the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Dec 2024
100-kyr ice age cycles as a timescale matching problem
Abstract. The dominant periodicity of the late Pleistocene glacial cycles is roughly 100 kyr, rather than other major astronomical periods such as 19, 23, 41, and 400 kyr. Various models explain this fact through distinct dynamical mechanisms, including synchronization of self-sustained oscillations and resonance in mono- or multi-stable systems. However, the variety of proposed models and dynamical mechanisms could obscure the essential factor for realizing the 100-kyr periodicity. We propose the hypothesis that the ice-sheet climate system responds to astronomical forcing at the ~100-kyr periodicity because the intrinsic timescale of the system is closer to 100 kyr than to other major astronomical periods. We support this idea with analyses and sensitivity studies of several simple ice age models with contrasting mechanisms.
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CC1: 'Some thoughts regarding Mitsui et al paper “100-kyr ice age cycles as a timescale matching problem”', Mikhail Verbitsky, 04 Jan 2025
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RC1: 'Comment on esd-2024-39', Holger Kantz, 09 Jan 2025
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The authors pose the hypothesis that out of the many models
for glacial cycles those which when driven with proper insolation data produce
particularly good matching with observations are those which possess an
intrinsic time scale close to 100kyr. This is a very interesting hypothesis.
The authors demonstrate this in detail for 3 models which represent very
different classes:
One with self-sustained oscillations, one which effectively is a
damped oscillator which without driving relaxes to a fixed point,
and one which is a bistable system where switches in either direction
involve some transition times the sum of which again is approximately 100kyr.
In order to verify the relevance of these intrinsic times, the authors
rescale time in all models while keeping the driving unaltered, thereby
showing that the models only exhibit the approximate 100kyr periodicity when
their intrinsic time scales are close to this. For about 10 more models they
check for their intrinsic time scales as well and show that all of these
satisfy their hypothesis. A relevant conclusion is that while the
existence of some intrinsic 100kyr time scale seems to be the key for good
performance, the dynamical mechanism by which this time scale is created can
be quite different.I consider this paper to be a very novel, interesting and relevant
contribution to the 100kyr problem.I missed (or may have overlooked the discussion of) only one aspect
in this issue of the 100kyr cycles: The lack of spectral power at 100kyr in the
65N insolation time series means that the driving signal lacks this frequency
component. Nonetheless they state in line 60 that 'proximity of the intrinsic
time scale .... to the 100kyr periodicity of the eccentricity cycles' is
relevant, i.e., they consider the 100kyr period of the driver to be due to
eccentricity. This seems to be in contradiction to the fact that in the PSD
of 65N insolation there is no enhanced power in this frequency band, and they
also cite Berger who proposed a kind of beating frequency
of the 23.7 and 19kyr modes to be responsible for the 100kyr cycle.
The fact that the eccentricity period of 95kyr is
close to the 100kyr, is this essential or just by chance? Perhaps the authors
can comment on this.purely technical minor issues:
line 25, "Hencefore, the $\approx$ 100 glacial cycles...": kyr is missing
line 117: "... the VCV18 model CANNOT be qualified as ... synchronization"???
line 155: What is the difference between I(t) and f(t)? In line 86 it is said
"I(t) is the standardized summer solstice insolation anomaly at 65N", as well
as in line 107. f(t) is defined in line 128 as '65N summer solstice insolation
anomaly'. Perhaps the authors can invest one more line to clarify this (also
where the mean over the past 1Myr appears and what f_1, f_2 are).Citation: https://doi.org/10.5194/esd-2024-39-RC1
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