Orbital forcing: Precession

Even before humans started affecting it, climate on Earth was not always the same. Climate changes occur due to internal processes, such as air-sea interactions (see ENSO under Present-day climate), or external factors, such as the eruption of volcanoes (see above). On very long timescales, ranging from thousands to millions of years, climate variability is caused by variations in the orbit of the Earth around the Sun. There are three aspects of this orbit that change: obliquity (also called tilt), precession, and eccentricity.
Precession indicates where along its orbit around the Sun the Earth is at what time of the year. This changes with a period of approximately 21,000 years. Because the orbit of the Earth around the Sun is not entirely circular (but rather elliptic), the Earth will be closer to the Sun in part of the year. Precession therefore determines the strength of the seasonal cycle in each hemisphere. With the current precession, the Earth is closest to the Sun in Northern Hemisphere winter, and furthest from the Sun in Northern Hemisphere summer. This means that compared to the Southern Hemisphere, Northern Hemisphere summers are cooler and winters are warmer. The seasonal cycle is thus stronger in the Southern Hemisphere than in the Northern Hemisphere. Approximately 11,000 years ago this was exactly the other way around.
The animation shows monthly incoming solar radiation for two extreme values of precession, 90° and 270°. The first value, 90°, is similar to 11,000 years ago. The seasonal cycle is stronger in the Northern Hemisphere, which means that compared to the Southern Hemisphere there is more insolation in Summer and less in Winter. The second value, 270°, is similar to today, with the largest seasonal cycle occurring in the Southern Hemisphere. It is interesting to note that precession does not change annual mean insolation, only the strength of the seasonal cycle.

Download animated gif file
Download SOS/MP tar file