Last modified on Mar 18, 2024 14:30

Contributors: Jacqueline Bannwart (University of Zurich), Inés Dussailan (University of Zurich), Frank Paul (University of Zurich), Michael Zemp (University of Zurich)

Issued by: UZH / Michael Zemp, Inés Dussaillant

Date: 09/01/2023

Ref: C3S2_312a_Lot4.WP3-TRGAD-GL-v1_202204_MC_TR_GA_i1.1

Official reference number service contract: 2021/C3S2_312a_Lot4_EODC/SC1

History of modifications

Related documents

Acronyms

General definitions

Glacier mass balance definitions and related terms as from the "Glossary of Mass Balance and Related Terms" (Cogley et al., 2011).

Altimetry
A remote-sensing technique in which surface altitudes (elevations) are estimated as a function of the travel time of a pulse

Digital elevation model (DEM)
An array of numbers representing the elevation of part or all of the Earth's surface as samples or averages at fixed spacing in two horizontal coordinate directions. Digital elevation models are now the preferred means of representing the elevation changes on which mass-balance measurements by geodetic methods are based. The elevation change is calculated by subtracting an earlier DEM from a later DEM.

Elevation change
Vertical change in glacier surface elevation (altitude), typically derived from two elevation measurements, adjusted if necessary for the difference of their respective datum surfaces, at the same (or nearly the same) horizontal coordinates.

Geodetic method
Any method for determining mass balance by repeated mapping of glacier surface elevations to estimate the volume balance; cartographic method and topographic method are synonyms. The conversion of elevation change to mass balance requires information on the density of the mass lost or gained, or an assumption about the time variations in density.

Glaciological method
A method of determining mass balance in-situ on the glacier surface by measurements of accumulation and ablation, generally including measurements at stakes and in snow pits; direct method has long been a synonym. The measurements may also rely on depth probing and density sampling of the snow and firn, and coring. They are made at single points, the results from a number of points being extrapolated and integrated to yield the surface mass balance over a larger area such as an elevation band or the entire glacier.

Gravimetric method
A technique in which glacier mass variations are calculated from direct measurements of Earth's gravity field. Satellite gravimetry is at present the most feasible method for determining glacier mass balance from changes in gravity. The Gravity Recovery and Climate Experiment (GRACE) consists of two polar-orbiting satellites separated by about 200 km along-track, and is the primary mission for this work to date.

Interferometry
Measurement of the interference of waves, particularly electromagnetic waves, from a common source such as a radar, with the aim of obtaining information about the topography, velocity field and other characteristics of the glacier surface.

Remote sensing
Measurement of surface properties with a sensor distant from the surface, such as on an airplane or satellite, or of subsurface properties with a sensor on or distant from the surface, either with a signal emitted by the sensor.

Further definitions for the present document

Brokered
No algorithms are involved in the transfer of datasets from one database to another.

"Brokered" use in this document: Indicates that independent observations on glacier elevation and mass changes derived from the geodetic and glaciological methods are brokered to the Fluctuations of Glaciers database (FoG) directly from the World Glacier monitoring Service (WGMS) database, and subsequently, brokered from the FoG directly to the Climate Data store (CDS) Glacier service.

Scope of the document

This report corresponds toto the Target Requirements and Gap Analysis Document (TRGAD) for the glacier essential climate variable (ECV) within the Land Hydrology and Cryosphere (LHC) service. We here provide users with the relevant information on the glacier change climate data record (CDR) product requirements and existing gaps on the current product version. A related document for the glacier area service is also available (Target Requirements and Gap Analysis Document, RD1).

The scope of this document is threefold: First, it defines the evolving target requirements for the glacier change CDR and the associated data products based on Global Climate Observing System (GCOS) requirements and Copernicus Climate Change Service (C3S) user needs. Secondly, it describes gaps and limitations to the data fitness-for-purpose according to the specific target requirements, identifying opportunities and needs to improve the glacier change dataset. Thirdly, it addresses limitations in the existing coverage, processing algorithms and methods for estimating uncertainties, identifying scientific research needs and opportunities for exploiting the new glacier change observations.

This document refers to the current glacier change products as provided in the Climate Data Store (CDS) from the FoG database versions 20191202 and 20200824.

Executive summary

This document provides gives an overview on the C3S glacier service products for the Glacier ECV, explains user requirements that force the evolution of the datasets and describes the gap analysis or existing limitations of the product and the opportunities for improvement.

