A conceptual framework linking Heinrich events and Dansgaard&ndash;Oeschger cycles

Heinrich, Hartmut; Lohmann, Gerrit

doi:10.5194/egusphere-2026-896

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https://doi.org/10.5194/egusphere-2026-896

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28 Aug 2026

| 28 Aug 2026

Status: this preprint is open for discussion and under review for Earth System Dynamics (ESD).

A conceptual framework linking Heinrich events and Dansgaard–Oeschger cycles

Hartmut Heinrich and Gerrit Lohmann

Abstract. Dansgaard–Oeschger (D–O) events and Heinrich events represent the dominant modes of millennial-scale climate variability during the last glacial period, yet their physical linkage remains incompletely understood. Here we propose a conceptual framework in which abrupt glacial variability emerges from the interaction of Atlantic Meridional Overturning Circulation (AMOC) dynamics, atmosphere–ocean CO₂ exchange, and dust-driven biogeochemical feedback operating within an orbitally modulated climate state. During stadial phases, enhanced dust transport and iron fertilization in the tropical Atlantic strengthen the biological pump and promote gradual atmospheric CO₂ drawdown, while reduced ocean ventilation facilitates the accumulation of respired carbon and heat in the ocean interior. Heinrich events represent extreme stadial states that amplify these imbalances through freshwater forcing, sea-ice expansion, and atmospheric circulation changes. The resulting buildup of subsurface heat and carbon may precondition the stratified North Atlantic for abrupt AMOC reinvigoration, releasing stored CO₂ and producing rapid interstadial warming. This framework provides a unified conceptual perspective on the asymmetry and recurrence of D–O cycles and suggests that orbital forcing modulates the probability of Heinrich events by influencing the background stability of the climate system.

Received: 15 Feb 2026 – Discussion started: 28 Aug 2026

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Hartmut Heinrich and Gerrit Lohmann

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RC1:
'Comment on egusphere-2026-896', Anonymous Referee #1, 23 Sep 2026 reply
The paper presents a conceptual framework for D/O- and H-events during the last glacial. This is a very active area of research with many contributions, both in proxy data and modelling, which has led to many hypotheses and speculations about possible mechanisms. Therefore, expectations are high for a study that promises a coherent framework for this variability. Unfortunately, this paper does not provide a convincing novel insight into the complex problem of D/O-H dynamics. The current version lists a few mechanisms that are undoubtedly involved in this natural climate variability of global scale. They have been proposed and investigated before in many papers, some of them comprehensively and recently. This paper does not provide novel information, offers no quantitative assessment of the highlighted processes (e.g., via feedback strengths), and presents no an analysis of such behavior in relevant model simulations (even only one would already be enlightening). Overall, this paper lacks substance and would need major revisions to be further considered.
Comments:
It is claimed that the presented framework provides a perspective on the asymmetry and recurrence of D/O cycles (e.g., lines 27, 361). This asymmetry is well captured in simulations across the entire model hierarchy, and it is thus a very well understood feature associated with heat accumulation below the ocean mixed layer and sea ice Atlantic formation during AMOC reduction or shut-down. I would argue that a new framework for this property would be useful if a novel mechanism were proposed that would also be underpinned by a model simulation.

The same holds true for the recurrence of D/O cycles. Here, the growing number of simulations showing self-sustained oscillations in climate models, again across the hierarchy, provides a dynamically consistent understanding of this variability. Again, the authors should give arguments for why a new framework for this feature is needed and what new understanding would result from it. The important new progress on self-sustained oscillations, and the influence of weather noise that destroys strict periodicity, is not given the appropriate attention in this paper.

There are two contributions (not cited) that already cover much of what the current article aims to do. Pedro et al (2018, QSR) and Buizert et al. (2026, Nature Geosci). The current study would certainly become more relevant if the progress relative to these two were explained and critically discussed.

The current versions of some of the figures are not helpful in supporting the text. For example, Fig. 1 uses most of the space for a Google Earth view of W Africa while rendering the stratigraphy and time series illegible. Figs 2 and 4 are not sufficiently informative by simply suggesting a sequence of processes without providing information regarding the strength of the causation between successive process. For instance, to what extent is increased Sahel dust input capable to reduce atmospheric CO2?

