over land and sea. This year's campaign is a huge logistical undertaking as airborne, helicopter and ground measurements are being taken simultaneously in three different locations - out on the floating sea-ice north of the Canadian Forces Station Alert, on the Devon ice cap in Canada and on the vast Greenland ice cap. A Twin Otter is carrying two key instruments: ASIRAS (Airborne SAR/Interferometric Radar System), a radar altimeter that mimics the radar altimeter onboard CryoSat-2, and a laser scanner which maps the surface beneath the plan, and a helicopter with an on-board sensor that measures sea-ice thickness. 9)
- In the spring of 2007, an international team of scientists stationed in Svalbard, Norway and two polar explorers are crossing the North Pole on foot. Both teams are part of a common effort to collect vital data on the ground and from the air in support of ESA's ice mission CryoSat-2. The expedition's two Belgian explorers, Alain Hubert and Dixie Dansercoer, 'stepped' onto the sea ice off the coast of Siberia on March 1, 2007 each pulling a 130 kg sledge holding supplies and equipment. A parallel campaign by scientists from Germany, Norway and the UK is unfolding in the extreme northern archipelago of Svalbard, Norway. As part of the CryoVEx 2007 campaign (CryoSat-2 Validation Experiment), they are spending one month making measurements of snow and ice properties along long transects that crisscross the ice sheet surface.
- As the ground experiments are carried out, measurements are also being taken from the air by the Alfred Wegner Institute (AWI), Bremerhafen, Germany. The Dornier-228 aircraft carries the ASIRAS (Airborne SAR/Interferometric Altimeter System) instrument, which is an airborne version of the radar altimeter instrument onboard CryoSat-2. By comparing the airborne data with ground measurements scientists will test and verify novel methods for retrieving ice-thickness change from the CryoSat-2 satellite mission ahead of the launch. - ASIRAS was built by Radar Systemtechnik (RST) of Switzerland with the support of the AWI and Optimare for the implementation and operation on an aircraft. It was test flown in March 2004 over the snow and ice expanses of Svalbard.
- The CryoVEx 2006 campaign took place April/May, 2006 and consisted of coordinated airborne and ground activities in support of CryoSat-2 validation goals over three land validation sites (Devon Island in Canada, Central Greenland and Svalbard, Spitzbergen, Norway) and a series of ice experiments over Alert / Ellesmere Island, Canada.
- LaRA (Laser and Radar Altimeter) campaign in the Arctic region of Greenland and Svalbard: The D2P (Delay-Doppler Phase-monopulse Radar) instrument of JHU/APL participated in this campaign which took place in May 2002 under joint NASA/ESA sponsorship to support calibration and validation activities, and science investigations in advance of the CryoSat and ICESat missions. The D2P radar altimeter was flown aboard the NASA-P3 aircraft along with the ATM (Airborne Topographic Mapper) laser altimeters to collect simultaneous laser and radar altimeter (hence, the LaRA campaign) measurements over land and sea ice.
- CryoVEx (CryoSat Validation Experiment) campaign: As a follow-on to the LaRA campaign, the D2P system was flown again in 2003 under joint NASA/ESA sponsorship as part of the CryoVEx field campaign. As in 2002, simultaneous laser and radar altimeter measurements were collected in the Arctic.
Such painstaking ground work is necessary to be able of measuring ice thickness down to centimeter level (1-3 cm average) from space. This in turn may lead to a better understanding of the impact that changing climate is having on the polar ice fields. See also the D2P and ASIRAS descriptions on the eoPortal (along with the campaigns for the validation of the SIRAL instrument).
The CryoSat-2 spacecraft is being built and integrated by EADS Astrium GmbH of Friedrichshafen, Germany, as prime contractor of a consortium. The spacecraft structure consists of a long rectangular main platform body, surmounted by fixed solar arrays in the form of a tent. The spacecraft has neither deployable appendages nor any other moving parts except for thruster valves. The lower surface of this structure is permanently earth facing. All electronics are mounted on the nadir plate acting as radiator. The antennas used for radio communication, and the Laser Retroreflector, are mounted on this surface; an emergency antenna for command and monitoring is also fitted on top of the satellite between the solar arrays. The two SIRAL instrument antenna dishes are mounted on a separate rigid bench in the forward section of the S/C. In addition, a dedicated SIRAL radiator is mounted at the nose tip. 10)
Figure 2: Illustration of the CryoSat-2 spacecraft with thermal covers on the SIRAL antennas (image credit: ESA)
Figure 3: Alternate view of the CryoSat-2 spacecraft (image credit: EADS Astrium)
The spacecraft is 3-axis stabilized. A slight nose-down attitude of the S/C (6º) is chosen (using magnetorquers and 10 mN cold-gas thrusters) to ensure minimum attitude correction due to gravity-gradient disturbances. The S/C has dimensions of 4.6 m x 2.34 m x 2.20 m. The S/C mass is about 720 kg (including 36 kg propellant), the design life is 3.5 years (goal of 5.5 years). Spacecraft power generation is provided by two triple-junction GaAs solar arrays with an efficiency of 27.5% (two oriented solar panels), each panel provides power of 850 W at normal solar incidence. A PCDU (Power Control and Distribution Unit) provides onboard distribution. The energy is stored by a lithium-ion battery (60 Ah capacity).
The pointing requirements are the main design drivers for the AOCS: 11)
• High precision cross-track pointing knowledge of < 10 arcsec for SARIn mode (SARIn refers to the SAR interferometric mode).
• S/C attitude maintenance with a pointing accuracy of < 0.2º per axis and a pointing stability of < 0.005º for 0.5 s in the nominal Earth-pointing phase of the mission
• Provision of very low disturbances due to thruster activity to meet the very high precise orbit determination (POD) accuracy of the CryoSat orbit.
• The AOCS (Attitude Orbit and Control Subsystem) comprises the following elements:
- A cold gas system (RCS) for attitude control and orbit transfer and maintenance maneuvers, 16 attitude control thrusters (10 mN) and 4 orbit control thrusters (40 mN). Nitrogen is used as propellant (132 l tank). 12)
- A set of 3 magnetorquers is used for compensation of environmental disturbance torques in support of RCS. The MT30-2-GRC, originally developed and qualified for the GRACE mission by ZARM Technik GmbH, has been selected for CryoSat.
- A set of three star tracker heads (also a part of the payload) providing autonomous inertial attitude determination for the spacecraft. The multiple configuration makes the sensor system one-failure tolerant, except for the rare occurrence of simultaneous sun and moon blinding of two heads, to which the system software is tolerant. Consequently, two camera heads are operated in parallel at all times to cope with sun-blinding. In its acquisition mode, which takes 2-3 seconds, the star tracker calculates a coarse attitude by matching triangle patterns of stars with patterns stored in its catalog. Subsequently, in attitude update mode it calculates precise attitude at a rate of 1.7 Hz.
The star tracker attitude serves also as reference for determining the orientation of the SIRAL interferometric baseline. The orientation of the interferometric baseline needs to be very accurately measured in-flight: small errors in knowledge of the roll-angle translate into substantial errors in the elevation of off-nadir points. The HE-5AS star tracker of Terma A/S, originally developed and qualified for the NEMO (Navy EarthMap Observer) and FCT (Foreign Comparative Test) projects, is selected for CryoSat. It is a fully autonomous star tracker capable of delivering high-accuracy inertial attitude measurements from a lost-in-space condition with no external attitude information. The EOL performance of the star tracker is < 3.2 arcsec in the lateral axes and < 16 arcsec about the roll axis under worst-case conditions. 13)
The star tracker baffle has been designed to meet the required sun exclusion angle of 30º and the moon exclusion angle of 25º. These exclusion angles ensure together with the star tracker accommodation on the antenna bench that during the whole mission sun and moon can only blind one star tracker head at any time.
Figure 4: Photo of one star tracker camera head unit (image credit: ESA)
• A DORIS receiver is part of an overall system, for real-time measurements of satellite position, velocity and time. DORIS measures the Doppler frequency shifts of UHF and S-band signals transmitted by ground beacons. Its measurement accuracy is < 0.5 mm/s in radial velocity allowing an absolute determination of the orbit position with an accuracy of 2-6 cm (see DORIS and LLR description under sensor complement). - The DORIS system comprises a network of more than 50 ground beacons, a number of receivers on several satellites in orbit and in development, and ground segment facilities. It is part of the International DORIS Service, the IDS, which also offers the possibility of precise localization of user beacons.
• CESS (Coarse Earth-Sun Sensor) of CHAMP and GRACE heritage (a patented design of Astrium GmbH). Provides attitude measurements (<5º) with respect to the sun and Earth for initial acquisition and coarse pointing. The FOV of CESS is a full spherical one, i.e. no special search maneuvers are necessary to find the Earth or the sun. Its measurement principle is based The concept is based on temperature differences measured by 6 omnidirectional arranged sensor heads (PT1000 thermistors).
• A set of three three-axis fluxgate magnetometers are used for magnetorquer control and as rate sensors. They provide a measurement range of at least ± 60.000 nT with an accuracy of better than 0.5 % full scale.
The AOCS provides high pointing accuracy (a few tens of an arcsecond), knowledge and stability in nominal Earth-pointing. It also has to perform the orbit changes between the science and validation phase orbits. The AOCS uses inertial attitude measurements from the set of 3 star tracker camera head units and DORIS real-time navigation to convert the inertial attitude into Earth referenced attitude (star sensor FOV of 22º x 22º, ). - The RCS (Reaction Control Subsystem), developed at PolyFlex Space Ltd. (a Marotta UK Ltd. company), is a cold gas propulsion system for auxiliary attitude control (in which it provides deadband protection around the axis defined by the instantaneous geomagnetic field) and for orbit transfer and maintenance maneuvers. It has 16 attitude control thrusters of 10 mN each and 4 orbit control thrusters of nominally 40 mN each. A single high-pressure tank stores 36.2 kg of nitrogen gas at 278.6 bar. 14) 15)
The CDMU (Control and Data Management Unit), consisting of a processor and a hardware-based fault detection system, handles all on-board command and control functions including telecommand decoding and the AOCS processing functions (the OBC is based on the ERC-32 microprocessor). A MIL-STD-1553B communications bus is used as payload interface (for SIRAL and DORIS). The on-board solid-state memory has a capacity of 2 x 128 Gbit.
Figure 5: Block diagram of the major elements od the CryoSat-2 spacecraft (image credit: ESA)
• Experimental Rate Sensor: CryoSat-2 carries a small technology experiment as a passenger. This device is an attitude rate sensor based on MEMS (Micro-Electro-Mechanical-Systems) technology in which microelectronics and mechanical devices (in this case a sensor) are fabricated on the same substrate. The MEMS sensor detects attitude rate to provide the same function as a more traditional gyro and is based on a device widely used in in-car navigation systems. Three orthogonal MEMS sensors are mounted in the experiment, to measure 3-axis attitude rates. The unit is called MRS (MEMS Rate Sensor) in the CryoSat context. The goal is to provide a low-cost rate-sensor or gyro. The device is provided free of charge to CryoSat-2 in exchange for the flight opportunity (Ref. 10).
The measurement data are not used on-board and only sent in housekeeping telemetry to the flight control centre. Here they will be used as an additional data type in monitoring satellite dynamics during attitude transitions.
