There are very few people in history, who, based upon just a few tidbits of DATA, came up with an amazing HYPOTHESIS that took scientists more than a half-century to confirm as a good working THEORY1; and, today, after more than a century of further confirmation of this theory, is still hardly recognized for their genius. Milankovitch was such a person.
In the 1910’s Milankovitch came up with a simple hypothesis:
Variations in the Earth’s orbit would cause periodic Ice Ages2.
He proposed this hypothesis at a time when probably no one even knew that Ice Ages were periodic, let alone what caused them3. These variations in the Earth’s orbit are now called the Milankovitch Cycles.
Figure 1 below shows the Earth’s temperature from the Vostok Ice Core DATA, and a sinusoidal representation of Elliptical Milankovitch cycle.
Figure 1. Vostok Ice Core Temperature DATA filtered with a 1/(1000 yr) low pass single-pole frequency filter, and a sinusoid representing the Elliptic Milankovitch Cycle.
Question re Figure 1
Paraphrasing Al Gore’s famous words4, I ask, “Do you think they might fit together?”. Fourier Analyses of the DATA from the Mud Cores and Ice Cores show the Elliptic to be the strongest peak in the spectrum, followed by the Obliquity, and finally the Precession Milankovitch frequency.
Of course, Milankovitch was “Wrong”! Or maybe he was “Right!”.
Today, in the words of Hays et al (footnote 1), the “Pacemaker of the Ice Ages” is operating on a 100,000 year cycle dominated by the Elliptic Cycle. Milankovitch predicted that the Pacemaker would operate on a 41,000 year cycle, with the Obliquity Cycle dominating. The Mud Core DATA show that at the onset of this present glaciation period about 3 million years ago, that the Ice Ages did indeed come every 41,000 years. Why about a million years ago the “Pacemaker” shifted its timekeeping from a 41,000 year cycle to a 100,000 year cycle is something I confidently prophesy will keep scientists (probably mostly graduate students) busy for 100,000 (or I could be wrong, perhaps instead 41,000) human-years.
Perhaps we can forgive Milankovitch for all the “errors” in his Hypothesis (after all, this was before he had the Mud and Ice Core DATA, and before computers), and celebrate his Genius!
I commemorate 28 May as the Official Holiday for Saint Milankovitch (his Birthday). On that day we can celebrate our luckiness to be living in one of the brief Inter-Glacial Periods (the Holocene) which he hypothesized!
Hays et al. confirmed the Milankovitch Cycles in their seminal 1976 publication in the journal, Science:
Variations in the Earth’s Orbit: Pacemaker of the Ice Ages (https://www.whoi.edu/cms/files/hays76sci_268464.pdf)
Previous Substack Postings about Milankotich:
I recently enjoyed reading through this simple explanation of the Milankovitch Cycles on a NASA webpage: Milankovitch (Orbital) Cycles and Their Role in Earth's Climate (By Alan Buis, NASA's Jet Propulsion Laboratory). This webpage also has nice graphics demonstrating the 3 Milankovitch Cycles.
With the DATA that we have today we know that Ice Ages are the rule, rather than the exception (See Figure 1). Perhaps instead of focusing on “Ice Ages”, we should focus on the brief periodic “Inter-Glacial” periods where life flourishes. Our human ancestors have lived though a few of these cycles, and Neanderthal Man (now extinct, though we have some of their genes) lived through about 10 of these cycles. So, we probably won’t all die when the next Ice Age comes any day (few thousand years from) now.
Snippets from Al Gore’s Film, An Inconvenient Truth, can be found in my post Hoisting Mr. Gore on his own Pitard. Yes, in today’s Orwellian world, we give Nobel Prizes to people (e.g. Al Gore) for promoting Mis- and Dis- Information!
Yes --
The earth's tilt has a much greater seasonal effect on insolation than the Earth-Sun distance, and cause the seasons. And, one might conjecture that seasonal variations in the Northern hemisphere might be somewhat ameliorated compared to the variations in the Southern hemisphere due to the effect you mentions. Perhaps one might even further conjecture that this might be the reason that continental sheets of glaciers during the Ice Ages are more of a Northern Hemisphere phenomenon.
I asked ChatGPT a few questions about sun-earth distance. Here is what came out. About 3 million mile difference The farthest distance between the Earth and the Sun occurs at a point in Earth's orbit known as aphelion. During aphelion, Earth is at its farthest point from the Sun. The distance at aphelion is approximately 94.5 million miles (about 152 million kilometers). This event typically takes place around early July of each year.
It's interesting to note that while there is a difference in distance between perihelion and aphelion, these variations have a relatively minor impact on Earth's climate and seasons. The primary driver of seasonal changes is the axial tilt of the Earth, which causes different parts of the planet to receive varying amounts of sunlight throughout the year.
ChatGPT makes an assumption about the effects of perihelion and aphelion. But simple physics and diminution of energy of 3million miles is significant.
The inverse square law states that the intensity of radiation is inversely proportional to the square of the distance from the source of radiation. In the case of sunlight reaching the Earth, this means that if the Earth is twice as far from the Sun (as it is at aphelion compared to perihelion), it receives only about one-fourth (1/2^2) of the solar radiation.
The actual difference in solar radiation between perihelion and aphelion is approximately 7%, with Earth receiving about 7% more solar energy at perihelion than at aphelion. The average temperature difference between an ice age and a warm period can vary, but it's generally estimated to be around 5 to 10 degrees Celsius (9 to 18 degrees Fahrenheit).