The Milakovitch Cycles: Tilt, wobble, eccentricity

Welcome back, readers!

The Milankovitch Cycles are named after Milutin Milankovitch, who correctly calculated the collective (and varying) amounts of solar insolation reaching the mid-latitudes (30-60 degrees) attributed to three specific orbital cycles. Rather than whirl around like a stationary seat on a merry-go-round, Earth actually wobbles like a top as it orbits the sun, sometimes more steeply tilted on its axis, and sometimes making more of an oval-shaped orbit instead of a perfectly circular orbit. These cycles are referred to as precession, obliquity, and eccentricity, respectively.

We humans cannot feel these astronomical movements because they take a long time to unfold. The shape of Earth’s orbit (eccentricity) becomes more ovular or more circular over the course of 100,000 years. This is because Jupiter and Saturn, the largest gaseous planets in the solar system, have a strong enough gravitational pull to warp our Earthly orbit.

“When Earth’s orbit is at its most elliptic, about 23% more incoming solar radiation reaches Earth at our planet’s closest approach to the Sun each year than does at its farthest departure from the Sun. Currently, Earth’s eccentricity is near its least elliptic (most circular) and is very slowly decreasing.” (

Meanwhile, Earth’s axial tilt (obliquity) ranges from 22.1 to 24.5 degrees every 41,000 years, which doesn’t sound like a lot to us, but makes a HUGE difference at the poles. When the Earth is less tilted (closer to 22.1 degrees), the sun’s rays hit the poles at a low, indirect angle. This promotes glaciation, i.e. the growth of continental ice sheets via the reduction of summer melt such that snow and ice can chronically accumulate. Twenty- to ten-thousand years ago, continental ice sheets covered North America down to New York and Washington state! In contrast, when Earth is tilted at 24.5 degrees, more direct sunlight can hit the polar regions, which contributes to summer melt such that it outpaces snow and ice accumulation, i.e. deglaciation. “Earth’s axis is currently tilted 23.4 degrees, or about halfway between its extremes, and this angle is very slowly decreasing.” (Ibid.)

Finally, Earth wobbles like a top over the span of 26,000 years. This has several effects. First, seasons become more extreme in one hemisphere and less extreme in the other. The northern hemisphere has more land mass for continental ice sheets to grow upon. Therefore, the amount of solar radiation reaching the northern hemisphere specifically drives the glaciation/deglaciation cycles. Milder seasons in the northern hemisphere promote ice growth. Second, axial precession also changes the timing of the seasons, causing them to begin earlier over time. Hey, our Gregorian calendar is just a social construct, after all! Third, precession causes us to point to new North Stars. Currently they are Polaris and Polaris Australis, but several thousand years ago, they were Kochab and Pherkad. (Ibid).

Don’t panic! The Milankovitch Cycles were easily the most difficult concept for me to grasp as an undergraduate. The main takeaway is that humans have absolutely no ability to influence our orbital movements, and these orbital movements drove the advance and retreat of the ice ages for millions of years before we even arrived on scene. What humans most assuredly DO have the ability to influence is atmospheric chemistry. We alter the carbon cycle every day on a massive scale. This is why we have extraordinarily fast warming, light-speed geologically speaking. We should be entering a gradual cooling period, slipping into the next ice age, but instead we are sky-rocketing in the opposite direction, with global CO2 and temperature increasing exponentially. We know precisely why.

On an unrelated note, I checked the Climate Prediction Center again. It looks like we have equal chances (50:50) forecasted for a normal precipitation year! Let’s hope we receive some of the La Niña moisture expected for the Pacific Northwest. We are technically part of the Cascades, overlying the subduction zone between the Juan de Fuca and North American tectonic plates. Did you know Lassen Peak is the southernmost Cascade volcano? I will continue to hope for plentiful precipitation this winter, ideally in the form of snow.

Be well in the meantime.

(5) comments

Trinity Bob

Wow! How very informative, Ms. Killeen.

I knew about some of this, but certainly did not know that Saturn and Jupiter affected Earth’s orbit around the sun because their mass exerts a pull on the mass of our own planet, although this makes obvious sense once you think about it.

So, if I understand this correctly, while we should be entering a cooling period, instead Earth is actually warming—quickly. This helps explain the great concern that you and other scientists share about how we are changing our climate and in a big, not-so-good way.

Thank you for enlightening us about these and other concepts in a way we can understand. Very much appreciated!

Truth First

I truly believe that in a hundred years or so, people will look back on our society in this era much the same way as we today view the sun worshippers of olden times. “Such a quaint, though well-meant, regimen those naive saps all put themselves through”, the people of the future will no doubt say about us. “But oh, how they wasted their lives. So adorable, weren't they? Thinking they had the power to truly alter nature, for better or worse. And that their finite existence, their puny little lives, actually mattered in the big scheme of things. Was it arrogance, or just ignorance? Perhaps we'll never know. If only they'd listened to those they dismissively referred to as the “deniers” of their time and not to the regimes and institutions who tried to control them through disinformation. If only they hadn't relied on incomplete “data” that was only a few decades old, instead of on millions of years (or thousands, if you're of Judeo-Christian persuasion) of natural history to know that the earth has evolved since its very creation; it pre-existed us, and it will go on with or without us. Simply put, we are irrelevant. The climate changes, yes, but it also changes back. The globe warms up, true, then it later cools, and then warms again. Polar ice caps and ozone layers erode, and eventually, they manage, like so many things in the natural world, to self-repair over time. Species evolve, and adapt to their ever-changing environments, and sometimes they die out completely. This is just nature. As it is, forever was, and always will be. The Yin and Yang of life, as they say. Things live. Things die....But even then, as one of the human males from back then, the one they called 'Springsteen', once said: 'Maybe everything that dies some day comes back...'”