In the context of the technology program in which the MRS has been developed, it is called SiREUS-FExp, for European Silicon Rate Sensor Flight Experiment. - SiREUS is a compact and lightweight solid-state MEMS rate sensor which was developed in the context of an ESA technology technology program. The UK development team consisted of the following partner organizations: AIS (Atlantic Inertial Systems - formerly BAE Systems of Plymouth), SEA (Systems Engineering & Assessment Ltd. of Bristol), and SELEX-GALILEO a Finemeccanica owned company (formerly BAE Systems of Edinburgh). This development is based on the established BAE SYSTEMS automotive MEMS detector, however significant developments were required to meet the performance requirements while achieving compatibility of the electronics to the space environment and ensuring low recurring price. The partnership with a significant commercial provider such as AIS should be emphasized as a critical aspect of the program. 16) 17) 18)
Table 1: MRS (MEMS Rate Sensor) key requirements and current (2008) status
The SiREUS unit has met or exceeded the key performance requirements set at the start of the program. The unit does not contain any software; all control loops are implemented digitally in an FPGA. The SiREUS unit is fairly compact, but its size is currently dominated by analog electronics, not the MEMS. It may be cost effective to achieve a significant reduction in mass and volume, if this results in a match with many more customer requirements.
SiREUS has demonstrated that it is possible to construct multi-lateral programs to spin-in technology from non space industry organizations and to make significant improvements in the performance of the 'spin-in' technology. There are positive signs for the wider application of this technology in 'spin-off' programs. The instrument has a size of 100 mm x 100 mm x 70 mm.
Figure 6: Top view of MRS FExp front end PCBs (left) and view of the MRS Exp unit on the CryoSat-2 nadir panel (right), image credit: SEA
Table 2: Overview of spacecraft parameters
Launch: The CryoSat-2 spacecraft was launched on April 8, 2010 on a Dnepr vehicle from the Baikonur Cosmodrome, Kazakhstan. The launch provider was ISC (International Space Company) Kosmotras. 19) 20) 21)
Note: The technical issue with the second stage of the Dnepr rocket that delayed the launch of ESA's Earth Explorer CryoSat-2 satellite in February 2010 has now been resolved – and the new launch date of 8 April has been set. The fuel reserve problem of the second stage surfaced a week before the scheduled launch date and after the 'space head module', encasing the CryoSat-2 satellite, had been mated to the rest of the rocket in the launch silo. Consequently, the space head was returned to the integration facilities pending an investigation and new launch date. 22)
The delay, from the planned launch date of Dec. 2009, is due to the limited availability of facilities at the Baikonur launch site in Kazakhstan, which is particularly busy at the moment. 23)
Satellite Orbit: Non sun-synchronous circular LEO orbit, mean altitude = 717 km, inclination = 92º, nodal regression of 0.25º per day. Ground track repeat cycle: 369 days (with 30 day pseudo subcycles). This configuration allows a sufficient coverage for the polar regions. The CryoSat mission requirements include:
• An orbit change is required during the mission with the objective to visit at least twice a validation orbit, approximately 6 km lower in altitude than the science phase orbit
• The payload must be operated in various modes, as a function of geographical region, such that the orbital operations, and data sets collected, on successive orbits are dissimilar
• The payload utilization demands very precise orbit and attitude restitution. Minimum operations of three years are required.
The CryoSat mission is aimed in part at gaining coincident coverage with the GLAS laser altimeter of the NASA ICESat mission. The following support phases are defined:
• Commissioning phase: The nominal duration is two months. During this phase the satellite and its payload are brought into a fully operating condition in its nominal orbit.
• Science phase: This includes a long-repeat cycle [a 369-day orbit (5344 revolutions) repeat phase will be used, with a 30-day subcycle]. The science phase is the nominal operational support mode of the mission. This orbit is designed to provide very dense orbit cross-overs above 72º of latitude, for use over the ice sheets. With coverage to 88º of latitude, all but a very small area of the land and marine ice fields will be within the coverage of the satellite. In addition, the 30 day subcycle provides approximately monthly coverage of the sea ice fluctuations.
• Validation phases: The objective is to conduct calibration or validation experiments that are at a fixed locations on Earth. In these phases the satellite may be placed into a 2-day repeat orbit. A validation phase may have a duration of up to 1 month, and there may be more than one during the mission lifetime. The measurements made by the satellite mission will need to be verified by ground-based experiments.
RF communications: The S-band link is used for all TT&C communications (2 kbit/s uplink and 8 kbit/s downlink). The physical downlink operates at 16 kbit/s but carries an overhead of error correction coding. The X-band downlink (center frequency of 8.100 GHz) provides a payload transfer rate of 100 Mbit/s. All onboard data are stored in the MMFU (Mass Memory and Formatting Unit) of 2 x 128 Gbit (EOL) capacity. Data arrive at the MMFU directly from the SIRAL instrument on a pair of fast IEEE 1355 standard serial links (SpaceWire for the two high-rate interferometric data channels) and via the MIL-STD-1553 bus for the low rate data channels. Data are also transferred from the CDMU and the DORIS over the MIL-STD-1553 bus. About 320 Gbit/day of onboard source data are being generated and transmitted to the ground.
Figure 7: The CryoSat-2 spacecraft and its instruments (image credit: ESA)
• April 8, 2020: Today marks 10 years since a Dnepr rocket blasted off from an underground silo in the remote desert steppe of Kazakhstan, launching one of ESA’s most remarkable Earth-observing satellites into orbit. Tucked safely within the rocket fairing, CryoSat had a tough job ahead: to measure variations in the height of Earth’s ice and reveal how climate change is affecting the polar regions. Carrying novel technology, this extraordinary mission has led to a wealth of scientific discoveries that go far beyond its primary objectives to measure polar ice. And, even at 10 years old, this incredible mission continues to surpass expectations. 24)
- The launch of a satellite is always a time to hold your breath, but CryoSat’s liftoff on 8 April 2010 was arguably more tense than most as it came less than five years after the original satellite was lost owing to a rocket malfunction.
- So important was the need to understand what was happening to Earth’s ice, the decision to rebuild was taken quickly – and thankfully, this day 10 years ago heralded the beginning of a mission that was set to advance polar science like no other.
- While other satellite missions can measure changes in the extent of Earth’s ice, CryoSat completes the picture by recording changes in ice height, which are used to work out changes in thickness and volume – key to understanding the total amount of ice loss.
- CryoSat was designed to observe two types of ice: the vast ice sheets of Antarctica and Greenland that rest on land, and the sea ice floating in the polar oceans.
- Not only do these two forms of ice have different consequences for our planet and climate, but they also pose different challenges when trying to measure their thickness.
- To do this, CryoSat carries the first spaceborne synthetic aperture interferometric radar altimeter, a sensor optimized to detect sea-ice floes as they drift in the ocean and to study the rugged glaciers that drain the polar ice sheets.
- In addition, CryoSat’s orbit reaches latitudes of 88° North and South, which takes it closer to the poles than all previous polar-orbiting altimetry satellites.
- ESA’s Director of Earth Observation Programs, Josef Aschbacher, said, “CryoSat is the epitome of an ESA Earth Explorer. It uses completely new technology to fill gaps in our scientific knowledge. The issue of diminishing ice linked to climate change is a real concern, and over the last 10 years this mission has been a game changer.
Figure 8: According to a new report, Greenland and Antarctica are losing ice six times faster than in the 1990s – currently on track with the Intergovernmental Panel on Climate Change’s worst-case climate warming scenario. - The findings, published in two articles in Nature, show that Greenland and Antarctica lost 6.4 trillion tons of ice between 1992 and 2017 – pushing global sea levels up by 17.8 mm. Of the total sea level rise, around 60% (10.6 mm) was due to Greenland ice losses and 40% was due to Antarctica (7.2 mm), image credit: CPOM (Center for Polar Observation and Modelling) located at the University of Leeds
- Andrew Shepherd from the University of Leeds, UK, added, “CryoSat’s contribution to polar science is truly astonishing. Not only do we now have a clear picture of how much ice Earth is losing, but its measurements have helped to improve the models we use to predict future climate change – information that is critical for society to adapt.”
- CryoSat has also revealed how the world’s 200,000 mountain glaciers have succumbed to climate change, thanks to advanced swath processing of its radar measurements, which allows small regions to be mapped in fine detail. This new technique takes the mission beyond its brief to study polar ice alone.
- Although changes in sea ice do not affect sea level directly because it is afloat, it plays a central role in the global climate system as it reflects solar radiation back into space, and because it moderates ocean heat transport around the planet by insulating the relatively warm water from the cold polar air. CryoSat has been instrumental in mapping changes in the thickness and volume of Arctic sea ice.
- Prof. Shepherd added, “Despite the long-term decline in the extent of Arctic sea ice, there have been significant year-to-year fluctuations in its thickness, and its volume has fallen in only seven of the past 10 years. But even with a decade of CryoSat measurements, the seasonal cycle of sea-ice growth and decay is still too large to confidently detect a long-term trend in volume, and so continued observation is essential.
- As well as fulfilling its primary role as a polar ice mission, CryoSat’s measurements have been put to good use in a wide range of alternative and innovative applications. During the winter, CryoSat has been able to record changes in the thickness of ice on lakes, and in the summer it has been used to monitor lake and river water levels across the globe – information that is important for travel and fishing, for example.
Figure 9: 2011–16 November Arctic sea-ice thickness. November Arctic sea-ice thickness as observed by CryoSat. Although November 2016 saw ice thicker than usual north of Canada, there is less ice overall in southerly regions such as the Beaufort, East Siberian and Kara Seas (image credit: CPOM/ESA)
- CryoSat’s measurements are now an important reference of global sea level in the polar regions and beyond, thanks to its high-inclination orbit and long-repeat cycle, allowing scientists to refine the long-term trend and to detect short-term fluctuations associated with ocean dynamics.
Figure 10: Gravity reveals seafloor. Scientists from Scripps Institute of Oceanography at University California San Diego used altimetry measurements from ESA’s CryoSat mission and from the CNES–NASA Jason-1 satellite to create a new marine gravity map – twice as accurate as the previous version produced nearly 20 years ago. CryoSat’s main task is to measure the elevation of the world’s ice but its altimetry measurements acquired over oceans measure sea-surface height, and this can be used to create global marine gravity models and, from them, eventually derive maps of the seafloor. The new gravity map exposes thousands of previously unchartered seamounts, ridges and deep ocean structures (image credit: Scripps Institution of Oceanography)
- And, it has even revealed what lies beneath the ocean surface thanks to its ability to detect tiny changes in marine gravity, which reflect the shape of the sea bed. CryoSat’s bathymetric charts are now an important tool for studying ocean dynamics, currents and tides, as well as for ship safety.
- ESA’s CryoSat Mission Manager, Tommaso Parrinello, said, “These are just some of CryoSat’s outstanding results and the mission is still going strong, but we will focus more on this at the CryoSat anniversary conference, which we’ve had to postpone until October because of the COVID-19 pandemic. In the meantime, however, I cannot praise the mission and all the people who have worked on it enough.”
- ESA’s Mark Drinkwater added, “Indeed, CryoSat is still giving us incredible data to advance science, and with its new synthetic aperture radar and interferometric capabilities it has also laid the foundation for the Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL) operational mission, which we are now developing on behalf of the ESA Member States and the European Commission.”
- CRISTAL will fill the recognized gap in sustained long-term monitoring of polar ice variability for the Copernicus Climate Change Service and Copernicus Marine Environment Monitoring Service, maritime security and international ice charting, and in support of the EU Integrated Arctic Policy and commitments to the Paris Agreement and Green New Deal.
• January 27, 2020: Ice loss from Pine Island Glacier has contributed more to sea-level rise over the past four decades than any other glacier in Antarctica. However, the way this huge glacier is thinning is complex, leading to uncertainty about how it is likely to raise sea level in the future. Thanks to ESA’s CryoSat-2 mission, scientists have now been able to shed new light on these complex patterns of ice loss. 25)
- Although Pine Island Glacier is one of the most intensively and extensively investigated glacier systems in Antarctica, different model projections of future mass loss give conflicting results; some suggesting mass loss could dramatically increase over the next few decades, resulting in a rapidly growing contribution to sea level, while others indicate a more moderate response.
