Wednesday, November 17, 2010

The Pleistocene Glaciations (Chapter 14)

Evidence for glaciations (p 272-276)

Milankovitch cycles (p 276-281)

Feedbacks (p 281-288)

CO2

CH4

T

For the past 500,000 years and longer, ice ages have

occurred with regularity that is explained by changes in

Earth’s orbit. Abundances of greenhouse gases like CO2

and methane have tracked the temperature changes

Key to Pleistocene ice ages – northern hemisphere summer

i.e. can you melt the snow from the previous winter?

If no, then ice sheet grows. If yes, ice sheet shrinks

Figure 14-1

Ice ages are an example of a two-state climate system

Figure 14-9

Factors that influence summer insolation

Tilt (obliquity) of earth’s orbit with respect to plane of rotation

about the sun (larger tilt, less sun in winter, but more sun in

summer)

• varies from 22 to 24.5

degrees (i.e. the arctic

circle oscillates north and

south)

• period is 41,000 years

Factors that influence summer insolation

Precession – where the orbital axis is pointed – presently, the

north star – influenced by gravity from Sun and Moon

• changes timing of summer

compared to distance from

sun

• Is also affected by Venus

and Jupiter (like torque

pulling on a ‘top’, making

it wobble)

• period is 19,000-23,000

years

Factors that influence summer insolation

Eccentricity – the shape (i.e., how circular it is) of the earth’s

orbit about the sun

• varies from 1.00 (circular)

to 1.06

• Currently 1.017 (nearly

circular, but closest to sun

in Dec., farthest from sun

in June)

• period is 100,000 years

• Small effect on total

insolation

Changes in climate due to changes in Earth’s orbit

Figure 14-8

Solar insolation in June, Northern Hemisphere

Amazingly, the prediction that the timing of ice ages would reflect

these three orbital parameters occurred BEFORE any ice cores

were ever drilled. It is pretty remarkable that the isotopes of water,

which reflect the amount of ice that is piled up in the northern

hemisphere ice sheet, oscillate at intervals that are a near-perfect

match for the orbital predictions.

These predictions were made by a Serbian geophysicist Milutin

Milanković, who, when World War I war broke out in 1914, was

interned by the Austro-Hungarian army. His interest was in solar

climates and temperatures prevailing on the planets. During his

internment in Budapest he was allowed to work in the library of

the Hungarian Academy of Sciences, where, by the end of the

war, he had finished a monograph which was published in 1920,

titled Théorie mathématique des phénomènes thermiques produits

par la radiation solaire (Mathematical theory of thermal

phenomena caused by solar radiation). (adapted from Wikipedia)

Next time (Monday, Nov. 29).

We need an amplifier to explain the large variations in

climate due to the 100,000 year eccentricity mode

CO2/biological pump

• Shelf-nutrient

• Iron fertilization

• Coral reef

• Cloud/albedo