Throughout the paper the terms such as “CO2 import”, “CO2 export”, ”oceanic import”, etc. are used. This is confusing and may suggest some kind of advective transport. Instead, probably CO2 uptake or release by the ocean is meant. But perhaps I am missing something.

Figure 5 is particularly confusing by packing too much information into it. Perhaps the idea was somehow mimic the temperature reconstruction along a series of D/O events based on the stable isotope records of Greenland ice cores. If understood correctly, the time for the part would progress from the right to the left. The “cycle” would then be terminated by an H event after which time jumps back to the right end of the figure. The use of “export” and “import” here is particularly confusing. Also, “ice sheet collapse” may be misleading, as an H-event is more likely associated with a rapid discharge of ice and IRD along some critical ice streams of the circum-Atlantic ice sheets.

The centennial to millennial variations of CO2 would provide an important constraint on the sequence of processes during D/O-H variations. The high-resolution reconstructions from Antarctic ice cores are not discussed here, although Figs. 2 and 4, as 5 do suggest a key role for CO2. Apart from jumps, CO2 variations during the last glacial are more gradual and resemble the Antarctic temperature reconstruction. How is this information reconciled in the proposed framework?

Figure 6 presents several records without referencing in the caption. This is important information for possible future studies. Referencing would also give explicit credit to the colleagues producing these data.

Section 4 is based on many quite old references with the latest from 2015 and 2016. The ref. from 2015 is not relevant, as it concerns the anthropogenic perturbations of the carbon cycle and marine ecosystems. This is fundamentally different from the natural physical (much larger than anthropogenic) and biogeochemical (much smaller than anthropogenic) variations during the last ice age. This section also contains some generic text, e.g. lines 312-315.

Section 5 opens with a very vague paragraph (lines 352-357). I miss the “testable predictions” announced in the section title.

Much information during the past decade concerns a palette of paleoceanographic tracers and their variations during the last ice age, particularly during MIS3. This provides crucial constraints to possible mechanisms and processes at work during D/O-H dynamics. None of this is mentioned and considered in this paper.

In summary, it not evident what new insight this paper will give to the community, in particular to the paleoclimate modelling community that would consider a new framework when setting up simulations and test specific hypotheses.

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Citation: https://doi.org/10.5194/egusphere-2026-896-RC1
RC2: 'Comment on egusphere-2026-896', Anonymous Referee #2, 12 Oct 2026 reply