- Identifying which is the more likely behavior is important for understanding future sea-level rise and how this vulnerable part of Antarctica is going to evolve over the coming decades.
- In a paper published in Nature Geoscience, scientists from the University of Bristol, UK, describe how they used information from CryoSat to help clarify the situation. They discovered that the pattern of ice loss is evolving in complex ways, both in space and time.
Figure 11: Pine Island Glacier thinning (image credit: University of Bristol)
- Rates in the fast-flowing central trunk of the glacier have decreased by about a factor of five since 2007 – and this is the opposite of what was observed prior to 2010.
- These new results suggest that rapid migration of the grounding line, the place where the grounded ice first meets the ocean, is unlikely over the next decades, without a major change in the role of the ocean in ice loss. Instead, the results support model simulations that imply that the glacier will continue to lose mass, but not at much greater rates than present.
- Lead author Prof. Jonathan Bamber from the University of Bristol’s School of Geographical Sciences, said, “This could seem like a ‘good news story’, but it’s important to remember that we still expect this glacier to continue to lose mass in the future and for that trend to increase over time, just not quite as fast as some model simulations suggested.
- “It’s really important to understand why the models are producing different behavior in the future and to get a better handle on how the glacier will evolve with the benefit of these new observations.
- “In our study, we didn’t make projections, but with the aid of these new data we can improve model projections for this part of Antarctica.”
- Tommaso Parrinello, ESA’s CryoSat mission manger, added, “In April, CryoSat will have been in orbit for 10 years, far exceeding its expected life. We continue to be proud of the contribution to science this extraordinary satellite mission is making.
- “And here, with the issue of sea-level rise a major global concern, the better equipped we are with facts the better it is. CryoSat has helped clarify the situation at Pine Island Glacier, which is not only important for our scientific understanding, but ultimately for society at large.”
• December 13, 2019: It is now almost 10 years since ESA’s CryoSat-2 mission was launched. Throughout its decade in orbit, this novel satellite, which carries a radar altimeter to measure changes in the height of the world’s ice, has returned a wealth of information about how ice sheets, sea ice and glaciers are responding to climate change. One of the most recent findings from this extraordinary mission shows how it can be used to map changes in the seaward edges of Antarctic ice shelves. 26)
- About three-quarters of the Antarctic coastline consists of ice shelves. They are permanent floating extensions of the ice sheet that are connected to and fed by huge ice streams draining the interior ice sheet. Ice shelves form as the ice sheet flows towards the ocean and detaches from the bedrock beneath. The advance or retreat of ice shelves is determined by a balance between mass gain from the flow of ice behind and snowfall on top, and mass loss through ocean melting at the base or iceberg calving at the edge.
- Ice shelves are important for the stability of the ice sheet because they act as buttresses, holding back the glaciers that feed them and slowing the flow of land ice into the ocean that contributes to sea-level rise.
- However, in recent years warming ocean waters and higher air temperatures are taking their toll on some of the ice shelves, causing them to thin, shrink or even collapse entirely. Therefore, mapping ice-shelf calving front locations is important for understanding and predicting future changes in the stability of the ice sheet.
Figure 12: The animation shows the gradual advance of the Filchner-Ronne ice shelf in Antarctica. Applying a new method, called ‘elevation edge’, to CryoSat data and computational theory has revealed that the entire Filchner-Ronne ice shelf advanced by more than 800 km2 per year between 2011 and 2018. The growth of the ice shelf was only interrupted by the calving of a 120 km2 iceberg in 2012 and a few smaller-scale events (video credit: ENVEO)
- A paper published recently describes how scientists have developed a novel approach of using CryoSat to generate a unique time series of ice front positions for the Filchner-Ronne ice shelf – the second largest ice shelf in Antarctica. 27)
- Jan Wuite, from ENVEO (Environmental Earth Information Technology GmbH) in Austria, said, “The detection of the calving front is based on the premise that the edge of an ice shelf is typically a steep ice cliff, with a drop of tens of meters to the ocean surface or sea-ice cover, which is clearly revealed by CryoSat.
- “Applying a new method, called ‘elevation edge’, to CryoSat’s data has revealed that the entire Filchner-Ronne ice shelf advanced by more than 800 km2 per year between 2011 and 2018. The growth of the ice shelf was only interrupted by the calving of a 120 sq km iceberg in 2012 and a few smaller-scale events.”
- Eventually, the advancing ice front is expected to break off as part of the natural ice shelf cycle, but these are rather episodic events that only happen every few years or sometimes decades. Many questions still need to be answered as to what is driving these calving events.
- Thomas Nagler, also from ENVEO, added, “Combining this new dataset with ice velocities derived from Copernicus Sentinel-1 data allows us to calculate changes in the thickness and area of the ice shelf, as well as the advance rates and iceberg calving rates, emphasizing the value of combining data from both satellite missions.”
- ESA’s Mark Drinkwater noted, “Understanding how the world’s ice shelves are changing is fundamental to assessing ice sheet stability, and the role of ice shelves in controlling ice-sheet contribution to sea-level rise.”
- “Just this week a paper was published in Nature stating that the Greenland ice sheet mass loss closely follows the IPCC high-end climate warming scenario – and the research was based on measurements from a number of different satellites.
- “Here, we see how using this innovative elevation edge method with CryoSat data is a welcome addition to standard calving front location detection techniques based on radar and optical satellite imagery. This is great news, as the more information we have the more confident we can be about what’s going on in the far reaches of the polar regions.”
- The new method provides calving front locations at regular intervals and can fill existing gaps in time and space. Moreover, it simultaneously provides ice-thickness measurements that are needed to calculate mass changes, and it also has a high degree of automation which removes the need for heavy manual intervention.
Figure 13: The Filchner-Ronne ice shelf in Antarctica. Applying a new method, called 'elevation edge', to CryoSat-2 data has revealed that the entire Filchner-Ronne ice shelf advanced by more than 800 km2 per year between 2011 and 2018. The growth of the ice shelf was only interrupted by the calving of a 120 km2 iceberg in 2012 and a few smaller-scale events (image credit: ENVEO)
- Dr Wuite added, “We fully expect that, in the future, altimetry data will deliver a systematic and continuous record of change in ice-shelf calving front positions around Antarctica.
- “With CryoSat set to remain in service and the future CRISTAL Copernicus Polar Ice and Snow Topography Altimeter mission – one of the Copernicus high-priority candidate missions – on the table for development, there are certainly excellent opportunities for satellite radar altimetry to deliver valuable new calving front location datasets to monitor the effects of climate change in Antarctica.”
• August 5, 2019: The rapidly changing climate in the Arctic is not only linked to melting glaciers and declining sea ice, but also to thinning ice on lakes. The presence of lake ice can be easily monitored by imaging sensors and standard satellite observations, but now adding to its list of achievements, CryoSat-2 can be used to measure the thickness of lake ice – another indicator of climate change. 28) 29)
Figure 14: The Great Bear Lake and Great Slave Lake in the Northwest Territory of Canada can be seen from this Copernicus Sentinel-3 image, captured on 21 May 2019 (image credit: ESA, CC BY-SA 3.0 IGO)
- CryoSat-2, one of ESA’s Earth Explorer satellites, carries the first radar altimeter of its kind. The instrument is traditionally used to determine the thickness of sea ice floating in oceans and to monitor changes in vast ice sheets on land, providing evidence of Earth’s diminishing polar ice.
- Lakes in North America’s Arctic and sub-Arctic regions cover between 15% and 40% of the landscape, and play an important role in the region’s climate. They are also a vital resource for both society and an important habitat for aquatic wildlife.
- Used as a platform for activities such as fishing, hunting and travel, knowledge of ice thickness is important for assessing safety. Monitoring changes in water volume and levels are also important for the supply of water for domestic, commercial and industrial use.
- For the first time, CryoSat-2’s altimeter has been used to measure the thickness of ice in the Great Slave Lake and the Great Bear Lake, in the Northwest Territory of Canada. Scientists from the University of Alberta and York University have documented their findings in a paper published in the Institute of Electrical and Electronics Engineers Transactions on Geoscience and Remote Sensing.
- The Great Slave Lake and the Great Bear Lake were chosen for their flat and smooth icy surfaces, and scientists were able to distinguish radar reflections from both ice-free and ice-covered areas. By subtracting the travel times of the radar signals between the ice surface and ice bottom, they were able to measure the thickness of the ice floating on the lake.
- The distance between the two reflections increased during winter, representing the seasonal thickening of the lake ice, and were then accurately validated with in situ drill-hole measurements.
- Christian Haas from the University of Bremen (formerly at York and Alberta), said, “Thanks to CryoSat-2, we are able to study seasonal changes and cycles of ice thickness, as well as volume and variability for many other smaller lakes in the sub-Arctic.
- “Although we have chosen to study the two largest lakes in the region, the same method can be applied to many other smaller lakes.
- “In addition to monitoring ice thickness, the method could also be used to retrieve lake water levels and volume throughout winter.”
- Jerome Bouffard, CryoSat-2’s data quality manager, says “We are delighted to see such results coming from CryoSat-2. Its data can provide key information over multiple surfaces that are critical to scientists’ understanding of the role that ice plays in the Earth system, at both local and global scales.”
• July 24, 2019: Our knowledge of the depth and shape of the Arctic Ocean floor – its bathymetry – is insufficient. Owing to year-round sea-ice coverage and the cost of research in this remote region, much of the Arctic Ocean’s bathymetry has remained a mystery, until now. 30)
Figure 15: Bathymetry of Chukchi Cap. Existing bathymetry map of the Arctic Ocean visible on the left, including the location of the Chukchi Sea and a ship sounding survey crossing the Chukchi Cap (red line). The figure to the right shows the difference between the predicted bathymetry and IBCAO. Only locations where bathymetry could be determined from altimetry are shown in the left figure (image credit: DTU Space)
- Bathymetry maps are crucial for studying ocean dynamics, currents and tides, as well as for ship safety. Several campaigns to map seafloor bathymetry through ship soundings have been proposed, but only small fractions of the Arctic Ocean have ever been covered.
- Scientists from DTU Space, Denmark’s national space research institute, have published a paper that reveals the first Arctic bathymetry map using marine gravity. 31)
- The surface of the ocean is not flat. Because of gravitational pull, the height of the ocean surface mimics the rise and fall of the ocean floor. Areas of greater mass such as underwater mountains have a higher gravity and therefore attract more water creating a rise in the sea surface.
- Fine-tuning the relationship between bathymetry and gravity in the Arctic Ocean has enabled scientists to calculate sea—floor bathymetry from satellite gravity measurements.
- By using ERS-1 and ERS-2, Envisat and seven years’ of CryoSat data, an altimetric gravity model has been developed by DTU Space. This has been combined with the existing IBCAO bathymetry map to create a new and improved hybrid bathymetry map of the Arctic Ocean.
Figure 16: How gravity and sea level interact. The gravitational pull of seamounts and the ‘bump’ visible on the ocean surface. Satellite radar measurements of sea level to detect these bumps in order to discover the unknown bathymetry of the ocean floor (image credit: NOAA)
- Fine-tuning the relationship between bathymetry and gravity in the Arctic Ocean has enabled scientists to calculate sea—floor bathymetry from satellite gravity measurements.
- By using ERS-1 and ERS-2, Envisat and seven years’ of CryoSat-2 data, an altimetric gravity model has been developed by DTU Space. This has been combined with the existing IBCAO bathymetry map to create a new and improved hybrid bathymetry map of the Arctic Ocean.
- CryoSat-2 was originally launched to measure sea-ice thickness, but data from the Earth-observing satellite have been exploited for other studies. Carrying a radar altimeter, the satellite can sense the gravity field at the ocean surface, so that seafloor characteristics are revealed – allowing it to map the global marine gravity field at a high spatial resolution.
