Ice ages have come and gone. And what's surprising is that there's a strong

pattern that explains why Ice Ages happen when they do. This is called the

Milankovitch cycle. Named after Milutin Milankovic, his theory explains how the

earth's climate changes over hundreds of thousands of years. His theory is based

on two key ideas: first, the Earth's climate is strongly affected by how much

sunlight the northern latitudes receive during the summer. Second, this amount of

sunlight varies based on changes in the Earth's orbit and rotation. Why are the

northern latitudes so important? It's because of ice. When sunlight hits the

ground, most of the energy is absorbed as heat. But if the ground is covered in ice,

most of the light reflects away because ice is white. This creates a positive

feedback loop. Ice forms when it's cold. But ice also reflects light, making it

colder which forms more ice. So ice is really important for climate. The

northern and southern hemispheres both contain lots of ice. But there's more ice

in the north because there's more land. Land has a

lower heat capacity than water which means that water doesn't change

temperature as easily as land does this is why coastal regions are generally

more mild and why ice forms more easily on land. Just look at the difference

between the northern and southern hemispheres. In the south, there are ice

caps that grow during its winter but not nearly as much as they do in the north.

During the winter the land above the Arctic Circle is covered in darkness

experiencing twilight 24 hours a day. It's very cold and lots of ice forms

during the winter. And this is true no matter what's going on with Earth's

orbit. The key variable here is how much ice melts during the summer.

This depends on how much sunlight there is during the summer. Now you might think

that this doesn't change, but it does. Milankovic showed that over hundreds

of thousands of years the amount of summer sunlight can shift plus or minus

15%. This can bring ice ages. This can end ice ages. How can the amount of summer

sunlight be changing? Well, first the distance from the earth to the Sun is

changing and second the earth's tilt is changing. The Earth's axis is currently

tilted at 23.5 degrees, but this changes. Other objects influence the

earth gravitationally nudging its tilt up and down. Every 41,000 years, it cycles

up and down. When the earth is more tilted there's more sunlight during the

summer. More summer sunlight means that more of our ice melts away. With less ice

on the ground less light is reflected away giving us a warmer climate. Earth is

unusual in that it's tilt doesn't change very much. Earth has a very large moon

which stabilizes its tilt. Mars has two tiny moons and so its tilt changes much

more dramatically. The next effect is the distance from the

earth to the Sun. The Earth's orbit is not a circle it's an ellipse. Every

fourth of July, we celebrate aphelion: the day that the earth is farthest from the

Sun. Then in January the earth moves closest to the Sun. Now the planets

Jupiter and Saturn both nudge the earth causing its orbit to shift slightly

becoming either more oval or more circular. This happens over period of

100,000 years. This effect is wildly exaggerated in this diagram. It

actually looks more like this. You can barely even see that the distance to the

Sun is changing, but this subtle change has important consequences for our

climate. Earth as a whole receives 6% more sunlight during January

than it does in July. The seasons change because the North Pole

sometimes tilts towards the Sun and sometimes tilts away. The change in the

distance to the Sun, this works against the change in the seasons. This moderates

the seasons in the north since the earth is farthest away in July, but this was

not always true. The Earth's axis is moving in a circle, it's spinning like a

top. This is called precession. In fact, I made an entire video about this and what

this means is that 13,000 years ago the tilt of the earth was reversed. When the

earth was closest to the Sun, it was summer in the North. The distance change

didn't oppose the seasons. It amplified seasons making them more extreme. Now

warmer summer means more melting. More melting means less reflection which

means the climate as a whole is warmer. The amount of summer sunlight is

affected by three long-term cycles: one changes the tilt, one makes our orbit

more circular or more oval, and one changes how the distance to

the Sun matches with the changing of the seasons. These three cycles powerfully

impact our climate. Scientists have measured the history of our climate

using ice cores. Now Earth's climate is complicated.

You can't just reduce it to a single input but the Milankovitch cycles have

played a key role in our climate for hundreds of thousands of years. For more

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