The C3S glacier change service provides time series of glacier elevation and mass change products, as available from the last updated Fluctuations of Glaciers (FoG) database. The FoG database is assembled from a collection of glacier observation datasets provided by numerous individuals worldwide, both from in-situ and remote sensing methods. These various glacier observations are collected and homogenized by World Glacier Monitoring Service (WGMS) (wgms.ch) on a yearly basis. Specific observations on glacier elevation and mass changes derived from the geodetic and glaciological methods, are then brokered directly to the FoG database (i.e. no algorithms involved). This document refers to the glacier change products as provided in the Climate Data Store (CDS) from the FoG database versions 20191202 and 20200824.

The target requirements for the C3S glacier elevation and mass change products are essentially in accord to the requirements established by international glaciological organisations (e.g. Integrated Global Observing Strategy (IGOS) and GCOS). We show how these requirements have evolved through time, mostly in line with advances in remote sensing technology and data availability. The evolving needs from the scientific community are also accounted.

Finally, the gap analysis section identifies current and future data availability, possible further development of algorithms, and opportunities to take full advantage of current, external research activities. Missing fundamental research lines are highlighted.

1. Product overview

1.1. Input and auxiliary data

The dataset that is provided for the CDS by the glacier change service is an extract from the FoG database that is brokered from WGMS (wgms.ch). It consists of time series of glacier elevation and mass changes in two separate csv files each with a shape file providing meta-information for each glacier listed in the related csv file. The mass balance time series represents the annually updated dataset from the field-based surveys (using the glaciological method). The elevation change time series presents remote sensing-based results (using the geodetic method) for region specific time periods (a few decades). The latter dataset has recently grown steadily and is much larger than the former in regard to the number of glaciers included.

Glacier mass changes derived from the glaciological method in the field do not require specific input data but some auxiliary data to compute glacier-specific results from the raw field measurements. To correct any possible biases in the glaciological time series (see Section 1.2), highly resolved geodetic estimates are usually used as auxiliary data for calibration (Zemp et al., 2013).

Glacier elevation changes derived from remote sensing with the geodetic method requires information about the elevation of the glacier surface (i.e. topography) from at least two points in time. These can be provided by (at least) two Digital Elevation Models (DEMs) acquired at different dates, repeat altimetry data, or a combination of the two. As glaciers are often small and located in steep mountain terrain, DEM differencing has the better performance and is most widely applied. Further auxiliary data are, in general, not required. However, for co-registration of the DEMs or uncertainty assessment (see Section 1.2), some authors additionally use Ice, Cloud and Elevation Satellite (ICESat) data or very high-resolution DEMs as validation datasets. Most of the datasets provided cover local, sub regional or regional (mountain range) scales but are rapidly being extended to a larger continental and global scale during the recent years.

1.2. Retrieval Algorithms and uncertainty estimation

Note that the dataset provided for the CDS by the glacier change service is an extract from the FoG database. For this reason, the retrieval algorithms and uncertainty estimation of the related products are inherent to the specific glaciological and geodetic methods used by the data providers. We do not perform any further processing of the datasets. Therefore, in this section we briefly explain the algorithms for the glacier mass and elevation change observations respectively. The uncertainty estimates are described in Section 3.3.

1.2.1. Glacier mass change observations:

Glacier mass change observations are retrieved by the glaciological method, consisting on in situ measurements of annual or sub-annual accumulated snow and surface wastage (i.e. ablation) at a series of points distributed along the glacier surface (Fountain et al., 1999; Cogley et al., 2011). Local mass balance at the end of the accumulation season is estimated by snow probe measurements along the glacier surface, together with density measurements at snow pits. During the ablation season, the emergence of long stakes installed the previous year allows determining the layer of snow or ice that has been lost. The annual glacier-wide surface mass balance is then obtained by extrapolation of the point measurements over a larger area such as an elevation band or the entire glacier. If measurements are performed repeatedly and during an extended period of time, the glaciological method will provide crucial information about the temporal variability of glacier changes. However, for practical reasons, only small and accessible glaciers can be monitored with this method, and the extrapolation to larger regions is challenging as the observed changes are not necessarily representative of the regional response.

