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The Cryosphere An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/tc-2020-320
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/tc-2020-320
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  21 Nov 2020

21 Nov 2020

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This preprint is currently under review for the journal TC.

20th century global glacier mass change: an ensemble-based model reconstruction

Jan-Hendrik Malles1,2 and Ben Marzeion1,2 Jan-Hendrik Malles and Ben Marzeion
  • 1Institute of Geography, Climate Lab, University of Bremen, Bremen, Germany
  • 2MARUM - Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany

Abstract. Negative glacier mass balances in most of Earth's glacierized regions contribute roughly one quarter to currently observed rates of sea-level rise, and have likely contributed an even larger fraction during the 20th century. The distant past and future of glaciers' mass balances, and hence their contribution to sea-level rise, can only be calculated using numerical models. Since independent of complexity, models always rely on some form of parameterizations and a choice of boundary conditions, a need for optimization arises. In this work, a model for computing monthly mass balances of glaciers on the global scale was forced with nine different data sets of near-surface air temperature and precipitation anomalies, as well as with their mean and median, leading to a total of eleven different forcing data sets. Five global parameters of the model’s mass balance equations were varied systematically, within physically plausible ranges, for each forcing data set. We then identified optimal parameter combinations by cross-validating the model results against in-situ mass balance observations, using three criteria: model bias, temporal correlation, and the ratio between the observed and modeled temporal standard deviation of specific mass balances. The goal is to better constrain the glaciers' 20th century sea-level budget contribution and its uncertainty. We find that the disagreement between the different ensemble members is often larger than the uncertainties obtained via cross-validation, particularly in times and places where few or no validation data are available, such as the first half of the 20th century. We show that the reason for this is that the availability of mass balance observations often coincides with less uncertainty in the forcing data, such that the cross-validation procedure does not capture the true out-of-sample uncertainty of the glacier model. Therefore, ensemble spread is introduced as an additional estimate of reconstruction uncertainty, increasing the total uncertainty compared to the model uncertainty obtained in the cross validation. Our ensemble mean estimate indicates a sea-level contribution by global glaciers (excluding Antarctic periphery) for 1901–2018 of 76.2 ± 5.9 mm sea-level equivalent (SLE), or 0.65 ± 0.05 mm SLE yr−1.

Jan-Hendrik Malles and Ben Marzeion

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Jan-Hendrik Malles and Ben Marzeion

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Latest update: 27 Nov 2020
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Short summary
To better estimate the uncertainty in glacier mass change modeling during the 20th century we ran an established model with an ensemble of meteorological data sets. We find that the total ensemble uncertainty, especially in the early 20th century, when glaciological and meteorological observations at glacier locations were sparse, increases considerably compared to individual ensemble runs. This stems from regions with a lot of ice mass, but little observations (e.g. Greenland).
To better estimate the uncertainty in glacier mass change modeling during the 20th century we...
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