Megan Killeen

It's so sad to me that you have no one in your life who loves you enough to prevent you from making these moronic comments online. What a sad, small, hateful life you lead, TF. We humans are KILLING OUR OFFSPRING. I will no longer refer to you as a climate change denier. I will refer to you as an ecocidal future-child murderer.

Truth First, if you had any semblance of maturity about you, you would find more productive ways to spend your time. You are an anonymous troll and your life is a waste.

Trinity Bob

“Simply put, we are irrelevant.”

Finally, something we can agree on! You’re so right, TF. Once we’ve wiped all life from this planet, Earth will repair itself and become whole once again. But only for a while.

Nature doesn’t need us. We need nature, but only until we’re dead. So, fugget about it?

One day, long after we are gone, as the sun ages, it will expand its diameter far beyond the orbit of Mars. Nothing here will remain. All will be consumed and obliterated and, finally, there will be peace on Earth again. So, truly, nothing we do in the here and now matters beyond that history-ending event.

Seems to me, you advocate that humankind’s end come sooner rather than later. If so, who am I to argue or to judge? Nothing matters. I guess on this issue, at least, we totally agree, though I believe it is in humanity’s best interest to continue our species for as long as possible, which requires cooperation and SOME smarts, applied. A measure of shared reality and values would help.

You speak often of God. Do you truly believe your philosophy of disregard for His creation aligns with His wishes? I could point you to some Old Testament texts, but I seriously doubt they would fit with your agenda, whatever that may be.

Is that little evil angel sitting on your shoulder whispering into your ear again?

Megan Killeen

TF, I feel embarrassed on your behalf every time you type. You claim to be for personal liberty, yet you screech like a banshee about the evils of cannabis. Shouldn't Americans be free to do what they like on their property, within the confines of the law? Mind you, cannabis is legal, but you appear to have not learned the lessons of prohibition at the turn of the century. Cannabis is out of the bag. It has been grown in this county for generations.

I am just as appalled at the environmental damage and human labor trafficking resulting from cartel operations and mega-grows. It’s scary and upsetting, I agree with you. But, frankly, the only way to fight it is with more money. And this county lacks funds.

The fact that you insist on killing legal weed proves that you would bite the hand that feeds you. Cannabis is the only way this county can generate revenue for taxes, aka services, aka utilities, healthcare, schooling, social services, and police protection. It is the only way this desperately destitute county will have a shot at a sustainable, long-term future. This country grows world-class cannabis and could rake in hundreds of millions of dollars more than it currently does in retail and tourism. Those dollars could go toward busting the worst environmental offenders and remediating the damage.

It is dangerous that you support the right of the people to possess weapons of death and destruction, but not to grow a specific plant on their own property. And it is dangerous that you would suggest that you are qualified to take justice into your own hands instead of supporting police to do the work they are trained to do. Are you for personal liberty, or only when it suits you and is on your terms? Are you of sound enough mind to be judge, jury, and executioner of your neighbors? You know my opinion: I don’t think you are of sound mind at all.

You mention Judeo-Christian teachings as if they are anything more than the mere mumblings of humans from thousands of years ago. Do you really think the thin, paltry languages our puny human brains wield are capable of describing the power of God? God is creation. God is nature. God is the cosmos. Science is the study of nature and of the cosmos. I look to the atoms to tell me what's real, not a book written by intellectually weak, limited humans in a language that we don't speak, that was then re-translated a dozen times. The fact that you mention religion at all tells me that you’re not ready or able to approach the topic intellectually. The fact that you won't accept the unshakable truth of mathematics and observation tells me how much of a waste of time it is bothering to write a retort at all. But I enjoy the sport.

Good job, TF! You said something real: Earth has existed for millions (actually, billions, but you seem to have not learned anything from my Climate Corner thus far) of years before us, and Earth will exist for billions of years after us, until the sun becomes a red giant and swallows all of the planet. But what you FAILED TO GRASP is that unlike the dinosaurs, wiped out by an unstoppable asteroid, WE HUMANS know EXACTLY why the planet is heating up. We are culpable. People who promote platitudes and false wisdom are especially culpable. We have influenced the carbon cycle on the scale of centuries instead of hundreds of millions of years, but so far you have been unable (or refused) to understand that fact.

I described the Milankovitch Cycles to prove your point: we have no influence on the orbital cycles driving our ice ages. And similar to the way that it seems you cannot grasp kinetic information in three dimensions, you likewise seem unable to grasp the concept of the future and the damage we are doing to our own children. You are promoting the continuation of a failed, broken, exploitative, extractive system that is unsustainable and is consequently causing the sixth mass extinction of Earth.

You don’t get to fall back on “Earth will be here after we’re gone.” And, “Extinctions have happened before.” WE, especially us here in the U.S., are causing this extinction. And YOU are cheering it on. Take your platitudes and false wisdom, swallow it all, and please be quiet.

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