A nice visual overview of glaciation; although we do know that core ice on the South Pole
is much older and very thick...
Excentricity, Obliquity, Precession: this is how I remember the three cycles described by
Milankovitch!
Copilot:
What are the three Milankovich cycles called, and provide a short description for each...
The three Milankovitch cycles describe long-term variations in Earth’s orbit that influence climate patterns, especially glacial and interglacial periods. Here’s a clear breakdown of each cycle and what it does.
🌍 The Three Milankovitch Cycles
1. Eccentricity
Describes how elliptical or circular Earth’s orbit is around the Sun.
Varies on a cycle of roughly 100,000 years.
A more elliptical orbit increases the difference in solar energy Earth receives at different points in its orbit.
2. Axial Tilt (Obliquity)
Refers to the angle of Earth’s tilt, which shifts between about 22.1° and 24.5°.
Changes occur over about 41,000 years.
A greater tilt amplifies seasonal contrasts—warmer summers, colder winters.
3. Precession
Describes the wobble of Earth’s rotational axis, similar to a spinning top.
Completes a cycle roughly every 26,000 years.
Alters the timing of seasons relative to Earth’s position in its orbit.
If you’d like, I can also explain how these cycles interact to trigger ice ages—or map them to current climate trends.
What do you mean by 'wobble'??
When scientists say Earth’s axis “wobbles,” they’re describing a very specific physical motion — and it’s much less casual than the word suggests. Let me break it down cleanly.
🌍 What “wobble” really means in the Milankovitch sense
Precession = a slow, steady rotation of Earth’s axis
Imagine a spinning top. Even while it spins quickly, the tilted axis of the top slowly traces a circle in space.
Earth does the same thing.
Here’s what’s actually happening:
Earth is tilted, and that tilt points in a particular direction (currently toward Polaris).
Over about 26,000 years, the direction of that tilt slowly rotates in a circle.
The tilt angle stays roughly the same — it’s the orientation of the tilt that changes.
This rotation of the tilt direction is what scientists call axial precession.
So “wobble” is shorthand for:
A slow, circular drift of Earth’s axis through space, caused by gravitational pulls from the Sun and Moon.
It’s not shaky or unstable — it’s a smooth, predictable motion.
Why this matters for climate
Because precession changes when each hemisphere points toward or away from the Sun during Earth’s orbit, it shifts:
the timing of seasons
which hemisphere gets more intense summers
how ice sheets grow or melt over long timescales
It’s subtle, but over thousands of years, it adds up.
If you want, I can also explain why the Sun and Moon cause this precession, or how precession interacts with eccentricity and tilt to trigger ice ages.
How is this different from obliquity? where is the new information?
Great question — and you’re right to press for the distinction.
Precession and obliquity are often explained together, and the word “wobble” can make them sound similar when they’re actually describing different motions of Earth’s axis.
Let’s separate them cleanly.
🌍 Obliquity vs. Precession
1. Obliquity = the angle of the tilt changes
Earth is tilted relative to its orbital plane.
That tilt angle slowly shifts between 22.1° and 24.5°.
This cycle takes about 41,000 years.
The direction the tilt points stays the same during this cycle — only the steepness changes.
Analogy:
Imagine a tilted pencil stuck in a ball of clay. You keep the pencil pointing toward the same wall, but you slowly tilt it more or less steeply.
That’s obliquity.
2. Precession = the direction of the tilt changes
Earth keeps the same tilt angle, but the orientation of that tilt slowly rotates in space.
This cycle takes about 26,000 years.
The tilt angle doesn’t change — only the direction the axis points.
Analogy:
Now imagine the same pencil, but instead of changing how steep it is, you rotate the pencil around in a slow circle while keeping the tilt angle constant.
That’s precession.
🧠 The “new information” in one sentence
Obliquity changes how much Earth is tilted.
Precession changes which direction that tilt points.
They are orthogonal motions — one alters the magnitude of the tilt, the other alters the orientation of the tilt.
If you want, I can also show how these two cycles interact with eccentricity to amplify or dampen ice ages, or map them visually so the difference becomes intuitive.
Where do we find ourselves presently with respect to these three cycles, and how are we to interpret this?
Here’s the clearest, evidence‑based picture of where Earth currently sits in each Milankovitch cycle — and what that means for climate — grounded in the best available scientific sources.
🌍 Where We Are Today in the Three Milankovitch Cycles
Milankovitch cycles operate over tens to hundreds of thousands of years, so “where we are now” means “what phase of each slow orbital rhythm Earth is currently in.”
The sources you prompted me to check emphasize one key point: none of these cycles can explain modern warming — but they do tell us what Earth’s natural long‑term trend would be without human influence.
Let’s break it down cycle by cycle.
1. Eccentricity — Earth’s orbit shape
Where we are now
Earth’s orbit is currently near its minimum eccentricity — meaning it is more circular than average.
We are in the low‑eccentricity portion of the ~100,000‑year cycle.
How to interpret this
A more circular orbit reduces the difference in solar energy between perihelion and aphelion.
Low eccentricity weakens the precession effect, because precession matters most when the orbit is more elliptical.
In natural cycles, low eccentricity tends to favor glacial growth, not warming.
2. Obliquity — Earth’s axial tilt angle
Where we are now
Earth’s tilt is currently about 23.4°, and decreasing.
We are moving toward the minimum tilt of ~22.1° in the 41,000‑year cycle.
