the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Aug 2024
Deglaciation and abrupt events in a coupled comprehensive atmosphere–ocean–ice sheet–solid earth model
Abstract. During the last 20,000 years the climate of the earth has changed from a state much colder than today with large ice sheets in North America and Northwest Eurasia to its present state. The fully-interactive simulation of this transition represents a hitherto unsolved challenge for state-of-the-art climate models. We use a novel coupled comprehensive atmosphere–ocean–vegetation– ice sheet–solid earth model to simulate the transient climate evolution from the last glacial maximum to preindustrial times. The model considers dynamical changes of the glacier mask, land–sea mask and river routing. An ensemble of transient model simulations successfully captures the main features of the last deglaciation, as depicted by proxy estimates. In addition, our model simulates a series of abrupt climate changes, which can be attributed to different drivers. Abrupt cooling events during the glacial and the first half of the deglaciation are caused by Heinrich-event like ice-sheet surges, which are part of the model generated internal variability. We show that the timing of these surges depends on the initial state and the model parameters. Abrupt events during the second half of the deglaciation are caused by a long-term shift in the sign of the Arctic freshwater budget, changes in river routing and/or the opening of ocean passages.
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RC1: 'Comment on cp-2024-55', Louise Sime, 08 Sep 2024
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Review of “Deglaciation and abrupt events in a coupled comprehensive atmosphere–ocean–ice sheet–solid earth model” by Uwe Mikolajewicz et al.
Manuscript summary: This study introduces a novel comprehensive coupled atmosphere-ocean-vegetation-ice sheet-solid earth model, MPI-ESM/mPISM/VILMA, designed to simulate the last deglaciation with high realism. This model is unique because it includes interactive Earth system components such as ice sheets, icebergs, solid earth, and dynamic river directions, allowing for a comprehensive exploration of deglacial processes. The findings highlight that the model can reproduce abrupt millennial-scale climate events and that the timing of these events is influenced by initial conditions and model parameters rather than solely by external forcing. Additionally, the model reveals that changes in Arctic sea-ice export and freshwater dynamics significantly affect the Atlantic Meridional Overturning Circulation (AMOC) and North Atlantic climate.
I find this study really quite exciting. It provides a breakthrough in understanding how different components of the Earth’s system interact over long timescales, particularly during the complex process of deglaciation. The ability of the new model to simulate abrupt climate events and reveal unexpected dynamics, including the influence of freshwater dynamics and ice sheet surges on ocean circulation, opens up new avenues for exploring past climate changes and understanding past climate tipping behaviours.
The analysis is mostly well-organised and constructed, and the paper is well-written. It effectively conveys the findings, highlights the novelty of the research, and provides a clear and concise overview of the study’s objectives, methods, results, and implications. With the caveats below about it being on the long side, and whether the authors could extend parts of the analysis and split it into two, it is clearly suitable for publication in CP.
Major points:
This is not necessarily a criticism, but I find there is really a lot of material/work in this manuscript. It might be more digestible to most readers if it were split in two, to allow more focus on some aspects. Perhaps with the material on abrupt changes placed into a second manuscript?
The simulations are generally run from 26ka to 1850, allowing a climatological mean PI for each simulation to be specified as the last 1,000 years or so of the run. However, we never see what the PI simulations look like. The manuscript would benefit from an appendix to show how the PI states differ for each version of the model, and some indication of how far they deviate from the observed PI, and a comment on this to be added to 2.2 and 3.1.
The description of the pre-26ka model spin-up seems to imply that the ocean is spun up for just 10 years, and then the first 1,000 years of each simulation are disregarded. This would give only 1,010 years to spin up the ocean. I think this may not be correct because the text also states that the simulations are initialized from pre-existing glacial simulations. However, this is not very clearly explained. Like the PI point above, spin-up is relatively important, so this should be carefully clarified in 2.1.
