Contributors: University of Zurich: Jacqueline Bannwart, Inés Dussailant, Frank Paul, Michael Zemp
Issued by: UZH/Inés Dussaillant, Michael Zemp
Date: 22/06/2023
Ref: C3S2_312a_Lot4.WP2-FDDP-GL-v1_202212_MC-PUGS-v4_i1.1
Official reference number service contract: 2021/C3S2_312a_Lot4_EODC/SC1
History of modifications
List of datasets covered by this document
Related documents
Acronyms
General definitions
Brokered Product: A brokered product is a pre-existing dataset to which the Copernicus Climate Change Service (C3S) acquires a license, for the purpose of including it in the Climate Data Store (CDS).
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 (Cogley et al., 2011).
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 (Cogley et al., 2011).
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 (Cogley et al., 2011).
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 (Cogley et al., 2011).
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 (Cogley et al., 2011).
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 (Cogley et al., 2011).
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 (Cogley et al., 2011).
Mass change components
Accumulation (C):All processes that add to the mass of the glacier (Cogley et al., 2011).
Surface ablation (𝐵𝑠𝑓𝑐): Ablation at the surface of the glacier, generally measured as the lowering of the surface with respect to the summer surface, corrected for the increase in density of any residual snow and firn and multiplied by the density of the lost mass (Cogley et al., 2011).
Internal ablation (𝐵𝑖𝑛𝑡): Loss of mass from a glacier by melting of ice or firn between the summer surface and the bed. Internal ablation can occur due to strain heating of temperate ice as the ice deforms. However, the largest heat sources for internal ablation are likely to be the potential energy released by downward motion of the ice and of meltwater (Cogley et al., 2011).
Basal ablation (𝐵𝑏𝑎𝑠): The removal of ice by melting at the base of a glacier (Cogley et al., 2011).
Frontal ablation (Af): Loss of mass from a near-vertical glacier margin, such as a calving front. The processes of mass loss can include calving, subaerial melting and subaerial sublimation, and subaqueous frontal melting (Cogley et al., 2011).
Calving (D): The component of ablation consisting of the breaking off of discrete pieces of ice from a glaciermargin into lake or sea water, producing icebergs, or onto land in the case of dry calving. Calving excludes frontal melting and sublimation, although in practice it may be difficult to measure the phenomena separately. For example subaqueous frontal melting may lead to the detachment of icebergs by undercutting or by encouraging the propagation of crevasses (Cogley et al., 2011).
Scope of the document
This document is the Product User Guide and Specification (PUGS) for the global gridded annual glacier mass change Climate Data Record (CDR) product provided to the Copernicus Climate Change Service (C3S) Climate Data Store (CDS).
Within C3S, glacier change products (versions 1 to 6) were provided to the CDS as two separate datasets of (i) glacier elevation and (ii) mass change time series as extracts from the Fluctuations of Glaciers (FoG) database brokered from the World Glacier Monitoring Service (WGMS). Since the first data cycle within C3S2, the glacier change product combines these two datasets to produce a completely new and unique product of globally gridded glacier mass changes at an annual temporal resolution.
The present PUGS document complements the corresponding submission of the glacier mass change CDR towards the end of the first data cycle within C3S2 (glacier change product version WGMS-FOG-2022-09, delivered in December 2022). The global gridded annual glacier-mass change product builds on the latest Fluctuations of Glaciers Database version released in September 2022 by the World Glacier Monitoring Service (WGMS, 2022)
Executive summary
This document provides a description of the CDR dataset provided by the C3S Glacier Change Service to the Climate Data Store (CDS). The Glacier Change Service addresses the essential climate variable (ECV) Glacier and provides a globally gridded product of annual mass changes with a spatial resolution of 0.5° x 0.5° covering the hydrological years from 1975/76 to 2020/21. The product was computed by combining the temporal variability from glaciological in-situ observations with multiannual to decadal trends from air- and spaceborne geodetic observation. As input data, our product builds on both glaciological and geodetic time series from individual glaciers as available from the latest version of the Fluctuations of Glaciers (FoG, WGMS, 2022) database of the World Glacier Monitoring Service (WGMS; https://wgms.ch). As auxiliary data, glacier area is used from the Randolph Glacier Inventory (RGI, version 6.0; RGI 2017) which is available from the CDS as brokered product (RD1, https://cds.climate.copernicus.eu/datasets/insitu-glaciers-extent?tab=overview).
