Rock debris can accumulate on glacier surfaces and dramatically reduce glacier melt. The structure of a debris cover is unique to each glacier and sensitive to climate. Despite this, debris cover has been omitted from global glacier models and forecasts of their response to a changing climate. Fundamental to resolving these omissions is a global map of debris cover and an estimate of its future spatial evolution. Here we use Landsat imagery and a detailed correction to the Randolph Glacier Inventory to show that 7.3% of mountain glacier area is debris covered and over half of Earth’s debris is concentrated in three regions: Alaska (38.6% of total debris-covered area), Southwest Asia (12.6%) and Greenland (12.0%). We use a set of new metrics, which include stage, the current position of a glacier on its trajectory towards reaching its spatial carrying capacity of debris cover, to quantify the state of glaciers. Debris cover is present on 44% of Earth’s glaciers and prominent (>1.0 km2) on 15%. Of Earth’s glaciers, 20% have a substantial percentage of debris cover for which the net stage is 36% and the bulk of individual glaciers have evolved beyond an optimal moraine configuration favourable for debris-cover expansion. Use of this dataset in global-scale models will enable improved estimates of melt over 10.6% of the global glacier domain.
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This study was funded by Northumbria University, privately by the first author, and by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme grant agreement no. 772751, RAVEN, Rapid mass losses of debris-covered glaciers in High Mountain Asia. S.H. thanks D. Quincey, M. Truffer, J. Brown and E. Miles for helpful comments and discussions.
The authors declare no competing interests.
Peer review information Primary handling editors: Stefan Lachowycz; Heike Langenberg
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
The footprint of each manually selected Landsat image is shown colored by the date of acquisition. The histogram inset shows the number of scenes per year differentiated by sensor and is broken down per region in the Supplementary Information.
Landsat image footprints over Alaska and Western Canada are outlined and colored by acquisition data illustrating the preference given to more recent imagery and the removal of overlapping image area.
Southern Andes as an example of the manual steps taken to identify and remove FP glacier error, area in shadow and cloud covered area; and identify and add FN errors in every RGI region. The wider shapes, or lassos, were used to tag enclave shapes initially mapped as ‘debris cover’ with a specific error or true (by omission) classification. This lasso approach enables the manual work conducted in this study to be used again in future inventories with any desired modifications.
Map view and cross-section cartoon illustrating the method used in this study to estimate the position of the equilibrium line for glaciers with 7% debris cover and/or >10 km2 debris-covered area. Equilibrium line is estimated by locating the upper-most debris exposure, extending the point of exposure to the full glacier width and adjusting the position up-glacier by a factor of d. d is the glacier specific distance between the true equilibrium line and the first down-glacier emergence of englacial debris.
Landsat image of glaciers in Svalbard (a) and corresponding results from this study (b) showing examples of where equilibrium line estimates (yellow line) derived from mapped debris cover fails. Equilibrium lines shown on grey glaciers did not meet the 7% debris cover and/or >10 km2 debris-covered area criteria and were not included in any further metric derivation or results. Glaciers shown as blue met the criteria to be included in the study but also are shown to have errors. The source for error include (1) sparse debris cover producing nonsensical equilibrium lines; (2) imperfect flow divides drawn in ambiguous cases within RGI v6.0 causing unphysical equilibrium line estimates; (3) the unusual case where debris cover is present up-glacier but is not sufficiently present at lower reaches of the glacier to be detected by the debris mapping algorithm; (4) a portion of a glacier’s ablation zone is debris free and big enough to cause the glacier width buffer to inaccurately extend the ablation zone area to encompass the full width of the glacier. While this sample region was selected due to a concentration of errors, (5) shows a location where equilibrium line location was predicted as intended.
Extended Data Fig. 6 Comparison between the global debris map presented here and the global debris map from (ref. 18).
Error, true positive rate and precision are calculated under the assumption that results from this study are correct. The basis of this assumption is the additional manual editing that was conducted within this study, where (ref. 18) used unaltered RGI v6.0. Greenland was excluded from the comparison due to the different spatial domains considered. The values used to make this figure are given in the Supplementary Information.
Two example locations are shown to illustrate a setting where heavy editing to the RGI was required (Central Asia) and almost no editing of the RGI was required (Alaska). The bottom two panels show the comparison of results from this study and those of (ref. 18). A clear undercounting of debris by (ref. 18) is apparent in both regions where in Alaska the methods of (ref. 18) are examined without influence from the RGI editing conducted in this study while the errors in Central Asia show a compound error in (ref. 18) composed of both an undercounting of true debris cover and an over counting where off-glacier area is erroneously classified as debris cover.
Extended Data Table 1 Global debris cover results and errors in RGI v6.0 presented as regional percentages.
All values are given for glaciers with a surface area greater than or equal to 1 km2 that fall within the considered spatial domain. ‘% RGI6.0 considered’ is unedited RGI v6.0 glacier area with i. glaciers smaller than 1 km2 removed and ii. area outside of the Landsat image composite (Extended Data Fig. 1) removed divided by entirely unaltered RGI v6.0 glacier area. ‘% SamRGI considered’ is the edited glacier area within the Landsat image composite divided by the edited glacier area inside and outside of the Landsat image composite.
Extended Data Table 2 Global debris cover results and errors in the RGI v6.0 presented as areas (km2).
Glacier area includes corrections to the RGI. These results exclude glaciers with a surface area less than 1 km2. Glacier area in shadow, including other visually uncertain shapes classified as debris, were removed from the debris map but not removed from the RGI.
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Herreid, S., Pellicciotti, F. The state of rock debris covering Earth’s glaciers. Nat. Geosci. 13, 621–627 (2020). https://doi.org/10.1038/s41561-020-0615-0