Earth’s tilted axis causes the seasons. The Earths rotational axis is Throughout the year, different parts of Earth tilted 23.5 degrees to the plane receive the Sun’s most direct rays. So, when the North Pole tilts toward the Sun, it’s summer in of orbit around the sun the Northern Hemisphere. And when the South Pole tilts toward the Sun, it’s winter in the Northern Hemisphere.

Perihelion: 147 Mkm

Aphelion: 152 Mkm

https://spaceplace.nasa.gov/seasons/en/ Axial Obliquity of Earth or Axial Tilt

Tilt

Tilt Current 23.5˚ Max 24.5˚

Min 22.1˚

41,000 Year Period. https://earthobservatory.nasa.gov/features/Milankovitch/milankovitch_2.php Axial Obliquity of Earth: • Recall that the tilt of the axis is responsible for the seasons. Presently it is 23.5 degrees • The tilt of the earths axis varies from 22.1 to 24.5 degrees over a period of 41,000 years or 1/100’th of a degree per century. • Imagine what would happen if tilt was greater? • As the axial tilt increases, the seasonal contrast increases so that winters are colder and summers are warmer in both hemispheres.

Colder winter

Warmer Summer What tilt extreme would tend to start the ice-albedo feedback?

Cooler summer

Warmer winter Consider that a lesser tilt than present would lead to less Northern sunlight during Boreal Summer resulting in cooler summer temperatures – a precondition for the initiation of an ice age! Atmos 1020 – Lecture 5 Long Term Variability

Obliquity is tending to decrease with the next minimum in about 10k years.

As obliquity decreases, how will the amplitude of the seasonal cycle change? Decrease- summers will be less warm and winters will be less cold • What is an ellipse? Eccentricity: a measure of how • Basically a stretched much an ellipse deviates from circle. circular. • A circle is a special type Eccentricity of a circle is 0. The of ellipse where both focal eccentricity of an ellipse which is points are the same. not a circle is greater than 0 but less than 1. Atmos 1020 – Lecture 5 Long Term Variability Keplers Laws of Motion 1. The orbit of every planet is an ellipse with the Sun at one of the two foci. 1571-1630 2. A line joining a planet 1546-1601 and the Sun sweeps out equal areas during equal intervals of time. 3. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit. Icarus: asteroid Potentially hazardous

Halley: comet visible from Earth every 75 years. Eccentricity: The earths orbital path around the sun varies from nearly circular to slightly elliptical on a 100,000 year timescale.

• Based on Keplers law, the earth moves in an elliptical orbit around the sun with the sun at one of the foci of the ellipse. This means the earth-sun distance varies over the course of a year. The more the eccentricity, the more this distance varies.

• If northern hemisphere summer were to occur when the earth is at aphelion in the elliptical orbit (like now), summer would tend to be warmer/cooler(?) than if summer occurred when the earth is at perihelion (closest to the sun).

Answer: cooler Atmos 1020 – Lecture 5 Long Term Variability

Earth-Sun Distance Anomaly (1 AU is the annual current average) Perihelion Aphelion Max .943 1.056 Now .983 1.0167

Lo So = 2 4π(re−s)

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http://earthobservatory.nasa.gov/Features/Milankovitch/milankovitch_2. php Direction of tilt at Aphelion or Perihelion. Precession is what Which makes Earth colder?? changes the direction Tilted towards the sun at aphelion. of tilt.

Northern hemisphere summer at aphelion will be the coolest summer. https://www.youtube.com/watch?v=dqk3FhtduYc

Atmos 1020 – Lecture 5 Long Term Variability

Min in Ice

future past

Berger and Loutre 1996

Max in Ice

Time

Berger 1978 Min in So Max in So http://oxfordreference.com/views/ENTRY.html?entry=t178.e0114&srn=2&ssid=546171388#FIRSTHIT Atmos 1020 – Lecture 5 Long Term Variability With a 26k year period for precession, summer occurred during perihelion about 13K years ago. In other words, this occurrence correlates with the end of the last ice age and the Yunger-Dryas – a few thousand years before the climatic optimum.

How will the tendency of precession effect seasons in the next half cycle? Presently, Northern summer occurs when the Earth is farthest away from the sun. In 13K years, summer will occur when the earth is closest to the sun. Summers will be warmer. Atmos 1020 – Lecture 5 Long Term Variability

How is eccentricity changing

As eccentricity decreases, how will the amplitude of the seasonal cycle change? Decrease- the contrast in the seasonal sunlight will be smaller. This figure shows examples of the three modes of orbital variation

What combination of the orbital variations would tend to cause Northern Summer to be coldest?

When summer occurs during aphelion with minimal obliquity (tilt) during a maximum in eccentricity.