Division of Energy
Global Climate Change: Effect of the Earth's Orbit
Variations in the earth's orbit and inclination toward the sun cause cyclical variations in solar energy received by the earth and on the spatial distribution of solar energy on the earth's surface. The variations are believed to be the cause of historic climatic changes as well as those larger pre-historic climatic changes which are recorded in the geologic record. The predominant climatic effects of orbital variations are changes in seasonality.
The Earth travels around the sun in an elliptical orbit that varies in shape, with time from nearly circular to distinctly elliptical. This, combined with the fact that the sun is not the centroid of the earth's orbit, causes the distance from the Earth to the sun to vary. For instance, the current annual variation of this distance is between 147 million kilometers and 152 million kilometers. The distances can be as much as 142 million kilometers for a minimum and 156 million kilometers for a maximum. The amount of solar energy received by the earth is greatest when the earth is nearest the sun. This phenomenon is called the eccentricity of the orbit and has a 100,000 year cycle. This factor, combined with the tilt of the earth's axis, is believed to cause seasonal climate changes which are out of phase in each hemisphere. For instance, northern hemisphere winters are currently milder and summers cooler than normal. The opposite situation, colder winters and hotter summers, is now occurring in the southern hemisphere.
The earth's rotational axis is tilted with respect to the sun. This is called obliquity and is defined as the angle between the earth's orbit and the plane of the earth's equator. The tilt is toward the sun in the summer hemisphere and away from the sun in the winter hemisphere. The tilt of the earth changes cyclically between 21 3/4 degrees and 24 1/4 degrees. The period of cyclicity is 42,000 years. A large tilt warms the poles and causes smaller temperature differences in the summer hemisphere. The current tilt is 23 1/2 degrees.
As the earth travels around the sun its rotational axis wobbles much as a child's toy top wobbles as it slows down. As the earth wobbles, its axis sweeps out an imaginary cone in space. This is known as precession of the earth's axis. It takes 21,000 years to complete a cycle through which the rotational axis departs from and then returns to its original position. The climatic intensity of the precessional cycle varies in complex cycles with the orbital eccentricity, which has a cycle of 100,000 years.
The history of glaciation during the ice age as gleaned from studies of deep sea cores support the orbital control theory of climate change. Ice age sediments record orbital climatic influences. Biological productivity as recorded by the fossil record, the record of large mud deposits caused by melting of ice during interglacials, and terrigenous glacial deposits reflect orbital cycles.
Changes in the intensity of sunlight (solar output) may also cause or contribute to cyclic changes in the earth's climate. It is believed sunspots that occur periodically on the sun's surface may increase solar output slightly. Evidence includes the fact that the Maunder Minimum (a period of very few sunspots) from 1645 to 1723 occurred at the height of the Little Ice Age, a period in which temperatures in Europe were colder than at any time since the end of the last ice age. In addition, the little climatic optimum, a period of significant warming in Europe, which began in the year 1050 and lasted more than 100 years, began with an increase in sunspot activity.
Historical sun-spot cycles are at 11, 22, and 720 years. Recently, evidence of a regular 125-year cycle of warming and cooling temperatures, believed to result from a 125-year sunspot cycle, has been discovered.
Although the effect of sunspot cycles on climatic cycles remains theoretical, we now have satellite instrumentation in space which should eventually verify whether or not the sunspot climate theory is valid.