- “The existing IBCAO (International Bathymetric Chart of the Arctic Ocean) bathymetry map of the Arctic Ocean is based purely on ship soundings and assisted by digital depth contours. Combining the IBCAO bathymetry with altimetry-derived marine gravity has resulted in a more accurate bathymetry map of the Arctic,” says Ole Baltazar Andersen from DTU Space.
- He continues, “The true value in the satellite data lies in the fact that it can help fill in data gaps between ship soundings, giving us a more complete picture of the Arctic bathymetry.”
- The value of the hybrid bathymetry has also been validated using recent independent ship sounding surveys accessed through the NOAA’s National Center for Environmental Information (NCEI). Over the Chukchi Cap in the Canadian Arctic, the hybrid bathymetry could improve the existing IBCAO model derived from sparse ship tracks in the region.
- “This mapping shows our satellites’ capability of providing us with new data, especially in more difficult areas such as the unknown Arctic waters,” says Jérôme Benveniste, senior advisor at ESA.
- Josef Aschbacher, ESA’s director of Earth observation programs, added, “And this is yet another piece of the jigsaw that adds to our understanding of the planet – and critically this kind of information can be used for maritime safety therefore benefiting society.”
Figure 17: Bathymetry vs ship sounding. Bathymetry of the Chukchi Cap in the Canadian Arctic, along the profile of a Healy Cruise ship sounding conducted in 2016 (image credit: DTU Space)
• July 11, 2019: We are all aware of the ebb and flow of the tide every day, but understanding tidal flow is important for a range of maritime activities and environmental monitoring, such as search and rescue operations, shipping routes and coastal erosion. The Arctic Ocean tides are particularly difficult to understand, but a new tidal model produced using ESA satellite data may shed some light on what is happening in this remote area. 32)
Figure 18: Tidal water elevations from non-linear tides displayed in meters over 48 hours in the Barents Sea, from 1 May 2019 until 2 May 2019. In this region, the displacement of water due to these non-linear tidal components can reach 40 cm, which represents approximately 30% of the local tidal signal (image credit: NOVELTIS/ESA)
- In many areas of the ocean, direct in situ measurements of ocean tides are rare. This means that tidal models need to be developed to fill in the gaps in observations.
- Owing to its location, the Arctic Ocean proves more difficult because of the scarcity of in situ observations, the frequent presence of sea ice and poorly-documented bathymetry. The bathymetry, or the depth and shape of the ocean floor, is crucial for studying ocean dynamics and for ship safety.
- Arctide2017 is a high-resolution tidal atlas of the Arctic Ocean. Developed by NOVELTIS (Labège, France), DTU Space (Lyngby, Denmark) and LEGOS (Toulouse, France), it combines altimeter data from ESA’s Envisat and CryoSat-2 satellites into the most complete dataset used in the Arctic region to estimate tidal information. 33)
- Satellite altimetry missions are often used to estimate ocean tide information, however this technique requires a long-time series of satellite data to derive accurate tidal estimates – usually more than 10 years.
- CryoSat-2 is traditionally used to determine changes in the thickness of ice, but its radar altimeter can also measure changes in sea level. Moreover, thanks to its unusually high-inclination orbit that takes it close to the poles, ESA’s CryoSat-2 mission can also provide tidal information in regions that are not sampled by other satellite altimetry missions.
- Also the number of altimeter observations increases towards the poles due to how the satellites circle Earth. This allows scientists to estimate tidal information over shorter periods of time, particularly in the Arctic Ocean.
- The Arctide2017 is based on modelling with data assimilation for the main linear tidal components, however non-linear tidal components have not yet been produced – until now.
- Tidal information extracted from satellite altimetry observations are generally reliable only for ‘linear tides,’ which describe the tide as the simple result of the gravitational pull of the Moon and Sun.
- However, other factors influence the tides such as bathymetry and coastlines, generating ‘non-linear’ tidal components. In shallower waters, a significant part of the tidal signal is due to these ‘non-linear’ components – causing very complex tides. One way of estimating these tides is through tidal modelling.
- Mathilde Cancet, a scientific engineer at NOVELTIS comments, “Thanks to the model’s grid resolution and the specific processing of the assimilated altimetry observations, the Arctide2017 regional tide atlas outperforms the concurrent regional and global models in most regions of the Arctic Ocean.”
- The Arctide tidal model will benefit satellite altimetry sea level measurements in the Arctic Ocean, as well as various end-users such as modelers and other stakeholders in the maritime industry.”
- This work was carried out in within the CryoSat Plus for Ocean (CP4O) project funded by ESA.
Figure 19: High-resolution tidal modelling. Resolution (in km) of the non-regular grid of the Arctide2017 regional tidal model. The grid of the model was specifically refined in the regions where the tides are more complex, in order to obtain a more reliable estimate (image credit: NOVELTIS)
Figure 20: Amplitude of the main tidal component in the Arctic Ocean. Amplitude (in m) of the main semi-diurnal tidal component (M2) estimated from satellite altimetry observations. Altimetry observations projected on a 1º x 3º grid in the Arctic Ocean to retrieve the ocean tide information. The data assimilation was performed using satellite altimetry observations from the Envisat and CryoSat-2 missions processed by DTU Space in order to estimate the ocean tides from the altimeter sea surface height (image credit: DTU Space/NOVELTIS)
• May 14, 2019: Ice is a hot topic when it comes to understanding and monitoring how this fragile component of the Earth system is being affected by climate change. Scientists, therefore, go to great lengths to study changes happening in the remote icy reaches of our planet – a subject that is being discussed in detail at this week’s Living Planet Symposium in Italy. Among the results being presented is a novel 3D dataset of Antarctica. Scientists from the University of Edinburgh, UK, created this new view by processing data from ESA’s CryoSat in a clever way. CryoSat carries a radar altimeter that measures the height of the world’s ice. Typically, the data are used to map the height of ice at single points. And, since it was launched in 2010, this has revealed much about how ice sheets, glaciers and sea ice are changing. 34)
- The technique is allowing scientists to better understand change and predict how ice sheets, glaciers and ice caps may behave as climate change takes a stronger grip. This is important with respect to global concerns such as sea-level rise.
- The team used this method to map Greenland in 2017, and now the Antarctica model is available. Both datasets can be downloaded from the CryoTop website.
- The CryoTop datasets contain surface elevation generated from swath processing of CryoSat-2 measurement. The CryoTop datasets also contain gridded products generated from the swath derived elevation, these are 2 Digital elevation models (500 m and 1 km posting) and 2 maps of rates of surface elevation change (500 m and 1 km posting) as well as associated errors. The swath elevation data are provided as NetCDF files following the naming convention of the original CryoSat-2 datafiles provided by the European Space Agency, the gridded products are provided as GeoTIFF files. The methodology and data format are described in the dataset user manual. 35)
Figure 21: Antarctica detailed in 3D. A technique called 'swath processing' takes the data to a new level. Scientists have used CryoSat-2’s novel ‘interferometric mode’ to produce whole swaths of data and in much finer detail and faster than is gained by conventional radar altimetry. The usual spatial resolution of a few kilometers has been improved to less than 1 km (image credit: University of Edinburgh)
• January 23, 2019: The CryoSat users are informed that the US government shutdown currently on-going is affecting the services that are responsible for providing NOAA's input data products used in CryoSat ice and ocean processing chains. 36)
- The CryoSat L1b and L2 products with dates of validity starting on 21 December 2018 are therefore affected by this situation. As a consequence, the quality of CryoSat related geophysical corrections and derived parameters (e.g. sea ice concentration in ice products and GDP+ wet tropospheric correction in GOP) are potentially degraded.
- Users will be informed at a later stage to what extent L1b/L2 parameters are directly or indirectly impacted and once the US services return to normal.
• September 15, 2018: After eight years in orbit, the status of the satellite is very good. Funds for operating the mission have been approved until the end of December 2019 and a further extension will be proposed to extend it until end of 2021 within the current Earth Observation Envelope Program (EOEP-5). With the exception of the power subsystem, which had to be switched to its backup system in October 2013, all other satellite subsystems are on the their initial primary hardware. The on-board consumables are sufficient to operate the satellite until at least 2025. 37)
- To maintain a reference orbit with an equidistant node crossings distribution and 1 km tolerance at equator, orbit housekeeping maneuvers are required on average once a month but more often during periods of high solar activity. They are always performed to minimize impact on data return and sometimes in conjunction with Collision Avoidance Maneuvers, required to avoid space debris in collision trajectory. The mission was originally not designed to maneuver away from space debris. However, the improvement of the space debris monitoring network and the ability to predict the threat well in advance, has allowed reconsideration of such operations and now they are part of the normal flight procedures. Since the launch, the satellite has performed thirteen Collision Avoidance Maneuvers and one of them was to avoid one piece of debris, which was at a radial distance less than 5 m from the satellite.
- The mission performance has surpassed the design specifications, delivering high quality data and providing unique contributions to several novel research and applications in Earth Science, both at global and regional scales.
- The mission has already generated data that has proven to be fundamental to the data records of sea-ice volume and ice sheet elevation changes. However, the large variety of challenges and scientific outcome emerging from the CryoSat-2 mission, identify this Earth Explorer as a classic example of a mission, scientifically intended for one domain that has successfully enlarged its portfolio of applications during its current lifetime including potential transit into an operational framework.
- Clearly, there is still much to be done. The mission will continue to determine decadal trends in ice sheets, ice caps, glaciers and sea ice mass to separate seasonal and interannual variability from long-term trends, and to robustly determine the impact of climate change on these trends. But new challenges are arising at the horizon such as the validation of the role of snow loading in the Arctic sea-ice, whose nature is now changing towards a system that is more similar to the Antarctic one. The mission is fit to continue observations within the framework it was designed for almost two decades ago and at the same time, it is committed to taking-up new challenges counting on synergies with existing and future missions like AltiKa, ICESat-2 and Sentinels.
• May 11, 2018: Thanks to ESA’s CryoSat-2 mission, a new map of Antarctica provides the most accurate 3D view ever of the continent’s vast ice sheet and floating ice shelves. This latest digital elevation model, which is available for download, is a result of research published recently in The Cryosphere. The model replaces the version published in March 2017. 38)
- Tom Slater from the UK CPOM (Center for Polar Observation and Modelling) said, “Our new model has several advantages over the previous one. It covers 350,000 km2 more of the continent’s surface and the resolution is twice as high, sampling the ice-sheet surface every kilometer.”
- CryoSat-2’s radar altimeter detects tiny variations in the height of the ice across the entire continent, including on the steeper continental margins where the vast majority of ice losses occur.
- This is about five million more than were used in the previous version, giving a snapshot of the height of the ice across 95% of the continent – a 3% increase on the 2017 version.
- Accurate knowledge of the current topography of Antarctica will allow scientists to better predict how the ice sheet will respond to a warming climate over the next decades.
- Andy Shepherd from CPOM added, “This model will also be useful to anybody wanting to know about the continent’s surface, whether they are planning scientific fieldwork, or modelling the ice sheet’s future behavior and potential sea level contribution.”
