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  • 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.

Combining the influences of the three orbital factor of obliquity, eccentricity, and precession,

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