MPT may lag a stronger ice-age feedback
A carbon-cycle oscillator locks ice-age periods to 40, 80, then 120 kyr as feedback grows, and the period jump can lag the parameter change by up to ~10 Myr.
Source
The Mid-Pleistocene Transition: a delayed response to an increasing positive feedback?
Study at a glance
- Design
- Computational / modelling — Calcifier-alkalinity carbon-cycle oscillator with growing weathering feedback under obliquity
- N
- Idealized dynamical-system experiments; not an observational sample N
- Population
- Modeled glacial–interglacial carbon-cycle oscillator under 40-kyr forcing
- Outcome
- Period locking to 40/80/120 kyr with delayed response after feedback strengthening
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Inspect source passages
MPT may lag a stronger ice-age feedback
These passages occur verbatim in the stored paper text. They support the points listed below; they do not verify every statement in the explanation.
What does a delayed period shift imply for MPT causes?
the fundamental change in the Earth system giving rise to the MPT may have occurred a significant time before the observed periodicity shift.
Should MPT explanations be limited to changes at ~1 Myr?
investigations into the cause of the MPT should not be limited to co-occurring changes in the system.
What they did
Authors added a bounded positive weathering feedback to the calcifier-alkalinity model, forced it with 40-kyr obliquity, and jumped the feedback strength to watch how locked cycle length responded.
What they found
Unforced oscillations last 14 kyr until a Hopf bifurcation; stronger feedback stretches the free period toward 120 kyr near γ≈0.12. With forcing, duration locks at 40, 80, and 120 kyr, but after an abrupt feedback jump the period can take ~1–10 Myr to move to the next multiple.
The limits
What it doesn't show
The 2-equation alkalinity toy model is not a GCM; the instantaneous γ jump is admitted to be unrealistic; this paper does not fit new paleo time series, so the delay is a mechanism, not a dated MPT cause.
Key terms
- Mid-Pleistocene Transition (MPT)
- Shift ~1 Myr ago from ~40-kyr to ~100-kyr glacial cycles.
- Ramping with frequency locking (RFL)
- Internal period grows until cycles lock onto multiples of orbital forcing.
- Calcifier-alkalinity model
- Two-variable oscillator of ocean alkalinity and calcifiers that sets CO2.
- Positive weathering feedback (γ)
- Alkalinity input rises at high alkalinity (e.g. exposed shelves).
- Hopf bifurcation
- Birth of a stable limit cycle as feedback strength increases.
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Quiz yourself
The MPT is described as a shift about 1 Myr ago from roughly:
Common questions
When did the MPT occur in the records they cite?
About 1 Myr ago, with dominant period lengthening from ~40 to ~100 kyr.
What forcing period do they use?
40 kyr, for obliquity, unless otherwise specified.
How long can the period delay last?
Around 10 Myr in one experiment and ~1 Myr in another.
Why look before the MPT for causes?
The period may lag the underlying Earth-system change.
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