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  • Over Earth's long history, there have been dramatic changes to our climate. The

  • 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

  • astronomical videos, please click to subscribe.

Over Earth's long history, there have been dramatic changes to our climate. The

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How Ice Ages Happen: The Milankovitch Cycles

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    王杰 posted on 2020/06/18
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