Figure 22: Thanks to ESA’s CryoSat-2 mission, a new map of Antarctica provides the most accurate 3D view ever of the continent’s vast ice sheet and floating ice shelves. This latest digital elevation model, which is available for download, replaces the version published in March 2017. The model uses about 250 million measurements that CryoSat-2 took between July 2010 and July 2016. This is about five million more than were used in the 2017 version, giving a snapshot of the height of the ice across 95% of the continent (image credit: CPOM)
• May 2, 2018: While ESA’s CryoSat-2 continues to provide clear insight into how much sea ice is being lost and how the Antarctic and Greenlandic ice sheets are changing, the mission has again surpassed its original scope by revealing exactly how mountain glaciers are also succumbing to change. Glaciers all over the globe are retreating – and for the last 15 years, glacial ice has been the main cause of sea-level rise. 39)
Figure 23: Apart from Antarctica, Patagonia is home to the biggest glaciers in the southern hemisphere, but some are retreating faster than anywhere else in the world. This is because the weather is relatively warm and these glaciers typically terminate in fjords and lakes, exacerbating surface melting and causing them to flow faster and lose ice as icebergs at their margins. Traditionally, it has been very difficult to map exactly how fast these glaciers are changing. However, a new way of processing ESA CryoSat-2 swath data now makes it possible to map these glaciers in fine detail. CryoSat-2 has revealed that between 2011 and 2017, there was widespread thinning, particularly in Patagonia’s more northern ice fields. The Jorge Montt glacier, which flows down to the ocean, retreated 2.5 km and lost about 2.2 Gt a year. In contrast, Pio XI, the largest glacier in South America, advanced and gained mass at a rate of about 0.67 Gt a year. However, over the six-year period, the glaciers overall lost mass at a rate of over 21 Gt a year. This loss is adding about 0.06 mm a year to sea level (video credit: ESA/Planetary Visions)
- Apart from Antarctica, Patagonia is home to the biggest glaciers in the southern hemisphere, but some are retreating faster than anywhere else in the world.
- This is because the weather is relatively warm and these glaciers typically terminate in fjords and lakes, exacerbating surface melting and causing them to flow faster and lose ice as icebergs at their margins.
- There is a clear need to monitor and understand glacial dynamics, not only in Patagonia but globally.
- However, with around 200,000 glaciers worldwide coupled with their remote rugged terrain, maintaining local monitoring systems is extremely difficult.
- Turning to space, satellite radar altimeters have been mapping ice loss from the large sheets for the last 25 years, but the footprint of this type of instrument is generally too coarse to monitor the smaller mountain glaciers.
- Fortunately, a new way of processing CryoSat-2 data now makes it possible to map these glaciers in fine detail.
Figure 24: The technique of swath processing differs from conventional radar altimetry. Using CryoSat-2’s novel interferometric mode, whole swaths, rather than single points, of elevations can be computed. This is yielding more detail that ever before on how glacial ice is changing (image credit: ESA/Planetary Visions)
- Noel Gourmelen from the University of Edinburgh said, “The technique of swath processing differs from conventional radar altimetry. Using CryoSat-2’s novel interferometric mode, we see how the radar wave front interacts with the surface.
- “We can then extract a whole swath of elevations rather than single elevation points. This is revolutionizing the use of CryoSat-2 over complex icy terrains, yielding more detail than we ever thought possible.”
- Luca Foresta, also from the University of Edinburgh, explained, “We’ve used CryoSat-2 to discover that between 2011 and 2017 there was widespread thinning, particularly in the northern part of the ice fields. For example, the Jorge Montt glacier, which flows down to the ocean, retreated 2.5 km and lost about 2.2 Gt of ice a year, and the Upsala glacier, which terminates at a lake, lost 2.68 Gt a year. In contrast, however, Pio XI, the largest glacier in South America, advanced and gained mass at a rate of about 0.67 Gt a year.”
- Over the six-year period, the Patagonian ice fields overall lost mass at a rate of over 21 Gt a year, which is equivalent to adding 0.06 mm to sea level. It is also a 24% increase compared to the amount of ice lost between 2000 and 2014.
• April 3 2018: ESA’s CryoSat-2 mission has revealed that, over the last seven years, Antarctica has lost an area of underwater ice the size of Greater London. This is because warm ocean water beneath the continent’s floating margins is eating away at the ice attached to the seabed. 41)
- Most Antarctic glaciers flow straight into the ocean in deep submarine troughs. The place where their base leaves the seabed and begins to float is known as the grounding line. These grounding lines typically lie a kilometer or more below sea level and are inaccessible even to submersibles, so remote methods for detecting them are extremely valuable.
- A paper published today in Nature Geoscience describes how CryoSat was used to map grounding-line motion along 16 000 km of Antarctic coastline. 42)
- Research led by Hannes Konrad from the CPOM ( Center for Polar Observation and Modelling) at the UK’s University of Leeds shows that between 2010 and 2017 the Southern Ocean melted 1463 km2 of underwater ice.
- The team tracked the movement of Antarctica’s grounding line thanks to CryoSat-2 and has produced the first complete map showing how this submarine edge is losing its grip on the seafloor.
Figure 25: By measuring changes in surface elevation, the retreat of glacier ground lines can be calculated. Information from ESA’s CryoSat-2 mission has revealed that, over the last seven years, Antarctica has lost an area of underwater ice the size of Greater London. This is because warm ocean water beneath the continent’s floating margins is eating away at the ice attached to the seabed (image credit: CPOM)
Figure 26: Rates of grounding line migration around Antarctica between 2010 and 2016. Most Antarctic glaciers flow straight into the ocean in deep submarine troughs. The place where their base leaves the seabed and begins to float is known as the grounding line. Information from ESA’s CryoSat mission has revealed that, over the last seven years, Antarctica has lost an area of underwater ice the size of Greater London. This is because warm ocean water beneath the continent’s floating margins is eating away at the ice attached to the seabed (image credit: CPOM) 43)
- The biggest changes are seen in West Antarctica, where more than a fifth of the ice sheet has retreated across the seafloor faster than the pace of deglaciation since the last ice age.
- Dr. Konrad said, “Our study provides clear evidence that retreat is happening across the ice sheet due to ocean melting at its base, and not just at the few spots that have been mapped before now. This retreat has had a huge impact on inland glaciers, because releasing them from the seabed removes friction, causing them to speed up and contribute to global sea-level rise.”
- Although CryoSat is designed to measure changes in the ice-sheet elevation, these can be translated into horizontal motion at the grounding line using the Archimedes principle and knowledge of the glacier and seafloor geometry.
- The researchers also found some unexpected behavior (Figure 27).
- Although retreat of the Thwaites Glacier in West Antarctica has sped up, at the neighboring Pine Island Glacier – until recently one of the fastest retreating on the continent – it has halted. This suggests that the ocean melting at its base has paused.
- Dr. Konrad added, “These differences emphasize the complex nature of ice-sheet instability across the continent, and being able to detect them helps us to pinpoint areas that deserve further investigation.”
- Co-author Andy Shepherd said, “We were delighted at how well CryoSat is able to detect the motion of Antarctica’s grounding lines. They are impossible places to access from below so it’s a fantastic illustration of the value of satellite measurements for identifying and understanding environmental change.”
- ESA CryoSat mission manager Tommaso Parrinello added, “Even though CryoSat is now approaching its eighth year in orbit – more than twice its intended lifetime – it’s wonderful to see that the mission is still making measurements of the highest quality and enabling new discoveries in polar science.”
Figure 27: Effect of grounding line on surface. By measuring changes in surface elevation, the retreat of glacier ground lines can be calculated. Information from ESA’s CryoSat-2 mission has revealed that, over the last seven years, Antarctica has lost an area of underwater ice the size of Greater London. This is because warm ocean water beneath the continent’s floating margins is eating away at the ice attached to the seabed (image credit: BAS–D. Vaughan) 44)
• October 11, 2017: We are all aware that Antarctica’s ice shelves are thinning, but recently scientists have also discovered huge canyons cutting through the underbelly of these shelves, potentially making them even more fragile. Thanks to the CryoSat-2 and Sentinel-1 missions, new light is being shed on this hidden world. 45)
- Antarctica is surrounded by ice shelves, which are thick bands of ice that extend from the ice sheet and float on the coastal waters. They play an important role in buttressing the ice sheet on land, effectively slowing the sheet’s flow as it creeps seaward.
Figure 28: Antarctic ice shelves may appear flat to the naked eye, but there can be huge changes going on underneath. Scientists have discovered that there are huge canyons cutting through the underbelly of these shelves, potentially making them even more fragile. Thanks to the CryoSat-2 and Sentinel-1 missions, new light is being shed on this hidden phenomenon (image credit: Noel Gourmelen)
- The ice sheet that covers Antarctica is, by its very nature, dynamic and constantly on the move. Recently, however, there has been a worrying number of reports about its floating shelves thinning and even collapsing, allowing the grounded ice inland to flow faster to the ocean and add to sea-level rise.
- While scientists continue to study the changing face of Antarctica, monitor cracks in the surface of the ice that might signal the demise of a shelf and learn how these changes are affecting the biology of coastal waters, they are also aware of dramatic changes taking place below the surface, hidden from view.
- There are huge inverted canyons in the underside of ice shelves, but little is known about how they form and how they affect the stability of the ice sheet.
- One type is thought to be caused by subglacial water that drains from beneath the ice sheet and runs into the ocean. In this region, the ocean water is stratified, with the warmer water at the bottom. However, as the colder meltwater pours down into the ocean it then rises because it is less dense than the seawater – but as it rises it drags up the warm bottom water which causes the underbelly of the floating ice shelf to melt.
- Another type is thought to be caused by the way ocean water circulates under the shelf.
- Scientists have been using ESA’s CryoSat-2 to study changes in the surface of the ice shelf and the Copernicus Sentinel-1 mission to study how shelves flow to learn more about what’s going on hidden from view. — Their focus has been on the Dotson ice shelf in West Antarctica.
- Noel Gourmelen from the University of Edinburgh said “We have found subtle changes in both surface elevation data from CryoSat-2 and ice velocity from Sentinel-1 which shows that melting is not uniform, but has centered on a 5 km-wide channel that runs 60 km along the underside of the shelf.
- “Unlike most recent observations, we think that the channel under Dotson is eroded by warm water, about 1°C, as it circulates under the shelf, stirred clockwise and upward by Earth’s rotation.
- ”Revisiting older satellite data, we think that this melt pattern has been taking place for at least the entire 25 years that Earth observation satellites have been recording changes in Antarctica.
- “Over time, the melt has calved in a broad channel-like feature up to 200 m deep and 15 km across that runs the entire length of the underside of Dotson ice shelf.
- “We can see that this canyon is deepening by about 7 m a year and that the ice above is heavily crevassed.
- “Melt from Dotson ice shelf results in 40 billion tonnes of freshwater being poured into the Southern Ocean every year, and this canyon alone is responsible for the release of four billion tonnes – a significant proportion.
- ”The strength of an ice shelf depends on how thick it is. Since shelves are already suffering from thinning, these deepening canyons mean that fractures are likely to develop and the grounded ice upstream will flow faster than would be the case otherwise.
- “It is the first time that we’ve been able to see this process in the making and we will now expand our area of interest to the shelves all around Antarctica to see how they are responding. We couldn’t do this without CryoSat-2 and the European Commission’s Copernicus Sentinel missions,” added Dr Gourmelen.
Figure 29: Dotson ice shelf from Sentinel-1. The Copernicus Sentinel-1 mission and ESA’s CryoSat-2 are being used to understand how a huge inverted canyon has formed in the underbelly of Antarctica’s Dotson ice shelf (image credit: ESA, the image contains modified Copernicus Sentinel data (2017), processed by A. Hogg/CPOM)
• July 5, 2017: All eyes are on Antarctica’s Larsen C ice shelf as a deep crack continues to cut across the ice, leaving a huge chunk clinging on. When it eventually gives way, one of the largest icebergs on record will be set adrift. Even before the inevitable happens, ESA’s CryoSat-2 mission can reveal some of the future berg’s vital statistics. 46)
Figure 30: CryoSat-2 reveals iceberg: Part of Antarctica’s Larsen C ice shelf will soon break away, spawning one of the largest icebergs on record. The crack in the ice shelf, which led to the birth of the iceberg, was monitored closely using radar images from the Copernicus Sentinel-1 satellites. ESA’s CryoSat-2 mission has been used to measure the thickness of the eventual berg: on average, it is 190 m thick, but at its thickest point it has a keel 210 m below the ocean surface, and it contains about 1155 km3 of ice (image credit: University of Edinburgh–N. Gourmelen)
Monitored by the Copernicus Sentinel-1 radar pair, the crack in the ice is now around 200 km long, leaving just 5 km between the end of the fissure and the ocean. While we wait for Sentinel-1 to tell us when this 6600 km2 iceberg is spawned, CryoSat-2 can reveal what the berg’s measurements will be.
