6.1E Milankovitch cycles (HL) Notes

1. Milankovitch cycles are long-term, predictable variations in Earth’s movement and orientation in space.
2. These cycles alter how much solar radiation (insolation) reaches Earth and how it is distributed across latitudes and seasons.
3. They operate over tens to hundreds of thousands of years, driving natural shifts between glacial and interglacial periods.
4. There are three primary cycles:
   1. Eccentricity (shape of Earth’s orbit)
   2. Obliquity (axial tilt)
   3. Precession (axial wobble)
5. Together, they determine changes in Earth’s energy budget, influencing ice sheet expansion, carbon dioxide concentrations, and global climate patterns

1. Eccentricity - Changes in the Shape of Earth’s Orbit

1. Earth’s orbit shifts between more circular and more elliptical shapes.
2. This cycle lasts approximately 96,000–100,000 years.
3. When the orbit is more elliptical, Earth’s distance from the Sun varies more widely during the year.
4. This means that the difference between the closest point (perihelion) and farthest point (aphelion) creates greater seasonal variations in insolation.
5. When the orbit is more circular, the solar radiation received is more evenly distributed throughout the year.

Climate Impacts of Eccentricity

1. A more elliptical orbit intensifies seasonal contrasts, especially in the Northern Hemisphere.
2. A more circular orbit reduces these seasonal differences, which can allow ice sheets to grow if other conditions also favour cooling.
3. When eccentricity aligns with lower tilt and certain precession conditions, glaciers expand over thousands of years.

2. Obliquity - Changes in the Tilt of Earth’s Axis

1. Earth’s axial tilt currently sits at 23.5°, but over a 41,000-year cycle, it varies between about 21.5° and 24.5°.
2. The tilt controls the intensity of seasons, because it determines how directly sunlight strikes different latitudes.

Climate Impacts of Obliquity

1. A greater tilt (closer to 24.5°):
   1. Stronger seasonal differences
   2. Hotter summers and colder winters
   3. Warmer summers melt more ice which promotes interglacial periods
2. A smaller tilt (closer to 21.5°):
   1. Weaker seasonal contrasts
   2. Cooler summers allow ice to persist
   3. Favourable conditions for glaciation

3. Precession - Wobble of Earth’s Axis

1. Earth’s axis slowly wobbles in a circular motion similar to a spinning top.
2. This cycle lasts about 21,000–26,000 years.
3. Precession changes the timing of seasons relative to Earth’s position in its orbit.

Climate Impacts of Precession

1. Currently, Earth is closest to the Sun during the Northern Hemisphere winter, which moderates winter temperatures and supports glacier growth.
2. When the cycle shifts so that Earth is closest to the Sun in the Northern Hemisphere summer, summers become hotter.
3. Hotter summers melt more ice, increasing warmth and reinforcing interglacial conditions.

Milankovitch Cycles and Climate Feedback Loops

1. Milankovitch cycles cause relatively small initial changes in energy received from the Sun.
2. However, these changes trigger powerful positive feedback loops that amplify climate shifts.

Cooling Feedback Loop

1. Reduced insolation → global cooling
2. More snow and ice → increased albedo (reflecting more sunlight)
3. Further cooling → greater ice sheet expansion
4. Oceans absorb more CO₂
5. Lower CO₂ further reduces warming capacity
6. Cooling intensifies → glacial period develops

Warming Feedback Loop

1. Increased insolation → global warming
2. Ice melts → albedo decreases
3. Darker surfaces absorb more heat
4. Warmer oceans release CO₂
5. Higher CO₂ enhances the greenhouse effect
6. Warming accelerates → interglacial period begins

Why Milankovitch Cycles Cannot Explain Current Warming

1. Milankovitch cycles operate on tens to hundreds of thousands of years.
2. Current warming has occurred over 150 years—a timescale far too short to be attributed to orbital forcing.
3. Present-day warming corresponds closely with the Industrial Revolution, rising CO₂ levels, and rapid increases in anthropogenic emissions.
4. According to current orbital positions, Earth should actually be slowly cooling, not warming.