Model results are often referred to as indices (Gruber, 2012; Boeckli et al., 2012) as the relationship with true extent is unknown. Permafrost extent, sometimes classified into zones such as ‘continuous permafrost’ or ‘extensive discontinuous permafrost’, is the most widely used permafrost data product.
Permafrost extent cannot be directly observed. Therefore, data products are based on heuristics or models (commonly relations with air temperature) resting on observations of ground temperature at few locations only, and the quality of data products is difficult to quantify. Relating permafrost extent (a spatial aggregation of a binary variable) to conditions at specific locations is difficult and usually requires additional guidance (see Boeckli et al., 2012). Simulating changes of permafrost extent on inter-annual to century scales usually involves extreme simplification (depth, ice content) that limits the interpretability of results. The difficulty inherent in defining and interpreting permafrost extent and zones is well known (e.g., Zhang et al., 2000; Heginbottom, 2002; Gruber, 2012; Gruber, 2016; Obu, 2021).
Maps of permafrost extent can communicate where permafrost may exist and approximately what proportion of the area is expected to have permafrost (see Gruber 2016). Permafrost extent can be aggregated into summaries of total permafrost area. This can inform teaching, policy making, and help decide if permafrost needs to be investigated in more detail during the early stages of land-use projects when the permafrost status of a location is unknown.
| name | source | data type | spatial domain | spatial resolution | temporal coverage | time step | data format | |
|---|---|---|---|---|---|---|---|---|
| Circum-Arctic Map of Permafrost and Ground-Ice Conditions, Version 2 | NSIDC | Map, manual delineation | Global | Polygons from 1:10,000,000 paper map | mid-late 20th century | None | Shapefiles | details |
| Permafrost Map of Canada | NRCan | Map, manual delineation | Canada | Polygons from 1:7,500,000 map | mid-late 20th century | None | details | |
| Permafrost Zonation Index | University of Zurich | Model based on reanalyses | Global | ~1 km x 1 km | equilibrium 1960–1990 | None | WMS; Binary | details |
| Circumpolar permafrost maps | Published in Scientific Data | Model based on reanalyses | North of N30º | ~1 km x 1 km | equilibrium 2000–2014 | None | GeoTIFF | details |
| Permafrost extent for the Northern Hemisphere, v3.0 | CEDA Archive/ESA Permafrost CCI+ | Model based on reanalyses | North of N30º | ~1 km x 1 km | A: 2003–2019B: 1997–2002 | Annual | NetCDF | details |
3.4.4.1 Data sources
Circum-Arctic Map of Permafrost and Ground-Ice Conditions, Version 2: https://nsidc.org/data/ggd318. This is a digitized version of Brown et al. (1997). The map itself is based on a compilation of heuristic, and mostly manual, delineation and some homogenizations of differing mapping systems (see Gruber, 2012). This is the most widely used spatial depiction of permafrost.
Permafrost Map of Canada: “Permafrost” 1:7,500,00 in the 5th Edition (1978 to 1995) of the National Atlas of Canada, https://ftp.maps.canada.ca/pub/nrcan_rncan/raster/atlas_5_ed/eng/environment/land/mcr4177.pdf
Global Permafrost Zonation Index Map (Gruber 2012): https://www.geo.uzh.ch/microsite/cryodata/pf_global/. This dataset casts rules used in Brown et al. (1997) – the Circum-Arctic Map of Permafrost and Ground-Ice Conditions – in a simple model and applies it to gridded mean annual air temperature for the period 1961–1990. Transient effects are not represented. In mountains, lapse rates are applied on a grid spacing of about 1 km.
Circumpolar permafrost maps (Karjalainen et al., 2018): https://doi.pangaea.de/10.1594/PANGAEA.893881. This dataset contains a baseline map for the period 2000–2014 at about 1 km resolution that has been derived based on statistical modelling with climate data and observed ground temperature.