In the present document, we provide a brief summary on the background of our data product in Section 1.1, followed by target requirements, product description and data usage information in Section 1.2. Information on data access is provided in Section 2.
The theoretical and methodological base of the CDR is described in the Algorithm Theoretical Basis Document (ATBD, RD2), corresponding requirements are provided in the Target Requirements and Gap Analysis Document (TRGAD, RD3), and product validations are discussed in the Product Quality Assessment Report (PQAR, RD4). The glaciological and geodetic time series used as input data are extensively discussed in (WGMS, 2022) and in Zemp et al. (2013, 2015, 2019).
Global gridded annual glacier mass-change product
1. Product description
1.1. Background: glacier changes from glaciological and geodetic methods
Glacier changes in elevation, volume, and mass can be observed using different methods. As such, in-situ measurements using the glaciological method (c.f Cogley et al., 2011) are carried out at a few hundred glaciers only (WGMS, 2021) but can provide the seasonal to annual variability of glacier mass changes (Zemp et al., 2019), which is well correlated over several hundred kilometers (Letréguilly and Reynaud, 1990; Cogley and Adams, 1998). Differencing of Digital Elevation Models (DEMs) from the geodetic method (c.f Cogley et al., 2011) using airborne and spaceborne sensors can provide glacier elevation and volume changes over multiannual to decadal periods for thousands of glaciers. Time series of glacier mass and elevation changes from glaciological and geodetic methods have been compiled and disseminated by the WGMS (2021, and earlier reports) from the glacier monitoring and research community.
In recent years, the scientific community developed automated processing chains (e.g. Girod et al., 2017) to apply the geodetic method over entire mountain ranges (e.g. Brun et al., 2017; Braun et al., 2019; Dussaillant et al., 2019; Menounos et al., 2019) and finally reached almost global coverage (Hugonnet et al., 2021). With the integration of these geodetic datasets into the WGMS database (WGMS, 2022) it became – for the first time – feasible to produce a global gridded glacier mass-change product with annual resolution from combining glaciological and geodetic methods. Inspired by previous methodological frameworks (Zemp et al., 2019, 2020), we developed a new approach to combine the temporal variability of the glaciological observations (at regional levels) with the long-term change rates (of individual glaciers) of geodetic observations. Further details of the methodology and algorithm can be found in RD2.
For C3S product version WGMS-FOG-2022-09, we computed a gridded, annually resolved, global product of glacier mass changes at a spatial resolution of 0.5°. This product is made available in the CDS as a CDR covering the hydrological years from 1975/76 to 2020/21. It is based on the glaciological and geodetic time series from the FoG database version from 2022-09-14 (WGMS, 2022, Figure 1) and uses the RGI version 6.0 (RGI Consortium, 2017; RD1) as auxiliary data. The almost complete observational coverage of the latest FoG database version is depicted in Figure 1, with the geodetic sample (blue dots) covering 96% of all worlds glaciers.
Figure 1: Distribution of glacier fluctuation records from the glaciological (red crosses) and geodetic (blue dots) samples over the 19 first order glacier regions defined by the Global Terrestrial Network for Glaciers (GTN-G, 2017). Glacier data from the latest version of the Fluctuations of Glaciers database in Sep 2022 by the World Glacier Monitoring Service (WGMS, 2022, https://doi.org/10.5904/wgms-fog-2022-09. Glacier regions from RGI6.0 (GTN-G, 2017) and country boundaries from Natural Earth.