This Earth Explorer satellite carries a radar altimeter to measure the height of the ice surface. In general, this information is used to work out how the thickness of sea ice and land ice is changing and, consequently, how the volume of Earth’s ice is being affected by the climate.
Noel Gourmelen from the University of Edinburgh said, “Using information from CryoSat-2, we have mapped the elevation of the ice above the ocean and worked out that the eventual iceberg will be about 190 m thick and contain about 1155 km3 of ice. “We have also estimated that the depth below sea level could be as much as 210 m.”
Icebergs calve from Antarctica all the time, but because this one is particularly large its path across the ocean needs to be monitored as it could pose a hazard to maritime traffic.
Again, Sentinel-1 and CryoSat-2 will play an important role in tracking the berg and keeping an eye on how it changes. Dr Gourmelen added, “We will continue to use CryoSat-2 to monitor how the berg changes as it drifts away from the ice shelf.”
A berg, similar in size, drifted around the Brunt ice shelf in December 2015, causing alarm for those stationed at the Halley research base, which sits on the floating section of the shelf.
Anna Hogg from the University of Leeds said, “Measurements from CryoSat showed that the Brunt berg was around 390 m, so too thick to come close to ‘shore’ since the sea is shallow here.
“As for this new Larsen C berg, we are not sure what will happen. It could, in fact, even calve in pieces or break up shortly after. Whole or in pieces, ocean currents could drag it north, even as far as the Falkland Islands. If so it could pose a hazard for ships in Drake Passage. What is certain, though, is that we shall continue to use CryoSat to keep a check on its progress.”
ESA’s Mark Drinkwater added, “Our historical effort to track large icebergs shows that those from the western Weddell Sea find their way out into the Antarctic Circumpolar Current or into the South Atlantic. It seems that only bergs from the Ross ice shelf stay in the westward coastal current and come close to Brunt ice shelf.”
The main purpose of CryoSat-2 is to give us information to understand how ice is changing to improve our understanding of Earth. The value of having satellites built to deliver for science and missions like Sentinel-1, which are built to deliver for everyday applications, is enormous.
In this case, the Copernicus Sentinel-1 mission and the ESA Earth Explorer CryoSat-2 mission complement each other, giving us a powerful tool to monitor changing ice sheets.
Figure 31: Six different satellite scatterometers are used to track icebergs around Antarctica. The image shows iceberg tracks from 1999 to 2010 (image credit: Scatterometer Climate Record Pathfinder) 47)
• March 24, 2017: Around 250 million measurements taken by ESA’s CryoSat-2 over the last six years have been used to create a unique 3D view of Antarctica, offering a snapshot of the undulating surface of this vast ice sheet. 48)
- CryoSat’s radar altimeter detects tiny variations in the height of the ice across the entire continent, including on the steeper continental margins where the vast majority of ice losses occur. Importantly, the satellite’s orbit takes it to latitudes within 200 km of the north and south poles – closer than other Earth observation satellites. Naturally, the mission is also used to map changes in the thickness of ice floating in the polar oceans, which is particularly important for the Arctic.
- The new ‘digital elevation model’ of Figure 32 was revealed at this week’s gathering of CryoSat scientists in Banff, Canada. Tom Slater, researcher at the UK CPOM (Centre for Polar Observation and Modelling), said, “We used around 250 million measurements taken by CryoSat between 2010 and 2016 to create the most comprehensive picture of Antarctic ice elevation currently available.” It offers wide range of applications – showing the surface of Antarctica in such detail means it can be used in anything from planning fieldwork to modelling the ice sheet.
- It also allows scientists to distinguish between changes in topography and ice motion when working with other satellite measurements, such as those used to calculate the balance between how much the ice sheet is gaining by accumulating snow and losing through melting and creating icebergs.
- The model will soon be freely available via the CPOM portal, which already provides information on sea-ice volume and thickness, ice velocity and, shortly, ice sheets. In the meantime, however, the model can be downloaded here.
- CPOM Director Andrew Shepherd added, “We want the digital elevation model to be accessible to anyone who uses ice-sheet surface topography measurements in their work. This should benefit not only studies of the Antarctic ice sheet, but also projections of future sea-level rise.”
- ESA’s CryoSat mission manager, Tommaso Parrinello, said, “We are hearing some great results from our mission at the meeting here in Banff.
Figure 32: Using around 250 million measurements taken by ESA’s CryoSat-2 mission between 2010 and 2016, scientists at the UK CPOM (Centre for Polar Observation and Modelling) have generated a unique 3D view of Antarctica (image credit: CPOM)
Figure 33: The most comprehensive picture to date of the height of the Antarctic ice sheet. With a resolution of 2 km, it provides an elevation measurement for 91% of the total ice on land and 97% of Antarctica’s floating ice shelves (image credit: CPOM)
• March 20, 2017: After the relative quiet of the long dark winter months, the Arctic will be a tad busier over the coming weeks as numerous researchers descend on this harsh, yet fragile environment. Their aim is not to disturb its beauty, but to join forces in an all-out effort to measure ice on land and sea. 49)
- Environmental changes in the Arctic are no longer only of interest to scientists. The need to understand and respond to dwindling polar ice is being given increasing importance at global climate discussions and vital for adopting strategies to mitigate and to adapt to change.
- Unequivocal evidence of changing polar ice comes largely from satellites. Since it was launched in 2010, ESA’s CryoSat-2, orbiting at an altitude of 700 km, has been measuring the height of the ice, both of that floating in the polar oceans and of the vast ice sheets covering Greenland and Antarctica. This provides essential information on how the thickness is changing and, in turn, how the volume of ice is changing.
- Over the last seven years, there have been several expeditions to the Arctic that involve taking measurements with a suite of sensors on aircraft and readings taken by hand actually on the ice to compare with those of CryoSat-2. By doing all this, scientists can ensure that ice-thickness maps created from satellite data are correct.
- This week sees the beginning of one of the largest Arctic expedition ever undertaken by ESA, the CryoVEx (CryoSat Validation Experiment) campaign. “We have scientists from around 10 agencies and institutes from all over the world converging in the Arctic,” explained Malcolm Davidson, head of ESA Earth observation campaigns. “We are pooling resources with other agencies such as NASA and other institutes to make our campaign a huge collaborative international effort.”
- Arne Olesen from DTU (Technical University of Denmark) added, “And, with so many people prepared to work for weeks in the most remote places on the planet and put up with the extreme cold and hazardous conditions, it just reflects how passionate and dedicated everyone is about polar science and getting the best data possible.”
- There is another purpose: to prepare for future satellite missions similar to CryoSat-2, but with even better measurement capabilities. Malcolm Davidson continued, “Our understanding of changing ice has improved enormously thanks to CryoSat-2, but we must prepare for the future now and test new types of sensors that may be able to give us even better information.
- “So, while we are out in the Arctic, we will be testing a new concept that involves a radar altimeter (Figure 34) that works with two different wavelengths instead of only one like on CryoSat-2. It’s always very exciting to be at the forefront of new technology. It is essential that we put in the groundwork to make sure a new concept will work – and, in this case, it means getting very cold and even the prospect of facing the occasional polar bear!”
- While the expedition gets underway, CryoSat-2 is also the focus of a conference in Alberta in Canada this week. Here, scientists have come together to discuss the latest results emerging from the mission.
Figure 34: Two antennas under the fuselage of the Twin Otter plane can be seen in this photograph. The bigger antenna (lower part of the image) belongs to the AS IRAS instrument and measures at Ku-band, the same frequency as that of CryoSat-2. The smaller antenna within the fuselage hole was built by MetaSensing BV and uses the higher-frequency Ka-band. Future dual-frequency satellites, which exist only on the drawing board at the moment, can be simulated from the air by combining the frequencies. The concept is being tested as part of an experiment campaign in the Arctic (image credit: ESA) 50)
• February 8, 2017: A novel way of using ESA’s CryoSat-2 mission has revealed how lakes beneath the Thwaites Glacier drained into the Amundsen Sea – potentially the largest such outflow ever reported in this region of West Antarctica. This new information is helping scientists understand more about what’s going on deep below the surface of the ice and what affects how fast the glaciers flow towards the ocean. Thwaites and its neighboring Pine Island Glacier are the fastest-receding glaciers on the Western Antarctic Ice Sheet. 51) 52)
- Although this huge sheet is some 2 km thick in places, much of its floor is well below sea level. This makes it particularly vulnerable to change, especially where the warmer ocean waters meet the underside of the floating terminus of the glacier.
- Understanding the movements of these glaciers is critical for predicting how the ice sheet may behave in the future and how it may affect sea level.
Figure 35: A novel way of using data from ESA’s CryoSat-2 mission has revealed how meltwater from lakes beneath the Thwaites Glacier drained into the Amundsen Sea – potentially the largest outflow from subglacial lakes ever reported in this region of West Antarctica (image credit: University of Edinburgh, N. Gourmelen)
Figure 36: This image from Sentinel-1 and geographic base map shows the speed of ice flow in West Antarctica. Reaching speeds of over 3 km per year, Thwaites and Pine Island are two of the fastest receding glaciers on the Western Antarctic Ice Sheet. Applying interferometric synthetic aperture swath processing techniques to CryoSat-2 data revealed that four lakes beneath Thwaites drained into the Amundsen Sea (image credit: ESA, the image contains modified Copernicus Sentinel data (2016)/CPOM University of Leeds–A. Hogg/University of Edinburgh–N. Gourmelen) 53)
- Lakes have been found under glaciers in many parts of Antarctica and are, indeed, commonly associated with fast-flowing glaciers. However, this is the first time they have been found and observed draining into the Amundsen Sea. In addition, this emptying is thought to happen only every 20–80 years.
- Water below the ice sheet plays an important role in how quickly glaciers flow towards the sea, thought to be because a layer of meltwater reduces friction between the ice and the bedrock. - In addition, when channels form under the ice they lubricate the glacier bed.
- Benjamin Smith from the University of Washington (Seattle, WA, USA) and lead author of the paper said, “This is first time we’ve been able to monitor both elevation changes and ice speed in this kind of detail over such a large area. Without a satellite like CryoSat, we would have probably have missed the lake draining and we would have had to guess how the lake drainage might have affected the ice speed. Together, they tell us about how water moving at the glacier bed affects ice speed, and what processes we need to understand so that we are better equipped to predict the future of Thwaites.”
- Noel Gourmelen from the University of Edinburgh explained, “Repeat observations from CryoSat-2 over Thwaites revealed that the surface of the ice subsided by several meters as water drained away from the four lakes under the ice. The lakes totalled an area of about 700 km2. On average, Thwaites carries about 135 km3 of ice to the sea every year, but drainage from these lakes released an extra 3.5 km3 of freshwater. In addition, the speed of the glacier increased by about 10% and would have contributed to a discharge of around 150 km3 a year between 2013 and 2014.”
- Drainage is estimated to have peaked at about 240 m3 per second, possibly the largest outflow of meltwater ever reported from subglacial lakes in this region. This peak rate is about four times faster than the River Thames in England discharges to the North Sea each year.
- Before this discovery, scientists had thought that this part of the ice sheet did not store water in lakes beneath the surface for very long because abrupt drainage had not been seen before in the area.
- Mark Drinkwater, head of ESA’s Earth observation mission science, said, “Previous studies have investigated if CryoSat-2 could be used for monitoring small vertical displacements associated with these events. The main issue has been the limited coverage of standard altimeter measurements. But thanks to new processing techniques, the capability of using CryoSat-2 to both discover and monitor Antarctic subglacial lakes has vastly increased.”
