the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 May 2022
A theory of abrupt climate changes: their genesis and anatomy
Abstract. We integrate our previous ice-sheet and climate models to examine abrupt climate changes pertaining to Heinrich event (HE), Dansgaard-Oeschger (DO) cycle as well as last deglaciation punctuated by Younger Dryas (YD). Since they are all accompanied by ice-rafted debris, we posit their common origin in the calving of ice sheet due to thermal switch at its bed. Such thermal switch would generate step-like freshwater flux and together with decadal ocean response, they would endow abruptness to these millennial climate signals, which need not involve ocean mode change, as commonly assumed. We distinguish thermal switches due to geothermal heat and surface melt, which would calve inland/marginal ice to drive HE/DO-cycle, respectively. As such, the glacial DO-cycle hinges on post-HE warmth that enables the ablation whereas the Holocene DO-cycle is self-sustaining. The ocean response to freshwater flux entails millennial adjustment to maximum entropy production (MEP), a process termed “MEP adjustment”. As its direct consequence, the termination of HE is accompanied by sudden warming followed by gradual cooling to exhibit saw-toothed H-cycle, and the cooling moreover would anchor DO-cycles to form the hierarchical Bond cycle. The meltwater produced during deglaciation, if rerouted to Hudson Bay, may augment the calving-induced freshwater flux to cause YD, the latter thus involves happenstance and did not materialize during penultimate deglaciation. By incorporating calving origin of the freshwater flux and MEP adjustment of the ocean, the theory has provided an integral account of these abrupt climate changes.
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RC1: 'Comment on egusphere-2022-208', Anonymous Referee #1, 20 Jun 2022
The manuscript by Ou presents a theory of abrupt glacial climate changes. The author postulates that Heinrich and Dansgaard-Oeschger events have the same nature and are related to abrupt changes in the ice calving into the ocean. In fact, the idea that abrupt climate changes of the glacial age were related to strongly variable freshwater flux into the ocean is by far not new, although one of many and not the most popular now. The only novelty of the proposed theory is that, according to the author, abrupt climate changes do “not involve ocean mode change, as commonly assumed“ (L. 11). Although the author repeated this statement a dozen times, he did not explain what he understands under “mode change”, and this is why it is difficult to assess how much the mechanisms proposed by the author differ from those that have been proposed in numerous previous studies.
The main problem of the manuscript under consideration is that it completely ignores a vast amount of recent studies on abrupt climate change. Excluding self-citations, only two (!) cited papers were published during the past ten years and the absolute majority of cited papers were published in the past century. This “statistic” is in odd with the drastic increase in the number of empirical and modelling studies of abrupt climate changes in recent times. This ignorance about the contemporary progress in understanding the mechanisms of past climate change results in the numerous erroneous statements and premises on which the theory is built, which made the manuscript under consideration totally inappropriate for publication in scientific journals.
General comments
The lack of up-to-date knowledge about the progress in the understanding of abrupt climate change gravely affects the entire manuscript. Normally, the introduction presents the current status of the progress in the related subject and the motivation for the presented study. Instead, the author used the introduction to introduce his own theory of abrupt climate changes. Already the first paragraph contains numerous inaccurate and erroneous statements:
The author claims that both Heinrich events and Dansgaard-Oeschger cycles “are all accompanied by ice-rafted debris (IRD) ... suggesting a common origin in the calving of ice sheet due to thermal switch at its bed“. The author referred here to a very old paper (Bond et al. 1997), based on a very coarse resolution record. In fact, the situation with IRD during Heinrich and non-Heinrich stadials is very complex. The late are found only in some locations and may imply better survival of icebergs in colder climates rather than an increase in their production. Moreover, there is a potential time lag of IRD compared to the onset of stadials (e.g. Barker et al., 2015), which contradicts the idea of freshwater forcing of stadial events. Thus, it is very likely that IRD during stadials represents the response rather than the forcing of abrupt climate changes.
The next sentence (L. 35) - ”since recurring time of calving is constrained by ice mass balance, the resulting freshwater flux is naturally availed the millennial timescale, a timescale not inherent to the ocean” - is even more problematic. First, the author does not explain why recurring time of calving is constrained by ice mass balance and why it should be millennia. The last part of the same sentence, namely, that the millennial time scale is not inherent to the ocean, is just wrong. Numerous recent studies demonstrate that the ocean alone can produce millennial-scale self-sustained oscillations resembling Dansgaard-Oeschger events (Peltier and Vettoretti, 2014; Brown and Galbraith, 2016; Klockmann et al., 2018). In fact, this finding is not so new: the existence of millennial-scale self-sustained AMOC oscillations has been demonstrated already in Winton and Sarachik (1993).