The global gridded annual glacier mass-change product uses annual mass balance observations obtained by the glaciological method and multiannual trends of glacier thickness change (i.e. elevation change) derived from the geodetic method available from the Fluctuations of Glaciers database as input datasets (illustrated in Figure 2 for glacier Hintereisferner in Austrian Alps). For more detail on these input datasets please refer to the previous versions of the C3S glacier product (versions 1 to 6) as well as to WGMS (2021) and Zemp et al. (2015).
Figure 2: Illustration of the annual mass balance observation from the glaciological method (left) and multiannual trends of glacier thickness change from the geodetic method (right) as available from the Fluctuations of Glaciers database. Results belong to glacier Hintereisferner, Austria. Source: WGMS (2022), https://doi.org/10.5904/wgms-fog-2022-09.
Future updates of the C3S global gridded annual glacier mass-change product will be made available as Intermediate Climate Data Record (ICDR), i.e. complementing the CDR with estimates from the latest hydrological years (e.g., 2021/2022), or as reprocessed CDR in order to profit from improved observational coverage in the FoG database or from advances in the algorithms used in our processing chain.
1.2. Target requirements
A detailed overview of the technical requirements for glacier observations in general is provided in the Appendix of the Integrated Global Observing Strategy Cryosphere Theme Report (IGOS, 2007) and updated in the posterior Global Climate Observing System (GCOS) implementation plan reports (GCOS, 2011, 2016 and 2022). Considering these documents, the target user requirements for the C3S glacier change service global gridded annual glacier mass-change product are described in the TRGAD [RD3]. These product exigencies 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 (annual and seasonal) resolution, allowing to produce final products in line with the evolving user needs.
Tables 1 and 2 delineate the dataset characteristics of the FoG annual mass balance and elevation change input datasets with respect to the glacier change related product requirements from the latest GCOS Implementation Plan report (GCOS, 2022). The related characteristics of the C3S global gridded annual glacier mass-change product are shown in Table 3. Our product is a major step forward – the first globally gridded product – and mostly fulfills the GCOS requirements at the level of its input data. However, the global product aggregates glacier-wide results at a spatial resolution of 0.5°, which fits the resolution of most other ECVs in the CDS but is lower than the GCOS requirements. An improvement of the spatial resolution would require that all input data is compiled and made available as gridded products at about 25x25m resolution (similar to Hugonnet et al. 2021). However, this would require a major increase of the financial and infrastructure resources of the WGMS.
Table 1: Comparison between the technical requirements for glacier elevation change observations (GCOS, 2022) and the characteristics of the glacier elevation change sample available from the latest version of the Fluctuations of Glaciers database (WGMS, 2022).
Glacier elevation change observations | GCOS 2022 - Glacier Elevation change ECV Product Requirements | FoG Glacier elevation change input data | |||
Unit | Value | Level* | Unit | value | |
Horizontal resolution | m | 1 | G | m | From 1 to 90 m (in general around 30-40m) depending on the DEM origin (maps, aerial photographs Lidar, spaceborne instruments) |
25 | B | ||||
90 | T | ||||
Vertical resolution | m | 0.01 | G | m | From 0.01 to 5 m depending on the DEM origin (maps, aerial photographs Lidar, spaceborne instruments) |
2 | B | ||||
5 | T | ||||
Temporal resolution | year | 1 | G | year | Multiannual to decadal for correct application of density conversion from volume to mass |
10 | T | ||||
Timeliness | In view of the low need for temporal sampling, the timeliness is not important | ||||
Required measurement uncertainty | m | 2 | B | m | Generally around 2 m |
Stability | m per decade | 2 (2m per decade = 0.2 m-2 a-1) | B | m per decade | Generally around 0.2 m-2 a-1 |
Standards and References | Huss (2013); Zemp et al., (2013, 2015) | WGMS (2022) | |||
Table 2: Comparison between the technical requirements for glacier mass change observations (GCOS, 2022) and the characteristics of the glacier mass change sample available from the latest version of the Fluctuations of Glaciers database (WGMS, 2022).
