Milankovitch Cycles (HL) (DP IB Environmental Systems & Societies (ESS)) : Revision Note

Milankovitch Cycles

• Milankovitch cycles describe three long-term changes in Earth's position and movement relative to the Sun

  • These cycles impact the amount of solar radiation Earth receives

  • This influences the climate over very long timescales (tens to hundreds of thousands of years)

• The cycles contribute to natural climate patterns such as:

  • Ice ages

  • Warmer interglacial periods

Three types of Milankovitch cycles

1. Eccentricity (shape of Earth’s orbit)

• Eccentricity refers to the shape of Earth's orbit around the Sun

• This orbit changes from a more circular shape to a more elliptical shape over a cycle of about 100,000 years

  • When the orbit is more elliptical, Earth’s distance from the Sun varies more

    • This creates larger differences in solar radiation received at different times of the year

  • When the orbit is more circular, the distance from the Sun remains more constant

    • This leads to smaller differences in seasonal temperatures

• Climate impact:

  • Lower eccentricity (circular orbit) results in milder seasonal differences

  • This milder climate can allow ice sheets to slowly build up over time, especially in polar regions

  • If the Earth’s tilt is also smaller, the poles receive less direct sunlight, making it harder for ice to melt

  • As a result, snow and ice accumulate

  • This can lead to the onset of a glacial period or ice age

2. Obliquity (tilt of Earth's axis)

• Axial tilt refers to the angle at which Earth's axis tilts in relation to its orbit around the Sun

  • The tilt angle changes from about 22.5° to 24.5° over a cycle of approximately 41,000 years

  • Greater tilt increases the intensity of seasons, making summers warmer and winters colder

• Climate impact:

  • A higher tilt (closer to 24.5°) creates stronger seasonal contrasts

    • This can reduce the buildup of polar ice by making summers warmer

  • A lower tilt (closer to 22.1°) reduces seasonal contrasts

    • This allows ice to persist at the poles and contributes to glaciation

3. Precession (wobble of Earth's axis)

• Axial precession is the wobble in Earth's rotational axis

  • This cycle takes approximately 26,000 years and causes the timing of seasons to shift

  • As Earth wobbles, the position of the North Star gradually changes

• Climate impact:

  • Precession affects the seasonal distribution of solar radiation

  • This can lead to warmer or cooler summers and winters

    • This depends on where Earth is in its orbit when each season occurs

Milankovitch cycles and climate change

• The combined effects of eccentricity, obliquity, and precession drive long-term climate shifts, like the Ice Ages

  • Positive feedback loops amplify these climate shifts

    • For example, cooling from reduced solar radiation can lead to glaciation

      • This snow and ice reflect sunlight (albedo effect) and cause further cooling

    • Warming increases carbon dioxide, enhancing the greenhouse effect and leading to interglacial periods

• Role of carbon dioxide:

  • Lower temperatures reduce carbon dioxide in the atmosphere, enhancing cooling

    • Cooler temperatures make the atmosphere less able to hold gases like carbon dioxide, which the oceans or soils then absorb

  • Higher temperatures release more carbon dioxide, enhancing warming

    • This occurs because warmer temperatures increase the rate at which carbon dioxide is released from sources like soils, oceans, and plant life

    • This increases atmospheric carbon dioxide levels

Real-world examples

• Last glacial maximum:

  • It occurred approximately 20,000 years ago when eccentricity, axial tilt, and precession aligned to favour cooling

  • Resulted in extensive ice sheets covering parts of North America and Europe

• Current interglacial period:

  • We are currently in a warmer interglacial period called the Holocene, which began around 11,700 years ago

  • This period has allowed the development of human civilisation due to the relatively stable and warmer climate