Milankovitch cycles describe the changes in the way the earth orbits the sun. These changes define the sequence of ice ages and warm periods.
Introduction to Milankovitch Cycles
The concept of the Milankovich cycles was developed in the 1930s by the Serbian mathematician Milutin Milankovitch. They take account of three variations in the orbit of the earth around the sun.
- The earth’s orbit changes from being nearly circular to slightly elliptical (eccentricity). This cycle is affected by other planets in the solar system and has a period of around 100,000 years.
- The angle of tilt of the earth’s axis changes from 22.1° to 24.5° (obliquity). This cycle has a period of 41,000 years.
- The direction of the tilt of the axis changes (precession) on a cycle of 26,000 years.
These changes influence the length of the seasons and the amount of solar radiation received by the earth. It is generally considered that the radiation received during July at a latitude of 65°N is the most sensitive indicator. Figure 1
shows the variation in radiation at 65° N in July, at 65° N averaged over a whole year and over the whole globe. In the latter case the values were cosine weighted to allow for the fact the bands of latitude cover a smaller area the further one is from the equator. As can be seen the variations in radiation are very different in the three cases; an amplitude of 24% for July at 65°N, an amplitude of 13% for the whole year at 65°N and an amplitude of 0.7% for the whole earth annual radiation.