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The 100 Kyr cycle. Agnes Barszcz. Who wants to be a climatologist?. 2 teams: The Aphelions The Perihelions Rules Joker. 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

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the 100 kyr cycle

The 100 Kyr cycle

Agnes Barszcz

who wants to be a climatologist
Who wants to be a climatologist?
  • 2 teams:
    • The Aphelions
    • The Perihelions
  • Rules
  • Joker
what i am going to talk about
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 it’s sensitivity to different parameters
  • Its flaws
  • What is the right theory
  • FUN: Giving out the price!!!
what is causing it
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!
question 1
Question 1:
  • Who can tell me in less then 2 minutes what the Milankovich theory is ?
milankovich and and why it is wrong
Milankovich and and why it is wrong!
  • 3 components
    • Eccentricity
    • Precession
    • Axial tilt
milankovich and and why it is wrong1
Milankovich and and why it is wrong!
  • 3 components: Eccentricity would be the one we are interested in!

-> Top five reasons that we should NOT……

slide11

Eccentricity changes are small

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

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

  • Well dated climate proxies show a 100 kyr cycle only over the last million of years
  • Double peak in frequency domain
  • Causality problem
suggested solution
Suggested solution:
  • Changes in the orbital inclination
methods
Methods
  • Use simple or complex models
pros and cons
Pros and cons
  • Complex models: Global circulation models
    • Pro: They take into account many parameters. They are more realistic
    • Cons: Require large computers $$$$$
    • -> Used to simulate equilibrium responses to various initial conditions
pros and cons1
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
what was used
What was used
  • A simple model
claim
Claim
  • Hezi Gildor and Eli triperman say the 100 kyr cycle is NOT related to the milankovich cycle!!!!!!!!
the answer
The answer!
  • Their hypothesis is:
    • The variation of the ice-albedo between glacial and interglacial periods
    • Variant of the precipitation-temperature feedback
what the autors used
What the autors used
  • A simple zonaly averaged box
the components of the model
The components of the model
  • Ocean meridional thermohaline circulation
  • Atmospheric temperature-humidity feedback
  • Land glaciers
  • Sea ice
what where how
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….
what where how1
What,where,how?
  • The atmosphere model:
    • Each box can have 4 types of lower surfaces:
      • Land
      • Ocean
      • Land Ice
      • Sea ice
      • -> All have different albedos
the technicalities
The technicalities
  • What is a leapfrog scheme?
why leapfrog
Why leapfrog?
  • Time reversible
  • Assures energy conservation
  • A better accuracy

http://www.lifelong-learners.com/pde/SYL/s2node4.php

http://einstein.drexel.edu/courses/CompPhys/Integrators/leapfrog/

how it goes
How it goes…

The crux of the problem

how it goes1
How it goes:

Ocean is ice free

  • Temperatures are mild
  • More precip then melting and carving

->->Land ice sheet grows

as ice sheet grows slowly
As ice sheet grows slowly
  • The albedo of the earth decrease
  • The sea temperature are below zero only in the polar boxes.

->->-> At year 90 kyr, the global SST reaches zero degrees

ice sheet rapidly grows
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.

->->-> Sea ice switch is ON

when the switch is on
When the switch is on..
  • Sea Ice Stop growing!!!

Why????

self limitation of sea ice
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

->->-> No more sea ice growth

the ice age
The ice age!
  • The glacier is at it’s maximum
  • The atmosphere is at it’s coldest

How do we get out of an ice age??

moisture feedback
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

->->-> Less moisture present in polar regions

we are loosing the icesheet
We are loosing the icesheet
  • The precipitaion rate is reduced by ½
  • The ablation rate stays constant

->->-> The glaciers retract

on the road to warm times
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.

-> How does that affect the land ice sheet?

further down on the road to warm times
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..

-> What is causing it to melt anyways?

close to interglaciation
Close to interglaciation..
  • The sea ice melts down because of the heat advected and diffused by the ocean, that is coming from the midlatitudes.

->What will cause the abrupt acceleration of the melt down of sea ice?

deep ocean
Deep Ocean
  • Because the deep ocean heats up:

But Why?

the deep ocean
The deep ocean
  • Because of the melt down of the land ice sheet
  • The switch is OFF
switch is off
Switch is “off”
  • All the ice melts down in about 40 years
  • Atmosphere and ocean temperatures rise again
and back
And back…
  • The temperatures are maximum
  • The ice sheet is minimum
  • The amount of precipitation is at it’s maximum

->We are back at the starting point!

the results
The Results
  • Simulated years from 170 kyr to 70 kyr
theoretically it makes sense
Theoretically it makes sense…

Ok so we have a scheme…

Does it practically?

so what about the thc
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?

the thc strength
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)
is it an ok resolution to have only 4 boxes
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 part’s 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
does it make sense to have a 100 kyr period
Does it make sense to have a 100 kyr period?
  • Let’s 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
land ice sink term
Land–Ice sink term
  • Reduce by 4 %
    • Reaches faster the critical value at which the ice starts growing
    • Slower deglaciation

-> Cycle is longer by 10 kyr

-> Less saw-tooth like shape

land ice sink term1
Land–Ice sink term
  • Increase by 20 %
    • Sink term always exceeds the precipitation rate

->>Glacier disappears!!!

albedo
Albedo
  • Increase both land ice and sea ice albedo

What will happen??

albedo1
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

-> Shorter growth/Longer meltdown

->Shorter but more symmetric cycle

albedo2
Albedo
  • Very significant increase in sea ice albedo

-> Permanent sea ice cover

->No land Ice

albedo3
Albedo
  • Very significant decrease in Sea ice albedo

-> Makes the sea-ice mechanism less effective as a switch

albedo4
Albedo
  • Increase in Land-ice albedo

->Reduces the amplitude of the oscillation and shortens the time scale

  • Converse is true
emissivity
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?

emissivity1
Emissivity
  • The emissivity will increase between a glacial minimum and a glacial maximum because there is a smaller water vapor concentration
how about the co2
How about the CO2?
  • Pelletier and Marshall:
    • P’=0.015*ln( 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.

-> Why longer?

how about the co21
How about the CO2?
  • Why longer?
    • Whole system is warmer,
    • We will need larger glaciers to turn the switch on!!
how about co2
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
ice sheet thickness1
Ice sheet thickness
  • Doesn’t change the cliamte!
    • Ice sheet play there role by
      • the albedo
      • Insulating properties
freezing temperature
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
problems
Problems
  • The model is
    • Quite simple
    • Highly idealized
    • Neglecting any zonal variations
    • Limited set of feed-backs
problems1
Problems….
  • It is not synchronous with observations
  • Fails to predict the synchronous deglaciation of the southern hemisphere

BUT

take home message
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.
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