Orbital forcing: Obliquity

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.
The axis of the Earth is at an angle with respect to the Sun. This angle is called the obliquity, and it causes the seasons: because of this angle we are closer to the Sun in summer, and further away in winter. At present the angle is 23.4°, but this value oscillates between 22.1° and 24.5° with a period of 41,000 years. If the angle increases, summers become warmer and winters become colder; in other words: the strength of the seasonal cycle increases.  At the same time, for high obliquity, high latitudes receive more solar radiation annually, while low latitudes receive less.
This animation first shows the effect of obliquity on annual mean insolation by showing the deviation from a medium obliquity value. Note how the high latitudes receive more insolation for high obliquity, while the low latitudes receive more for low obliquity. Next the animation shows the effect of obliquity on the strength of the seasonal cycle. Note how it is decreases as obliquity decreases.

Download animated gif file
Download SOS/MP tar file