Combining the influences of the three orbital factor of obliquity, eccentricity, and precession,
one can reconstruct a history of solar insolation, the amount of solar energy reaching Earth
per unit area.
Solar insolation oscillates with the periods of its components; 41,000 years obliquity,
100,000 years eccentricity, and 21,000 years precession.
At the beginning of the twentieth century estimates of these periods and those of Earth's
glacial cycles became sufficiently accurate to suggest a causal relationship between Earth's
radiation balance and climate.
Solar insulation in the northern hemisphere has a greater effect than that of the Southern
Hemisphere because the northern hemisphere currently has about 65 percent of Earth's
land mass.
And land absorbs much more solar energy than ocean.
Therefore, when the northern hemisphere receives more solar energy, so does the planet as a
whole.
The Serbian astrophysicist, Melanin Melankovich 1879-1958, proposed that when the solar insulation
in the northern hemisphere was relatively high during the December, January period and
relatively low during the June, July period, more snow accumulated during the winter, and
less melted during the summer causing glaciers to advance and an ice age to occur.
How well does this theory fit the observed climatic variations?
The Delta eighteen oxygen of sea bottom sediment cores a temperature proxy, oscillates with
periodicity that reflects changes in obliquity and eccentricities.
The influence of precession on Delta eighteen oxygen is less evident.
Changing the alignment between the sun's rotational axis and the gravitational center of the solar
system produces intense fluctuation in vertical magnetic fields of the sun.
These divert heat flow from deeper layers in the sun and generate patches of fluctuating
temperatures on the surface that manifest as sunspots.
Although Chinese astronomers recorded the presence of sunspots as early as 28 B.C.,
systematic counts of sunspots began with the invention of the optical telescope.
They show that the number of sun spots varies with about an eleven year period as well as
with some less predictable longer cycles.
Measurements from spacecrafts since 1978 affirm that total solar energy oscillates with 0.05
percent to 0.07 percent in synchrony with sunspot number.
Variation in solar energy of this magnitude should account for only about a 0.03 degree
Celsius change in global temperatures.
Nonetheless sunspot number and mean surface temperature of the terrestrial northern hemisphere
are positively correlated.
For example the period from 1645 to 1750 the middle of the little ice age, when Europe,
North America, perhaps much of the world suffered bitterly cold winters was largely devoid of
sunspots.
Do sunspots significantly affect earth's climate and might they be responsible for some of
the recent climate anomalies?
The current consensus is that the influence of sunspots is far smaller than that of many
other forcing factors such as Earth's atmospheric composition.
This segment has focused on external forcing factors.
Some major points were: number one, external forcing factors include galactic variations,
solar orbital variations and sunspots.
Number two, galactic variations occur over hundreds of millions of years and thus are
uncertain.
Sunlight impinging on Earth depends on the Cosine law, Kepler's Second law and Inverse
Square law.
Sunlight varies with obliquity, eccentricity and precession of the solar orbit.
Number five, Solar orbital variations may explain glacial and interglacial periods.
Lastly, number six, sunspots have a surprisingly strong influence on climate.