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Lecture 14Astronomical Control of
InsolationIntro to Orbital-Scale Climate
Change

• Earths orbit is variable
• a major control on climate
• time scales of 10s to 100s of thousands of
  years
• Well focus on past 3 Myr
• Insolation solar energy received at top of
  Earths atmosphere

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Tilt of Earths Spin Axis, I

• Explains
• Tropics of Cancer Capricorn (23.5 latitude)
• Where sun directly overhead at noon on longest
  day
• Artic, Antarctic Circles (66.5 latitude)
• No sunlight at higher latitudes, shortest winter
  day

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Tilt of Earths Spin Axis, II

• Angle and direction of tilt constant in short
  term
• not true over longer term of 1000s of years
• Responsible for seasons
• Solstices shortest longest days of year
• 6/21 and 12/21
• Equinoxes when length of days nights equal
• 3/20 and 9/22

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Earths Elliptical Orbit

• Effect on insolation is small (3)
• Seasons mostly due to tilt
• Timing of equinoxes not symmetric

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Sidebar Cycles, Simple and Complex

• Use wave terminology for cycles
• Wavelength, period, frequency, amplitude
• Modulation refers to cyclic change in amplitude

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Change in Tilt Through Time, I

• What is effect of changing tilt? Seasonality
• Easiest to think of extremes (left)
• Reality is more subtle (right)
• Greater tilt higher insolation to summer poles,
  lower to winter poles.

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Change in Tilt Through Time, II

• Tilt cycles between 22.2 and 24.5
• Cause is gravity of larger planets
• Occurs with period of 41,000 years
• Today, tilt is at midpoint and decreasing

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Change in Orbit Through Time

• Shape of Earths orbit not constant
• Main cycles have periods of 100,000 years and
  413,000 years

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Precession of Solstices Equinoxes, I

• Axial Precession Earths spin axis wobbles like
  a top.
• A matter of direction, not amount of tilt

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Precession of Solstices Equinoxes, II

• Precession of the Ellipse Earths elliptical
  orbit precesses through space.
• i.e., perihelion occurs at different points in
  orbit (fixed frame of reference)

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Precession of Solstices Equinoxes, III

• Over time, Earth-sun distance has varied for each
  season
• Net result is precession of equinoxes around
  orbit, with period 23,000 years

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Precession of Solstices Equinoxes, IV

• Get many different combinations for
  distance-from-sun and timing-of-solstice
• Cyclic effects on insolation, seasonality

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Eccentricity ModulatesTilt Precession Cycles

• Net effect is a modulation, not a cycle

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Changes in Insolation Depend on Latitude

• Insolation can vary as much as 12 around mean
  value
• Tilt matters at high lat.
• Precession matters at low lat.

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Time Series, I

• Complex signals not intuitive
• Spectral analysis is a mathematical means of
  deciphering wiggly time series
• Filtering can be used to isolate a single period

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Times Series, II Aliasing

• Aliasing false trends created by undersampling a
  time series
• Need at least two samples per cycle

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Remaining Questions

• Do orbital influences show up in the climate
  record?
• Why do they show up?
• Things we arent covering in Ch. 8
• Figure 8-12, Box 8-2, Figures 8-18 through 8-20
• The related text about phasing of insolation