This video segment begins with an examination of climate forcing factors.
Forcing factors such as sunlight and greenhouse gases are responsible for climactic variations
that Earth has experienced throughout its history.
To predict the future verse climate requires a thorough understanding of forcing factors
and how they are likely to change.
Forcing factors fall into two categories, external and internal.
External forcing factors involve agents outside of Earth and its atmosphere.
By contrast, internal forcing factors originate right here on Earth.
Variations in sunlight for example are external, where variations in greenhouse gas concentrations
are internal.
This segment focuses on external forcing factors.
Its major points are: number one, external forcing factors include galactic variations,
solar orbital variations, and sunspots.
Number two, galactic variations occur over 100's of millions of years and thus are uncertain.
Number three, sunlight impinging on Earth depends on the Cosine Law, Kepler's Second
Law, and the Inverse Square Law.
Four, sunlight varies with the obliquity, eccentricity, and precession of the solar
orbit.
Number five, solar orbital variations may explain glacial and interglacial periods.
Last, sunspots have a surprisingly strong influence on climate.
Earth is a planet orbiting the Sun, a yellow dwarf star in a spiral galaxy called the Milky
Way.
Here's a photograph of our galaxy.
Who took this picture?
Actually it's a photograph of a galaxy that is similar in size and shape to the Milky
Way.
The Milky Way rotates around a central axis, completing a rotation every 150 to 350 million
years.
The quantity and quality of energy reaching Earth from nearby star systems and the interstellar
medium undoubtedly varies during this rotation.
Several major climactic events such as ice ages, the KT boundary, the Great Dying, and
Ordovician and Silurian extinction are separated by about 150 to 190 million years; might be
coincidental with a period of galactic rotation.
Yet the galactic period is so long and so uncertain so as to obscure its influence on
Earth's climate.
The major driver of Earth's climate is the amount of sunlight that it receives.
Three laws- Cosine law, Kepler's second law, and Inverse Square law relate the amount of
sunlight impinging on Earth to it's orientation with the sun.
The first is the Cosine law.
Sunbathers instinctively obey this law in orienting themselves perpendicular to the
sun, to maximize the interception of solar rays, or more parallel to the sun to decrease
interception.
The sharper the angle between the sunbather and the sun, the greater the area over which
a given amount of solar energy is distributed and thus, the smaller amount of energy per
unit area.
Mathematically, I equals I sub zero cosine theta, where I equals the solar energy per
unit area.
I sub zero equals the incoming solar energy and theta is the angle of incidence.
In the sunbathing example, the sun bather perpendicular to the sun is I equals I sub
zero, cosine angle of zero degrees equals I sub zero.
The sunbather parallel to the sun is I equals I sub zero, cosine 90 degrees which is equal
to zero.
The cosine law explains many of the differences in climate between the equator and the poles.
At the spring and autumn equinoxes, March twentieth and September twenty-second the
sun is at midday, directly over the equator, and focused on a relatively small area.
Whereas near the poles it is at an oblique angle and spread over a relatively large area.
Moreover at the oblique angle near the poles.
Solar energy passes a longer distance through the atmosphere and is more likely to be reflected
or absorbed before striking the surface.
The reflectance of many materials increase that of an oblique angle.
For example, sunsets reflecting off the water.
For all of these reasons, the poles receive less solar energy per unit area than the equator.
The cosign law also explains differences among the seasons.
Earth rotates daily around an axis that is oriented at an angle of 23.4 degrees away
from its plane of the sun.
At the equinoxes the sun is directly over the equator.
But at the solstices, June twenty-first or December twenty-first, the sun is directly
above the Tropic of Cancer, 23.4 degrees north, or the Tropic of Capricorn 23.4 degrees south.
Therefore on June twenty-first, the Tropic of Cancer receives full sunlight, and the
Tropic of Capricorn receives 68.5 percent of full sunlight.
On December twenty-first, the reverse applies.