The first paragraph ends with another erroneous statement: “variation in the sea-surface temperature (SST) remains well short of mode change except during deglaciation”. The authors did not explain which model is meant here, but I guess this is the “ocean mode.” Then, the author explains what “short" means: “SST variation is considerably smaller, ranging in low single digits” (L. 228). In fact, SST variations in the Northern Atlantic during DO events were about 5oC (e.g. Martrat, 2007; Alonso-Garcia et al., 2011) which is fully consistent with what climate models simulate in response to the ocean mode change, namely a complete AMOC shutdown (e.g. Jakson et al., 2015)
In the next paragraph, the author postulates that H-cycle is related to “to calving of the inland ice”, whilst “for DO-cycle it is the surface melt over the ablation zone that causes calving of the marginal ice”. Unfortunately, the author does not explain why he believes thy HE events are associated with calving of “grounded ice” and what “marginal ice” means in the case of DO events.
As far as a very stimulating McAyeal’s concept of binge-purge oscillations is concerned, the author should be aware that it has been advanced over the past decades significantly from 1-dimension to 3-dimensional case and evolved apart from geothermal and frictional heat, also strain heating, basal hydrology, activation/deactivation waves, etc., (Calov, 2002, 2010; Roberts et al., 2016; Feldmann and Levermann, 2017; Schannwell et al., 2022).
On pages 11 and 14, the author, at last, mentioned alternative mechanisms of abrupt climate changes. He wrote: “H-cycle has been modelled as ocean mode change (Paillard 1995; Ganopolski and Rahmstorf 2001), which is unsupported by observation”. Which observations do not support “mode change”, the author did not explain. In turn, I am aware of numerous paleoclimate records which support qualitative AMOC changes during Heinrich events and non-Heinrich stadial (e.g. Lippold et al., 2009; Böhm et al., 2015). In the next sentence, based on Equation (16), the author claims that SST change should be “proportional to the freshwater perturbation” (L. 313). This, however, contradicts a vast amount of modelling studies which show a strongly nonlinear response of AMOC and SST to freshwater perturbation. They also demonstrate that abrupt climate changes can be caused by a very gradual forcing or even without any external forcing. Moreover, simulated self-sustainable oscillations resembling DO events have typically periodicity of one to several millennia. Since I have no reason to believe in Equation (16) more than in the results of realistic climate models, I cannot consider the proposed theory as a valid alternative to the modern concepts of abrupt climate changes.
Three pages later, the authors criticized results obtained with very simplistic models (Sakai and Peltier, 1999; Schulz and Paul, 2002), but similar results have been later obtained with much more realistic climate models (see above). Interestingly, here the author explicitly assumes that there were Dansgaard-Oeschger events during the Holocene, which is, of course, wrong - the last DO event (Bolling-Allerod) occurred well before the onset of the Holocene.
As far as the explanation of the deglaciation is concerned, one only can wonder why the author placed meltwater pulses 1A and 1B ca. 3000 years later than they happened in reality (Fig. 10).
References
Alonso-Garcia, M., et al. Ocean circulation, ice sheet growth and interhemispheric coupling of millennial climate variability during the mid-Pleistocene. Quaternary Science Reviews 30, 3234-3247, 2011.
Barker S., et al. Icebergs not the trigger for North Atlantic cold events. Nature, 520, 333-336, 2015.
Böhm, E., et al. Strong and deep Atlantic meridional overturning circulation during the last glacial cycle. Nature, 517, 73-76, 2015.
Brown, N., and Galbraith, E. D. Hosed vs. unhosed: interruptions of the Atlantic Meridional Overturning Circulation in a global coupled model, with and without freshwater forcing. Climate of the Past, 12, 1663-1679, 2016.
Calov, R., et al. Results from the Ice-Sheet Model Intercomparison Project – Heinrich Event INtercOmparison (ISMIP HEINO), J. Glaciol., 56, 371–383, 2010.
Feldmann, J. and Levermann, A. From cyclic ice streaming to Heinrich-like events: the grow-and-surge instability in the Parallel Ice Sheet Model, The Cryosphere, 11, 1913–1932, 2017.
Jackson, L. C., et al. Global and European climate impacts of a slowdown of the AMOC in a high resolution GCM. Climate dynamics, 45, 3299-3316, 2015.
Klockmann, M., et al. Two AMOC states in response to decreasing greenhouse gas concentrations in the coupled climate model MPI-ESM. Journal of Climate, 31, 7969-7984, 2018.