1.2.2. Elevation change observations:

The geodetic method is the basic processing used to derive elevation change observations over glaciers during a specific period of time by comparing repeated surveys of its surface topography. It needs at least two, or more, different topographic maps or DEMs of the glacier surface obtained at different dates. Important pre and post processing steps have to be applied. First, the DEMs have to be co-registered on a horizontal and vertical basis, for example following the semi-automated method proposed by Nuth and Kääb (2011). When glacier-wide mean values are to be computed, data voids and artefacts have to be considered and their impact reduced (McNabb et al., 2019). In many cases, further biases derived from the satellite sensor characteristic need to be corrected (e.g. across and along track biases, biases due to different DEM resolution, Synthetic Aperture Radar (SAR) penetration issues). After differencing both bias corrected topographies, the resulting elevation changes are integrated over the entire glacier area. To ensure a reliable elevation change estimation, a relatively complete coverage of the entire glacier surface is a key aspect of the input DEMs, as high biases may arise if only a fraction of the glacier is surveyed (e.g. only the glacier tongue, Berthier et al., 2010)). The precision of the results is also a function of the time separation between DEMs, where time periods larger than 5 or 10 years are usually preferred to minimize uncertainties. Finally, changes on stable terrain off glaciers are calculated to determine the product uncertainty. The uncertainties of the geodetic method will depend mostly on the precision of the DEM. Consequently, the characteristics of the input data and the technique selected to elaborate the DEMs will determine to a great extent the final accuracy of the final product.

2. User Requirements

Several requirements for the elevation and mass change products of the ECV Glaciers have already been listed and described in previous documents from international organizations, foremost in the Integrated Global Observing Strategy Cryosphere Theme Report (IGOS, 2007) and the Global Climate Observing System reports (GCOS, 2011, 2016) identifying glaciers as the Product T.3.2: "Elevation change of glaciers and ice caps, from geodetic methods, in regions where outlines are available". These requirements represent the international community needs and therefore apply as well to the C3S glacier service.

2.1. IGOS 2007

A detailed overview of technical requirements for glacier observations is provided in the Appendix of the (IGOS, 2007). The relevant entries for the elevation (i.e. topography) and mass balance products are shown in Table 1.

Table 1: Target requirements for glaciers according to IGOS (2007).

C: Current Capability, T: Threshold Requirement (Minimum necessary), O: Objective Requirement (Target), L: Low end of measurement range, U: Unit, H: High end of measurement range, V: Value, mo: month, yr: year.

The values listed for measurement range and accuracy still reflect the current target requirements. The topography parameter refers to the quality requirements of DEMs used to determine glacier volume and mass changes with the geodetic method (DEM differencing). Given the current vertical DEM accuracies ranging between 2-8 m, an accuracy of 0.1 m can only be achieved with DEMs acquired 20-80 years apart. This is still unrealistic for optical stereoscopic and interferometric radar sensors but will be possible soon. Altimetry sensors such as ICESat, can reach accuracies of 0.2 m/yr for cross-over points (Moholdt et al. 2010) but observations are spatially limited, as only points are measured and spatial extrapolation introduces other uncertainties.

Glacier mass balance measurements require in situ measurements. Determining mass balance from space is not (yet) possible as the evolution of the density of the snowpack cannot be determined remotely. The proposed combination of a process model with SAR-based measurements is a theoretical possibility to determine snow water equivalent (SWE) but has so far not materialized as the required SAR sensors have not been launched.
Hence, for the time being the strategy is to:

1. Ensure the continuity of annual/seasonal in-situ mass balance measurements on selected glaciers.
2. Ensure good representativity of mass balance measurements in all glaciated regions
3. Validate and calibrate these carefully with the geodetic methods on a decadal time scale.
4. Determine the representativeness of the measured glaciers for the entire mountain range from DEM differencing (e.g. Paul and Haeberli, 2008; Le Bris and Paul, 2015) to improve spatial up-scaling.

The two parameters stated in IGOS, 2007 i) mass balance, representing annual and seasonal variability of glacier changes and (ii) topography, representing the quality of the DEMs that allow to calculate glacier elevation changes, remain key in any glacier change related exercise and therefore still apply for the glaciers ECV within the Copernicus glacier service.

2.2. GCOS report 2011, 2016

Similar to IGOS, 2007 the GCOS, 2011 report provides an overview of the technical requirements that have been adopted for glacier change related products. In the latest GCOS Implementation Plan (GCOS, 2016) the requirements for the glacier mass and elevation changes have been updated to overcome the potential confusion of glacier elevation changes with glacier topography (Table 2).

It is assumed here that measurement uncertainties for annual field observations of mass changes are two times better (10 cm/y) than for satellite derived elevation changes (20 cm/year).

It is important to state that the user requirements listed in the IGOS and GCOS documents are critical in ensuring the evolution towards better quality glacier change related estimations, both at the spatial (better resolved and complete dataset and related products) and temporal resolution (annual and seasonal), allowing to produce final products in line with the evolving user needs. Some of the direct benefits are listed below.

