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.