Lippold, J. et al. Does sedimentary 231Pa/230Th from the Bermuda Rise monitor past Atlantic meridional overturning circulation? Geophys. Res. Lett. 36, L12601, 2009.
Martrat, B., et al. Four climate cycles of recurring deep and surface water destabilizations on the Iberian margin. Science 317, 502-507, 2007.
Peltier, W. R., and Vettoretti, G. Dansgaard-Oeschger oscillations predicted in a comprehensive model of glacial climate: A “kicked” salt oscillator in the Atlantic, Geophys. Res. Lett., 41, 2014.
Roberts, W. H. G., et al. The role of basal hydrology in the surging of the Laurentide Ice Sheet, Clim. Past, 12, 1601–1617, 2016.
Schannwell C., et al. Sensitivity of Heinrich-type ice-sheet surge characteristics to boundary forcing perturbations. Climate of the Past Discussion, 2022.
Winton, M., and Sarachik, E. S. Thermohaline oscillations induced by strong steady salinity forcing of ocean general circulation models. Journal of Physical Oceanography, 23, 1389-1410, 1993.
Citation: https://doi.org/10.5194/egusphere-2022-208-RC1 -
AC1: 'Reply on RC1', Hsien-Wang Ou, 28 Jun 2022
-
AC2: 'Reply on AC1', Hsien-Wang Ou, 28 Jun 2022
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-208/egusphere-2022-208-AC2-supplement.pdf
-
AC2: 'Reply on AC1', Hsien-Wang Ou, 28 Jun 2022
-
AC1: 'Reply on RC1', Hsien-Wang Ou, 28 Jun 2022
-
RC2: 'Review of egusphere-2022-208', Anonymous Referee #2, 23 Jul 2022
Ou uses simple ice-sheet and climate models to propose a theory related to Dansgaard/Oeschger (D/O), Heinrich (H) and Younger Dryas variability.
The author makes hypotheses related to the origin and processes involved in this millennial-scale variability. These hypotheses guide the choice of parameters used in the simple models (heating parameters, duration of events…). The author explores 2D regime diagrams of a few physical variables (e.g. subpolar temperature, density surplus, MOC) and then concludes that all the millennial-scale variability is due to meltwater input, and distinguish the processes (geothermal heat trapping, surface melt of the ablation zone, meltback of the LIS by interglacial warmth) leading to this meltwater input for D/O, H and YD. The author further concludes that this millennial-scale variability does not involve “ocean mode change”, noting that the meaning of “ocean mode change” is unclear.
The main issue with the manuscript is that the conclusions are unsubstantiated.
The tools used for the purpose of the study are not appropriate to study the origin of D/O, Heinrich or YD variability. Similarly, the simplicity of the climatic model used does not allow conclusions related to the role of ocean and sea-ice variability associated with these abrupt climate change events.
I also note that most of the literature published over the last 20 years related to processes involved in D/O, Heinrich and YD variability has been ignored.
Citation: https://doi.org/10.5194/egusphere-2022-208-RC2
Status: closed
-
RC1: 'Comment on egusphere-2022-208', Anonymous Referee #1, 20 Jun 2022
The manuscript by Ou presents a theory of abrupt glacial climate changes. The author postulates that Heinrich and Dansgaard-Oeschger events have the same nature and are related to abrupt changes in the ice calving into the ocean. In fact, the idea that abrupt climate changes of the glacial age were related to strongly variable freshwater flux into the ocean is by far not new, although one of many and not the most popular now. The only novelty of the proposed theory is that, according to the author, abrupt climate changes do “not involve ocean mode change, as commonly assumed“ (L. 11). Although the author repeated this statement a dozen times, he did not explain what he understands under “mode change”, and this is why it is difficult to assess how much the mechanisms proposed by the author differ from those that have been proposed in numerous previous studies.
The main problem of the manuscript under consideration is that it completely ignores a vast amount of recent studies on abrupt climate change. Excluding self-citations, only two (!) cited papers were published during the past ten years and the absolute majority of cited papers were published in the past century. This “statistic” is in odd with the drastic increase in the number of empirical and modelling studies of abrupt climate changes in recent times. This ignorance about the contemporary progress in understanding the mechanisms of past climate change results in the numerous erroneous statements and premises on which the theory is built, which made the manuscript under consideration totally inappropriate for publication in scientific journals.