• Support for the instrumental data record of climate by providing climate-related information, further back in time, in remote areas and at higher altitude than meteorological stations.
• Input to regional climate models and the validation of impact assessment and climate scenarios on a regional scale.
• Computation of glacier melt contribution to regional hydrology and global sea-level rise; and
• Support the in-situ mass-balance measurements to assess their representativeness for entire mountain ranges as well as to extend data coverage in space and time.

Table 2: Target requirements for glaciers according to GCOS (2016).

3. Gap Analysis

3.1. Description of past, current and future glacier change observations

3.1.1. Historic development of glacier change observations

For more than a century, the WGMS and its predecessor organizations have been compiling and disseminating standardized data on glacier fluctuations. The historical development of this service as well as of the related datasets and science are summarized in Haeberli (2008), Zemp (2012) and Zemp et al. (2014). The main variables currently observed in standardized formats are changes in glacier mass, elevation and volume, area, and length (front variations). Glacier changes are observed using in-situ and remote sensing methods. The glaciological mass balance is obtained from ablation stake and snow pit measurements and provides seasonal to annual information on glacier contribution to runoff. Geodetic methods from in-situ, airborne and space borne platforms provide multi-annual to decadal information on glacier elevation changes. Based on assumptions on the density of snow, ice and firn, the observed geodetic elevation changes can be converted to mass balance and runoff contribution (e.g. Huss, 2013). Glacier elevation change and mass balance are a relatively direct reaction to the atmospheric conditions. They are thus relatively easy to interpret but comparably difficult to measure. Glacier front variations on the other hand, are an indirect and delayed reaction to climate change that are thus more difficult to interpret but easy to measure (from both in-situ and remotely sensed observations). Their much longer time series allow the extension of the observational series back into the Little Ice Age period.

3.1.2. Presently incomplete glacier change observations

Zemp et al. (2015) provide a detailed overview of the available datasets and discuss the potential and the shortcomings for scientific assessments. The Global Glacier Change Bulletin (WGMS, 2021, https://wgms.ch/ggcb/) and the Global Terrestrial Network for Glaciers (GTN-G) Global Glacier Browser (http://www.gtn-g.org) provide a periodically updated overview and access to all data products, respectively. Figure 1 provides a visual representation of global glacier distribution over the 19 Randolph Glacier Inventory (RGI) first order glacier regions and the actual state distribution of glacier change observations from the glaciological and geodetic (elevation change) samples available in the FoG database version 20200824. Similar to in-situ measurements, geodetic elevation change observations derived from DEM differencing also have a globally inhomogeneous spatial distribution. They cover mostly local glaciers, several sub-regions and only few specific regions completely. Moreover, time periods and areas covered vary with the available datasets (input DEMs).

Past periods (i.e. before the boom of the satellite era in the 2000s) are generally scarcely sampled, in part because historical national elevation data (i.e. derived from airborne imagery) is only commercially distributed and its sharing is prohibited. Only in some exceptional cases national DEMs are freely available (e.g. DEMs from 1960s to 1980s for the United States and Canada) and have been used in related studies to derive elevation and mass changes (e.g. Larsen et al., 2007; Berthier et al., 2010). As the quality of the national DEMs differ (in general older ones have a lower accuracy), also the quality of the derived elevation changes differs. In part, the lower quality is compensated by the longer time period of observation, i.e. a DEM from 1960 with an elevation uncertainty of 8 m is as good as a DEM from 1990 with a 2 m uncertainty (0.2 m/yr) when both are subtracted from a year 2000 DEM (such as SRTM). Spy satellites (such as Korona and Hexagon) have only been declassified recently and efforts to use them to estimate past glacier changes are still under development (Belart et al., 2020; Dehecq et al., 2020). 

Figure 1: Distribution of glacier fluctuation records from the glaciological (red crosses) and geodetic (blue dots) samples over the 19 RGI first order regions. Glacier fluctuation data from WGMS (FoG 20200824 or 08-2020), glacier regions from RGI6.0 and country boundaries from Natural Earth.

