Chapter 7:
Orbital Scale Climate Change
•Changing time
scales – Tectonic
to Orbital
•Earth’s orbit and
it variations
•Insolations –
Radiation received
from Sun
•Times series data
and its analysis
Earth’s Rotation and Revolution
• The 23.5o axial tilt results in seasons
– Increase tilt angle stronger seasonal variations
Elliptical Orbit
• Eccentricity – measure of the “flatness” of an
ellipse. Greater eccentricity means greater
difference between perihelion and aphelion
Variations in Earth’s Orbit
Axial Tilt
• Obliquity Range : 22.2o to
24.5o ; current 23.5o
• 41,000 year cycle
• Amplifies or suppresses
seasonal variation
– Great tilt means stronger
seasonal variation
• Most noticeable at high
latitudes
• Wobbles with a 26,000 year
period
Variations in Earth’s Orbit
Elliptical Path
• Eccentricity - .005 to .06 (0 a circle; 1 a line);
currently .017
• Controls the difference between perihelion
and aphelion distance
– can range from 1/3 of today up to 4 time today
• 100,000 and 413,000 year cycles
Variations in Earth’s Orbit
Precession of the Equinoxes
• Direction of axial tilt changes (not just angle)
• Direction of the elongation of ellipse changes
• The two combine to cause “Precession of the
Equinoxes”
– Do solstices or equinoxes occur at perihelion and
aphelion? Current we are closer to solstices.
• Cycle is 23,000 years
Period Cycles to Remember
• 23,000 years – precession of the equinoxes
• 41,000 years – axial tilt
• 100,000 and 413,000 – eccentricity
– 100,000 dominates and the 413,000 amplifies
every 4th one
• Extreme points
– Smaller seasonal variations: circular orbit, low tilt
angle, equinoxes at perihelion and aphelion
– Largest seasonal variations: elongate orbit, high
tilt, solstices at perihelion and aphelion
Precessional Index
(don’t sweat the math on this one)
• Combine all the variation
to get the Precessional
Index.
– Dominated by the 23,000
and 100,000 year cycles
Insolation
• Amount of solar
radiation that
strike the top of
Earth’s
atmosphere
• Varies with
latitude, season
and as orbital
parameters
change
Isolation Changes and Orbital Changes
• Between 45N and 45S latitude the 23,000 year
precessional cycle controls insolation (tilt
variation have opposite effects in each
hemisphere and cancel each out globally)
• At high latitudes the 41,000 year axial tilt cycle
controls insolation – high tilt, both poles have lots
of sun in summer and none in winter; low tilt,
both poles have less extremes – both poles have
extremes or milds seasona at the time in the
cycle – can cause global effects.
Longer term orbital changes
(millions of years)
• Moon is slowly moving away from Earth (gravitation
drag effect)
• This cause Earth’s rotation to slow (day length
increases)
• Precession and axial title cycles related to rotation
rate, so they have also slowed
• Tilt cycle today: 41 k; 400 MYA: 31 k
• Precession cycle today: 23 k; 400 MYA: 20 k
• So data from the distant past will have to be
analyzed with these cycle variations in mind
Data (Time Series) Analysis
Fourier Analysis
• Climate data is indexed to time
• We look for patterns (cycles) in these data
• All cycles will be combined or modulated
(tangled together)
– Like hearing the symphony with all the different
instruments
• To find the individual cycles with must
“untangle” the signals – Fourier Analysis
Fourier Analysis
Untangling complex signals