The 100 Kyr cycle

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The 100 Kyr cycle

• Agnes Barszcz

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Who wants to be a climatologist?

• 2 teams
• The Aphelions
• The Perihelions
• Rules
• Joker

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What I am going to talk about

• Various hypotheses for 100 kyr cycle
• Why Milankovich is wrong
• Suggest a new hypothesis
• See if it is reasonable
• Look at its sensitivity to different parameters
• Its flaws
• What is the right theory
• FUN Giving out the price!!!

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What is the 100 kyr cycle?
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What is causing it?

• Many hypothesies
• The milankovitch cycle
• Isostatic adjustmets of the litosphere under the
  weight of the glacier
• Feedback between atmosphere ocean and Co2
• ..
• WE ARE STILL MISSING A SOLUTION!

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Question 1

• Who can tell me in less then 2 minutes what the
  Milankovich theory is ?

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Milankovich and and why it is wrong!

• 3 components
• Eccentricity
• Precession
• Axial tilt

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Which one was key for us?
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Milankovich and and why it is wrong!

• 3 components Eccentricity would be the one we
  are interested in!
• -gt Top five reasons that we should NOT

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• Eccentricity changes are small

• Orbital calculation when caried out with greater
  presision show a major cycle of 400 Kyr

• Well dated climate proxies show a 100 kyr cycle
  only over the last million of years

• Double peak in frequency domain

• Causality problem

• Spectrum of 100-kyr glacial cycle Orbital
  inclination,not eccentricity. By RICHARD A.
  MULLER AND GORDON J. MACDONALD
• http//www.pnas.org/cgi/reprint/94/16/8329.pdf ,
  1995

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Suggested solution

• Changes in the orbital inclination

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Find a suitable solution
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Methods

• Use simple or complex models

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Pros and cons

• Complex models Global circulation models
• Pro They take into account many parameters. They
  are more realistic
• Cons Require large computers
• -gt Used to simulate equilibrium responses to
  various initial conditions

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Pros and cons

• Simple models
• Pro Require less computer power, and run
  faster..
• Con Yield less realistic results. We do not see
  the influence of all the small parameters that we
  have neglected

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What was used

• A simple model

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Claim

• Hezi Gildor and Eli triperman say the 100 kyr
  cycle is NOT related to the milankovich
  cycle!!!!!!!!

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The answer!

• Their hypothesis is
• The variation of the ice-albedo between glacial
  and interglacial periods
• Variant of the precipitation-temperature feedback

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What the autors used

• A simple zonaly averaged box

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The components of the model

• Ocean meridional thermohaline circulation
• Atmospheric temperature-humidity feedback
• Land glaciers
• Sea ice

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What,where,how?

• The ocean model
• 4 surface boxes (400 m)
• 2 polar
• Water may be covered with sea ice of variable
  extent
• Land may be covered with land ice of variable
  extent
• 2 midlatitude boxes
• 4 Deep water boxes (4000 m)
• Important to note that the ablation rate of
  glacier stays constant with time.

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What,where,how?

• The atmosphere model
• Each box can have 4 types of lower surfaces
• Land
• Ocean
• Land Ice
• Sea ice
• -gt All have different albedos

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The technicalities

• What is a leapfrog scheme?

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Leapfrog
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Why leapfrog?

• Time reversible
• Assures energy conservation
• A better accuracy
• http//www.lifelong-learners.com/pde/SYL/s2node4.p
  hp
• http//einstein.drexel.edu/courses/CompPhys/Integr
  ators/leapfrog/

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How it goes
The crux of the problem
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How it goes

• Ocean is ice free
• Temperatures are mild
• More precip then melting and carving
• -gt-gtLand ice sheet grows

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How does that affect the temperature?
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As ice sheet grows slowly

• The albedo of the earth decrease
• The sea temperature are below zero only in the
  polar boxes.
• -gt-gt-gt At year 90 kyr, the global SST reaches
  zero degrees

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Whats special about the moment that the SST
reaches zero?
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Ice sheet rapidly grows

• As the SST attained a critical value, sea ice
  grows very rapidly!
• In 20 year all polar box is covered in sea ice.
• -gt-gt-gt Sea ice switch is ON

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When the switch is on..

• Sea Ice Stop growing!!!
• Why????

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Self-limitation of sea ice

• The sea cools by giving out heat to the
  atmosphere
• When it is covered by ice, there is no more
  exchange
• The warm midlatitude waters mix with the polar
  waters
• -gt-gt-gt No more sea ice growth

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The ice age!

• The glacier is at its maximum
• The atmosphere is at its coldest
• How do we get out of an ice age??

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Moisture feedback

• There is less moisture captured by cold air, so
  less northward moisture transport
• Because of the ice cap, there is also less
  evaporation in the polar region
• -gt-gt-gt Less moisture present in polar regions

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We are loosing the icesheet

• The precipitaion rate is reduced by ½
• The ablation rate stays constant
• -gt-gt-gt The glaciers retract

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On the road to warm times

• As the ice sheet retract, the albedo is
  decreasing.
• Atmospheric and ocean temperatures rise slowly.
• As long as there are ice sheets in polar region
  the ocean temperature in the region in below zero
  sea ice is present.
• -gt How does that affect the land ice sheet?

