A Climate Change Primer: Solar and Orbital Variation

Published June 1, 2003

In Part One of this three-part series, Lehr and Bennett defined and described the “greenhouse effect”; summarized temperature observations and reported that average global temperature has increased roughly 0.6º C over the past 120 years; and described the methods used to measure temperature. In this Part Two, they explain how the Earth’s movement around the sun affects climate.

Weather varies from day to day or season to season, while climate is the long-term trend of temperature or rainfall. Climate may properly be considered the sum of a series of weather events.

In defining the tremendous impact the sun has on climate, one must understand the movement of the Earth around the sun. There are three variables–orbit shape, tilt, and wobble–that profoundly affect weather patterns.

The Earth’s orbit does not form a circle as it moves around the sun: It forms an ellipse passing further away from the sun at one end of the orbit than it does at the other end. During a 100,000-year cycle, the tug of other planets on the Earth causes its orbit to change shape. It shifts from a short broad ellipse that keeps the Earth closer to the sun, to a long flat ellipse that allows it to move farther from the sun and back again.

At the same time the Earth is orbiting, it also spins around an axis that tilts lower and then higher during a 41,000-year cycle. Close to the poles the contrast between winter and summer is greatest when the tilt is large.

The Earth wobbles because it is spinning around an axis that tilts back and forth. Thus temperatures drop in the Northern Hemisphere when it tilts away from the sun. Then the same things happens in the Southern Hemisphere, and again in the North in a 22,000-year cycle.

This means that about 11,000 years from now, the northern midwinter will fall in July instead of January, and the glaciers may return. It also means summer temperatures peak in the tropics twice as often as the concentrated heat of the sun passes back and forth across the equator.

Tens of Thousands of Years

The sun drives the climate. Even if the sun’s output of energy did not vary (but it does), the amount of sunlight reaching different areas of the Earth would still change because of the way the Earth moves around the sun.

Climate drives the ebb and flow of glaciers and vegetation. During cold periods, ice sheets spread and shrink within a 100,000-year cycle (orbital change); glaciers dominate the land for 60,000 to 90,000 years during the cold phase of the cycle, and then they all but disappear during the warm phase.

A Yugoslavian astrophysicist, Milutin Milankovitch, discovered the connection between cycles of Earth rotation and climate, called the Milankovitch Cycle. (1) Short warm gaps, or interglacials, break up glacial ages. These occur when summer temperatures rise in the north.

The last interglacial ended about 122,000 years ago. The interglacial in which we now live, the Holocene, began about 10,000 years ago and is approximately half over. The warming trend that melted the glaciers started much earlier, but it peaked about 8,000 years ago.

Magnetism and Radiation

Because many forces influence climate, temperatures do not rise and fall uniformly. Small cycles occur within larger cycles, producing cold or warm periods. Cycles in the energy output of the sun and shifting ocean currents contribute to these swings in the Earth’s temperature. The changes in the sun’s radiation are particularly important.

One natural factor in climate change may be variation in the brightness of the sun, over decades to centuries, that are in step with changes in the sun’s magnetism. These changes occur over cycles of roughly 11 years–what is known as the sunspot cycle. Climate models suggest changes of roughly 0.5 percent in the sun’s brightness would produce global average temperature changes of about 0.5º C over a century or so. (2)

The almost perfect correlation between the sun’s magnetic activity and the Earth’s temperature is too close to be readily dismissed as coincidence. (3) This magnetic activity is caused by strong magnetic fields that erupt on the sun’s surface in sunspots–bursts of energetic particles and radiation.

The changes in the surface magnetic fields do not in themselves transfer enough energy to the Earth and its atmosphere to have a direct impact on climate. However, satellite observations of the sun have shown that when its surface magnetic activity goes up, its energy output increases. When the surface magnetic activity diminishes, its energy output decreases. It is safe to conclude that energy output from the sun is the major factor in changes in global temperature.

In the July issue of Environment & Climate News: Computer models and the need for continued climate research.

Jay Lehr is science director for The Heartland Institute. Richard Bennett is president of The Society of Environmental Truth in Corpus Christi, Texas.


(1) J. Imbrie and K.P. Imbrie, Ice Ages, Solving the Mystery (Short Hills, NJ: Enslow Publishers, 1979).

(2) W. H. Soon, E.S. Posmentier, and S. Baliunas, “Inference of Solar Irradiance Variability from Terrestrial Temperature Changes, 1880-1993: An Astrophysical Application of the Sun-Climate Connection,” Astrophysical Journal 472, 891-902 (1996).

(3) D. Raymond, “Carbon Dioxide Content and Temperature,” Science 259, 926 (1993).