- 4035 - SUN - cycles and changes? Cycles also play key roles in Earth’s short-term weather and long-term climate. A century ago, Serbian scientist Milutin Milankovitch hypothesized the long-term, collective effects of changes in Earth’s position relative to the Sun are a strong driver of Earth’s long-term climate, and are responsible for triggering the beginning and end of glaciation periods (Ice Ages).
---------------------------- 4035 - SUN - cycles and changes?
- Our lives literally revolve around cycles:
series of events that are repeated regularly in the same order. There are
hundreds of different types of cycles in our world and in the universe.
-
- Milankovitch examined how variations in
three types of Earth orbital movements affect how much solar radiation (known
as insolation) reaches the top of Earth’s atmosphere as well as where the
insolation reaches. These cyclical orbital movements, which became known as the
Milankovitch cycles, cause variations of up to 25 percent in the amount of
incoming insolation at Earth’s mid-latitudes, the areas of our planet located
between about 30 and 60 degrees north and south of the equator.
-
- The shape of Earth’s orbit, known as
eccentricity. The angle Earth’s axis is
tilted with respect to Earth’s orbital plane, known as obliquity; and the
direction Earth’s axis of rotation is pointed, known as precession.
-
--------------------- Eccentricity – Earth’s annual pilgrimage
around the Sun isn’t perfectly circular, but it’s pretty close. Over time, the
pull of gravity from our solar system’s two largest gas giant planets, Jupiter
and Saturn, causes the shape of Earth’s orbit to vary from nearly circular to
slightly elliptical. Eccentricity measures how much the shape of Earth’s orbit
departs from a perfect circle. These variations affect the distance between
Earth and the Sun.
-
- Eccentricity is the reason why our seasons
are slightly different lengths, with summers in the Northern Hemisphere
currently about 4.5 days longer than winters, and springs about three days
longer than autumns. As eccentricity decreases, the length of our seasons
gradually evens out.
-
- The difference in the distance between
Earth’s closest approach to the Sun (known as perihelion), which occurs on or
about January 3 each year, and its farthest departure from the Sun (known as
aphelion) on or about July 4, is currently about about 3.2 million miles, a
variation of 3.4 percent. That means each January, about 6.8 percent more
incoming solar radiation reaches Earth than it does each July.
-
- When Earth’s orbit is at its most elliptic,
about 23 percent more incoming solar radiation reaches Earth at our planet’s
closest approach to the Sun each year than does at its farthest departure from
the Sun. Currently, Earth’s eccentricity is near its least elliptic (most
circular) and is very slowly decreasing, in a cycle that spans about 100,000
years.
-
- The total change in global annual insolation
due to the eccentricity cycle is very small. Because variations in Earth’s
eccentricity are fairly small, they’re a relatively minor factor in annual
seasonal climate variations.
-
------------------- Obliquity, the angle Earth’s axis of
rotation is tilted as it travels around the Sun is known as obliquity.
Obliquity is why Earth has seasons. Over the last million years, it has varied
between 22.1 and 24.5 degrees with respect to Earth’s orbital plane. The
greater Earth’s axial tilt angle, the more extreme our seasons are, as each
hemisphere receives more solar radiation during its summer, when the hemisphere
is tilted toward the Sun, and less during winter, when it is tilted away.
-
- Larger tilt angles favor periods of
deglaciation (the melting and retreat of glaciers and ice sheets). These
effects aren’t uniform globally, higher latitudes receive a larger change in
total solar radiation than areas closer to the equator.
-
- Earth’s axis is currently tilted 23.4
degrees, or about half way between its extremes, and this angle is very slowly
decreasing in a cycle that spans about 41,000 years. It was last at its maximum
tilt about 10,700 years ago and will reach its minimum tilt about 9,800 years
from now.
-
- As obliquity decreases, it gradually helps
make our seasons milder, resulting in increasingly warmer winters, and cooler
summers that gradually, over time, allow snow and ice at high latitudes to
build up into large ice sheets. As ice cover increases, it reflects more of the
Sun’s energy back into space, promoting even further cooling.
-
--------------------- Precession, as Earth rotates, it wobbles
slightly upon its axis, like a slightly off-center spinning toy top. This
wobble is due to tidal forces caused by the gravitational influences of the Sun
and Moon that cause Earth to bulge at the equator, affecting its rotation. The
trend in the direction of this wobble relative to the fixed positions of stars
is known as axial precession. The cycle of axial precession spans about
25,771.5 years.
-
- Axial precession makes seasonal contrasts
more extreme in one hemisphere and less extreme in the other. Currently
perihelion occurs during winter in the Northern Hemisphere and in summer in the
Southern Hemisphere. This makes Southern Hemisphere summers hotter and
moderates Northern Hemisphere seasonal variations.
-
- In about 13,000 years, axial precession
will cause these conditions to flip, with the Northern Hemisphere seeing more
extremes in solar radiation and the Southern Hemisphere experiencing more
moderate seasonal variations.
-
- Precession does affect seasonal timing
relative to Earth's closest/farthest points around the Sun. However, the modern
calendar system ties itself to the seasons.
The Northern Hemisphere winter will never occur in July. Today Earth’s
North Stars are Polaris and Polaris Australis, but a couple of thousand years
ago, they were Kochab and Pherkad.
-
- There’s also “apsidal precession”. Not only
does Earth’s axis wobble, but Earth’s entire orbital ellipse also wobbles
irregularly, primarily due to its interactions with Jupiter and Saturn. The
cycle of apsidal precession spans about 112,000 years. Apsidal precession
changes the orientation of Earth’s orbit relative to the elliptical plane.
-
- The combined effects of axial and apsidal
precession result in an overall precession cycle spanning about 23,000 years on
average.
-
- The small changes set in motion by
Milankovitch cycles operate separately and together to influence Earth’s
climate over very long timespans, leading to larger changes in our climate over
tens of thousands to hundreds of thousands of years.
-
- Milankovitch combined the cycles to create
a comprehensive mathematical model for calculating differences in solar
radiation at various Earth latitudes along with corresponding surface
temperatures. The model is sort of like a climate time machine: it can be run
backward and forward to examine past and future climate conditions.
-
- Milankovitch assumed changes in radiation
at some latitudes and in some seasons are more important than others to the
growth and retreat of ice sheets. In addition, it was his belief that obliquity
was the most important of the three cycles for climate, because it affects the
amount of insolation in Earth’s northern high-latitude regions during summer.
-
- He calculated that Ice Ages occur
approximately every 41,000 years. Subsequent research confirms that they did
occur at 41,000-year intervals between one and three million years ago. But
about 800,000 years ago, the cycle of Ice Ages lengthened to 100,000 years,
matching Earth’s eccentricity cycle.
-
- Milankovitch’s work was supported by other
researchers of his time, and he authored numerous publications on his
hypothesis. But it wasn’t until about 10 years after his death in 1958 that the
global science community began to take serious notice of his theory. In 1976,
using deep-sea sediment cores found that Milankovitch cycles correspond with
periods of major climate change over the past 450,000 years, with Ice Ages
occurring when Earth was undergoing different stages of orbital variation.
-
- Several other projects and studies have
also upheld the validity of Milankovitch’s work, including research using data
from ice cores in Greenland and Antarctica that has provided strong evidence of
Milankovitch cycles going back many hundreds of thousands of years.
-
June
4, 2023 SUN
- cycles and changes? 4035
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--- Sunday, June 4, 2023
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