It is only during the last decades that the increased efforts on the exploitation of remote sensing data have allowed us to picture the distribution of glacier changes over increasingly larger regions with improved precision and homogeneity of the estimates. Since the year 2000, DEMs from numerous satellite missions are increasingly being used (e.g., Satellites Pour l'Observation de la Terre (SPOT) Stereoscopic survey of Polar Ice: Reference Images & Topographies (SPIRIT), ASTER, ArcticDEM, Reference Elevation Model of Antarctica (REMA), Advanced Land Observing Satellite (ALOS) World 3D-30m (AW3D30), TanDEM-X) to determine elevation changes at the mountain range, continental and even global scales (e.g. Braun et al., 2019; Dussaillant et al., 2019; Shean et al., 2020; Hugonnet et al., 2021). Integrating the results of these and other forthcoming studies into the FoG database is one key goal for the future C3S glacier service.

3.1.3. Future of satellite missions

The future of satellite missions able to measure glacier elevation changes is uncertain. ASTER-Terra satellite, providing an extensive archive of stereoscopic images over glaciated regions with free availability (thanks to the efforts from the Global Land Ice Measurements from Space, GLIMS, Raup et al. (2000)), has already exceeded its predicted lifespan and is expected to be decommissioned soon. Most of the alternative spaceborne datasets are commercially distributed and allow free exclusive use only to some national parties (e.g. Pleiades and SPOT in French National Centre for Space Studies (CNES)-airbus, TanDEM-X in German Aerospace Center (DLR)). Future planned missions are scarce, with uncertain launching dates and most of them planned for commercial use. Ensuring the continuation of open source spaceborne datasets with extensive acquisitions tasking planned over glaciated regions is crucial for ensuring the good quality of future glacier product, and one of the greatest gaps in the quality and continuation of the glacier services delivered to C3S.

3.2. Development of processing algorithms

Note that the C3S glacier service mass and elevation change products are directly brokered from the FoG database, produced annually by the WGMS. Therefore, we do not apply any algorithm to the datasets provided. However, we follow closely the further development of processing algorithms to retrieve glacier mass and elevation change observations. These developments are performed as part of the Climate Change Initiative extension (CCI+) project but also by the science community. In the case improvements appear in the literature, we will test if the methods are sufficiently robust for the products we wish to provide for C3S. For the brokered datasets we will also analyse the methods used to generate them before forwarding them to the CDS. For example, the currently applied automated processing lines for optical stereo images have created much more robust and homogeneous elevation change trends with complete coverage (regional/global) and improved uncertainty estimation compared to earlier calculations (e.g. Dussaillant et al., 2019; Menounos et al., 2019; Hugonnet et al., 2021).

3.3. Accuracy assessment for glacier changes

3.3.1. Glaciological method uncertainty estimation

The uncertainties of the glaciological method lie mostly on the problems of sampling: i) the use of a limited number of point measurements, which are not necessarily able to capture the spatial variability of surface mass balances along the glacier. (ii) The difficulties in measuring certain zones of the glacier like highly steep slopes or strongly cracked zones. And (iii) the errors on every specific measure. Therefore, glacier-wide mass balance estimations by the glaciological method can be biased and need to be calibrated with the other methodologies. Total uncertainties on the annual glaciological mas balance lie between 0.2 and 0.4 m w.e. yr^-1 (e.g. Sicart et al., 2007; Thibert et al., 2008; Zemp et al., 2013). Geodetic measurements are usually preferred for calibration (Thibert and Vincent, 2009, Zemp et al 2013), but attention must be taken, as both methods measure different quantities: the geodetic method integrates the subaerial part of frontal ablation. In glaciers where frontal ablation is not a negligible part of the mass loss (Truffer and Motyka, 2016), large discrepancies can be observed between the surface mass balance measured by the glaciological method and the geodetic mass balance.

3.3.2. Geodetic method uncertainty estimation

DEMs from various sources can be used for glacier elevation changes assessment and their characteristics (e.g. time stamp, spatial sampling, optical/radar) have to be considered to obtain meaningful uncertainties (Figure 2). In some mountain regions appropriate data might not be available and a more qualitative description of DEM uncertainties might have to be used. 

Figure 2: Changes in the tongue of Findelengletscher (Switzerland) with the areas and hillshaded elevation models in (a) 2005 (red) and (b) 2010 (blue). The middle row shows (c) the elevation change in metres from the 1 m resolution airborne laser scanning (ALS) DEMs and (d) an example of 50 m contour lines (2005: black, 2010: green) with yellow areas showing the area differences used for the volume change calculations. The bottom row shows the elevation change at (e) 30 m resolution (similar to Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Map (GDEM)) and (f) 90 m resolution (similar to Shuttle Radar Topography Mission (SRTM) DEM), with resampled DEMs using the same elevation change colours as the 1 m ALS DEM. Numbers in the legends are in meters of elevation change. (source: Joerg and Zemp, 2014)

Table 3 provides an overview on the measures for uncertainty assessment that can be applied to the elevation change product (DEM differencing). The table does not consider uncertainties introduced during post-processing, e.g. the method selected to fill data voids and reduce artefacts. The mandatory step to be performed in any case is image co-registration (e.g. following Nuth and Kääb (2011)), horizontally as well as vertically if this is sensible and both datasets have the same geodetic datum. As a further minimum requirement and first quantitative descriptor of product accuracy the elevation differences over stable ground should be given. Stable means outside of glaciers, water bodies and forests.