General comments
The lack of up-to-date knowledge about the progress in the understanding of abrupt climate change gravely affects the entire manuscript. Normally, the introduction presents the current status of the progress in the related subject and the motivation for the presented study. Instead, the author used the introduction to introduce his own theory of abrupt climate changes. Already the first paragraph contains numerous inaccurate and erroneous statements:
The author claims that both Heinrich events and Dansgaard-Oeschger cycles “are all accompanied by ice-rafted debris (IRD) ... suggesting a common origin in the calving of ice sheet due to thermal switch at its bed“. The author referred here to a very old paper (Bond et al. 1997), based on a very coarse resolution record. In fact, the situation with IRD during Heinrich and non-Heinrich stadials is very complex. The late are found only in some locations and may imply better survival of icebergs in colder climates rather than an increase in their production. Moreover, there is a potential time lag of IRD compared to the onset of stadials (e.g. Barker et al., 2015), which contradicts the idea of freshwater forcing of stadial events. Thus, it is very likely that IRD during stadials represents the response rather than the forcing of abrupt climate changes.
The next sentence (L. 35) - ”since recurring time of calving is constrained by ice mass balance, the resulting freshwater flux is naturally availed the millennial timescale, a timescale not inherent to the ocean” - is even more problematic. First, the author does not explain why recurring time of calving is constrained by ice mass balance and why it should be millennia. The last part of the same sentence, namely, that the millennial time scale is not inherent to the ocean, is just wrong. Numerous recent studies demonstrate that the ocean alone can produce millennial-scale self-sustained oscillations resembling Dansgaard-Oeschger events (Peltier and Vettoretti, 2014; Brown and Galbraith, 2016; Klockmann et al., 2018). In fact, this finding is not so new: the existence of millennial-scale self-sustained AMOC oscillations has been demonstrated already in Winton and Sarachik (1993).
The first paragraph ends with another erroneous statement: “variation in the sea-surface temperature (SST) remains well short of mode change except during deglaciation”. The authors did not explain which model is meant here, but I guess this is the “ocean mode.” Then, the author explains what “short" means: “SST variation is considerably smaller, ranging in low single digits” (L. 228). In fact, SST variations in the Northern Atlantic during DO events were about 5oC (e.g. Martrat, 2007; Alonso-Garcia et al., 2011) which is fully consistent with what climate models simulate in response to the ocean mode change, namely a complete AMOC shutdown (e.g. Jakson et al., 2015)
In the next paragraph, the author postulates that H-cycle is related to “to calving of the inland ice”, whilst “for DO-cycle it is the surface melt over the ablation zone that causes calving of the marginal ice”. Unfortunately, the author does not explain why he believes thy HE events are associated with calving of “grounded ice” and what “marginal ice” means in the case of DO events.
As far as a very stimulating McAyeal’s concept of binge-purge oscillations is concerned, the author should be aware that it has been advanced over the past decades significantly from 1-dimension to 3-dimensional case and evolved apart from geothermal and frictional heat, also strain heating, basal hydrology, activation/deactivation waves, etc., (Calov, 2002, 2010; Roberts et al., 2016; Feldmann and Levermann, 2017; Schannwell et al., 2022).
On pages 11 and 14, the author, at last, mentioned alternative mechanisms of abrupt climate changes. He wrote: “H-cycle has been modelled as ocean mode change (Paillard 1995; Ganopolski and Rahmstorf 2001), which is unsupported by observation”. Which observations do not support “mode change”, the author did not explain. In turn, I am aware of numerous paleoclimate records which support qualitative AMOC changes during Heinrich events and non-Heinrich stadial (e.g. Lippold et al., 2009; Böhm et al., 2015). In the next sentence, based on Equation (16), the author claims that SST change should be “proportional to the freshwater perturbation” (L. 313). This, however, contradicts a vast amount of modelling studies which show a strongly nonlinear response of AMOC and SST to freshwater perturbation. They also demonstrate that abrupt climate changes can be caused by a very gradual forcing or even without any external forcing. Moreover, simulated self-sustainable oscillations resembling DO events have typically periodicity of one to several millennia. Since I have no reason to believe in Equation (16) more than in the results of realistic climate models, I cannot consider the proposed theory as a valid alternative to the modern concepts of abrupt climate changes.
Three pages later, the authors criticized results obtained with very simplistic models (Sakai and Peltier, 1999; Schulz and Paul, 2002), but similar results have been later obtained with much more realistic climate models (see above). Interestingly, here the author explicitly assumes that there were Dansgaard-Oeschger events during the Holocene, which is, of course, wrong - the last DO event (Bolling-Allerod) occurred well before the onset of the Holocene.
As far as the explanation of the deglaciation is concerned, one only can wonder why the author placed meltwater pulses 1A and 1B ca. 3000 years later than they happened in reality (Fig. 10).
