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?
Many hypothesies:The milankovitch cycle Isostatic adjustmets of the litosphere under
the weight of the glacierFeedback between atmosphere ocean and
Co2…..WE ARE STILL MISSING A SOLUTION!
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……
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
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
Pros and cons
Complex models: Global circulation modelsPro: They take into account many
parameters. They are more realisticCons: Require large computers $$$$$ -> Used to simulate equilibrium responses
to various initial conditions
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
Claim
Hezi Gildor and Eli triperman say the 100 kyr cycle is NOT related to the milankovich cycle!!!!!!!!
The answer!
Their hypothesis is:
The variation of the ice-albedo between glacial and interglacial periods
Variant of the precipitation-temperature feedback
The components of the model
Ocean meridional thermohaline circulation
Atmospheric temperature-humidity feedback
Land glaciers
Sea ice
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,how?
The atmosphere model:Each box can have 4 types of lower surfaces:
LandOceanLand IceSea ice
-> All have different albedos
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:
Ocean is ice free
Temperatures are mild
More precip then melting and carving
->->Land ice sheet grows
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
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
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 glacier is at it’s maximum
The atmosphere is at it’s coldest
How do we get out of an ice age??
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
The precipitaion rate is reduced by ½
The ablation rate stays constant
->->-> The glaciers retract
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..
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..
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?
Switch is “off”
All the ice melts down in about 40 years
Atmosphere and ocean temperatures rise again
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!
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
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?
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?
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/sminumum = 0.03 e6 m3/s
S-Ablation rate
Land–Ice sink term
Reduce by 4 %Reaches faster the critical value at which
the ice starts growingSlower deglaciation
-> Cycle is longer by 10 kyr
-> Less saw-tooth like shape
Land–Ice sink term
Increase by 20 %Sink term always exceeds the precipitation
rate
->>Glacier disappears!!!
Albedo
Increase both albedosShorter cycleLess land ice needed to reduce SST under
the threshold valueGlaciers 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
Albedo
Very significant decrease in Sea ice albedo
-> Makes the sea-ice mechanism less effective as a switch
Albedo
Increase in Land-ice albedo
->Reduces the amplitude of the oscillation and shortens the time scale
Converse is true
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?
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?
Pelletier and Marshall:P’=0.015*ln( CO2 / CO2 ref)
So for a 30% CO2 changeP changes by - 0.003
Induces an increase in the time scale of the oscillation of about 9 kyr.
-> Why longer?
How about the CO2?
Why longer? Whole system is warmer, We will need larger glaciers to turn the
switch on!!
How about CO2?
If we change P by +0.003Cycle 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 thickness
Doesn’t change the cliamte!
Ice sheet play there role by the albedo Insulating properties
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
The model isQuite simpleHighly idealizedNeglecting any zonal variationsLimited set of feed-backs
Problems….
It is not synchronous with observations
Fails to predict the synchronous deglaciation of the southern hemisphere
BUT