Earths Climate Past and Future

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Earths ClimatePast and Future

• Ch. 8
• Orbital-Scale Climate Change

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Astronomical Control of Solar Radiation

• Earths Orbit Today
• The geometry of Earths present-day orbit forms
  the basis for understanding past changes in
  Earth-Sun geometry.
• The larger frame of reference for understanding
  Earths orbit is the plane in which it moves
  around the Sun, the plane of the ecliptic (Figure
  8-1).

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Figure 8-1Earths tilt Earths rotational (spin)
axis is currently tilted at an angle of 23.5
away from a line perpendicular to the plane of
its orbit around the Sun.
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8-1 Earths Tilted Axis of Rotation and the Seasons

• Two fundamental motions describe the present-day
  orbit. First, Earth spins around on its axis once
  every day.
• Earth rotates around an axis (or line) that
  passes through its poles (Figure 8-1).
• This axis is tilted at an angle of 23.5,
  referred to as Earths obliquity, or tilt.

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• The second fundamental motion that describes
  Earths present-day orbit is its once-a-year
  revolution around the Sun.

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• We experience the results of this kind of motion
  as seasonal shifts between long summer days, when
  the Sun rises high in the sky and delivers
  intense radiation, and short winter days, when
  the Sun stays low in the sky and delivers weaker
  radiation.

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• These seasonal differences culminate at the
  summer and winter solstices, which mark the
  longest and shortest days of the year (June 21
  and December 21 in the northern hemisphere, the
  reverse in the southern hemisphere).

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• During each yearly revolution around the Sun,
  Earth maintains a constant angle of tilt (23.5)
  and a constant direction of tilt in space.
• When a hemisphere (northern or southern) is
  tilted directly toward the Sun, it receives the
  more direct radiation of summer.
• When it tilts directly away from the Sun, it
  receives the less direct radiation of winter.
• At both times (and at all times of the year) it
  keeps the same 23.5 tilt.

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• Earths 23.5 tilt also defines the 66.5
  latitude of the Arctic and Antarctic circles by
  this relationship 90 - 23.5 66.5.
• Because of Earths 23.5 tilt away from the Sun
  in winter, no sunlight reaches latitudes higher
  than 66.5on the shortest winter day (winter
  solstice).
• Between the winter and summer solstices, during
  intermediate positions in Earths revolution
  around the Sun, the lengths of night and day
  become equal in each hemisphere on the equinoxes
  (which means equal nights that is equal to
  the days).

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• The two equinoxes and two solstices are referred
  to as the four Cardinal points in Earths orbit.

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8-2 Earths Eccentric Orbit Changes in the
Distance between Earth and Sun

• Earths actual orbit (Figure 8-2) is not a
  perfect circle it has a slightly eccentric or
  elliptical shape. The noncircular shape of
  Earths orbit is the result of the gravitational
  pull of other planets on Earth as it moves in
  space.

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• Figure 8-2

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• Basic geometry tells us that ellipses have two
  focal points, in contrast to the single central
  focus of a circle.
• In earths case. The Sun lies at one of the two
  focal points in its elliptical orbit, as required
  by the physical laws of gravitation.
• The other focus is empty (Figure 8-2).

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• In this elliptically shaped orbit, the distance
  to the Sun changes with Earths position in its
  orbit.
• As you might expect, this changing distance has a
  direct effect on the amount of solar radiation
  Earth receives.
• The simplest way to describe this elliptical
  orbit is to examine its two extremes.

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• The position in which Earth is closest to the Sun
  is called perihelion (the close pass position,
  from the Greek meaning near the Sun).
• The position farthest from the Sun is called
  aphelion (the distant pass position, from the
  Greek meaning away from the Sun).

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• On average, Earth lies 155.5 million kilometers
  from the Sun, but the distance ranges between 153
  million km. at perihelion and 158 million km. at
  aphelion.
• The occurrence of this close-pass position in
  January causes winter radiation in the northern
  hemisphere and summer radiation in the southern
  hemisphere to be slightly higher than they would
  be in a perfectly circular orbit.

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• The effect of Earths elliptical orbit on its
  seasons is small, enhancing or reducing the
  intensity of radiation received by only a few
  percentage points.

