The effect of a Holocene climatic optimum on the evolution of the Greenland ice sheet during the last 10 kyr

Publisher's version (útgefin grein)The Holocene climatic optimum was a period 8–5 kyr ago when annual mean surface temperatures in Greenland were 2–3°C warmer than present-day values. However, this warming left little imprint on commonly used temperature proxies often used to derive the climate forcing for simulations of the past evolution of the Greenland ice sheet. In this study, we investigate the evolution of the Greenland ice sheet through the Holocene when forced by different proxy-derived temperature histories from ice core records, focusing on the effect of sustained higher surface temperatures during the early Holocene. We find that the ice sheet retreats to a minimum volume of ~0.15–1.2 m sea-level equivalent smaller than present in the early or mid-Holocene when forcing an ice-sheet model with temperature reconstructions that contain a climatic optimum, and that the ice sheet has continued to recover from this minimum up to present day. Reconstructions without a warm climatic optimum in the early Holocene result in smaller ice losses continuing throughout the last 10 kyr. For all the simulated ice-sheet histories, the ice sheet is approaching a steady state at the end of the 20th century.This work is supported by the Danish National Research Foundation under the Centre for Ice and Climate, University of Copenhagen and Villum Investigator Project IceFlow. Brice Noël and Michiel van den Broeke (IMAU, Utrecht University) are thanked for providing the RACMO2.3 Greenland SMB, precipitation and temperature data. B. Vinther is thanked for providing the Holocene accumulation reconstruction for the GRIP site. We are grateful for computing resources provided by the Danish Center for Climate Computing, a facility build with support of the Danish e-Infrastructure Corporation and the Niels Bohr Institute. Development of PISM is supported by NASA grants NNX13AM16G and NNX13AK27G. We thank the anonymous reviewers and Ralf Greve for their helpful suggestions which substantially improved the paper.Peer Reviewe

A Bayesian hierarchical model for glacial dynamics based on the shallow ice approximation and its evaluation using analytical solutions

Bayesian hierarchical modeling can assist the study of glacial dynamics and ice flow properties. This approach will allow glaciologists to make fully probabilistic predictions for the thickness of a glacier at unobserved spatiotemporal coordinates, and it will also allow for the derivation of posterior probability distributions for key physical parameters such as ice viscosity and basal sliding. The goal of this paper is to develop a proof of concept for a Bayesian hierarchical model constructed, which uses exact analytical solutions for the shallow ice approximation (SIA) introduced by Bueler et al. (2005). A suite of test simulations utilizing these exact solutions suggests that this approach is able to adequately model numerical errors and produce useful physical parameter posterior distributions and predictions. A byproduct of the development of the Bayesian hierarchical model is the derivation of a novel finite difference method for solving the SIA partial differential equation (PDE). An additional novelty of this work is the correction of numerical errors induced through a numerical solution using a statistical model. This error-correcting process models numerical errors that accumulate forward in time and spatial variation of numerical errors between the dome, interior, and margin of a glacier.The Icelandic Research Fund (RANNIS) is thanked for funding this research.Peer Reviewe

A Hierarchical Spatiotemporal Statistical Model Motivated by Glaciology

This is a post-peer-review, pre-copyedit version of an article published in Journal of Agricultural, Biological and Environmental Statistics. The final authenticated version is available online at: http://dx.doi.org/10.1007/s13253-019-00367-1In this paper, we extend and analyze a Bayesian hierarchical spatiotemporal model for physical systems. A novelty is to model the discrepancy between the output of a computer simulator for a physical process and the actual process values with a multivariate random walk. For computational efficiency, linear algebra for bandwidth limited matrices is utilized, and first-order emulator inference allows for the fast emulation of a numerical partial differential equation (PDE) solver. A test scenario from a physical system motivated by glaciology is used to examine the speed and accuracy of the computational methods used, in addition to the viability of modeling assumptions. We conclude by discussing how the model and associated methodology can be applied in other physical contexts besides glaciology.Icelandic Centre for Research (152457).Peer reviewe

The geodetic mass balance of Eyjafjallajökull ice cap for 1945–2014: processing guidelines and relation to climate

Publisher's version (útgefin grein)Mass-balance measurements of Icelandic glaciers are sparse through the 20th century. However, the large archive of stereo images available allows estimates of glacier-wide mass balance in decadal time steps since 1945. Combined with climate records, they provide further insight into glacier-climate relationship. This study presents a workflow to process aerial photographs (1945-1995), spy satellite imagery (1977-1980) and modern satellite stereo images (since 2000) using photogrammetric techniques and robust statistics in a highly automated, open-source pipeline to retrieve seasonally corrected, decadal glacier-wide geodetic mass balances. In our test area, Eyjafjallajökull (S-Iceland, ~70 km2), we obtain a mass balance of <![CDATA[$, with a maximum and minimum of and , respectively, attributed to climatic forcing, and , mostly caused by the April 2010 eruption. The reference-surface mass balances correlate with summer temperature and winter precipitation, and linear regression accounts for 80% of the mass-balance variability, yielding a static sensitivity of mass balance to summer temperature and winter precipitation of-2.1 ± 0.4 m w.e.a-1K-1 and 0.5 ± 0.3 m w.e.a-1 (10%)-1, respectively. This study serves as a template that can be used to estimate the mass-balance changes and glaciers' response to climate.This study was funded by the University of Iceland (UI) Research Fund. Collaboration and travels between IES and LEGOS were funded by the Jules Verne research fund. We thank David Shean and two anonymous reviewers for their valuable comments, which greatly improved the manuscript. We thank Carsten Kristinsson at LMÍ for scanning the aerial photographs, Oleg Alexandrov for his helpful tips and advice on ASP, Luc Girod for his help in the MicMac forum and Deirdre Clark and Ken Moxham for the Englishlanguage editing of the manuscript. Pléiades images were acquired at research price thanks to the CNES ISIS program (http://www.isis-cnes.fr). This study uses the lidar mapping of the glaciers in Iceland, funded by the Icelandic Research Fund, the Landsvirkjun research fund, the Icelandic Road Administration, the Reykjavík Energy Environmental and Energy Research Fund, the Klima-og Luftgruppen research fund of the Nordic Council of Ministers, the Vatnajökull National Park, the organization Friends of Vatnajökull, LMÍ, IMO and the UI research fund. This study uses the GLIMS database of the outlines of Icelandic glaciers. E.B. acknowledges support from the French Space Agency (CNES) through the TOSCA program.Peer Reviewe