Evolving Glacial Climate Dynamics

1
Evolving Glacial Climate Dynamics

• Ruth Chambers

2
Continuous Obliquity Pacing

• The Mid-Pleistocene transition (MPT) is when
  there was a shift from 40 to 100kyr glacial
  cycles.
• Huybers (2006) believes that there wasnt really
  a shift but the observed change was due to an
  increase in the skipping of obliquity beats.
• This resulted in 80 or 120kyr cycles which on
  average gives the 100kyr variability.

3
Evidence

• Major deglaciation features are identified over
  the last 2Myr and a remarkable 33 out of 36 occur
  when the Earths obliquity is anomalously large.

4

• During the early Pleistocene, the stability of
  the obliquity phase is significant at the 99
  level (R0.7), as expected, given that early
  Pleistocene glacial cycles are known to have a
  40ka timescale.
• Late Pleistocene deglacial events have R0.8
• Thus, the late Pleistocene 100ka world actually
  shows greater obliquity phase stability than the
  40ka world. Mental.
• R Rayleighs R measure of phase coupling.

5
(No Transcript)
6
Why does obliquity skip?

• There does not appear to be any systematic
  relationship between the time when obliquity
  beats are skipped and the amplitude of obliquity
  cycles
• Obliquity cycle skipping results from internal
  climatic factors

7
The Pleistocene Progression

• Changes in glacial variability are not marked by
  any single transition so much as they exhibit an
  approximately linear trend over the last 2Ma.
• Because
• 1) As is true for nearly all geophysical
  measurements, Pleistocene climate records show
  variability at all times and timescales.
• 2) There were 80kyr cycles before the MPT and
  40ka cycles as late as 0.6Ma.
• 3) The d18O stack reflects aggregate changes in
  both ice volume and temperature.
• i.e. It was a gradual rather than sudden
  transition.

8

• The mean, variance, skewness, and timescale
  associated with the glacial cycles all exhibit an
  approximately linear trend over the last 2Myr.

9
Trends and Transitions in Glacial Cycle Dynamics

• However, Lisiecki and Raymo (2006) believe that
  there were significant changes in the dynamics of
  the climate system.
• They identify two major transitions in climate
  dynamics
• 1) The onset of the Northern Hemisphere
  glaciation at approximately 2.7Ma.
• 2) The Early Pleistocene transition, 1.4Ma.
• But, the causes still remain uncertain.

10

• Also, their results suggest that precession
  responses correlate with modulations in forcing
  for the last 5Myr, but 41kyr response is
  sensitive to obliquity modulation only before
  1.4Ma.
• They say that glacial cycles are orbitally forced
  rather than being self sustained or paced by
  orbital changes

11
(No Transcript)
12
Asymmetry

• Changes in climate dynamics are also suggested by
  changes in the shapes of glacial cycles.
• The development of this asymmetry is most likely
  due to a change in the internal dynamics of the
  climate system because asymmetry is not found in
  any orbital or insolation curves.

13

• But, Huyber says
• 1) This could be the result of sediment
  composition or accumulation rates covarying with
  other climate changes
• 2) Or, numerous explanations have been put
  forward for the asymmetry between rates of
  glaciation and deglaciation e.g. ice-sheet
  instabilities, decreases in the albedo of aging
  snow and changes in accumulation caused by rapid
  expansion of sea ice - and there appears no
  fundamental reason why any of these physical
  mechanisms could not evolve gradually.

14
Milankovitch Forcing the Pacemaker of Glacial
cycles

• Tziperman et al 2006, put forward the theory that
  Milankovitch cycles drive Glacial cycles via
  nonlinear phase locking.
• Nonlinear phase locking can determine the timing
  of major deglaciations, nearly independently of
  the specific mechanism or model that is
  responsible for these cycles as long as the
  mechanism is suitably nonlinear.
• A consequence of this is that the fit of a
  certain model output to the observed ice volume
  record cannot be used as an indication that the
  glacial mechanism in this model is necessarily
  correct.
• i.e it doesnt matter what initial conditions you
  have, the pacemaker will cause glacial cycles
  to develop in accordance to Milankovitch forcing.
  

15
(No Transcript)
16

• The most important condition for nonlinear phase
  locking to occur is that oscillations may change
  their period as a function of their amplitude.
• E.g in the case of a pendulum the period of the
  oscillation is longer when it makes larger
  swings.
• We have some idea of the nonlinearities in the
  equations governing the oceans, atmosphere, ice
  sheets and even vegetation dynamics. However, we
  cannot determine which is any of these
  nonlinearities is the critical one.

17

• Raymo (1997) and Ridgewell et al (1999) suggested
  that the glacial period is quantized into
  multiples of the precession or obliquity periods.
  This result arises from the phase locking
  condition, which states precisely that the
  glacial oscillation period is quantized by that
  of the Milankovitch forcing.
• This explains the relatively recent skipping of
  cycles.
• This may be due to the non uniform character of
  the Milankovitch forcing.