Earth Sun Geometry
• Define/describe aphelion, perihelion, North Star, plane of the ecliptic
• Trace the path of the perpendicular rays of the sun over the surface
of the Earth over a year
• Explain how tilt generates the march of the seasons
• Explain how tilt shapes sun angle and the length of daylight
• Explain how sun angle controls the intensity and amount of
insolation
• Describe how insolation patterns vary for the tropics, the
midlatitudes, and the poles over a year for both the Northern and
Southern hemispheres
• Sketch a conceptual model for the march of seasons
• Define the components of Milankovitch orbital cycles
• Define/describe interglacial, glacial, Pleistocene, Holocene
• Explain how the last glacial maximum and Holocene warming have
shaped sea levels
• Describe how weak seasonal contrasts create glacial conditions
• Describe how strong seasonal contrasts create interglacial
conditions
Earth-Sun Geometry
• Driving variable of environmental processes on
Earth
• Geometry determines the amount and intensity
of incoming solar radiation (insolation) reaching
particular earth
• Geometry (and how it changes) determines:
– Seasonality (1 yr)
– Glacial (cold) and interglacial (warm) periods (1000’s
of years)
Which determines march of the
seasons?
• Tilt?
• Aphelion and
perihelion?
Tilt is more relevant for
the march of the seasons.
What determines the march of
the seasons?
•
•
•
•
Tilt and solar radiation
Headlights effect
Flashlight effect
Variation in day
length over a year
• Changing sun angles
over a year
• Greater heating of the
tropics
• Radiation imbalance
Ultimate causes
Proximate causes
North Star
Southern Cross
In conjunction with the curvature of the Earth, tilt
determines the concentration and distribution of insolation
striking the earth
Flashlight effect
Headlights effect
Flashlight effect
TROPICS
MIDLATITUDES
High sun angle:
Larger concentration
of insolation per area
Intermediate sun angle
HIGH LATITUDES
Low sun angle:
Smaller concentration of
insolation per area.
Headlights effect
As the Earth revolves around the
Sun, tilt creates variation in day
and night lengths over the duration
of a calendar year
Circle of illumination
What day of the year is this?
Over the course of a calendar year, the Earth’s tilt
creates variation in the angle between the horizon and
the Sun. Maximum sun angles occur summer,
minimums in winter.
Solar declination: latitude that receives direct overhead (90 degrees) insolation.
Declination, migrates annually from Tropic of Cancer (+23.5° N) to Tropic of
Capricorn (-23.5°S). Locations between these two latitudes are the only locations
on Earth that get sunlight directly overhead at some point during the year.
But what is a more proximate explanation
for the march of the seasons?
…the energy
imbalance
between tropics
and poles drives
circulation of
atmosphere and
ocean.
As this radiation shifts back forth over a year (in
response to the Earth revolving around the Sun), we
experience it as the march of the seasons – the change
from warm to cold, from summer to winter
http://www.vets.ucar.edu/vg/thornton/
movies/ustmax1997.mpg
Long-term variations in Earth-Sun
geometry
• Contribute to alternating
climates:
– Interglacial (warm)
– Glacial (cold)
• Have profound effects on
sea-level
• Pleistocene: approximately 2
million year period of glacials
and interglacials
Pleistocene was not an “Ice
Age”, but a two million year
period of oscillating warm and
cold climates.
Interglacials and glacial period
Maximum extent of glaciation during the
Pleistocene - 1/3 of the Earth’s land surface
Important dates for natural climate
change
• Wisconsin glaciation
– Last glacial maximum
in North America
– Peaked 8,000 ybp
(years before present)
– Boreal forests
extended as far south
as Atlanta and
Birmingham, Alabama.
Important dates for natural climate
change
• Holocene interglacial
(10,000 ybp)
– Return of warmer
conditions
Milankovitch Cycles
• Name of the geometric changes that
influence Earth-Sun geometry over long
temporal scales
• Three components:
– Orbital eccentricity
– Obliquity
– Precession
Milankovitch cycles shape
interglacials and glacial periods
1. Orbital eccentricity
• Distance between Earth & Sun changes
over scale of ~100,000 years
• This changes length of seasons
2. Obliquity
• Tilt varies between 22-24.5 degrees over a
time scale of ~40,000 years.
• Present tilt of 23.5 degrees can be
considered unchanging from your scale of
observation
3. Precession
• Earth wobbles on its
axis
• Physics of a spinning
object
• Point in orbit where
aphelion and
perihelion varies
slightly
• Occurs over times
scales of ~20,000
years
Components of
Milankovitch Cycles
Mechanism?
Milankovitch cycles create weaker or
stronger seasonal contrasts.
• Weaker seasonal contrasts►glacial
climates dominate
• Stronger seasonal contrasts►interglacial
climates dominate
Interglacial (warm) epoch
• Strong seasonal contrasts
– Highly elliptic orbit and large tilt
– Cold winters: less evaporation: less snow: less glacial
accumulation
– Hot summers: more glacial melting
– Net loss of glacial extent and warmer temperatures
– Sea levels rise
Glacial (cold) epoch
• Weak seasonal contrasts
– Less elliptic orbit and small tilt
– Warm winters: more evaporation: more snow: more
glacial accumulation
– Cool summers: less glacial melting
– Net gain of glacial extent and cooler temperatures
– Sea levels drop
Visualization of impacts of
Milankovitch cycles on
total solar radiation over
15,000 year period
Large contrasts in color
across the globe reflect
greater seasonality – and
interglacial conditions
When the color is uniform,
seasonal contrasts are low –
conditions for a glacial period
Note weak contrasts in
seasonality roughly 18,000
years ago (last pulse of glacial
expansion during the
Wisconsin glaciation) and
strong contrasts in seasonality
10,000 years ago (beginning
of warming that marked start
of Holocene)