- Tommaso Parrinello, ESA’s CryoSat-2 mission manager, added, “CryoSat again is proving what a versatile satellite it is. Now we also have the Copernicus Sentinel-1, with both providing powerful tools for developing further understanding of the relationship between lake drainage and ice dynamics in Antarctica.”
Figure 37: One of four lakes under the glacier (image credit: University of Edinburgh, N. Gourmelen)
• December 16, 2016: Although not designed to deliver information on ice, ESA’s Earth Explorer SMOS satellite can detect thin sea-ice. Since its cousin, CryoSat-2, is better at measuring thicker ice scientists have found a way of using these missions together to yield an even clearer picture of the changing Arctic. 54)
- Carrying a radiometer, SMOS was designed capture images of brightness temperature. While these images can be turned into information on soil moisture and ocean salinity to improve our understanding of the water cycle, it turns out that these data can also be used to measure sea ice.
- In contrast, CryoSat-2 carries a radar altimeter that measures freeboard of sea ice, which is the distance between the waterline and the top of the ice.
- This is being used to work out how the thickness of sea ice is changing and, in addition, how the volume of Earth’s ice is being affected by the climate.
- Despite the two missions being very different, scientists from the University of Hamburg and the AWI (Alfred Wegener Institute) in Bremerhaven, Germany, who are involved in both Earth Explorer missions, have found a way of combining data from both satellites to gain a more complete picture of changes in the thickness of ice floating in Arctic waters. — While the accuracy of measurements from CryoSat-2 increases with increasing ice thickness, SMOS data are more accurate when the sea ice is relatively thin, less than about a meter.
Figure 38: The animation shows how data from CryoSat-2 and SMOS have been combined to yield a more accurate and comprehensive view of sea-ice thickness in the Arctic (image credit: AWI)
Figure 39: Although not designed to deliver information on ice, ESA’s Earth Explorer SMOS satellite can detect thin sea-ice. By combining measurements from SMOS with measurements from CryoSat-2 the two different satellites missions are yielding an even clearer picture of the changing Arctic. SMOS is also helping to improve the accuracy of sea-ice forecasts, which could help marine traffic operators determine the safest and most economic routes through waters such as the Northwest Passage and the Northern Sea Route as the ice becomes thinner owing to climate change (image credit: ESA, M. Drusch)
- CryoSat measurements yield high-spatial resolution information and cover the Arctic every month. While SMOS offers daily images, they are a much coarser resolution than those of CryoSat-2. Robert Ricker from AWI said, “By combining ice-thickness estimates from CryoSat-2 and SMOS, we obtain a more accurate and comprehensive view on the actual state of Arctic sea ice. Users need timely information across the entire Arctic and we can meet their needs by combing information from these two different, but complementary satellite missions.”
- The University of Hamburg is already using SMOS to provide daily maps of Arctic sea-ice thickness during the winter. These maps are produced within 24 hours of the measurements being taken in space. SMOS is also helping to improve the accuracy of sea-ice forecasts, which could help marine traffic operators to determine the safest and most economic routes through waters such as the Northwest Passage and the Northern Sea Route as the ice becomes thinner owing to climate change.
- In addition, both missions’ archived data have been merged to generate information on thin sea-ice going back to 2010.
Figure 40: Sea-ice change from SMOS: Based on measurements from the SMOS mission, the animation shows changes in sea-ice thickness during November between 2010 and 2016. Although designed to improve our understanding of Earth’s water cycle, SMOS is now being used to provide accurate measurements of thin sea-ice, complementing the CryoSat mission (image credit: University of Hamburg)
- This will make an important contribution to studies into the fragile component of the Earth system and help to understand annual variations and climate change. Lars Kaleschke, from the University of Hamburg, emphasized, “It is good see how information from two different types of measurements can be combined into one product to advance science and improve operational applications. It has now been demonstrated that using ice thickness information from SMOS improves the model computations and forecasts. It will be interesting to see how ocean current and air temperature models will benefit from a better understanding of the sea-ice fields.”
• November 30, 2016: ESA’s CryoSat-2 satellite has found that the Arctic has one of the lowest volumes of sea ice of any November, matching record lows in 2011 and 2012. Early winter growth of ice in the Arctic has been about 10% lower than usual. - CryoSat carries a radar altimeter that can measure the surface height variation of ice in fine detail, allowing scientists to record changes in its volume with unprecedented accuracy. These observations are vital for tracking climate change and are an essential resource for maritime operators who increasingly navigate the icy waters of Earth’s polar regions. 55)
Figure 41: November Arctic sea-ice thickness as observed by CryoSat. Although November 2016 saw ice thicker than usual north of Canada, there is less ice overall in southerly regions such as the Beaufort, East Siberian and Kara Seas (image credit: CPOM/ESA)
- The US NSIDC (National Snow and Ice Data Center) reported that the area of the Arctic covered by sea ice fell to 4.1 million km2 in September this year – slightly less than the sea-ice extent in September 2011. - But CryoSat-2 shows that the ice was thicker at the end of summer than in most other years, at 116 cm on average. This means there was substantially more ice this year than in 2011.
- Thicker ice can occur if melting is lower, or if snowfall or ice compaction is higher. However, the Arctic usually gains about 161 km3 of ice per day in November, but this year’s growth has been about 10% lower, at 139 km3 per day, with a total ice volume estimated to have accumulated to 10,500 km3 by the end of the month.
- This would essentially tie with conditions in the Novembers of 2011, when levels were at their lowest on record for this time of the year. Although sea ice in the central Arctic is currently thicker than it was in 2011, there is far less ice in more southerly regions such as the Beaufort, East Siberian and Kara Seas.
- “Because CryoSat can measure Arctic sea ice thickness in autumn, it gives us a much clearer picture of how it has fared during summer,” said Rachel Tilling, at the UK’s CPOM (Center for Polar Observation and Modelling), who came to these conclusions. “Although sea ice usually grows rapidly after the minimum extent each September, this year’s growth has been far slower than we’d expect – probably because this winter has been warmer than usual in the Arctic.”
- As demand for information on Arctic conditions increases, CryoSat-2 has become an essential source of information for polar stakeholders, ranging from ice forecasting services to scientists studying the effects of climate change. “In its short, six years of life, we have learnt more about Arctic sea ice from CryoSat-2 than from any other satellite mission,” commented CPOM Director and principal scientific advisor to the CryoSat mission, Professor Andrew Shepherd. “To understand the role that sea ice plays in the climate system, and the restrictions it places on maritime operations, we must ensure that its measurements are continued into the future.”
- CPOM plans to release a complete assessment of 2016 sea ice conditions in the coming weeks.
Figure 42: 2011–16 November Arctic sea-ice volume. Early-winter Arctic sea-ice volume as observed by CryoSat-2. Sea-ice growth in November 2016 has been about 10% lower than usual, and ties with November 2011 and 2012 as a record low (image credit: CPOM/ESA)
• July 26, 2016: Trying to measure sea levels around rugged coastlines is not always an easy task. ESA’s CryoSat-2 satellite is making a difference with its radar altimeter. Sea level is a very sensitive indicator of climate change, reflecting components of the climate system such as heat, glaciers and the melting of ice-sheets. Precisely monitoring changes in the average level of oceans is vitally important for understanding not only climate but also the social and economic consequences of any rise in sea level, especially in coastal zones. Previous radar altimeters have been aimed at measuring oceans and land, but CryoSat’s is the first sensor of its kind designed for ice, and able to map sea levels with unprecedented accuracy. 56)
- Scientists from the Norwegian University of Life Sciences also discovered that CryoSat had the potential to map sea level closer to the coast. Using satellite altimeters in coastal zones is notoriously difficult. Norway boasts the world’s second longest coastline of some 100,000 km, comprising many islands, steep mountains and deep, narrow fjords. The rugged coastline means that other altimeters produce confused readings close to the coast, showing differences of 10 cm or more. By contrast, CryoSat’s results compare favorably with the Stavanger tide gauge in southwestern Norway, provided by the Norwegian Mapping Authority.
- While classical altimetry offers a few tens of observations over a five-year period, more or less near a tide gauge, some 7000 measurements very close to the gauge are obtainable with CryoSat-2. The result is a better affinity with the Stavanger data, to within 7 cm for CryoSat-2, contrasting with the 10–15 cm for classical altimetry. This demonstrates the superior accuracy of CryoSat-2 in the coastal zone, where the regional impact of sea level rise is more important for humans.
- “Conventional altimeters on satellites like Envisat and Jason-3 typically have 10–30 times larger footprint than the new altimeters on CryoSat and Sentinel-3,” comments Ole Baltazar Andersen, senior scientist at the National Space Institute and DTU Space of Denmark. “Hence, the radar pulse used to measure the sea-surface height is more frequently disturbed in the coastal zone. Therefore, you have to go further from the coast to obtain accurate observations with conventional altimeters. “Consequently, you are not measuring the sea-surface height right at the coast, as we now can with CryoSat and Sentinel-3.” CryoSat-2 also gives favorable results along the remaining Norwegian coast. In comparison with the Kabelvåg tide gauge in the Lofoten area in northern Norway, differences as low as 5.4 cm were obtained. There are now great expectations from the more recent Sentinel-3 mission, which carries a similar altimeter.
• July 21, 2016: After six years in orbit, the status of the satellite remains very good. Funds for operating the mission have been approved until the end of February 2017 and a further extension is foreseen into the next phase of the EOEP-5 (Earth Observation Envelope Program). With the exception of the power subsystem, which switched to its redundant side in October 2013, all other subsystems on the satellite remain in their prime. The onboard consumables are sufficient to operate the satellite until 2025. Recently, the onboard startracker software has been upgraded to improve robustness and performance ready for the next mission extension phase. 57)
- Since launch, the CryoSat ground segment has evolved significantly and has included new products in response to requirements coming from various scientific communities, mainly from areas such as oceanography, marine gravity and hydrology. Two major versions of CryoSat product portfolio have been released to users since launch and a new one is expected by the end of 2017. - The number of CryoSat users has more than tripled during its first six years in orbit, confirming the high level of interest of the worldwide scientific community in this mission.
• April 21, 2016: Sea ice physicists from the AWI (Alfred Wegener Institute) / Helmholtz Center for Polar and Marine Research (Bremerhaven, Germany), are anticipating that the sea ice cover in the Arctic Ocean this summer may shrink to the record low of 2012. The scientists made this projection after evaluating current satellite data about the thickness of the ice cover. The data show that the arctic sea ice was already extraordinarily thin in the summer of 2015. Comparably little new ice formed during the past winter. Today Marcel Nicolaus, expert on sea ice, has presented these findings at a press conference during the annual General Assembly of the European Geosciences Union in Vienna. 58)
- Predicting the summer extent of the arctic sea ice several months in advance is one of the great challenges facing contemporary polar research. The reason: until the end of the melting season the fate of the ice is ultimately determined by the wind conditions and air and water temperatures during the summer months. Foundations are laid during the preceding winter, however. This spring, they are as disheartening as they were in the negative record year of 2012. Back then, the sea ice surface of the Arctic shrunk to a record low of 3.4 million km2.
- “In many regions of the Arctic, new ice only formed very slowly due to the particularly warm winter. If we compare the ice thickness map of the previous winter with that of 2012, we can see that the current ice conditions are similar to those of the spring of 2012 – in some places, the ice is even thinner,” Marcel Nicolaus, sea ice physicist at AWI, said today at a press conference during the EGU (European Geosciences Union) General Assembly in Vienna. 59)
- Together with his AWI colleague Stefan Hendricks, they evaluated the sea ice thickness measurements taken over the past five winters by the CryoSat-2 satellite for their sea ice projection. Seven autonomous snow buoys, which the AWI researchers had placed on floes last autumn, supplied additional important clues. In addition to the thickness of the snow cover on top of the sea ice, the buoys also measure the air temperature and air pressure. A comparison of their temperature data with the AWI long-term measurements taken on Spitsbergen, has shown that the temperature in the central Arctic in February 2016 exceeded average temperatures by up to 8ºC.
