Orbital forcing: Eccentricity

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.
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. Eccentricity describes how elliptical exactly the orbit of the Earth around the Sun is. The more elliptical the orbit is, the larger the difference in insolation between being closer to the Sun and further away from it. The eccentricity changes on really long timescales, from 100,000 to 400,000 to 1.2 million years. The effect of eccentricity on annual mean insolation is negligible, but it plays an important role in modifying the strength of the precessional cycle. Precession is more effective when eccentricity is high.
This is illustrated in the animation, which shows the seasonal cycle of insolation for different values of eccentricity and precession. Note how the value of precession hardly affects the strength of the seasonal cycle when eccentricity is low (0.0002).

Download animated gif file
Download SOS/MP tar file