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 DeepSeek on Milankovitch cycles:

Describe to me the three Milankovich cycles...

Of course. The Milankovitch cycles (note: often anglicized as "Milankovitch" but originally Milanković) are three cyclical changes in Earth's orbital and rotational characteristics that combine to influence long-term climate patterns, especially the timing of glacial and interglacial periods over hundreds of thousands of years. They are named after Serbian geophysicist and astronomer Milutin Milanković, who mathematically described them in the early 20th century.

Here are the three main cycles, from shortest to longest period:

1. Eccentricity (Orbital Shape Cycle)

What it is: The change in the shape of Earth's orbit around the Sun, from more circular to more elliptical (eccentric).

Time Period: Approximately 100,000 and 400,000 years for the main cycles.

Mechanism: The gravitational pull of other planets, primarily Jupiter and Saturn, slowly stretches and squeezes Earth's orbit.

Climate Effect: Alters the total solar radiation (insolation) Earth receives over a year. When the orbit is highly elliptical, there is a significant difference in solar energy received at perihelion (closest approach) versus aphelion (farthest point). Currently, Earth's orbit is mildly elliptical, with about a 6% difference in solar receipt. At maximum eccentricity, this difference can be around 23%.

2. Obliquity (Axial Tilt Cycle)

What it is: The change in the tilt of Earth's rotational axis relative to its orbital plane. Think of it as the "wobble" in the tilt.

Time Period: Approximately 41,000 years.

Mechanism: The tilt oscillates between about 22.1 degrees and 24.5 degrees (it is currently 23.4 degrees and decreasing).

Climate Effect: This is the main driver of seasonal contrast. A greater tilt means more extreme seasons—warmer summers and colder winters at high latitudes. This is critical for ice sheet growth/decay: cooler summers in the high northern latitudes (where most land is) are key to preventing summer melt of ice, allowing glaciers to advance.

3. Precession (Axial Wobble & Orbital Precession)

What it is: A dual motion often called the "wobble" of the spinning top.

Axial Precession: The slow, circling wobble of Earth's rotational axis (like a spinning top winding down), completing a full circle.

Apsidal Precession: The slow rotation of Earth's elliptical orbit itself.

Time Period: Approximately ~23,000 years for the full cycle (often rounded to 26,000 for axial precession alone, but the combined climatic effect cycle is ~23,000 years).

Mechanism: Caused by gravitational tugs from the Sun and Moon on Earth's equatorial bulge.

Climate Effect: Changes the timing of the seasons relative to Earth's position in its orbit. It determines whether a given hemisphere's summer or winter occurs at perihelion (closest to the Sun) or aphelion (farthest). For example, about 11,000 years ago, Northern Hemisphere summer was at perihelion (making it warmer). Today, Northern Hemisphere summer is at aphelion, making it slightly milder.

How They Combine to Drive Ice Ages

The key concept is that these cycles do not cause large changes in the total annual solar energy, but they dramatically redistribute it by latitude and, crucially, by season. The dominant theory is that glaciations are triggered by cool summers in the high northern latitudes, which allow winter snow to persist and accumulate into continental ice sheets over thousands of years.

The 41,000-year obliquity cycle was dominant in climate records until about 1 million years ago.

For the last million years, the 100,000-year eccentricity cycle has become dominant in pacemaking the major ice ages, although its direct effect is the weakest. This suggests a strong nonlinear feedback within the climate system (e.g., ice-albedo feedback, CO₂ release from oceans) that amplifies the small initial trigger.

Important Caveats

Pacemaker, Not Driver: Milankovitch cycles provide the consistent pace for the glacial-interglacial cycles, but they are not the sole cause. The initial orbital change is small; it must be amplified by feedbacks (like changes in greenhouse gas concentrations—CO₂ and CH₄—and ice-albedo feedback) to produce the large temperature swings we see in the paleoclimate record.

Not Relevant for Modern Climate Change: These cycles operate on timescales of tens of thousands of years. The current, rapid global warming is happening 100 times faster than warming at the end of past ice ages and is unequivocally driven by human emissions of greenhouse gases, not by these slow orbital changes.

In summary: The Milankovitch cycles are the long-term astronomical "clockwork" of Earth's climate, altering the geographical and seasonal distribution of solar energy, which, when amplified by Earth's own climate feedbacks, sets the rhythm of the Ice Ages.

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Precession is when a the axis of a rotation rotates in turn to another set of forces...