1. Climates of Geologic Time
Current Weather
Overview and Historical Perspective
The Pleistocene and Holocene
Milankovitch Theory
For Next Class: Read Ch. 9 (pp )
Move Exam to October 16, 18, or 20?

2. Climates of Geologic Time
Geologic time scale: standard division of Earth history into eons, eras, periods, and epochs based on large-scale geological events
© AMS

3. Definitions
What is the difference between a glacial and interglacial?

4. Climates of the Pleistocene Ice Age
Climate and Glaciers
Glacial climates favor positive mass balance
Interglacial climates favor negative mass balance
Climate shifted between
glacial and interglacial numerous times during the Pleistocene
© AMS

5. Climates of the Pleistocene Ice Age
Chronology and Temperature Trends
Last major glacial climatic episode began about 27,000 years ago and reached its peak about 20,000 to 18,000 years ago
© AMS

6. Climates of the Pleistocene Ice Age
© AMS

7. Climates of the Pleistocene Ice Age
Younger Dryas: relatively cool episode from 11,000 to 10,000 years ago
Began abruptly when glacial ice lobes disrupted drainage patterns
Weakened the meridional overturning circulation, which in turn diminished the warm water flowing into the central and northern North Atlantic
Cooled surrounding lands
© AMS

8. Climates of the Holocene
The Holocene Epoch is the present interglacial
Glacial ice withdrew from North American Great Lakes region about 10,500 years ago
Civilization and agriculture developed
Holocene has had spatially and temporally variable temperature and precipitation
© AMS

9. 8.2 ka Cooling Event
Another period of abrupt climate change occurred approximately 8,200 years ago and is hence termed the 8.2 ka cooling event.
Temperatures were lower and much of the world was much drier.
Climate returned to warmer conditions characteristic of the present interglacial after about 150 years.

10. Possible Causes of 8.2 ka Event

12. Causal Mechanisms
Significant influx of freshwater into the North Atlantic appears to have caused both the Younger Dryas cooling and the 8.2 ka cooling event.
Freshwater influx changed the density structure associated with meridional overturning circulation (or thermohaline circulation) and caused a slowing or stopping of Gulf Stream circulation.

13. Orbital Influences on Insolation
Variations in axial tilt (41,000 year cycle)
Axial precession (25,700 year cycle)
Changes in Earth’s eccentric orbit (100,000 year cycle)
Precession of the ellipse (23,000 year cycle)

14. Earth’s Tilt Actually Varies
Varies between 22 and 24°, on a 41,000 year cycle

15. No Tilt vs. 90° Tilt
How does the annual pattern of solar radiation vary at the Equator and at the Poles in the two examples on the right?
Which scenario would be most favorable for polar ice sheet growth? Tropical glacier advance?

16. Axial Precession
A rotating body like the Earth is subject to a slow wobble.

17. Changes in Eccentric Orbit
Eccentricity, or the shape of Earth’s path around the sun, changes on a 100,000 year cycle
Causes changes to the distance away from the sun during the
perihelion and aphelion, resulting in subtle differences in insolation.

18. Precession of the Ellipse
The entire ellipse is also subject to slow rotation.

19. Precession of the Equinoxes
Axial precession and precession of the ellipse results in the precession of the equinoxes. A strong cycle is evident on a 23,000 year cycle.

20. Milankovitch Theory
“. . . Ice ages are triggered by minima in summer insolation near 65°N, enabling winter snowfall to persist all year and therefore accumulate to build NH [Northern Hemisphere] glacial ice sheets” (IPCC AR4 Ch. 6, p. 445).
Onset of last ice age (~116,000 yrs ago) was associated with mid-June insolation about 40 w m-2 lower than today.