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
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The procession of the equinox in which ancient cultures followed religiously.
The variable tilt in the earth's axis is the wobble in axial rotation.
Too perfect. This has to be a simulation.
No mention of solar cycle or cyclical crustal displacement pole shifts.
Sirius!