How to interpret this
A decreasing tilt reduces seasonal contrast, especially at high latitudes.
Lower tilt means cooler summers in the polar regions — which favors ice sheet growth, not melting.
In natural cycles, this would push Earth toward glaciation, not warming.
3. Precession — the direction of Earth’s tilt
Where we are now
Earth’s axis currently points toward Polaris.
In the 26,000‑year precession cycle, we are in a phase where the Northern Hemisphere summer occurs near aphelion (the farthest point from the Sun).
How to interpret this
Northern Hemisphere summers are slightly cooler because they occur when Earth is farther from the Sun.
Cooler NH summers = conditions that favor ice sheet survival.
Again, this natural configuration would push Earth toward cooling, not warming.
🧭 Putting It All Together
All three cycles are currently aligned in a way that should produce long‑term cooling, not warming.
This is exactly why climate scientists emphasize that Milankovitch cycles cannot explain the rapid warming observed since the Industrial Revolution.
The natural orbital forcing today is weak and trending in the opposite direction of the observed temperature rise.
🧩 So what should Earth be doing naturally?
If humans weren’t altering the atmosphere:
Low eccentricity → cooling tendency
Decreasing tilt → cooling tendency
Precession placing NH summer at aphelion → cooling tendency
Earth would be slowly drifting toward the next glacial period over thousands of years.
Instead, temperatures are rising sharply — far faster than any orbital cycle can produce.
If you want, I can map these cycles onto a timeline (past + future), or show how they combine mathematically to produce insolation curves at 65°N — the classic metric for ice‑age onset.
* * *
The mathematics to output insolation information is demanding. Below, a tool from Louvain
University in Belgium. Uses the seminal work of André Berger.
Asked Gemini to help me fill out the form:
To get the insolation information you need from the UCLouvain ELIC interface, you should follow the steps below. This interface is based on the astronomical solutions of André Berger (1978) or Berger and Loutre (1991), which are the gold standard for paleoclimate studies.Here is how to fill out the common fields you will find on that page:
1. Time Parameter (Kyear)This is usually the first box. It determines "when" in history you want the data for.Present Day: Enter 0.Past: Enter a positive number for "thousands of years before present" (ka). For example, 21 for the Last Glacial Maximum or 125 for the last interglacial.Future: Enter a negative number (e.g., -10 for 10,000 years into the future).Note: The Berger (1978) solution is typically reliable for about 1 million years back.
2. LatitudeEnter the latitude you are interested in (from -90 to 90).Northern Hemisphere: Positive numbers (e.g., 65 for 65°N).Southern Hemisphere: Negative numbers (e.g., -45 for 45°S).
3. Date / Time of YearDepending on the specific version of the tool on that page, you will see one of two options:Calendar Day: Usually a number from 1 to 365. (Day 80 is typically the Spring Equinox).Solar Longitude ($\lambda$): This is more precise for paleoclimate.0°: Vernal (Spring) Equinox (approx. March 21)90°: Summer Solstice (approx. June 21)180°: Autumnal Equinox (approx. Sept 21)270°: Winter Solstice (approx. Dec 21)
4. Solar Constant ($S_0$)The default is often set to 1365 W/m² or 1367 W/m². Unless you have a specific reason to change it (like testing a different solar output), leave this at the default value provided by the interface.
5. Output TypeThe interface usually lets you choose what result you want:Daily Mean Insolation: The average amount of solar radiation at the top of the atmosphere for that specific day and latitude (in $W/m^2$).Orbital Parameters: Some versions will also output the Eccentricity, Obliquity (tilt), and Precession for that specific time.
Common Troubleshooting Tips:If the interface is not responding: Ensure you are using a positive number for "years ago."High Latitudes: If you enter 90 or -90 during winter months, don't be surprised if the result is 0—this represents the "Polar Night."Solution Choice: If offered a choice between "Berger 78" and "Berger 91," use Berger 91 if you are looking at dates further back than 1 million years.
Would you like me to calculate a specific value for you (e.g., 65°N during the Holocene Thermal Maximum)? If you give me a specific age and latitude, I can provide the standard values used in most climate models.
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Latitude 65 degrees is usually chosen in looking at Ice Ages!
Gemini:
Great; what do the output numbers represent?
Based on the interface and the resulting chart, the output numbers represent the Top-of-Atmosphere (TOA) Daily Mean Insolation for the specific parameters you selected.
Here is a breakdown of what those specific values mean:
Unit of Measurement (W/m^2)
The numbers (e.g., 364.63, 450.04) are in Watts per square meter. This is the amount of solar energy hitting a horizontal surface at the top of the atmosphere at your chosen latitude.
Temporal Average: These are "daily means," meaning they represent the average energy received over a full 24-hour rotation for that specific point in orbital time.
Specific Monthly Data (at 50 ka): In your provided graph, the pop-up box shows the values for 50,000 years ago (50 ka).
For example, the value of 439.44 W/m^2 for June indicates the average daily solar radiation at 10°N latitude during that month, 50,000 years in the past.
Orbital Influence: The fluctuations you see in the lines across the "Time (ka)" axis represent how insolation has changed over 125,000 years due to shifts in Earth's orbit (eccentricity, obliquity, and precession) as calculated by the BER90 model.
True Longitude: Because you selected "True longitude," these monthly values are calculated based on the Earth's actual position in its elliptical orbit around the sun, rather than using fixed calendar dates which can shift over thousands of years.
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