Table 3: List of measures to determine uncertainty of glacier elevation changes from DEM differencing.

At the next level, DEM elevations can be compared to ICESat (L1a) and ICESat data can be integrated in the co-registration process to determine the vector sum of the residuals (L1b). If a high-quality DEM or ground control points are available (L2), elevation differences over stable terrain for the DEMs used can be calculated. Finally (L3), it is also possible that change rates are directly compared to an independent dataset that, at best, should have been available for the same period. If not, differences due to timely variable change rates might occur. As a note of caution: it is required to adjust all datasets compared to the same geodetic datum before they are compared, as this is not always WGS84 (e.g. for ICESat and national DEMs, or different SRTM products).

3.3.3. User needs in glacier products uncertainty assessment

Until recently, comprehensive uncertainty assessments have rarely been carried out and mass balance and elevation change data have often been assessed using rough error estimation or even without consideration of errors. Based on an expert workshop, Zemp et al. (2013) proposed a framework for reanalysing glacier mass balance series that includes conceptual and statistical toolsets for assessment of random and systematic errors, as well as for validation and calibration (if necessary) of the glaciological mass balance observations with the geodetic mass balance results. These are widely accepted and applied by the mass balance observers (e.g. Huss et al., 2015; Andreassen et al., 2016; Thomson et al., 2016). However, these efforts still remain scarce and have been applied only at local glacier scales. Recent developments on the geodetic method have allowed to assess glacier changes for larger regions (e.g. Brun et al., 2017; Braun et al., 2019; Dussaillant et al., 2019; Shean et al., 2020) and even globally (Hugonnet et al., 2021). The geodetic method provides a great opportunity to tackle this challenge at a larger scale. However, a basic requirement is a sound uncertainty estimate for such geodetic change assessments. A framework for good practices in geodetic uncertainty estimate assessments has only been published recently (Hugonnet et al., 2022) and will certainly help ameliorate future glacier elevation change uncertainties.

3.4. Opportunities to improve quality and fitness-for-purpose of the CDRs

In the 5^th Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC), glacier mass budgets were reconciled by combining traditional observations (i.e., results from glaciological and geodetic measurements) with satellite altimetry and gravimetry to fill regional gaps and obtain global coverage (Vaughan et al., 2013). However, this approach is challenged by the relatively small number and inhomogeneous distribution of field measurements and their often-unknown representativeness for the related mountain range as well as by scale issues of satellite altimetry (point data) and gravimetry (coarse resolution) missions.

Geodetic surveys from air and space borne sensors have a great potential for (i) the reconstruction of glacier elevation changes, (ii) the validation and calibration of direct measurements using the glaciological method, (iii) assessing glacier elevation changes over entire mountain ranges, and (iv) determination of the representativeness of the field measurements for respective mountain ranges. Whereas long-term in-situ measurements provide the temporal variability of glacier mass changes with annual or seasonal resolution, differencing of high-resolution DEMs, such as from airborne (national) surveys or TanDEM-X, bear the potential to assess elevation changes for thousands of individual glaciers over entire mountain ranges on a decadal time scale. In combination, the calibrated field measurements can be used to determine elevation changes over entire mountain ranges at high confidence. A first step into this direction with the data available so far has been taken by Zemp et al. (2019) for a global assessment of glacier contributions to sea-level rise (Figure 3). In this study the datasets from WGMS (annual mass balance measurements) were complemented with an additional 70,873 geodetic volume change observations computed for 6,551 glaciers in Africa, Alaska, the Caucasus, Central Asia, Greenland's periphery, Iceland, New Zealand, the Russian Arctic, Scandinavia and Svalbard. The obvious next step is to update the results of this study considering the new available regional and global geodetic estimates (Hugonnet et al., 2021). The improved temporal resolution of this observational based results can then be used to reconcile satellite altimetry and gravimetry products (IPCC SROCC 2019, IPCC 2021). 