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Further down, on the road to warm times..

• As long as the ice sheet is there, the land
  glaciers retreat fast
• The sea ice, does melt by sloooowly
• The meltdown is slow because the SSt is close to
  zero..
• -gt What is causing it to melt anyways?

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Close to interglaciation..

• The sea ice melts down because of the heat
  advected and diffused by the ocean, that is
  coming from the midlatitudes.
• -gtWhat will cause the abrupt acceleration of
  the melt down of sea ice?

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Deep Ocean

• Because the deep ocean heats up
• But Why?

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The deep ocean

• Because of the melt down of the land ice sheet
• The switch is OFF

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Switch is off

• All the ice melts down in about 40 years
• Atmosphere and ocean temperatures rise again

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And back

• The temperatures are maximum
• The ice sheet is minimum
• The amount of precipitation is at its maximum
• -gtWe are back at the starting point!

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The Results

• Simulated years from 170 kyr to 70 kyr

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Theoretically it makes sense
Ok so we have a scheme
Does it practically?
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So what about the THC?!?

• THC Thermohaline circulation
• It was included in the model
• It is rather an effect the a cause of the 100 kyr
  cycle
• What changes the strength of the THC?

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The THC strength

• When glaciers melt, there is a lot of fresh water
  relised minimal THC (12Sv)
• At interglacial periods, there is a lot of
  evaporation Salier water strong THC(16Sv)
• When glaciers form less evaporation weaker
  THC(13Sv)

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Is it an ok resolution to have only 4 boxes?

• How many did we use in the model we made in
  class??
• It is only the upper part of the ocean that has
  to cool significantly for glaciation. (The lower
  parts role is to provide delayed responses to
  various forcing)
• It was already demonstrated in previous papers
  that this can be achieved in only a few tens of
  years

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Does it make sense to have a 100 kyr period?

• Lets do some simple math!!!!!
• Volume of land glaciers
• ?V V max V min
• 2.4 e16 m3
• M-Accumulation rate
• maximum 0.09 e6 m3/s
• minumum 0.03 e6 m3/s
• S-Ablation rate

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Sensitivity to different parameters
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LandIce sink term

• Reduce by 4
• Reaches faster the critical value at which the
  ice starts growing
• Slower deglaciation
• -gt Cycle is longer by 10 kyr
• -gt Less saw-tooth like shape

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LandIce sink term

• Increase by 20
• Sink term always exceeds the precipitation rate
• -gtgtGlacier disappears!!!

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Albedo

• Increase both land ice and sea ice albedo
• What will happen??

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Albedo

• Increase both albedos
• Shorter cycle
• Less land ice needed to reduce SST under the
  threshold value
• Glaciers need to become smaller then before to
  enable a temperature increase that results in ice
  melting
• -gt Shorter growth/Longer meltdown
• -gtShorter but more symmetric cycle

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Albedo

• Very significant increase in sea ice albedo
• -gt Permanent sea ice cover
• -gtNo land Ice

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Albedo

• Very significant decrease in Sea ice albedo
• -gt Makes the sea-ice mechanism less effective
  as a switch

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Albedo

• Increase in Land-ice albedo
• -gtReduces the amplitude of the oscillation and
  shortens the time scale
• Converse is true

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Emissivity

• The long wave emissivity represents
• Cloud cover
• Humidity
• Land cover
• Topography
• Aerosols
• CO2
• How will the emissivity evolve between a glacial
  minimum and maximum?

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Emissivity

• The emissivity will increase between a glacial
  minimum and a glacial maximum because there is a
  smaller water vapor concentration

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How about the CO2?

• Pelletier and Marshall
• P0.015ln( CO2 / CO2 ref)
• So for a 30 CO2 change
• P changes by - 0.003
• Induces an increase in the time scale of the
  oscillation of about 9 kyr.
• -gt Why longer?

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How about the CO2?

• Why longer?
• Whole system is warmer,
• We will need larger glaciers to turn the switch
  on!!

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How about CO2?

• If we change P by 0.003
• Cycle is longer by 12 kyr
• If we change P by 0.005
• Cools the climate enough so there is a permanent
  sea-ice cover

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Ice sheet thickness?
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Ice sheet thickness

• Doesnt change the cliamte!
• Ice sheet play there role by
• the albedo
• Insulating properties

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Freezing temperature

• Similar effect as with the emissivity
• Lowering the freezing T by 0.5o
• Time scale is longer by 20 kyr
• Increasing the freezing T by 0.5o
• Time scale is shorter by 20 kyr

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Problems

• The model is
• Quite simple
• Highly idealized
• Neglecting any zonal variations
• Limited set of feed-backs

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Problems.

• It is not synchronous with observations
• Fails to predict the synchronous deglaciation of
  the southern hemisphere
• BUT

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Take home message

• The aim of a such a simple model is NOT to be
  accurate with observations but just to explore
  potentially IMPORTANT effects and feedback of the
  climate.