B1 Intermediate US 21 Folder Collection
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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.
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How Ice Ages Happen: The Milankovitch Cycles

21 Folder Collection
王杰 published on June 18, 2020
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