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