Twentieth-century contribution to sea-level rise from uncharted glaciers

Abstract

Global-mean sea-level rise (GMSLR) during the twentieth century was primarily caused by glacier and ice-sheet mass loss, thermal expansion of ocean water and changes in terrestrial water storage1. Whether based on observations2 or results of climate models3,4, however, the sum of estimates of each of these contributions tends to fall short of the observed GMSLR. Current estimates of the glacier contribution to GMSLR rely on the analysis of glacier inventory data, which are known to undersample the smallest glacier size classes5,6. Here we show that from 1901 to 2015, missing and disappeared glaciers produced a sea-level equivalent (SLE) of approximately 16.7 to 48.0 millimetres. Missing glaciers are those small glaciers that we expect to exist today, owing to regional analyses and theoretical scaling relationships, but that are not represented in the inventories. These glaciers contributed approximately 12.3 to 42.7 millimetres to the historical SLE. Additionally, disappeared glaciers (those that existed in 1901 but had melted away by 2015, and that therefore cannot be included in modern global glacier inventories) made an estimated contribution of between 4.4 and 5.3 millimetres. Failure to consider these uncharted glaciers may be an important cause of difficulties in closing the GMSLR budget during the twentieth century: their contribution is on average between 0.17 and 0.53 millimetres of SLE per year, compared to a budget discrepancy of about 0.5 millimetres of GMSLR per year between 1901 and 1990. Although the uncharted glaciers will have a minimal role in sea-level rise in the future, and are less important after 1990, these findings imply that undiscovered physical processes are not required to close the historical sea-level budget.

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Fig. 1: Frequency density of glaciers as a function of glacier size.
Fig. 2: Annual glacier mass loss time series.

Data availability

The RGIv5 dataset used for glacier area distribution data are available from GLIMS at https://www.glims.org/RGI/randolph50.html with identifier doi:10.7265/N5-RGI-50. The updated glacier model output is available from the corresponding author upon reasonable request. The SGI is described in ref. 22 with identifier doi:10.1657/1938-4246-46.4.933, and data was available from the authors on reasonable request. The data generated for this paper is not provided owing to the difficulty of representing a collection of matrices indexed by glacier size class and year in a simple CSV file in a way that is easily readable, but the data is available from the corresponding author on reasonable request.

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Acknowledgements

This research is funded by the Austrian Science Fund (FWF) project P25362.

Reviewer information

Nature thanks W. Pfeffer and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Authors

Contributions

D.P. and B.M conceived and designed the study. B.M. performed the glacier model experiments. D.P. then developed and applied the upscaling techniques and performed the analysis. D.P. wrote the manuscript with contributions by B.M.

Corresponding author

Correspondence to David Parkes.

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Extended data figures and tables

Extended Data Fig. 1 RGIv5 glacier change statistics by size class.

a, Mean specific glacier mass balance by glacier size class. The fact that this graph is relatively flat suggests that the differing mass balance between small glaciers and larger glaciers is not a driver for small glaciers (and by extension missing glaciers) contributing a large amount to SLE mass loss relative to their current ice mass. Glacier size does not strongly affect mean specific mass balance, and this weak dependence is also shown in observations from the literature25. b, Mean proportion of 1901 mass lost between 1901 and 2015 as a function of glacier size class. The smallest glaciers that exist in 2015 typically lost almost all of their 1901 mass, with the proportion dropping consistently as 2015 glacier size increases, up to the largest glaciers in 2015, which have seen an average of less than 10% of their mass disappear since 1901.

Extended Data Fig. 2 Glacier distribution for Switzerland.

We believe that in Switzerland, the RGIv5 (solid red) has a much better representation of small glaciers. The Swiss Glacier Inventory22 (SGI) (solid black), which is based on high-resolution orthophotographs, and which is therefore believed to have better representation of small glaciers than is available globally, shows good agreement with the RGI. The power laws for the RGI and SGI (dashed red and dashed black respectively) are calculated for the 10−2 to 100 km2 range, and show that a credible power law exists in this region down to the smallest glacier sizes, albeit with reduced exponents (1.26 and 1.16 respectively).

Extended Data Table 1 Distribution of unmodelled glaciers

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Parkes, D., Marzeion, B. Twentieth-century contribution to sea-level rise from uncharted glaciers. Nature 563, 551–554 (2018). https://doi.org/10.1038/s41586-018-0687-9

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Keywords

  • Small-sized Glaciers
  • Global Mean Sea Level Rise (GMSLR)
  • Glacier Size
  • Glacier Contribution
  • Peripheral Glaciers

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