Glacier mass change observations | GCOS 2022 - Glacier mass change ECV Product Requirements | FoG Glacier mass change input data | |||
Unit | Value | Level* | Unit | value | |
Horizontal resolution | N/A | ||||
Vertical resolution | m | 0.01 | B | m | 0.01 m or 10 kg m-2 (precision of ablation stake and snow pit readings at point locations) |
0.05 | T | ||||
Temporal resolution | month | 1 | G | Month or year | From 6 to 12 months “seasonal to annual”, generally from measurement campaigns which are carried out at the time of maximum accumulation (spring) and of maximum ablation (end of hydrological year) |
3 | B | ||||
12 | T | ||||
timeliness | day | 365 | T | day | 365 days, the WGMS grants a one-year retention period to allow investigators time to properly analyze, document, and publish their data before submitting the data. |
Required measurement uncertainty | kg m-2 a-1 | 0.2 2-sigma (200 kg m-2 a-1 = 0.2 m w.e. m-2 a-1) | B | mm w.e. a-1 ** | Between 0.1 and 0.5 which is the lowest requirement in glaciology |
0.5 | T | ||||
Stability | kg m-2 per decade | 2 (recommended better than 300 kg m-2 a-1) | B | mm w.e. per decade ** | Around 200 mm or less. In some few cases assessed by validation calibration of a glaciological times series with decadal results from the geodetic method. |
Standards and References | Zemp et al., (2013, 2015, 2019) | WGMS (2022) | |||
* Goal (G): an ideal requirement above which further improvements are not necessary. Breakthrough (B): an intermediate level between threshold and goal which, if achieved, would result in a significant improvement for the targeted application. The breakthrough value may also indicate the level at which specified uses within climate monitoring become possible. It may be appropriate to have different breakthrough values for different uses. Threshold (T): the minimum requirement to be met to ensure that data are useful.
** The mass balance unit kg m-2 is equivalent to mm water equivalent (mm w.e.)
Table 3: Characteristics of the C3S global gridded annual glacier mass-change product from the latest version of the Fluctuations of Glaciers database (WGMS, 2022).
Data product characteristic | C3S global gridded annual glacier mass-change product CDR Product Requirements | |
Unit | Value | |
Horizontal resolution | Gridded (WGS-84) | 0.5° |
Vertical resolution | N/A | |
Temporal resolution | year | 1 |
timeliness | year | Updated on a yearly basis After homogenization and integration of new elevation and mass change observations collected by the WGMS and brokered to the FoG |
Required measurement uncertainty | Gt per grid-point | between 0.02-0.2 |
Standards and References | Zemp et al., (2019, 2020); WGMS (2022); Dussaillant et al., (in prep) | |
1.3. Product Data description
For C3S product version WGMS-FOG-2022-09, we computed a gridded, annually resolved, global product of glacier mass changes at a spatial resolution of 0.5°. This product is made available in the CDS as a CDR covering the hydrological years from 1975/76 to 2020/21. It is based on the glaciological and geodetic time series from the FoG database version from 2022-09-14 (WGMS, 2022) and uses the RGI version 6.0 (RGI 2017; RD1) as auxiliary data.
Our algorithm produces a global gridded product of glacier mass-changes in four processing steps summarized in Figure 3. First, we estimate for each glacier of the RGI 6.0 its temporal mass-change variability (calculated as the mean annual anomaly with respect to a given reference period) from nearby glaciological time series. Second, we calibrate this mean annual anomaly to the long-term trend from the different geodetic surveys available for the corresponding glacier. Third, we produce an observationally calibrated annual mass change time series, or i.e. one time series for each glacier calculated as a weighted mean of all calibrated time series, considering the uncertainty as well as of the temporal coverage of the geodetic surveys. Finally, we aggregate the time series of all glaciers as area-weighted mean for each grid cell. A more detailed description of the algorithms involved in the different processing steps is described in the ATBD document (RD2).
