the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Disparate energy sources for slow and fast Dansgaard-Oeschger cycles
Diederik Liebrand
Anouk T. M. Bakker
Heather J. H. Johnstone
Charlotte S. Miller
Abstract. During the Late Pleistocene, Dansgaard-Oeschger (DO) cycles triggered warming events that were as abrupt as the present-day human-induced warming. However, in absence of a periodic forcing operating on millennial time scales, the main energy sources of DO cycles remain debated. Here, we identify the energy sources of DO cycles by applying a bispectral analysis to the North Greenland ice core project (NGRIP) oxygen isotope (δ18Oice) record—a 123-thousand-years (kyr) long proxy-record of air-temperatures (Tair) over Greenland. For both modes of DO cyclicity—slow and fast—we detect disparate energy sources. Slow-DO cycles are marked by multi-millennial periodicities in the 12.5 to 2.5 kyr bandwidth and receive energy from astronomical periodicities. Fast-DO cycles have millennial periodicities in the 1.5 ± 0.5 kyr range and receive energy from centennial periodicities. We propose cryospheric and oceanic mechanisms that facilitate the transfer of energy from known sources to slow- and fast-DO cycles, respectively. Our findings stress the importance of understanding energy-transfer mechanisms across a broad range of time scales to explain the origins of climate cycles without primary periodic energy-sources.
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Diederik Liebrand et al.
Status: open (until 21 Apr 2023)
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RC1: 'Comment on cp-2023-6', David De Vleeschouwer, 12 Mar 2023
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Peer review „Disparate energy sources for slow and fast Dansgaard-Oeschger cycles”
Our climate system is full of rhythmical features on a wide variety of time-scales: tidal cycles, the daily cycle, the yearly cycle, sunspot cycles on decadal and centennial timescales, and Milankovic forcing on timescales of several ten thousands of years, ... For most of the rhythmical features in our climate system, we have a straight-forward explanation and the driving mechanism is typically astronomical in nature. However, for some cyclicity observed in the Earth climate history, the explanation is not that straight-forward. The most notable example is the 100-kyr problem, referring to the discrepancy between the 100-kyr rhythm in late Pleistocene glacial/interglacial cycles and the absence of a 100-kyr cycle in the insolation forcing. The Dansgaard-Oeschger cycles constitute a similar problem: The last glacial period was characterized by major climate variability on millennial timescales (stadial/interstadial variability), yet there is no obvious forcing mechanism at hand. To provide deeper insight into nature of this cyclicity, the authors apply bispectral analysis to the North Greenland ice core project (NGRIP) δ18Oice record. They conclude that “slow” Dansgaard-Oeschger cycles (3.5 – 7 kyr periods) are driven by the energy-transfer from astronomical frequencies. They conclude that “fast” Dansgaard-Oeschger cycles (1 – 2 kyr periods) are driven by the energy-transfer from centennial sunspot cycles (e.g., the 200-year de Vries cycle). In other words, the authors provide evidence for an external forcing for the Dansgaard-Oeschger cycles.
After reviewing the manuscript, I very much appreciate the novelty of the statistical approach the authors have applied. I commend them for explaining the approach in a clear and concise manner to an audience that is not yet very familiar with bispectral analysis. The figures presented in the manuscript require some effort to fully understand. Yet, they are well designed, easy to read and convincingly illustrate the claims made by the authors. Furthermore, I believe this work has the potential to become an important stepstone towards a more comprehensive understanding of Dansgaard-Oescher events, which have been the subject of long-standing debates (internal vs. external forcing). The authors' fresh and original view provides new insights into this complex phenomenon and will be of interest to the Quaternary paleoclimate community.
That being said, I have two major comments/questions that need to be addressed before the manuscript can be accepted for publication.
- The 100-kyr eccentricity cycle is presented as a net source of energy for “slow” Dansgaard-Oeschger cycles (Fig. 3b). However, the 100-kyr glacial-interglacial cycle itself only exists by the energy-transfer of precession and obliquity to the 100-kyr band. It is because of this major comment that I started my review by making the analogy between the 100-kyr problem and the Dansgaard-Oeschger cycles. Most paleoclimate researchers now agree that the 100-kyr glacial-interglacial cycle is the result of a non-linear response of the Earth System to high-latitude insolation forcing. Thereby, an energy transfer occurs from the precession band towards the 100-kyr eccentricity band. Hence, I am surprised that this energy-transfer does not show up in Fig 2. To the contrary, the lowermost purple horizontal band in Figure 2 is entirely showing up in orange and red colors. This implies that the bispectral analysis is suggesting that the 100-kyr cycles (f2 in this case on Fig. 2) is providing the energy to “fuel” climate variability in the obliquity band, the precession band, and the “slow” D-O band. The same observation is illustrated in Fig. 4a (100-kyr transferring energy to obliquity/precession) and Fig. 4c (astronomical frequencies including the 100-kyr cycle transferring energy to the “slow” D-O cycles). Can the authors comment on this -to -me- surprising outcome of the bispectral analysis?
- I agree with the authors that the sawtooth-shaped D-O cycles are an indication for the nonlinear transfer of energy across the power spectrum. Again, very similar to the 100-kyr cycles. However, I would like to read a little bit more details on potential mechanisms facilitating this energy transfer. The authors mention the regeneration time of grounded ice in the Hudson Bay to explain the slow cooling during a D-O cycle, and link the catastrophic collapse of this ice volume with the abrupt warming. They emphasize that the asymmetrical build-up and collapse of the cryosphere would have also influenced ocean circulation (lines 160 – 167). While I agree that the ocean and the cryosphere are likely at the origin of the nonlinear climate behavior, I don’t consider this paragraph as a sufficient and satisfactory explanation for this paper. I do not understand how energy can be transferred from astronomical frequencies to the slow D-O band via these processes. I miss some kind of conceptual model. I am explicitly thinking about the Imbrie and Imbrie model for the 100-kyr cycles, quantitatively explaining how the asymmetry between ice buildup and ice melt causes a transfer of energy from the high frequencies to the low frequencies. The authors propose an energy transfer in the reverse direction, from low astronomical frequencies to higher D-O frequencies. To me, such a low-to-high transfer seems more difficult to explain... This is why I would encourage the authors to provide somewhat more details on possible mechanistic pathways that could explain this transfer. Even when such a discussion may be rather speculative, I think this is important to make this paper a completer and more comprehensive story.
Overall, this manuscript has the potential to make an important contribution to the field and with the necessary revisions, could be a valuable addition to the literature.
Citation: https://doi.org/10.5194/cp-2023-6-RC1
Diederik Liebrand et al.
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