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First, Earth spins around on its axis once every
day è The Tilt. |
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Second,
Earth revolves around the Sun once a year è The
shape of the Orbit. |
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Both the tilt and the shape of the orbit have
changed over time and produce three types of orbital variations: |
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(1)
obliquity variations |
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(2)
eccentricity variations |
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(3)
precession of the spin axis. |
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At present-day, the axis is tilted at an angle
of 23.5°, referred to as Earth’s “obliquity”, or “tilt”. |
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The Sun moves back and forth through the year
between 23.5°N and 23.5°S. |
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Earth’s 23.5° tilt also defines the 66.5°
latitude of the Artic and Antarctic circles. No sunlight reaches latitudes
higher than this in winter day. |
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The tilt produces seasons!! |
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The position in which the Earth is closest to
the Sun is called “perihelion”. |
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Perihelion means “near the Sun” in Greek. |
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The position in which the Earth is farthest to
the Sun is called “aphelion”. |
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Aphelion means “away from the Sun” in Greek. |
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Seasons |
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Solstices: mark the longest and shortest days of the years (June 21
and December 21 in the northern hemisphere, the reverse in the southern) |
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Equinoxes: the length of night and day become equal in each
hemisphere. |
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At the present-day orbit, the winter and summer
solstices differ from the aphelion and perihelion by about 13 days. |
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Over time, the tilt angle varies in a narrow
range. |
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These variations are caused by the gravitational
tug of large planets, such as Jupiter. |
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The present-day value of the tile is decreasing. |
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Cyclic changes in the tilt angle occur at a
period of 40,000 years. |
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Today’s eccentricity is 0.0167, lies well toward
the lower end of the variation range of Earth’s eccentricity (closer to
circular). |
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The long-term variations in orbital eccentricity
are concentrated at two periods: 100,000 years and 413,000 years. |
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There are two kinds of precession: (1) the
precession of the spin axis and (2) the precession of the ellipse. |
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Earth’s wobbling motion is called the axial
precession. It is caused by the
gravitational pull of the Sun and Moon. |
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The combined effects of these two precessions
cause the solstices and equinoxes to move around Earth’s orbit, completing
one full 360° orbit around the Sun every 23,000 years. |
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Ice sheets reacted strongly to insolation
changes. |
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Summer insolation control the size of ice sheet
by fixing the rate of ice melting. |
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Milankovitch suggested that the critical factor
for Northern Hemisphere continental glaciation was the amount of summertime
insolation at high northern latitudes. |
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Low summer insolation occurs during times when
Earth’s orbital tilt is small. |
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Low summer nsolation also results from the fact
that the northern hemisphere’s summer solstice occurs when Earth is
farthest from the Sun and when the orbit is highly eccentric. |
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A closer look of the last 150,000 years of the d18O
record shows 23,000-year and 41,000-year cycles. |
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The longest orbital-scale record of CO2 changes
comes from the Vostok ice core drilling site. |
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The CO2 record shows a series of regular
oscillation between CO2 values as high as 280-300 ppm and as low as 180-190
ppm over the last 400,000 years. |
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The dominant period of CO2 variations is about
100,000 years. |
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The Vostok ice record shows a series of cyclic
variations in methane concentration, ranging between 350 to 700 ppb (part
per billion). |
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Each Ch4 cycle takes about 23,000 years. |
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Air moves freely through snow and ice in the
upper 15 m of an ice sheet. |
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Flow is increasingly restricted below this
level. |
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Bubbles of old air are eventually sealed off
completely in ice 50 to 100 m below the surface. |
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The best place on an ice sheet to take ice cores
is at the top. |
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Ice cores can be dated by counting annually
deposited layer (or ice flow model). |
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Annual layering is recorded in several
properties of ice cores, the most obvious of which are layers of dust
easily visible to the eye. |
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Dust is usually deposited at the end of cold,
dry windy winters. |
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One of the most famous ice core drilling is at a
site high on the Antarctic ice sheet, which is called the Vostok ice
record. |
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