1. Earth Sun Geometry

2. Our Objectives Utah Earth Science Core Curriculum 9.3.3b:
Explain how Earth’s climate has changed over time and describe the natural causes for these changes.
The most widely accepted idea regarding this is Milankovitch Theory
You’ll be expected to know the three variables in Milankovitch Theory.

3. Earth-Sun Geometry
Driving variable of environmental processes on Earth
Geometry determines the amount and intensity of incoming solar radiation (insolation) reaching Earth
Geometry (and how it changes) determines:
Seasonality (1 year)
Glacial (cold) and interglacial (warm) periods (1000’s or millions of years) 3

4. Which determines when ice ages happen?
Aphelion (away) and perihelion?
Tilt?

5. Tilt is most relevant for the change of seasons.

6. Can tilt change on smaller time scales?
Yes, but very, very little
The Earth tilted about an inch (2.5 centimeters) on its axis, and day length decreased by 3 millionths of a second from the 2004 Indian Ocean earthquake (9.1 RS)
Above: Tsunami

7. In conjunction with the curvature of the Earth, tilt determines the concentration and distribution of insolation striking the earth
Flashlight effect

8. Flashlight effect TROPICS High sun angle: Larger concentration
Same amount of insolation is spread over a larger area—more diffuse. Flashlight effect
TROPICS
High sun angle:
Larger concentration
of insolation per area
MIDLATITUDES
HIGH LATITUDES
Intermediate sun angle
Low sun angle:
Smaller concentration of
insolation per area.

9. What day of the year is this?
As the Earth revolves around the Sun, tilt creates variation in day and night lengths over the duration of a calendar year
What day of the year is this?

11. But what more directly makes the weather change?
The energy imbalance (difference in solar energy)
between tropics and poles drives circulation of
atmosphere and ocean 11

12. How you experience the changing of the seasons in terms of Earth-Sun geometry
Sun’s altitude: angle between horizon and sun is different throughout the year. Sun is not always directly overhead at a 90° angle at noon. Max sun angles in summer, lower in winter
Day/night length: Maximum contrast at solstices, while perfectly equal (12 hours day/12 hours night) across the globe on the equinoxes.
Declinations are used for other things as well—magnetic declination, for example.

13. Long-term variations in Earth-Sun geometry
Contribute to alternating climates:
Interglacial (warm)
Glacial (cold)
Have profound effects on sea-level
Pleistocene: approximately 2 million year period of glacials and interglacials

15. Interglacial and glacial period15

19. Bering Straits—human migration; land mass of eastern seaboard, Bahamas, Gulf, California.

20. Important dates for natural climate change
Wisconsin glacial period (ended 15,000 ybp)
Boreal (what are conifers?) forests extended as far south as Atlanta and Birmingham, Alabama.

21. Important dates for natural climate change
Holocene interglacial then started at the end of the Wisconsin glacial period (15,000 ybp)
Warming marked the start of the Holocene. 21

22. Milankovitch Cycles
Name of the geometric changes that influence Earth-Sun geometry over long and repeating scales
Three components:
Orbital eccentricity
Obliquity (tilt)
Precession (wobble of Earth’s axis)

23. 1. Orbital eccentricity
Distance between Earth & Sun changes over scale of ~100,000 years
This changes length of seasons
Remember the laws of planetary motion?
What is eccentricity?

24. 2. Obliquity (Tilt)
Tilt varies between degrees over a time scale of ~41,000 years.
Present tilt of 23.5 degrees is decreasing but can be considered unchanging from your scale of observation (one human lifetime)

25. 3. Precession Earth wobbles on its axis
Also sort of wobbles in its orbit around the sun so the point in orbit where aphelion and perihelion varies slightly
Occurs over times scales of ~26,000 years 25

26. Components of
Milankovitch Cycles

27. Milankovitch cycles shape interglacial and glacial periods

28. Mechanism? So what do Milankovitch cycles do?
Milankovitch cycles create weaker or stronger seasonal contrasts.
In theory:
Weaker seasonal contrasts►glacial climates dominate
Stronger seasonal contrasts►interglacial climates dominate

29. Interglacial (warm) period
Strong seasonal contrasts
Highly elliptic orbit and large tilt
Cold winters: less evaporation: less snow: less glacial accumulation
Hot summers: more glacial melting
Net loss of glacial extent and warmer temperatures
Sea levels rise

30. Glacial (cold) period Weak seasonal contrasts
Less elliptic orbit and small tilt
Warm winters: more evaporation: more snow: more glacial accumulation
Cool summers: less glacial melting
Net gain of glacial extent and cooler temperatures
Sea levels drop

32. Our Objectives Utah Earth Science Core Curriculum 9.3.3b:
Explain how Earth’s climate has changed over time and describe the natural causes for these changes.
The most widely accepted idea regarding this is Milankovitch Theory
You’ll be expected to know the three variables in Milankovitch Theory.