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

Look at it’s sensitivity to different parameters

Its flaws

What is the right theory

FUN: Giving out the price!!!

What is the 100 kyr cycle?

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!

Question 1:

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

Milankovich and and why it is wrong!

3 componentsEccentricityPrecessionAxial tilt

Which one was key for us?

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

Suggested solution:

Changes in the orbital inclination

Find a suitable solution…

Methods

Use simple or complex models

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

What was used

A simple model

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

What the autors used

A simple zonaly averaged box

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

The technicalities

What is a leapfrog scheme?

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…

The crux of the problem

How it goes:

Ocean is ice free

Temperatures are mild

More precip then melting and carving

->->Land ice sheet grows

How does that affect the temperature?

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

What’s special about the moment that the SST

reaches zero?

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..

Sea Ice Stop growing!!!

Why????

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?

Deep Ocean

Because the deep ocean heats up:

But Why?

The deep ocean

Because of the melt down of the land ice sheet

The switch is OFF

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!

The Results

Simulated years from 170 kyr to 70 kyr

Theoretically it makes sense…

Ok so we have a scheme…

Does it practically?

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

Sensitivity to different parameters

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 land ice and sea ice albedo

What will happen??

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 increase in sea ice albedo

-> Permanent sea ice cover

->No land Ice

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?

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

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.