Permafrost extent for the Northern Hemisphere, v3.0: https://catalogue.ceda.ac.uk/uuid/6e2091cb0c8b4106921b63cd5357c97c. This dataset contains annual maps of permafrost extent north of 30°N at about 1 km resolution. They are derived from a transient thermal model and fractional coverage of seven land cover classes (Westermann et al., 2020). For 2003–2019 this is based on MODIS Land Surface Temperature (LST) merged with ERA5 air temperature and for 1997–2002 it is based on ERA5 air temperature and bias correction with the MODIS LST 2003–2019. Annual permafrost extent in this product is derived from ground temperature at 2 m depth, and as such, permafrost can persist at depth in higher fractions than indicated for years to centuries.
3.4.4.2 Strengths and Limitations of Datasets
Manual delineation: Expert knowledge (i.e., perceived permafrost extent and its relationship with physiography and climate) is used as a heuristic for delineation of polygons in the generation of national and international maps. The rules and entities used (see Heginbottom 2002; Gruber, 2012) for drawing lines differ and are difficult to compare between regions, or to reproduce accurately. The use of permafrost zones is well established. Such maps enjoy great popularity and often trust, even though it is unclear in many areas how reliable the mapping is. Transient effects are not represented and the period for which this data is valid is not specified explicitly.
Gridded model: Quantitative data from reanalysis, interpolated observations, or remote sensing is used as input for model simulations. Relying on quantitative spatial data makes these types of data reproducible and more consistent spatially than those derived from manual delineation. The models used contain assumptions about the subgrid characteristics of microclimate, snow, land surface, and subsurface. The appropriateness of these assumptions is difficult to ascertain.
References – Permafrost Extent
Boeckli, L., A. Brenning, S. Gruber, and J. Noetzli, 2012: Permafrost Distribution in the European Alps: Calculation and Evaluation of an Index Map and Summary Statistics. The Cryosphere, 6(4), 807-820, doi.org/10.5194/tc-6-807-2012.
Brown, J., O. Ferrians, J.A. Heginbottom, and E. Melnikov, 1997: Circum-Arctic Map of Permafrost and Ground-Ice Conditions. U.S. Geological Survey in Cooperation with the Circum-Pacific Council for Energy and Mineral Resources, Washington, D.C., doi.org/10.3133/cp45.
Cao, B., S. Gruber, D. Zheng, and X. Li, 2020: The ERA5-Land Soil Temperature Bias in Permafrost Regions. The Cryosphere, 14(8), 2581-2595, doi.org/10.5194/tc-14-2581-2020.
Gruber, S. 2012. Derivation and Analysis of a High-Resolution Estimate of Global Permafrost Zonation. The Cryosphere, 6(1), 221-233, doi.org/10.5194/tc-6-221-2012.
Gruber, S., 2016: Specification of a Permafrost Reference Product in Succession of the IPA Map (Action Group Report), Retrieved from: https://ipa.arcticportal.org/images/stories/AG_reports/IPA_AG_SucessorMap_Final_2016.pdf.
Heginbottom, J.A. 2002. Permafrost Mapping: A Review. Progress in Physical Geography, 26(4), 623-642, doi.org/10.1191/0309133302pp355ra.
Karjalainen, O., J. Aalto, M. Luoto, S. Westermann, V.E. Romanovsky, F.E. Nelson, B. Etzelmüller, and J. Hjort, 2018: Circumpolar Raster Grids of Permafrost Extent and Geohazard Potential for Near-Future Climate Scenarios, PANGAEA, doi.org/10.1594/PANGAEA.893881.
Obu, J. 2021. How Much of the Earth’s Surface is Underlain by Permafrost? Journal of Geophysical Research: Earth Surface, 126(5), 1-5, doi.org/10.1029/2021JF006123.
Westermann, S., A. Bartsch, and T. Strozzi, 2020: Permafrost - Algorithm Theoretical Basis Document. European Space Agency Contract Report: 1–30, Retrieved from https://climate.esa.int/en/projects/permafrost/key-documents/.
Zhang, T., J.A. Heginbottom, R.G. Barry, and J. Brown, 2000: Further Statistics on the Distribution of Permafrost and Ground Ice in the Northern Hemisphere 1. Polar Geography, 24(2), 126-131, doi.org/10.1080/10889370009377692.