Figure 3: The new estimate of specific (colour-coded) and cumulative glacier mass balances (numbers in Gt) over the 1961-2016 period by Zemp et al. (2019). Values are obtained from statistically combining the annual mass balance measurements in the field with the decadal observations of the geodetic mass balance as derived from satellite data (e.g. from DEM differencing).

3.5. Community needs: Glacier elevation and mass changes

Since the CDS glacier change service is an extract from the FoG database, similar user applications are expected. Users have historically employed the data provided by the FoG database as is, or after converting / aggregating them on a diversity of spatial (e.g. per RGI region) and temporal (e.g. decades) scales to obtain averages for comparison with other datasets (e.g. Vaughan et al., 2013). Apart from calculating the sea-level contribution of glaciers or detection of climatic trends and variability (e.g. Huss et al., 2014), a key application is run-off contribution and water resources assessment (e.g. Bliss et al., 2014). For many, in particularly drier climatic regions, the meltwater from glaciers released during the hot summer months is key for agriculture and livelihood (e.g. Kaser et al., 2010). Also, direct economic impacts of dwindling glaciers are relevant (Vergara et al., 2007) and the transformation of a landscape hosting glaciers to a desert of rocks and unconsolidated debris is both dangerous and heart breaking. While it is unclear how documenting these changes can help in slowing down its progress, the awareness and sensibility it has created for a larger problem (global temperature rise) across the population is incalculable.

Even though the FoG glacier fluctuation database stands as the only exhaustive glacier change data compilations, it still holds large shortcomings, mostly related to the reduced spatio-temporal coverage of observations. Naturally, the CDS glacier change products will inherit the same flaws.

3.5.1. Spatially and temporally incomplete glacier change observations

The glaciological method provides annually to seasonally resolved information on glacier evolution, but only for a small sample of the world glaciers. Only a few glaciers have direct (and at least) annual measurements of mass balance over more than 30 years (about 40 out of more than 200'000). These are used as reference glaciers for regional to global scale applications.

Today, a distinction in use of glacier fluctuation data by the science community and international organizations is required. The mean elevation or mass changes per individual glacier (measured in m water equivalent per unit area) is the only value that is globally comparable and allows applications such as up-scaling to derive regional-scale mass changes. However, scientists are generally more interested in raw measurements (mass balance at a stake-point) as these are not disturbed by any spatial interpolation and can thus be used for validation of numerical models that are calculating mass balance. These datasets require detailed documentation (location, geodetic projection / datum, date, etc.) to be useful and are now only available for about 200 glaciers. The spatio-temporal restrictions of data availability in some regions like the Andes and High Mountain Asia, have resulted in a wide range of modelling applications to close these gaps and use the scarce available data for model calibration and validation.

Satellite-based measurements are increasingly used for model calibration purposes since the geodetic method (DEM differencing) allows to measure glacier elevation changes over large glacierized regions. As consequence, the scientific endeavours to ameliorate these datasets have increased during the recent years, such as the best way to interpolate data voids in the DEMs (e.g. McNabb et al., 2019), correctly calculate uncertainties (Hugonnet et al., 2022) and efforts on how to best consider the largely unknown radar penetration of the SRTM (C-band) or TanDEM-X (X-band) into snow and firn (e.g. Rankl and Braun, 2016). However, due to the high uncertainty of density transformation of snow and firn, high accuracy can only be achieved for multiannual/decadal time periods (Huss, 2013). The largest, and still unsolved limitation of geodetic estimates lies on its coarse temporal resolution.

3.5.2. Data format

Glacier-wide elevation and mass change results are presently available from FoG in a standardized format allowing easy implementation in models or spread-sheets (Table 4). For C3S, all datasets are provided in similar formats (see Product User Guide and Specification Document, RD2). Each product version consists of two CSV files (one for elevation change series and one for mass change series) and a subfolder containing a series of ESRI ARCGIS shapefiles for each of the two products.

Table 4: C3S Glacier product dataset format and description

For science applications it is clear that users always want to have raw data to have full control over the further processing (e.g. methods and error propagation), but this generally results in incomparable results. During recent years, the interest of the glaciological and other scientific communities for regional and global glacier mass change products at improved spatial and temporal resolution has considerably increased. The needs spread from comparing results from spaceborne gravity and altimetry, as well as for model calibration and validation. Examples are the Horizon 2020 Global Gravity based Groundwater product (G3P) project (https://www.g3p.eu/); The International Association of Cryospheric Sciences working group on "Regional Assessments of Glacier Mass Change" (IACS-WG-RAGMAC, https://cryosphericsciences.org/activities/wg-ragmac/) and the European Space Agency (ESA) Glacier Mass Balance Intercomparison Exercise project (GlaMBIE, https://glambie.org/); and the Glacier Model intercomparison Project (GlacierMIP, https://climate-cryosphere.org/glaciermip/).