Some clarification/justification for modifications to ocean mixing/vertical diffusion would be useful, both in the context of expected glacial-interglacial ocean mixing changes (due to bathymetric and stratification effects on the dissipation of internal tides/waves). Alongside this, a strengthened discussion of how AMOC and mixing may be affecting the results from this model, given abrupt changes results are relatively strongly dependent on the strength of the AMOC & mixing i.e. discussion of would we expect other GCMs/ESMs to behave similarly (if they also had the same extra component coupled onto them, and were run for similar experiments), or are these unique to this model?
The section on Abrupt Events is very sensibly laid out; I like the analysis. However, I find it slightly surprising that the focus is solely on simulated abrupt cold events, given there is considerable community interest in the possibility of abrupt warmings too, and some deglacial events are indeed abrupt warmings, rather than solely coolings. If the authors intend to retain the focus solely on abrupt coolings, then it would be helpful to have a sentence or two added to the Introduction and Section to better justify this focus. Otherwise, the focus could perhaps be broadened to also include abrupt warmings, which are also visible in the timeseries provided. See also the first main comment about splitting this manuscript in two.
Minor points:
The model name is a currently a bit of a mouthful, have the authors also considered giving the model a shorter name too MPI-ESM-extended or similar, perhaps?
L18, missing refs, for very different rates of changes
L19-22, greater and lesser ‘volumes’ rather than changes?
L34 abrupt ‘AMOC’ changes, or what events?
L34, ‘the quantification of’ rather than ‘the exact changes’
L54, and after, clarify what ‘CMIP-style’ means. Either CMIP models, or perhaps models that use CMIP-atmosphere (AMIP?) models, or perhaps that are run at common CMIP atmosphere-ocean resolutions? Either way, replace ‘CMIP-style’ descriptions with something more meaningful.
L75, split this into two sentences.
L113, clarify what is meant here by radial directions – by depth, but not by lat or long?
Section 2.2 and 3.1, please see above first two main points.
Table 1 headings, spell out the headings better. Information about spin-up is not clear. Better to replace the last column ‘Parent run’ with a much clearer verbal description. Exp file names can be omitted.
~L125, consider to add a short subsection or para in 2.1 which describes the water tracer/dye methodology, including precise conditions for tracing/dying water, and how tracers/dye is reset (presumably without resetting on exposure, the ocean would eventually saturate?).
L193, maximum?
L200-202, better to replace this with an Appendix that more carefully shows what the PI states look like themselves (not just the LGM-PI anomalies).
Figure 3, obscures more than it shows. It might be more instructive to see the LGM-PI anoms subtracted from the equivalent Anna et al and Osman et al anoms.
Figure 5, I really like these dye/tracer sections.
Figure 5 and Figure 6, and thereabouts in text, please ensure there is sufficient information in the text to reassure the reader that these results are not due to spin-up issues. See also main points, and minor point aboves about better clarification on spin-up procedures and ocean run durations.
Page 6, and generally dye/tracer results. Please clarify how dye/tracer is removed. Is it reset to 0 on exposure to the atmosphere? Or something else?
L295, it is interesting that these coupled model simulations do not help much with this Antarctic ice sheet extent problem, possibly it is worth highlighting that this results supports the idea that the problem is in the representation of ice sheet physics in PISM rather than in the forcing/coupling?
L310-311, rewrite this sentence – it is very hard to understand.
L375, missing punctuation/sentence issue.
Table 2, this table would be easier to digest if the rows were shaded to reflect whether the simulated even occurs earlier, later, or (within the uncertainties) at commensurate with the proxy evidence for a similar event.
L529, salinity twice
L531, ‘varies significantly’ rather than the significantly varies
L581, see first comment on model name
L602, remove ‘also’?
L607-610, there are some odd clause orders in here. Check ordering for English, and improve the sentences.
L632, either clarify what is meant by unexpected, or possibly rewrite this sentence to focus on the successfully model simulation of hereto uncaptured processes? (river rerouting, arctic freshwater sign changes, strait flow impacts, and other ice-sheet change related climate-land surface-ocean related processes.).
Citation: https://doi.org/10.5194/cp-2024-55-RC1
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