Wednesday, December 2, 2009

The Pleistocene Glaciations (Chapter 14)

Evidence for glaciations (p 270-274)

Milankovitch cycles (p 274-279)

Feedbacks (p 279-287)

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 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

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

Example - Iron Fertilization Hypothesis

Iron abundances limit primary productivity (new growth) in certain

regions of the ocean (where the other limiting nutrients are plentiful).

Iron is supplied by wind-blown dust (Sarahan, Gobi deserts – wait

until next week or the week after!)

Lower CO2 during glaciations would imply more wind-blown dust, so

either dryer climate or greater average winds.

Surface temp temp gradient winds iron CO2

Evidence supports this feedback – windblown dust in

ocean sediments is higher during glacials

More detail next time!

Figure 14-15

Iron Fertilization Hypothesis