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• The main cause of the seasons is the direction of
  tilt of Earths axis in its orbit around the Sun
  (Figure 8-1).

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8-3 Changes in Earths Axial Tilt through
Time
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• We have already seen that the tilt of Earths
  axis creates our seasons.
• Another way of understanding why this is so is to
  consider two hypothetical cases, using the
  simplifying assumption that Earth has a perfectly
  circular orbit around the Sun.

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• For both cases, we examine the two seasonal
  extremes in Earths orbit the summer solstices,
  when solar radiation is most direct in each
  hemisphere, and the winter solstices, when solar
  radiation is least direct.

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• First we consider the case in which Earths axis
  is not tilted (Figure 8-3A).
• With no tilt, incoming solar radiation is always
  directed straight at the equator throughout the
  year, and it always passes by the poles at a 90
  angle.

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• With no tilt, no seasonal changes occur in solar
  radiation received at any latitude, including the
  polar regions.
• As a result, solstices and equinoxes do not even
  exist.
• Therefore . . . .

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• A TILTED Axis
• Is necessary
• For Earth to
• Have
• Seasons!

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• Next we consider the opposite extreme a maximum
  tilt of 90 (Figure 8-3B).
• In this case, solar radiation is directed
  straight at the summer-season pole, while the
  winter-season pole lies in complete darkness.

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• Six months later, the two poles have reversed
  position, moving between full-time darkness and
  sunlight.
• The difference between the two configurations
  shown in Figure 8-3 suggests that seasonal
  differences in solar radiation due to tilt should
  be particularly striking at polar latitudes.

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• Over time, the angle of the Earths tilt actually
  varies in a narrow range, cycling back and forth
  between values as small as 22.2 and as large as
  almost 24.5 (Figure 8-4).
• Figure 8-4 ?

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• The present-day tilt (23.5) is near the middle
  of the range, and the value is decreasing.
• Cyclic changes in tilt angle occur mainly at a
  period of 41,000 years, the time interval
  separating successive peaks or successive
  valleys.
• The cycles are fairly regular, both in period and
  amplitude.

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• Changes in tilt cause long-term variations in
  seasonal solar insolation received on Earth, with
  the largest changes at high latitudes.
• The main effect of these changes is to amplify or
  suppress the seasons
• Increased tilt amplifies seasonal differences,
  decreased tilt reduces them.

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• Larger tilt angles turn the summer-hemisphere
  poles more directly toward the Sun and increase
  the amount of solar radiation received (Figure
  8-5).

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• The increase in tilt that turns the North pole
  more directly toward the sun at its summer
  solstice on June 21st also turns the South pole
  more directly toward the Sun at its summer
  solstice six months later (December 21st).
• On the other hand, the increased angle of tilt
  that turns each polar region toward the Sun in
  summer also turns each winter-season pole away
  from the Sun, reducing the amount of solar
  radiation received (Figure 8-5).

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8-4 Changes in Earths Eccentric Orbit
through Time
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• The shape of an ellipse can be described by
  reference to its two main axes The major (or
  longer) axis and the minor (or shorter) axis
  (Figure 8-6).

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Long-term Changes in Eccentricity

• Eccentricity (E) has varied over time between
  values of 0.005 and nearly 0.0607 (Figure 8-7),
  and todays value of 0.0167 lies well toward the
  lower end of this range (close to circular).

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• These changes in orbital eccentricity are
  concentrated mainly at two periods, both of which
  are far more irregular than the 41,000-year tilt
  cycle studied earlier.
• One eccentricity cycle shows up clearly as
  variations between maxima and minima at a period
  near 100,000 years (Figure 8-7).

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• Figure 8-7

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• This cycle actually consists of four cycles of
  nearly equal strength and width periods ranging
  between 95,000 and 131,000 years, but these
  cycles blend and form a single cycle year near
  100,000 years.
• The second major eccentricity cycle is a cycle at
  a wavelength of 413,000 years.

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• This longer cycle is somewhat more difficult to
  see, but it shows up in the tendency of clusters
  of 100,000-year cycles to alternate between
  larger and smaller amplitudes (Figure 8-7).

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Figure 8-7