IB Environmental systems and societies 6.1E Milankovitch Cycles Notes

Orbital Cycles That Drive the Ice Ages

1. Milankovitch cycles are long-term, predictable variations in Earth's movement and orientation in space.
2. They alter how much solar radiation (insolation) reaches Earth and how it is spread 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 change Earth's energy budget, influencing ice-sheet growth, CO₂ concentrations, and global climate.

Eccentricity: The Shape of Earth's Orbit

1. Earth's orbit shifts between more circular and more elliptical shapes.
2. This cycle lasts about 100,000 years.
3. When the orbit is more elliptical, Earth's distance from the Sun varies more across the year.
4. This widens the gap between the closest point (perihelion) and farthest point (aphelion), increasing seasonal variation in insolation.
5. When the orbit is more circular, radiation is spread more evenly through 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 seasonal differences, letting ice sheets grow if other conditions also favour cooling.
3. When low eccentricity aligns with low tilt and certain precession conditions, glaciers expand over thousands of years.
4. During the last glacial maximum (about 20,000 years ago), Earth's orbit was near-circular, reinforcing long-term cooling.

Obliquity: The Tilt of Earth's Axis

1. Earth's axial tilt is currently 23.5°, but over a 41,000-year cycle it varies between about 22.1° and 24.5°.
2. The tilt controls the intensity of seasons by setting how directly sunlight strikes each latitude.

Climate Impacts of Obliquity

1. A greater tilt (closer to 24.5°):
   1. creates stronger seasonal differences,
   2. with hotter summers and colder winters,
   3. so warmer summers melt more ice and promote interglacial periods.
2. A smaller tilt (closer to 22.1°):
   1. creates weaker seasonal contrasts,
   2. so cooler summers let ice persist,
   3. favouring glaciation.

Precession: The Wobble of Earth's Axis

1. Earth's axis slowly wobbles in a circle, like a spinning top.
2. This cycle lasts about 21,000–26,000 years.
3. Precession changes the timing of the 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 winters and supports glacier growth.
2. When the cycle shifts so Earth is closest to the Sun in the Northern Hemisphere summer, summers become hotter.
3. Hotter summers melt more ice, reinforcing interglacial conditions.

How Feedback Loops Amplify the Cycles

1. Milankovitch cycles cause only small initial changes in the energy Earth receives.
2. These trigger powerful positive feedback loops that amplify the shift.

Cooling Feedback Loop

1. Reduced insolation causes global cooling.
2. More snow and ice raise albedo, reflecting more sunlight.
3. Further cooling expands the ice sheets.
4. Colder oceans absorb more CO₂.
5. Lower CO₂ reduces warming further.
6. Cooling intensifies until a glacial period develops.

Warming Feedback Loop

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

Why Milankovitch Cycles Cannot Explain Current Warming

Timescale and Emissions Evidence

1. Milankovitch cycles operate over tens to hundreds of thousands of years.
2. Current warming has spanned only about 150 years, far too short to be caused by orbital forcing.
3. It matches the Industrial Revolution, rising CO₂, and rapid anthropogenic emissions.
4. By current orbital positions, Earth should actually be slowly cooling, not warming.