The manuscript by Heinrich and Lohmann presents a novel conceptual framework linking abrupt glacial climate changes, such as Dansgaard–Oeschger and Heinrich events, to millennial-scale variations in the global carbon cycle and orbital forcing. It also provides an extensive review of the hypotheses proposed to explain the nature of abrupt glacial climate changes and their possible interconnections. The effort to synthesize a large body of previous research by formulating a clear and testable hypothesis is generally welcome. However, in its current form, the manuscript has a number of important issues that need to be addressed before its central hypothesis can be adequately evaluated.
The main idea of the proposed concept is that abrupt glacial climate changes interact with the carbon cycle by modifying dust deposition over the tropical Atlantic, which is explained by changes in atmospheric circulation induced by the AMOC weakening during stadials. While this is an interesting hypothesis, there are two significant problems with it. Firstly, even at present, the tropical Atlantic is not considered an area of iron-limited marine productivity; rather, it is nutrient-limited. While a number of previous studies have considered the impact of changes in dust deposition on iron fertilisation as a potential mechanism for glacial-interglacial CO₂ variability, this mechanism has been attributed to the Southern Ocean, where marine biological productivity is at least partly iron-limited (e.g. Martin, 1990).
The second problem with the proposed hypothesis is that it assumes an increase in dust deposition during stadial conditions leads to a drawdown of atmospheric CO₂ through enhanced oceanic carbon uptake, but this is not supported by existing paleoclimate records. Curiously, the authors extensively discuss CO₂ changes during abrupt glacial climate changes, yet they do not present any CO₂ records or cite relevant papers. Such records do exist, however, and are crucial for evaluating the mechanism proposed by the authors. In particular, a high-resolution atmospheric CO₂ record from the West Antarctic Ice Sheet Divide (Bauska et al., 2021) shows that, during Heinrich stadials (except H3 and H2), CO₂ increased by 10–15 ppm, whereas no significant changes in CO₂ occurred during non-Heinrich stadials. Similar CO₂ changes are also evident during previous glacial cycles (Nehrbass-Ahles et al., 2020). A number of papers discuss the potential cause of the CO₂ rise during Heinrich stadials, often attributing it to CO₂ outgassing through enhanced Southern Ocean ventilation (Menviel et al., 2015; Gottschalk et al., 2016; Skinner et al., 2020; Willeit et al., 2025).
The proposed link between orbital forcing and the timing of Heinrich events in Section 3 is also not entirely convincing. While there is no doubt that orbital forcing affects millennial-scale climate variability directly and through the temporal evolution of ice sheets and CO₂, it is unclear why the authors believe that low-latitude insolation can affect ocean heat storage. Furthermore, it is even unclear what the authors mean by 'low-latitude insolation', given that their Figure 6 shows 'boreal summer insolation' averaged between 30°S and 30°N, the relationship of which to climate variability is not obvious at all.
While the manuscript cites many relevant publications, not all of these are consistent or properly interpreted, and some relevant publications are missing. For example, the 'binge-purge' oscillation hypothesis by MacAyeal (1993) is cited several times, but it is unclear from the manuscript whether this hypothesis was tested using real ice sheet models. In fact, self-sustained oscillations associated with ice surges have been successfully simulated using increasingly complex and spatially resolved models (Payne, 1995; Calov et al., 2002; Roberts et al., 2016; Ziemen, 2019, etc.). Conversely, Alley and MacAyeal (1994) did not discuss the mechanism of marine ice sheet instability (line 295). Furthermore, the relevance of the publications by Hendy and Kenett (2003) and Davitian and Bard (2023) to the destabilisation of ice sheets by subsurface warming is unclear.
Bauska, T.K., et al. 2021. Abrupt changes in the global carbon cycle during the last glacial period. Nature Geoscience, 14(2), 91-96.
Calov, R., et al. 2002. Large-scale instabilities of the Laurentide ice sheet simulated in a fully coupled climate-system model, Geophys. Res. Lett., 29.
Gottschalk, J. et al. 2016. Biological and physical controls in the Southern Ocean on past millennial-scale atmospheric CO2 changes. Nat. Commun. 7, 11539.
Menviel, L., et al. 2015. Contribution of enhanced Antarctic Bottom Water formation to Antarctic warm events and millennial-scale atmospheric CO2 increase. Earth Planet. Sci. Lett. 413, 37–50.
Nehrbass-Ahles, C., et al., 2020. Abrupt CO2 release to the atmosphere under glacial and early interglacial climate conditions. Science, 369, 1000-1005.
Payne, A. J., 1995. Limit cycles in the basal thermal regime of ice sheets, J. Geophys. Res., 100(B3), 4249– 4263.
Roberts, W. H. G. et al., 2016. The role of basal hydrology in the surging of the Laurentide Ice Sheet, Clim. Past, 12, 1601–1617.
Skinner, L., et al., 2020. Southern Ocean convection amplified past Antarctic warming and atmospheric CO2 rise during Heinrich Stadial 4. Commun. Earth Environ. 1, 1–8.
Willeit, M. et al., 2025. Earth system response to Heinrich events explained by a bipolar convection seesaw. Nature Geoscience, 18, 1159-1166.

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Citation: https://doi.org/10.5194/egusphere-2026-896-RC2

Hartmut Heinrich and Gerrit Lohmann

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Short summary

To date, models have not been very successful in linking Heinrich events (He) with Dansgaard-Oeschger (D-O) cycles, nor in identifying the driver. Here, we present a concept based on CO₂ exchange between the atmosphere and the ocean, and on the input of Fe-rich dust during the cold phases of the systems. Cooling trend during a D-O cycle is interrupted by a precessionally maximized Atlantic circulation causing a He. The subsequent strong release of CO₂ from the ocean triggers a D-O new cycle.


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