Until now glacier products have been provided to the CDS as separated glacier elevation and mass change datasets. Yet, there is still space to explore the best ways of providing a user friendly and easily comparable distributed glacier change product to the community (this is currently done in the framework of the Glaciers_cci project). The most feasible possibility until now is to provide results in a grid format (netcdf or geotiff) with a user pre-scribed spatial resolution.

3.6. Opportunities from exploiting the Sentinels and any other relevant satellite

Repeat DEMs from the same sensor and with global coverage as currently planned for the extended TerraSAR-X / TanDEM-X mission will substantially increase the possibility to determine glacier elevation and mass changes regularly. In combination with sensors such as ICESat-2 to determine residual effects of radar penetration, uncertainties of the related products can be calculated. With the now available automated processing lines for optical stereo images, future high-resolution sensors (along-track or across-track) will allow the calculation of elevation changes more frequently and robustly (e.g. elevation trends derived from many rather than only two DEMs).

The glacier change products, as provided by the C3S CDS, are based on glaciological and geodetic methods. The glaciological method provides glacier (surface) mass changes at annual resolution from in-situ observations of a few hundred glaciers worldwide. The geodetic method provides glacier thickness and volume changes from differencing of DEMs, which were computed from air and spaceborne surveys. Such geodetic observations provide results at multi-annual to decadal time resolution from currently tens of thousands of glaciers and will soon reach close to global coverage as Figure 4 reveals (Hugonnet et al., 2021). Both products are combined based on the approach by Zemp et al. (2019, 2020) to regional and global estimates of glacier contributions to mean-global sea-level rise as reported in the related European State of the Climate report (ESOTC) Cryosphere section (https://climate.copernicus.eu/climate-indicators/cryosphere).

Improvements in global glacier mass-change assessments are still possible and necessary. The observational database needs to be extended in both space and time. The spatial gap has been recently overcome, with the geodetic sample boosting from a largely incomplete coverage (9% of the world glaciers in Zemp et al. (2019)) to an almost complete sample (96% of glaciers in Hugonnet et al. (2021)). However, because of volume to mass conversion issues in short time periods  (Huss, 2013), the geodetic sample is not able to solve for the temporal variability of glacier mass changes at periods shorter than 5-10 years. There is still a large potential to improve the temporal variability to annual and even seasonal temporal resolution by calibrating the geodetic sample with the glaciological sample following the approach by Zemp et al. (2019, 2020).

In regions with large glacier covers, the present estimates can be complemented with estimates from spaceborne gravimetry and altimetry observations. Spaceborne altimetry determines the surface elevation by measuring the two-way travel time of pulses between glacier and sensor using radar (e.g. CryoSat-2, Jakob et al., 2021) or laser (e.g. IceSAT, Treichler et al., 2019) technology. Spaceborne gravimetry is used to detect changes in the Earth’s gravimetric force by measuring the distance between two satellites on the same orbit (e.g., Gravity Recovery and Climate Experiment (GRACE)). From the observed variations in the gravity fields, numerical modelling is used to estimate related changes in the terrestrial water storage and related components, such as glacier mass changes (Wouters et al., 2019; Ciracì et al., 2020).

The variety of methods now available opens up new opportunities for regional evaluation of results from different methods as well as for reconciled global assessments of glacier mass changes and related contributions to sea-level rise. The strong community determination to reconcile the measurements from all methods is clear given the recently accepted ESA project for a Glacier Mass Balance Intercomparison Exercise (GlaMBIE). At the same time, the glacier research and monitoring community is facing new challenges related to data size, formats, and availability as well as new questions with regard to best practises for data processing chains and for related uncertainty assessments. These questions are currently addressed by the working group on “Regional Assessments of Glacier Mass Change” of the International Association of Cryospheric Sciences (RAGMAC/IACS; https://cryosphericsciences.org/activities/wg-ragmac/).

Figure 4: The new estimate of mean elevation change rates over the 2000-2019 period by Hugonnet et al., (2021). Surface elevation changes at a high spatiotemporal resolution are obtained over all of Earths glaciers by DEM differencing from a largely untapped satellite archive. In this study a 97% of the global glacierized terrain present valid observations (or 96% of the world glaciers). The percentage of area observed by year and region is displayed on the time friezes at the bottom of the disks.

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