Everything Everywhere Daily: History, Science, Geography & More - Milankovitch Cycles

Episode Date: January 26, 2021

The Earth takes a year to go around the sun, and a day to turn on its axis. It is tilted 23.5 degrees which is what causes the seasons. All of these facts which you learned in school are true, but they are not permanent. They change, very slowly, over time. One astrophysicist in the 1920s figured out that all of these cycles could interact with each other, affecting the long term climate of the Earth. Learn more about Milankovitch Cycles on this episode of Everything Everywhere Daily. Learn more about your ad choices. Visit megaphone.fm/adchoices

The Earth takes a year to go around the sun and a day to turn on its access. It's tilted 23.5 degrees, which is what causes the seasons. All of these facts you learned in school are true, but they're not permanent. They change very slowly over time. One astrophysicist in the 1920s figured out that all of these cycles could interact with each other affecting the long-term climate of the Earth. Learn more about Milankovic cycles on this episode of Everything Everywhere Daily. What if your perceptions about the past,

were wrong. ThruLine is a podcast that takes you back in time to uncover the parts of the story that may have gone unnoticed. It effectively turned day into night and how it shaped the world now. Time travel with us every week on the ThruLine podcast from NPR. This episode is sponsored by the Travel Photography Academy. In 2007, I sold my home to travel around the world. I bought an expensive camera that I didn't know how to use and took a bunch of terrible photos. Several years, 100 countries, and tens of thousands of photos later, I'd improve my photography to the point where I was winning national awards, being named Travel Photographer of the year three times in North America. I created the Travel Photography Academy, so you don't have to

spend the many years that I did to improve your photography. Even though we can't easily travel right now, you can still work on improving your photography at home and in your own community. Just go to travel photography academy.com or click on the link in the show notes to start improving your photography today. To understand Milankovic cycles, we have to understand each of the cycles which the earth goes through individually. There are several of them, and it's going to take a bit of visualization to get the concept via a podcast where there are no visual aids, but it shouldn't be too difficult. We'll start by going through the cycles that the earth itself goes through. To understand these, you need to think of the earth as a spinning top. When the top spins, it's usually not perfectly upright.

The top will be tilted somewhat. Just like a top, the earth has a tilt to its rotation. Currently, the earth tilts 23.5 degrees, and that is what is responsible for the seasons. However, that tilt, known as obliquity, isn't static. It actually wobbles back and forth between 22.1 degrees and 24.5 degrees. Right now, we're in the middle of such a cycle. The time it takes to complete one full cycle, of going from 22.1 degrees to 24.5 degrees and back again is 41,000 years. The greater the tilt,

the more sun the polar regions will get in the summer, and the more extreme the seasons are. The next part of the cycle is axial procession. If you can imagine the spinning top again, as it's spinning, the axis of the top is rotating in a circle. It isn't just tilting in one direction. On the earth, the direction of our axis in the north currently points to the side. the North Star, or Polaris. However, this is temporary. Over the course of 25,771.7155 years, the Earth's axis will go in a circle. That means that not only will the North Star not be the North Star at some point, but 25,771 years from now, it will be the North Star again. While the Earth is going about its cycles on its wobbling and spinning axis, there are also things happening to

the Earth's orbit itself. For this part, instead of a spinning top, I want you to visualize a spinning plate. The edge of the spinning plate would be the orbit of the Earth, and at the center of the plate would be the Sun. The first orbital cycle is the orbital eccentricity cycle. The orbit of the Earth around the Sun isn't a perfect circle. It's slightly elliptical. The shape of that ellipse changes over time, and how much it deviates from a circle is known as eccentricity. The eccentricity cycles between 0.0034, which is almost perfectly circular, to 0.058, which is more slightly elliptical. The change is due to the gravitational pull of large planets like Jupiter and Saturn. This cycle takes about 100,000 years. The next cycle is called

obsidial procession. If you can imagine that plate again, this time imagine it as more of an oval plate. As the Earth is going around its orbit around the edge, the plate itself is rotating. That means the closest and farthest point that the Earth is from the sun will change over time. This cycle is about 112,000 years. Finally, there is a cycle for orbital inclination. That rotating plate isn't flat. It actually tilts, and the tilt changes over time as well. This cycle is about 100,000 years as well, and is very close to the same length as the orbital eccentricity cycle. Each of these cycles involves relatively small changes over long periods of time. However, they can compound each other or they can mitigate each other. All of these cycles have been

known for a while. Some were known back as far as antiquity and others were more recently discovered in the 19th and 20th centuries. In the 1920s, a Serbian astrophysicist named Molotan Milankovic put all the pieces together. He realized that these cycles closely followed the patterns of ice ages. In particular, three of these, axial tilt, eccentricity, and procession, all affected the amount of sunlight that would fall on the northern hemisphere. These cycles could either cancel each other out to moderate the strength of seasons, or they could compound each other, making the seasons even more powerful. In particular, what really mattered was the amount of sunlight falling on the northern hemisphere in the summer. Why the northern hemisphere? That is where most of the land is.

Sixty-eight percent of the land on Earth is in the northern hemisphere. land can't store heat as well as water, which means that ice can form on it easier. Ice reflects sunlight, which can cause further cooling. During an ice age, most of the ice accumulates in the northern hemisphere. In the south, ice can only accumulate to a point before it hits warmer water, and the ice will cleave off to form icebergs. Glaciers depend on how much of the ice melts during the summer. When the Earth is at its maximum tilt, more sun is hitting the northern hemisphere in the summers.

If the orbit of the Earth is such that it's at its closest point to the sun when this happens, summers will be very intense and ice will melt. When the opposite happens, when the tilt is at a minimum, and the Earth is farther away in the summers, ice will not melt as much and glaciers will grow. All of these factors individually are rather small. At its closest point to the sun, which currently happens on January 4th, remember back to my episode on why we celebrate New Year's Day when we do,

There's only about 6% more solar radiation hitting the Earth than when we are at the farthest point. Likewise, the axial tilt of the Earth only changes a few degrees. However, these effects can be big enough when they work in conjunction to cause an ice age. The observed strength of ice ages is usually found to be stronger than the Milankovych cycles would suggest, leading some climatologists to think that there might be a positive feedback mechanism at work, something which causes the planet to cool faster than expected. The timing of ice ages is still being worked. worked on. Milankovych predicted that ice ages would be about 41,000 years apart. And that was true,

up until about 1 million years ago. Since then, ice ages have come at about 100,000 years, which corresponds to the eccentricity cycle. Milankovych cycles aren't just unique to Earth. Like the Earth, Mars has all the cycles I just mentioned, except that the timing and the extent of the cycles are different. Researchers estimate that Mars has had between 6 and 20 ice ages over the last 800 million years. The Martian Milankovych cycle might bring about an ice age every 400,000 to 2.1 million years. Some of you might be wondering if Milankovic cycles are responsible for the recent climatic changes measured over the last several decades. And the answer is no. Milankovic cycles take thousands, if not tens of thousands of years, to change. Their effects can't be noticed over periods

as short as a decade. So the next time you think about the Earth as a spinning ball in space, realize that the spinning and the orbit isn't a static, unchanging thing. It's always slowly changing, and there are cycles, within cycles, within cycles. Executive producer of Everything Everywhere Daily is James McAla. The associate producer is Thor Thompson. Remember to leave a five-star review to get your review read on the show. They can be left at Apple Podcasts, Podcasts, Podcast Republic, or wherever you listen to the show. Also, you can help support the show over at patreon.com.

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