the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Sep 2025
A sea ice free Arctic: Assessment Fast Track abrupt-127k experimental protocol and motivation
Abstract. Given that the Arctic could be ice-free in summer within the next ten to twenty years, accurately predicting low-ice states is of crucial importance. Paleo-evidence shows that the strong orbitally-induced high latitude insolation anomaly at 127,000 years ago (127 ky), of around +70 W m−2 in the Arctic during spring-summer, led to warm conditions and an Arctic that was occasionally or often ice-free during summer. Building on two Coupled Model Intercomparison Projects (CMIPs): the Sea-Ice Model Intercomparison Project and the Paleoclimate Modelling Intercomparison Project, we propose an Assessment Fast Track experiment, abrupt-127k, focusing on this seasonally ice-free, or near ice-free, Arctic at 127 ky. The abrupt-127k experiment is initialised from a piControl simulation and abruptly imposes observed values for the insolation distribution and greenhouse gas forcing at 127 ky. It provides a new opportunity to evaluate models used to compute climate projections, both against paleo-evidence and each other, during a known low Arctic sea ice state. As CMIP models are not usually tuned to paleo observations, abrupt-127k represents a true ‘out-of-sample’ test. The abrupt-127k experiment has four key scientific objectives, to: ascertain the simulated Arctic sea ice state, including the presence and characteristics of last-ice areas; evaluate the simulated climates using Arctic paleo-evidence; characterise the central Arctic surface energy budget; and analyse the ice budget including ice melt, growth, and transport. We show that a large Arctic ice response will manifest within the first 30 years of the simulation, thus a single 100-year long run is sufficient for these objectives. Modelling groups are requested to follow standard CMIP output protocol for analysis, including the use of standard ‘fixed-length’ output. Given Fast Track abrupt- 127k is similar in setup to abrupt-2xCO2 and abrupt-4xCO2 experiments, combined analysis of these abrupt-experiments will facilitate understanding of the impacts of instantaneous radiative forcing in the Arctic.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-3531', Anonymous Referee #1, 23 Nov 2025
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RC2: 'Comment on egusphere-2025-3531', Anonymous Referee #2, 22 Jan 2026
General Comments
This manuscript presents an experimental protocol for a “rapid” Last Interglacial simulation to assess how CMIP7 models would perform under 127 ka orbital forcing. The experimental protocol is well motivated by the fact the Arctic Ocean in the LIG likely had some periods of being completely sea ice free, or close to it. Investigating that phenomenon could help constrain how well CMIP7 models might represent future sea-ice free summers in the Arctic. As it’s mostly about designing new experiments, I have mostly minor comments.
The one area which I found a bit disappointing was the lack of requirement for ocean metrics. Since this is an evaluation of fully coupled GCMs, it would seem sensible to have a decent collection of ocean variables as a standard feature of these experiments. The authors only request an ‘optional’ variable of daily sea surface temperature (‘tos’ in CMIP terms). I would recommend having a requirement for monthly outputs of enough variables to enable a decent analysis of the ocean, for example:
- Temperature (3D)
- Salinity (3D)
- Velocities (3D)
- Horizontal transport vectors (3D) – since these can be difficult to reconstruct directly from velocities
- Barotropic stream function
- Meridional overturning circulation
- Sea surface height
- Surface fluxes on the ocean grid
- Evaporation and precipitation
- Runoff
(It's at the authors' discretion to think of a suitable set of ocean metrics, but I think there should be some ocean metrics.)
Part of the motivation for such metrics is that sea ice formation (and destruction) is closely tied to near-surface stratification. Sea ice formation requires the near-surface to be salt-stratified. If the near-surface becomes too salty this can inhibit sea ice. Thus, 3D outputs of temperature and salinity are essential to gain an understanding of the interplay. Likewise, the horizontal circulation and the meridional overturning circulation are critical to understanding how the North Atlantic and Arctic regions behave in terms of temperature and salinity.Line Comments
L57: “Analyse supports assessment…” Grammar is wrong
L127: “ice decline is rapid (Fig. 3)”. This should be Fig. 2.
L141-142: Here one wonders, what about the ocean variables? As in my general comments.
L146: “1 °latitude.” Here there should be space between the degree symbol and “latitude”.
L191-214: I think all these numbered objectives ought to have new lines to make them clearer. Perhaps also use bold type on the letter and numbering to help clarify that is a numbered item. Having the objectives somewhat written like normal paragraphs, while being interrupted all the time by the “O3.1” and “O4.1” etc doesn’t fit very well in my opinion, as if it’s only going half-way to a proper list.
Figure 5 caption, first line: “for example HadGEM3…”, I think “for the example…” might be helpful.
Figure 5 caption, “We use years 50-100”. I thought it stated clearly in the text that it’s years 51-100, not 50-100.
Figure 6 caption, again “years 50-100”, I thought this should have been 51-100?
L237: Acronyms SIA and SIM are defined only the Figure captions and the appendix. I would also re-define them when using for the first time in the main text.
L239: “Gt x 103”. It seems very strange to put the power of 10 before the unit. I would state this as “10-15 x 103 Gt”.
L249: “the (i) the …” repeated “the”.L250-251: “SAT increase values for which the model…” seems a bit garbled grammatically, please clarify this sentence.
In keeping with my general comment, there are no Figures from HadGEM3 presented for Arctic Ocean near-surface stratification (e.g. temperature and salinity) and I think those are quite pertinent to the sea ice story.
Citation: https://doi.org/10.5194/egusphere-2025-3531-RC2
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This manuscript provides a clear and well-organized protocol for the CMIP7 Assessment Fast Track abrupt-127k experiment. The scientific motivation, experimental design, and HadGEM3 example are well presented. I have two brief suggestions:
1. The manuscript distinguishes abrupt-127k from the quasi-equilibrium lig127k experiment, but it would benefit from clearer guidance on their complementary roles and how their results should be jointly interpreted.
2. The protocol includes only insolation and greenhouse gas forcings. Given the availability of the lig127k experiment, additional forcings—such as Greenland Ice Sheet changes, meltwater fluxes, and sea-level variations—should also be considered, as they are important for Arctic sea-ice conditions.