the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Jul 2026
ICE-NADI-1: A new deglacial ice sheet thickness history (25–1 ka) for North America using empirically-constrained ice margins with implications for global sea-level contributions, relative sea level, and vertical land motion
Abstract. We generate three new empirically-based deglacial ice sheet histories (ice sheet thickness reconstructions) of the North American Ice Sheet Complex (ICE-NADI-1, ICE-NADI-1_min, and ICE-NADI-1_max) from 25 ka to 1 ka in 500-year intervals using a simple, steady-state numerical ice sheet model. To build the ice sheet histories, we adopt the North America Deglaciation Isochrones (NADI-1; Dalton et al., 2023). Our new ICE-NADI-1 ice sheet thickness reconstruction contains a peak North American sea-level equivalent (SLE) ice volume of 64 m of at the Last Glacial Maximum, 25 % smaller than the North American ice volume contained within ICE-7G (Roy & Peltier, 2017). North American ice saddle merging contributes 13 m SLE to global sea-level fall leading into the Last Glacial Maximum. Across Meltwater Pulse 1a (14.5–14 ka), our ICE-NADI-1 ice sheet thickness reconstruction contributes 6.3 m SLE, and the peak ice loss rate (13 mm SLE/year) is less than half the rate of ice loss in ICE-7G.
Glacial isostatic adjustment simulations show that our new ICE-NADI-1 ice sheet thickness reconstruction fits relative sea-level observations across North America as well or better than previous reconstructions. Predicted present-day vertical land motion with ICE-NADI-1 differs from previous predictions by up to 9 mm per year. Because NADI-1 ice margins capture geologic uncertainty in ice margin, we are able to quantitatively assess the sensitivity of glacial isostatic adjustment predictions (relative sea level, paleotopography, and present-day vertical land motion) to ice geometry, which previously has not been possible. We find this uncertainty comparable in magnitude to that associated with unknowns in Earth structure. Ultimately, our ice sheet thickness reconstructions illustrate how 2-D ice margins inform the history of ice volume evolution, granting new insight into North American Ice Sheet Complex dynamics and the associated changes in global sea level, relative sea level, and vertical land motion.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-4374', Anonymous Referee #1, 04 Sep 2026
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RC2: 'Comment on egusphere-2026-4374', Lauren Gregoire, 12 Oct 2026
The manuscript presents a new reconstruction of the North American ice sheet evolution during the deglaciation. Its approach is to apply a very simple ice sheet model (ICESHEET) which computes ice thickness given only ice sheet margins and a basal shear stress map. The model computes an ice sheet profile assuming the ice is perfectly plastic and at equilibrium. There is no memory in the ice sheet model; each ice margin is simulated independently. Ice dynamical processes and the influence of climate through the surface mass balance are neglected in this model. Glacial isostatic adjustments are computed with a sea level model, and I think this is done in a way that retains the memory of the ice sheet and solid earth evolution, though this is worth clarifying further in the methods.
I think this work is of significant value, providing an alternative reconstruction of the North American ice sheet, using a method that is in principle sounds but requires a few more sensitivity tests to assess the robustness of results. Indeed, the simulated ice thickness is strongly dependent on the basal sheet stress inputs used, as well as the solid earth parameters. Here, only solid earth parameters and ice margin positions are varied, without discussing how basal sheet stress inputs could affect the results. Since dynamical processes and surface mass balance are not represented in this model, the effects of these on ice thickness distribution need to be incorporated in the basal sheer stress input. Pollard et al. (2023) demonstrated how to incorporate these effects into the ICESHEET model inputs and run ensembles of simulations spanning the uncertainty inputs to quantify uncertainty.
Currently, the manuscript presents a very detailed evaluation of the results, looking at volume changes during very complex events such as the North American Saddle collapse. These results are interesting and well described, but I think some are overinterpreted, given that the model lacks key ice-dynamics and surface-mass-balance processes active during this event. In particular, I would not trust the ice profile computed as the Cordilleran and Laurentide ice sheets separate. I also think that the effect of ice margin positions on ice volume are likely to be overestimated by the model particularly in the latter part of the deglaciation when the ice sheet is very far from equilibrium.
I therefore request that the authors perform sensitivity tests to assess the robustness of the results to uncertainty in the input sheet stress, particularly the processes introduced in the sheet stress map by Pollard et al. (2023) that mimic the effects of ice dynamics, climate, and disequilibrium. Setting the shear-stress input purely based on geological information is inadequate for simulating the transient evolution of the ice sheet during deglaciation. The paper should therefore discuss in more detail the choices made in setting up the model and their implications for the results. Finally, the results need much more careful interpretation that accounts for the model's limitations. We need to have a clear understanding of what robust conclusions can be drawn from this work and which ones are an artefact of the simplicity of the ice sheet model used.
Citation: https://doi.org/10.5194/egusphere-2026-4374-RC2
Data sets
Data from: ICE-NADI-1: A new deglacial ice sheet thickness history (25-1 ka) for North America using empirically-constrained ice margins with implications for global sea-level contributions, relative sea level, and vertical land motion E. Lucas et al. https://doi.org/10.5281/zenodo.21464139
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Please find my detailed referee report attached. I consider the manuscript scientifically worthwhile, but substantial revisions are required before publication. I therefore recommend major revision.