Earth Sun Geometry

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.

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

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

Tilt is most relevant for the change of seasons.

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: 2004 Tsunami

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

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.

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?

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

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.

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

Interglacial and glacial period 15

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

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.

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

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)

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?

2. Obliquity (Tilt) Tilt varies between 22.2-24.5 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)

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

Components of Milankovitch Cycles

Milankovitch cycles shape interglacial and glacial periods

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

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

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

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.