Climate Cycles and Cycles in Earth’s Orbit
Cycles give predictability because they repeat at regular intervals. One of the major
climate discoveries is that for the past 800,000 years, ice ages have repeated at more or
less regular intervals roughly every 100,000 years and that these cycles match one of
the cycles in the Earth’s orbit about the Sun. Earth’s orbit changes slowly over time as
the Moon, Sun, and Jupiter tug on it. The orbital changes alter the amount and
distribution of sunlight striking Earth. For example, at present, Earth is closest to the Sun
on January 3 but 11,000 years ago, Earth was closest to the Sun in July and the North
Hemisphere received 7% more sunlight per summer day than it does now.
The critical factor that starts and ends ice ages appears to be the strength of the Sun in
summer at high latitudes (about 65º) in the North Hemisphere because that is where the
major ice sheets grow and shrink. When the summer Sun of the North Hemisphere is
strong, winter snows melt entirely and ice sheets melt. Only when the summer Sun of
the North Hemisphere is weak does weather in the summer remain cold enough so that
the winter snows do not completely melt, and therefore can accumulate.
The three cyclic variations of Earth’s orbit are:
1: Precession of the Equinoxes (Season when Earth is closest to the Sun)
2: Obliquity of the Ecliptic (Tilt of Earth’s Rotation Axis)
3: Eccentricity of Earth’s Orbit
The next slides show sediments with cyclic variations due to the Ice Age cycles and an
EKG showing cycles of the human heart.
http://en.wikipedia.org/wiki/Astronomical_theory_of_paleoclimates
1. Precession of the Equinoxes. As the Earth spins and circles the Sun it precesses or
gyrates like a tilted top. As a result, the day Earth is nearest to the Sun slowly advances
with respect to the equinoxes. Right now we are closest to the Sun on January 3, but in
roughly 60 years it will be Jan 4. In about 5500 years Earth will be closest to the Sun on
April 3, and 11,500 years from now it will be closest to the Sun on July 4. Precession has
a complex cycle about 23,000 years long, but at least one simple climate impact--it
made the Sahara a fertile grassland up to 5500 years ago!
2. Obliquity of the ecliptic. This is the fancy term for the tilt of the Earth’s axis of
rotation. Right now the tilt is 23.47°, but it gets as small as 22° and as large as 24.5°,
repeating with a cycle of about 41,000 years. The tilt is now decreasing and will reach a
minimum about 10,000 years from now. The smaller the tilt, the smaller the differences
between the seasons and the cooler the high latitudes in summer.
3. Eccentricity. Earth’s orbit is an ellipse that can be more circular or more eccentric.
Eccentricity is how much the distance varies from average. The eccentricity of Earth’s
orbit is now 1.67% because its distance from the Sun varies by 1.67% (2.5 million km)
from the average of 149.5 million km. Eccentricity varies from almost 0% to as much as
5%, repeating with two cycles, one about 108,000 years and the other, 412,000 years. It
is now decreasing and will reach a minimum in about 25,000 years.
Distance from the Sun also affects the length of the seasons. When Earth is closest to
the Sun it moves fastest in its orbit, and the seasons are shorter. At present there are
186 days in the summer half of the year (March 21 to September 23) and only 179 days
in the winter half when Earth is closest to the Sun. But 11,500 years from now, summer
vacations in the North Hemisphere will be shorter.
Some sedimentary rocks
have cycles of deposition,
due to cycles of climate.
A normal EKG (electrocardiograph) for a 26 year-old male. This shows the
complex but regular cycle of heartbeats. Ice age cycles are also complex but
not quite so regular.
Record of the Vostok Ice Core including atmospheric CO2, CH4, 18O (determined from
trapped air bubbles in the ice) compared with solar Insolation in June at 65ºN latitude.
A 5.5 Million year record of 18O of shells in a sediment core from the sea floor in the
tropics. Because the temperature of tropical waters haven’t changed that much, this is
essentially a record of the volume of the ice sheets on land. When the blue curve is
lower (more enriched 18O in the ocean) ice volume is larger. Note that ice volume
began increasing about 3 million years ago and had high frequency variations. At first
these variations repeated with rough cycles about 41,000 years but for the past
800,000 years, the cycles have slowed to about 100,000 years. The match of climate
with a 100,000 year cycle is shown by the red dashed lines. It is certainly not perfect.
Eccentricity of Earth’s Orbit  100,000 and 400,000 year cycles
Obliquity of the Ecliptic = Tilt of the Earth’s Axis of Rotation
 41,000 year cycle
Precession:  23,000 Year Cycle
Polaris
Vega
Now
Polaris
Vega
11,500 kY
11000 BP
PRESENT
Past and Future
Milankovitch Cycles
We can predict past and
future orbital parameters with
great accuracy. ε is obliquity
(axial tilt). e is eccentricity. ϖ
is longitude of perihelion.
esin(ϖ) is the precession
index, which together with
obliquity,
controls
the
seasonal cycle of insolation.
QAVG(day) is the calculated
daily-averaged insolation at
the top of the atmosphere, on
the day of the summer
solstice at 65 N latitude.
Benthic forams and Vostok
ice core show two distinct
proxies for past global sea
level and temperature, from
ocean sediment and Antarctic
ice respectively. Vertical gray
line is current conditions, at 2
ky A.D.
Ice Age Cycles and Summer Sunshine
Insolation around the Summer Solstice at about 65º N latitude has been identified as a
critical driver of Ice Ages. Ice Ages end when summer sunshine is large and cools to
Ice Age conditions when summer sunshine is small. Summer in the Northern
Hemisphere is now as cool as it will be for the next 50,000 years. Since it is apparently
not cool enough to allow an ice age to start (possibly because we have added CO2 and
CH4), it is unlikely that the next Ice Age will start for at least another 50,000 years.
Would it be cool enough to have started an ice age now if we hadn’t increased CO2
and CH4? That is a tough question because the present minimum value of summer
Insolation is nowhere near as low as 23,000 years ago.
Right Now
Post Ice Age Melting
Depth of Last Ice Age
Next Chance for an Ice Age
How long should it take to melt
the ice sheets?
Absorbed Solar
Irradiance
Melts Ice
Sheets
When sunlight shines on ice, much is
reflected but some is absorbed. After the
absorbed heat warms the ice to the
melting point it begins to melt the ice.
Each extra 1 Wm-2 of absorbed sunlight
will melt 0.0086 m of ice (with density,
900 kg m-3) per month.
To estimate the time to melt an ice sheet
2 km thick assume that in the millennia
after the last ice age that ended 20KBP,
solar irradiance increased  20 W m-2
during the 5 spring and summer months
(Apr – Aug). If the albedo of the dirty,
melting ice was 50% then 10 W m-2 was
used to melt the ice. Then the monthly
melting rate was 0.086 m and the annual
melting rate was 50.086 m = 0.43 m yr-1.
Height
Rate, R 
t
H 2000
t

 4650 y
R 0.43