References
Alonso-Garcia, M., et al. Ocean circulation, ice sheet growth and interhemispheric coupling of millennial climate variability during the mid-Pleistocene. Quaternary Science Reviews 30, 3234-3247, 2011.
Barker S., et al. Icebergs not the trigger for North Atlantic cold events. Nature, 520, 333-336, 2015.
Böhm, E., et al. Strong and deep Atlantic meridional overturning circulation during the last glacial cycle. Nature, 517, 73-76, 2015.
Brown, N., and Galbraith, E. D. Hosed vs. unhosed: interruptions of the Atlantic Meridional Overturning Circulation in a global coupled model, with and without freshwater forcing. Climate of the Past, 12, 1663-1679, 2016.
Calov, R., et al. Results from the Ice-Sheet Model Intercomparison Project – Heinrich Event INtercOmparison (ISMIP HEINO), J. Glaciol., 56, 371–383, 2010.
Feldmann, J. and Levermann, A. From cyclic ice streaming to Heinrich-like events: the grow-and-surge instability in the Parallel Ice Sheet Model, The Cryosphere, 11, 1913–1932, 2017.
Jackson, L. C., et al. Global and European climate impacts of a slowdown of the AMOC in a high resolution GCM. Climate dynamics, 45, 3299-3316, 2015.
Klockmann, M., et al. Two AMOC states in response to decreasing greenhouse gas concentrations in the coupled climate model MPI-ESM. Journal of Climate, 31, 7969-7984, 2018.
Lippold, J. et al. Does sedimentary 231Pa/230Th from the Bermuda Rise monitor past Atlantic meridional overturning circulation? Geophys. Res. Lett. 36, L12601, 2009.
Martrat, B., et al. Four climate cycles of recurring deep and surface water destabilizations on the Iberian margin. Science 317, 502-507, 2007.
Peltier, W. R., and Vettoretti, G. Dansgaard-Oeschger oscillations predicted in a comprehensive model of glacial climate: A “kicked” salt oscillator in the Atlantic, Geophys. Res. Lett., 41, 2014.
Roberts, W. H. G., et al. The role of basal hydrology in the surging of the Laurentide Ice Sheet, Clim. Past, 12, 1601–1617, 2016.
Schannwell C., et al. Sensitivity of Heinrich-type ice-sheet surge characteristics to boundary forcing perturbations. Climate of the Past Discussion, 2022.
Winton, M., and Sarachik, E. S. Thermohaline oscillations induced by strong steady salinity forcing of ocean general circulation models. Journal of Physical Oceanography, 23, 1389-1410, 1993.
Citation: https://doi.org/10.5194/egusphere-2022-208-RC1 -
AC1: 'Reply on RC1', Hsien-Wang Ou, 28 Jun 2022
-
AC2: 'Reply on AC1', Hsien-Wang Ou, 28 Jun 2022
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-208/egusphere-2022-208-AC2-supplement.pdf
-
AC2: 'Reply on AC1', Hsien-Wang Ou, 28 Jun 2022
-
AC1: 'Reply on RC1', Hsien-Wang Ou, 28 Jun 2022
-
RC2: 'Review of egusphere-2022-208', Anonymous Referee #2, 23 Jul 2022
Ou uses simple ice-sheet and climate models to propose a theory related to Dansgaard/Oeschger (D/O), Heinrich (H) and Younger Dryas variability.
The author makes hypotheses related to the origin and processes involved in this millennial-scale variability. These hypotheses guide the choice of parameters used in the simple models (heating parameters, duration of events…). The author explores 2D regime diagrams of a few physical variables (e.g. subpolar temperature, density surplus, MOC) and then concludes that all the millennial-scale variability is due to meltwater input, and distinguish the processes (geothermal heat trapping, surface melt of the ablation zone, meltback of the LIS by interglacial warmth) leading to this meltwater input for D/O, H and YD. The author further concludes that this millennial-scale variability does not involve “ocean mode change”, noting that the meaning of “ocean mode change” is unclear.
The main issue with the manuscript is that the conclusions are unsubstantiated.
The tools used for the purpose of the study are not appropriate to study the origin of D/O, Heinrich or YD variability. Similarly, the simplicity of the climatic model used does not allow conclusions related to the role of ocean and sea-ice variability associated with these abrupt climate change events.
I also note that most of the literature published over the last 20 years related to processes involved in D/O, Heinrich and YD variability has been ignored.
Citation: https://doi.org/10.5194/egusphere-2022-208-RC2
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