Figure 3: Summary illustration of the main processing steps to produce the global gridded annual glacier mass-change product since the hydrological year 1975/76 spatially distributed in a global regular grid. (a) Visualization of the temporal component of the global gridded annual glacier mass-change product and associated uncertainties. (b) Visualization example of the gridded netCDF 4.0 glacier change product for the hydrological year 2018/19. (c) Distribution of glacier fluctuation records from the glaciological (red crosses) and geodetic (blue dots) observation samples. (d) close up example of the gridded product for Icelandic glaciers.
The final product is provided in NetCDF 4.0 file format as annual individual files containing glacier changes and related uncertainties as variables (in Gigatonnes per year), and time (year), latitude and longitude as dimensions. Files are gridded in a global regular grid with grid-point naming convention as the center of the grid-point. Table 4 shows an overview of the C3S global gridded annual glacier mass-change product output data fields and characteristics. A visualization example of both the spatial (0.5° regular grid) and temporal (annual temporal resolution) components of the global gridded annual glacier mass-change product and its relative uncertainties is presented in Figures 4 and 5, respectively. For information about the quality of the product against data requirements we refer to the Product Quality Assessment Report (PQAR, RD4).
Table 4: Overview of C3S global gridded annual glacier mass-change product output data fields.
Horizontal coverage | Global |
Horizontal resolution | 0.5° (latitude - longitude) regular grid |
Spatial gaps | Glacier related grid-point artefact in polar regions (see PUGS document) |
Vertical coverage | Surface |
Vertical resolution | Single level |
Temporal coverage | Hydrological years from 1975/76 to 2020/21 |
Temporal resolution | Annual, hydrological year |
Temporal gaps | N/A |
Update frequency | Annual |
File format | NetCDF 4.0 annual files (i.e. 46 files from 1975/76 to 2020/21) |
Conventions | NetCDF 4.0 convention CF version CF-1.8 |
Available versions | Version WGMS-FOG-2022-09 Provided as a global gridded annual glacier mass-change product from FoG database version from 2022-09-14 (WGMS (2022), https://doi.org/10.5904/wgms-fog-2022-09) Note that Versions 1 to 6 of the glacier change product were provided to the C3S as two separate datasets of glacier elevation and mass change time series from previous versions of FoG. |
Projection | Geographic Coordinate System: GCS_WGS_1984 Datum: D_WGS_1984 |
Data format | Gridded NetCDF 4.0 file Variables: glacier change and uncertainties (Gt) as variables of the same file Dimensions: time, latitude and longitude grid-point naming convention: latitude, longitude at the middle of the grid-point (e.g. 52.25, -176.25) |
Figure 4: Global gridded annual glacier mass-changes and uncertainties (in Gt per year). Visualization example of the gridded netCDF 4.0 glacier change product (top) and related uncertainties (bottom) for the hydrological year 2016/17, spatially distributed in a global regular grid of 0.5° (latitude/longitude).
Figure 5: Annually resolved global glacier mass changes covering the hydrological years from 1975/76 to 2020/21. Visualization of the temporal component of the global gridded annual glacier mass-change product and associated uncertainties.
1.4. Known limitations of the global gridded annual glacier mass-change product
The known issues and limitations for the global gridded annual glacier mass-change product are provided as brief points in the following section. All issues and limitations are fully addressed in the product ATBD (RD2). Quality and validation of the product is addressed in the PQAR document (RD4). Since this is the first version of gridded global glacier mass-changes CDR product we lack information about practical usage constraints resulting from direct user feedback. We encourage users to contact us through the C3S user support portal (https://confluence.ecmwf.int/site/support) to include them in future product version documents.
