| 26 Sep 2025
Stabilizing feedbacks allow for multiple states of the Greenland Ice Sheet in a fully coupled Earth System Model
Abstract. The Greenland Ice Sheet (GrIS) will experience substantial mass loss and might even disappear if elevated global-mean temperatures are maintained over the next millennia. Previous studies indicated that once melted, the GrIS might not regrow even under subsequently lowered temperatures.
Here, we use a newly developed complex fully-coupled climate-ice sheet model to explore a potential multistability of the GrIS. This model system is more complex and includes more critical feedbacks relevant for the stability of the GrIS than previously used models. In a set of steady state simulations, we find that at least four steady states exist under a pre-industrial (PI) climate: Besides a state with a large GrIS that is similar to the PI state, we find steady states with GrIS volumes of about 48 %, 28 % and 19 % of the PI volume. These steady states are stabilized through several feedback processes, such as the melt-elevation and melt-albedo feedback. In the smaller states, ice sheet expansion is further limited by a redistribution of precipitation, a Föhn effect and additional warming driven by atmospheric circulation changes due to the reduced blocking of a smaller GrIS. The southern part of the GrIS is controlled by alterations of the sea-surface temperature of the Irminger Sea and the Nordic Seas. We also show that interactions between the GrIS and the Antarctic Ice Sheet (AIS) impact the transient behavior of the GrIS. Our results highlight the importance of climate-ice sheet feedbacks in maintaining multiple steady states of the GrIS. Such multistability has implications for assessing the consequences of global warming. Our simulations indicate that if the GrIS volume drops below a critical threshold of 83–70 % of its PI volume, at least half of its current volume will be irreversibly lost even if we return to global PI temperatures through a reduction in CO2 concentrations.
Review of “Stabilizing feedbacks allow for multiple states of the Greenland Ice Sheet in a fully coupled Earth System Model” by M. Andernach et al.
This manuscript investigates the potential multi-stability of the Greenland Ice Sheet (GrIS) using a fully coupled climate–ice sheet model under pre-industrial climate conditions. The existence of multiple steady states of the GrIS is not new, but this study provides a fresh and valuable contribution by employing a fully coupled model configuration and identifying four distinct equilibrium states at approximately 100%, 48%, 28%, and 19% of the pre-industrial ice volume.
The paper is well written, clearly structured, and scientifically solid. It is thoroughly embedded in the existing literature and successfully highlights both the consistency with, and the departures from, earlier work. The study thus adds important nuance to our understanding of Greenland Ice Sheet stability and the role of climate–ice sheet feedbacks.
I recommend acceptance with minor revisions. The manuscript is already strong, and the suggestions below are primarily aimed at clarification, readability, and strengthening the framing around stabilizing feedbacks.
## General Comment ##
Focus on stabilizing feedbacks and suggested summary table: The title emphasizes stabilizing feedbacks as key mechanisms allowing for multiple steady states. Given this framing, the paper would benefit from a clearer and more systematic presentation of which feedbacks dominate and how they differ among the identified equilibria.
I suggest including a summary table (e.g. in Section 4) listing the four steady states and the corresponding stabilizing feedbacks that maintain each. If the same mechanisms apply across all states, this could be explicitly stated. Such a synthesis would align the manuscript with its title and improve clarity for readers.
## Specific Comments ##
L7–8: “These steady states are stabilized through several feedback processes, such as the melt-elevation and melt-albedo feedback.”
Please clarify whether the melt–elevation and melt–albedo feedbacks are indeed stabilizing. These processes are usually considered positive feedbacks (destabilizing). Are they stabilizing only in certain states, depending on basin of attraction? A brief explanation of when and how their sign changes would be useful.
L12: “highlight the importance of climate–ice sheet feedbacks”
Consider adding “fully coupled”, as this aspect is a major strength of the study.
L61–69: You mention stabilizing feedbacks via isostatic adjustment and freshwater release into the North Atlantic. Could you clarify whether these are active in your simulations and, if so, whether they appear among the feedbacks constraining your steady states? If they are not significant here, a short note acknowledging that would be helpful.
L85–86: You talk of previous studies neglecting interactions with components such as the AMOC, vegetation, and isostatic adjustment. Since these interactions were previously neglected, it would strengthen the discussion (in Section 4 and perhaps already here) to comment briefly on whether they are important in your results—e.g., does the AMOC play a stabilizing or destabilizing role for any of the steady states?
L95–96: “we identify which feedbacks or combination of feedbacks constrain each steady state of the GrIS.” This is central to your paper’s theme but remains somewhat implicit. A concise table summarizing which feedbacks constrain which state would help make this claim more concrete.
L127: “the asynchronous coupling method has no impact on the results.” This phrasing feels too strong. Consider softening it to something like “We find no significant impact on the results or conclusions from the asynchronous coupling method.”
L129–150 This paragraph is long and dense. Consider splitting it into smaller paragraphs to improve readability.
L130 “five simulations starting from different GrIS volumes (0%, 21%, 43%, 70%, and 100% of the PI value; Tab. 1).” The list of initial conditions does not match Table 1 (which lists 0%, 33%, 70%, 100%). This creates confusion. Either align the lists or move the table reference to where the consistent set appears.
L200–201: “the dynamic growth of grass and shrubs in the unglaciated areas, which leads to strongly positive melt-albedo feedback.” Please clarify whether vegetation expansion is itself what you refer to as the melt-albedo feedback. Typically, the melt-albedo feedback refers to darkening of snow/ice by melt rather than vegetation. If the vegetation effect is distinct, please rephrase accordingly.
L242–243: When describing how the SG state becomes unstable and transitions to the MG state (paraphrasing: Above a certain threshold it becomes unstable), consider mentioning which physical processes cause this instability.
L290: You mention “the inertia of the ice sheet.” Please clarify what is meant by “inertia.” In a physical sense, ice sheets have relatively slow response times but limited true dynamical inertia; a short explanation would avoid confusion.
L342: “Below 70–68%, even further parts of the GrIS are lost”. It is unclear where these threshold numbers (70 – 68%) come from. Please specify.
## Editorial and Typographical Comments ##
L193: Suggest to revise to: “Only in the mountains are temperatures cold enough…”
L263–264: Revise to: “does an ice cover in the northwest become stable”
L273:“disintegrates” (add final s)