1. Long-term climate change & Short-term climate variability

2. Long-term climate change

3. Solar radiation Most important forcing mechanism for climate
Amount of solar radiation that the Earth surface (or any part thereof) receives is slowly but continuously changing
Changes are related to variations in the orbit of the Earth about the Sun
As a result, climate will also inevitably change
Climate change is perfectly normal

4. Milankovitch theory
Proposed in 1920 by Milutin Milankovitch to explain the cycle of glacial & interglacial periods (widely accepted today)
3 important orbital parameters:
eccentricity of Earth’s orbit
tilt of Earth’s axis of rotation
precession of Earth’s orbit
In combination, these influence the amount & distribution of solar radiation reaching the Earth, which changes with both latitude & season
Each parameter has a unique periodicity

5. Eccentricity of Earth’s orbit
Present orbit
Orbit 50,000 years ago
This graph shows the variation in the eccentricity of the Earth's orbit over the last 750,000 years. The primary periodicity is approximately 100,000 years. The blue line traces the eccentricity with respect to time. The orange line shows today's value for comparison.
Horel and Geisler, p. 78

6. Tilt of Earth’s axis Tilt = 0 Tilt = 23.5
This graph shows the variation in the tilt of the Earth's axis over the last 750,000 years. The tilt varies from 21.6o to 24.5o, today’s tilt is 23.5o. The primary periodicity is approximately 41,000 years. The blue line traces the tilt with respect to time. The orange line shows today's value for comparison.
Horel and Geisler, p. 79

7. Precession of Earth’s orbit
At present, the time of the perihelion occurs near the winter solstice.
In 11,500 years, the time of the perihelion will occur near the summer solstice.
Horel and Geisler, p. 80

8. Precession of Earth’s orbit
This graph shows the precession of the equinox over the last 750,000 years. The combined effect of changing aphelion and perihelion and equinoxes has periodicity of about 22,000 years. The precession is expressed as the longitude of the perihelion from the vernal equinox. The blue line traces the precession with respect to time; the orange line shows today's value for comparison.

9. Combination of periodicities
Sum
Precession
Tilt
Eccentricity
Horel and Geisler, p. 80

10. Vostok ice core, Antarctica