Seasonal Changes in Climate Are Caused by What?

Earth's Orbit Around the Sun

• It takes 365 and 1/4 days for Earth to complete its orbit around the sun. The path Earth takes is not a perfect circle, and in January of every year, Earth is closer to the sun than it is at other times of the year, a phenomenon known as perihelion. Because perihelion occurs during the northern hemisphere's winter season, northern winters are slightly milder than those in the south. Earth's orbit varies slightly over long periods of time, however and 11,000 years ago, perihelion occurred in July, causing more extreme variations between summer and winter seasons.

Earth's Tilted Axis

• Earth's axis, or the imaginary line connecting the North and South poles, is slightly tilted in relationship to its orbit at an angle of about 23 degrees. As Earth moves through its orbit, the hemisphere that is tilted toward the sun experiences summer, while it is winter in the hemisphere that is tilted away. If it weren't for this tilt, there would be no seasonal variations. This is because there would be no variations in the amount of atmospheric solar energy absorption throughout the year. Spring and fall occur when both the northern and southern hemispheres receive about the same amount of energy from the sun.

Solar Energy Distribution

• Variations in the intensity of solar energy are the fundamental cause of seasonal changes. When the northern hemisphere, for example, is angled away from the sun, the sunlight is scattered over a greater surface area than when it is angled toward the sun. When solar energy is spread out this way, that hemisphere absorbs less heat, causing winter. At the same time, in the southern hemisphere, the sunlight that reaches the surface is more concentrated, and more heat energy is absorbed per unit of area.

The Milankovitch Theory

• The severity of the seasons is determined by the tilt of Earth's axis, and because that tilt varies over time, Earth's climate slowly changes as a result. A Serbian astronomer named Milutin Milankovitch calculated the variations in Earth's orbit and its axis's tilt over a cycle of 41,000 years. The Milankovitch Theory says that these gradual changes, which are reinforced by feedback systems in Earth's climate, cause a cycle of ice ages and warmer periods.