Buoy data show: the sea ice did not melt during the winter, but it grew only slowly.
- Contrary to a report published by US researchers, this warmth did not result in the thinning of the sea ice cover in some regions over the course of the winter. “According to our buoy data from the spring, the warm winter air was not sufficient to melt the layer of snow covering the sea ice, let alone the ice itself,” Marcel Nicolaus explains. During the past winter, the growth of the arctic sea ice was significantly slower than the scientists had expected.
- In previously ice-rich areas such as the Beaufort Gyre off the Alaskan coast or the region south of Spitsbergen, the sea ice is considerably thinner now than it normally is during the spring. “While the landfast ice north of Alaska usually has a thickness of 1.5 meters, our US colleagues are currently reporting measurements of less than one meter. Such thin ice will not survive the summer sun for long,” Stefan Hendricks, AWI sea ice physicist, explained.
Large amounts of thick pack ice will be carried away by Arctic sea currents before the autumn.
- Examining the CryoSat-2 sea ice thickness map for this spring, Stefan Hendricks further explained: “The Transpolar Drift Stream, a well-known current in the Arctic Ocean, will be carrying the majority of the thick, perennial ice currently located off the northern coasts of Greenland and Canada through the Fram Strait to the North Atlantic. These thick floes will then be followed by thin ice, which melts faster in the summer. Everything suggests that the overall volume of the arctic sea ice will be decreasing considerably over the course of the coming summer. If the weather conditions turn out to be unfavorable, we might even be facing a new record low,” Stefan Hendricks said.
- According to the AWI scientists, the extent of the ice loss will be great enough to undo all growth recorded over the relatively cold winters of 2013 and 2014. AWI researchers observed a considerable decrease in the thickness of the sea ice as early as the late summer of 2015, even though the overall ice covered area of the September minimum ultimately exceeded the record low of 2012 by approximately 1 million km2. The unusually warm winter has thus contributed to the likely continuation of the dramatic decline of the Arctic sea ice throughout 2016.
Figure 43: This map shows in which regions the Arctic sea ice in Feb. 2016 was thinner (blue) or thicker than in Feb. 2012 (image credit: AWI, Stefan Hendricks)
Figure 44: Plot of the CryoSat-2 sea ice thickness data for February 2016 (image credit: AWI, Stefan Hendricks)
• March 2016: Urgent call for a follow-up mission. A group of 179 researchers is concerned the ageing CryoSat-2 mission could die in orbit at any time. They have urged the EC (European Commission) and ESA (European Space Agency) to start planning a replacement. "The mission is now central to international efforts to monitor the state of the cryosphere," they write in a letter to top officials at the EC and ESA. 60)
- CryoSat-2 was launched in 2010 on what was initially supposed to be just a one-off, three-and-half-year observation of marine and land ice - to get a snapshot of any gains and losses. But the performance of the spacecraft's mapping instrument SIRAL ( SAR Interferometer Radar Altimeter) - has exceeded all expectations, and made for some compelling data sets. The satellite has delivered the first complete assessment of Arctic sea-ice thickness and volume, as well as the most precise measurements yet of the volume and mass of the great ice sheets covering Antarctica and Greenland.
- "Over recent years, the ESA CryoSat-2 satellite has significantly improved our understanding of how polar ice sheets - in particular, the WAIS (West Antarctic Ice Sheet) - are changing and contributing to current global sea-level rise," said letter signatory Prof David Vaughan, the director of science at the British Antarctic Survey. "Many of the recent improvements in the models we use to predict the future of the WAIS were driven by the requirement to accurately simulate CryoSat's observations. So maintaining the record of ice-sheet change in future decades will be vital if we are to achieve the most rapid possible improvements in future projections of sea-level rise," he told BBC News.
- How long CryoSat-2 can keep working is anyone's guess. It has enough fuel to sustain itself into the early 2020s but component failure in the harsh environment of its orbit, 720 km above the Earth, is an ever-present risk.
- If there is to be a CryoSat-3, it will not come directly out of the ESA stable. The agency's job is to develop new technologies; its remit does not extend to funding ongoing, repeat missions. This means a successor would fit better within the Copernicus series of satellites - known as the Sentinels - which are currently being rolled out by the European Commission, paid for by EU member states; ESA participates only as the technical advisor.
- One of these new platforms, Sentinel-3, can do some work in polar regions: it has a radar altimeter to sense ice surfaces, too. But the spacecraft's orbit does not reach the same heights, meaning its data contains a 1,860 km wide "hole" at northernmost and southernmost latitudes. This makes it blind to most Arctic sea-ice, for example. Additionally, Sentinel-3's radar does not operate in a so-called interferometric mode. This is the capability that allows CryoSat to measure the slopes and ridges at the edges of the ice sheets, where losses in Antarctica and Greenland have been most pronounced.
- "The Copernicus program is a phenomenal achievement for Europe and Sentinel-3 will be doing vital work, especially over the oceans, but we'd really like to see Copernicus incorporate a proper polar Sentinel," said Prof Andy Shepherd, the principal scientific advisor to the CryoSat mission.
- The scientists are hoping for a swift process with a positive outcome. They want to avoid the gap in observations that would arise if CryoSat-2 fails and a successor is not ready. "A continuation assures a consistency in the estimates of the contribution of ice sheets to sea level change using altimetry," explained Prof Angelika Humbert from the Alfred Wegener Institute for Polar and Marine Research, Germany. "The highly sophisticated processing scheme and the error estimates are established already. In short, we know the sensor well; we've already got quite far with whatever we can squeeze out from its signals and the most benefit, with most efficiency, would come from continuing the mission."
• December 11, 2015: The satellite age has revolutionized our understanding of Earth, giving us accurate information to help critical agreements on climate change such as at the current COP21 (Conference of Parties 21) in Paris , also known as the 2015 Paris Climate Conference within the UNFCCC (UN Framework on Climate Change). Diminishing polar ice is one of the most visible indicators of change, but how much have we learnt over the last decades? 61)
- Spectacular feats of polar exploration actually go back to the 1800’s when early expeditions offered a rare glimpse into these icy regions. However, it is only relatively recently that we have understood the importance of ice in the climate system and have evidence that these frozen expanses are becoming a casualty of climate change.
- Arctic sea ice, for example, is particularly sensitive to our warming climate and is often cited as a barometer of global change. Ice that forms and melts in the ocean only has a very tiny effect on sea level – the melting of ice sheets and glaciers that overlie land are the main causes of sea-level rise, along with the thermal expansion of the water.
- However, sea ice does affect how much sunlight is reflected back out to space, it affects global heat transport by insulating the relatively warm ocean from the cold polar atmosphere, and it significantly influences ocean circulation patterns, which play a role in our global climate system.
- Because of the remoteness, extreme cold and hostile conditions of the Arctic, it is impossible to acquire frequent all-weather measurements any other way than from space. Each year, the polar oceans experience the formation and then melting of vast amounts of sea ice. Around the North Pole, an area roughly the size of Europe melts every summer and then freezes again the following year.
- Scientists have been using radar measurements from satellites such as ERS-1, ERS-2 and Envisat for more than 25 years to study this seasonal change in ice extent. They have found that since 2000 the area of the Arctic Ocean covered by ice in the summer has reduced drastically.
- For example, in September 2007, it was discovered that the sea ice had shrunk to its lowest level since satellite measurements began, opening up the Northwest Passage, a long-sought shortcut between Europe and Asia that had been historically impassable.
- The extent of ice reached the lowest on record in September 2012. However, the area of the ocean covered by ice is only part of the story. It is also essential to have measurements of the thickness of the ice to work out how the actual volume is changing.
- Launched in 2010, ESA’s CryoSat-2 satellite has shed new light on diminishing polar ice. ESA’s CryoSat mission manager, Tommaso Parrinello, said, “By measuring the height of the ice, both of that floating in the polar oceans and of the vast ice sheets covering Greenland and Antarctica, CryoSat is providing essential information on how the ice thickness is changing.”
Figure 45: Arctic sea-ice thickness in October–November 2015 as measured by ESA’s CryoSat-2 mission (image credit: ESA/CPOM)
• July 20, 2015: Measurements from ESA’s CryoSat-2 satellite show that the volume of Arctic sea ice increased by a third following the unusually cool summer of 2013. This new finding suggests that ice in the northern hemisphere is more sensitive to changes in summer melting than it is to winter cooling. 62)
- Scientists at University College London (UCL) and the University of Leeds in the UK used 88 million sea-ice thickness measurements taken by CryoSat between 2010 and 2014. The study, published today in Nature Geoscience, shows a 14% reduction in the volume of summer sea ice between 2010 and 2012, but the volume of ice jumped by 41% in 2013, when the summer was 5% cooler than the previous year. Lead author Rachel Tilling, from CPOM (Centre for Polar Observation and Modelling) at UCL, said, “The summer of 2013 was much cooler than recent years, with temperatures typical of those seen in the late 1990s. “This allowed thick sea ice to persist northwest of Greenland because there were fewer days when it could melt. Although models have suggested that the volume of Arctic sea ice is in long-term decline, we know now that it can recover by a significant amount if the melting season is cut short.” 63)
- The team say although the first five years of CryoSat-2 measurements have revealed important information on the state of Arctic sea ice, the record is still short to establish a long-term trend. The team now plans to use the measurements of CryoSat-2 of changing sea-ice thickness to help improve the models that are used to predict future climate change, and also to assist maritime activities in the Arctic region, which can be dangerous and costly to navigate.
Figure 46: Changes in autumn Arctic sea-ice observed by CryoSat-2 during the period 2010-2014 (image credit: UCL/CPOM/University of Leeds)
• May 22, 2015: A recent acceleration in ice loss in a previously stable region of Antarctica has been detected by ESA’s ice mission. The latest findings by a team of scientists from the UK’s University of Bristol show that with no sign of warning, multiple glaciers along the Southern Antarctic Peninsula suddenly started to shed ice into the ocean starting in 2009 at rate of about 60 km3 each year. 64)
- This makes the region one of the largest contributors to sea-level rise in Antarctica, having added about 300 km3 of water into the ocean in the past six years. Some glaciers along the coastal expanse are currently lowering by as much as four m each year. Prior to 2009, the 750 km-long Southern Antarctic Peninsula showed no signs of change.
- “It appears that sometime around 2009, the ice-shelf thinning and the subsurface melting of the glaciers passed a critical threshold that triggered the sudden ice loss,” said Bert Wouters from the University of Bristol, who led the study. “However, compared to other regions in Antarctica, the Southern Peninsula is rather understudied, exactly because it did not show any changes in the past, ironically.”
- The study includes five years of measurements from ESA’s ice mission, CryoSat-2, which employs an advanced radar altimeter that can measure the surface height variation of ice in fine detail, allowing scientists to record changes in its volume with unprecedented accuracy. The ice loss in the region is so large, that it has even caused small changes in Earth’s gravity field, detected by the US/German GRACE mission (Figure 48). — Climate models show that the sudden change cannot be explained by changes in snowfall or air temperature. Instead, the team attributes the rapid ice loss to warming oceans.
Figure 47: Southern Antarctic Peninsula ice loss since 2009 (image credit: University of Bristol)
Legend to Figure 48: Changes in Earth’s gravity field resulting from loss of ice from West Antarctica between November 2009 and June 2012 (mE = 10–12 s–2). A combination of data from ESA’s GOCE mission and NASA’s GRACE satellites shows the ‘vertical gravity gradient change’.