1.4.1. Grid-point artifacts in polar regions
For mass change purposes a glacier must be considered as a whole; an all-in-one system which cannot be divided in parts. The best glaciologically correct solution to integrate glacier changes into a grid-point is to consider a glacier belonging to a grid-point when its geometric centroid lies within the grid-point. In polar region above 60° latitude, grid-points are smaller in surface and individual glaciers can be larger than the 0.5° grid. This directly derives into a biased centroid grid-point mass change, and consequent neighbor glacierized grid-points without mass change estimate. Our product is consistent with regard to total glacier mass change at global to regional scales, e.g. 19 Global Terrestrial Network for Glaciers (GTN-G) Glacier Regions. However, it is not fully able to represent the local to regional mass change distribution in regions where glaciers are larger than the pixel resolution. A solution to this issue would require an increase in the spatial resolution of the input data from (currently) glacier-wide averages to gridded mass-change fields, which currently is not feasible for all input datasets.
1.4.2. Calendar year vs Hydrological year
The global gridded annual glacier mass-change product (version WGMS-FOG-2022-09) netCDF 4.0 files represent glacier changes for the hydrological years from 1975/76 to 2020/21. For glaciers, the hydrological year varies between regions (South and North Hemispheres and Tropics) and is not equal to the calendar year. Note that this issue– inherited from the input data – introduces some inconsistencies and uncertainties that might need to be considered by the user. As such, annual values from a pixel or region on the northern hemisphere are temporally not fully consistent with annual values from a pixel or region on the southern hemisphere. For cumulative values over longer time periods, these differences are less important. A solution of this issue would require to increase the temporal resolution of the input data to monthly observations, which currently is not feasible.
Hydrological year periods for the different grid-points:
- Starting 1st October first year to 30th September second year, for grid-points located in RGI glacier regions of the Northern Hemisphere
- Starting 1st April first year to 31th March second year, for grid-points located in RGI glacier region of the Southern Hemisphere
- Calendar years, starting 1st January to 31st December, for grid-points located in the the Low Latitude RGI region.
2. Data access information
2.1. Global gridded annual glacier mass-change product
Glacier data will be made available through the Copernicus Climate Data Store (CDS) web-based service, which is the sole data distributor (https://cds.climate.copernicus.eu). A free of charge registration is required to access the CDS and all its toolbox software. Data can be downloaded directly from the website and used under the License to Use Copernicus Products (also included on download page). Data may also be viewed directly online. All requests for information or further data should be channelled through the CDS Knowledge Base at https://confluence.ecmwf.int//display/CKB/.
2.2. Input data (FoG glaciological and geodetic time series)
Previous C3S glacier change products (versions 1 to 6) were provided to the CDS as two separate datasets of glacier elevation and mass change time series as extracts from the Fluctuations of Glaciers (FoG) database brokered from the WGMS. These former datasets are still accessible within the C3S download package and described in the related documentation provided. They can also be found on the WGMS website (https://wgms.ch/data_databaseversions/) in their original form.
The current C3S product version WGMS-FOG-2022-09 represents a completely new global gridded annual glacier mass-change product that builds on the latest updated version of the Fluctuations of Glaciers Database. The present FoG database version (WGMS, 2022) is available for download from the WGMS website (https://wgms.ch/data_databaseversions/). We note that the present and future FoG database versions will no longer be provided through the CDS webpage. A generalised overview on the available datasets is also possible using the GTN-G browser that can be found at https://www.gtn-g.ch/data_browser. For questions regarding the FoG or any other GTN-G dataset, please contact the WGMS staff via wgms@geo.uzh.ch.
2.3. Auxiliary data (RGI)
The various versions of the glacier distribution outlines are available from the Copernicus Climate Data Store (CDS) and are described in related documentation that is provided in the download package. RGI version 6.0 is also available for download from the Global Land Ice Measurements from Space (GLIMS) website (https://www.glims.org/RGI/). Shapefiles can be visualised by a range of commercial (e.g. ESRI ArcGIS) and freely available (e.g. QGIS) software packages.
2.4. Data citation requirement
Citation of the FoG input datasets:
WGMS (2022): Fluctuations of Glaciers Database. World Glacier Monitoring Service, Zurich, Switzerland. DOI:10.5904/wgms-fog-2022-09.
Citation of the new globally gridded product:
(to be updated later)
References
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