Throughout geological time, the Earth’s climate has varied drastically, primarily driven by through energy transfer from the sun to the Earth’s surface. This position and the interaction between the Earth and the sun (orbital variation) has varied over the Earth’s 4.6 billion year history, controlling the climate.
What is the Milankovitch theory?
The Milankovitch theory is used to explain the impacts of the Earth’s movements on its climate. It was developed by Serbian geophysicist and astronomer Milutin Milanković during the early 1900s. Using mathematical theory, he hypothesised that variations in the Earth’s orbit were the primary drivers of global climate patterns. These ‘orbital forcing’ mechanisms are centred on three parameters: eccentricity, obliquity, and precession (see below). These parameters interact with one another, each with a different periodicity (or cycle). The Milankovitch theory addresses the impacts of these three factors on the Earth’s climate.
Similar hypotheses were also developed much earlier, by James Croll and Joseph Adhémar during the 19th Century. However, at that time, geological dating methods were not sufficiently advanced to provide reliable dated evidence in support of the theory. In fact, despite the evidence presented by Milankovitch, it wasn’t until the publication of the pioneering work of Hays, Imbrie and Shackleton in 1976 that the Milankovitch theory attained widespread acceptance. Hays, Imbrie and Shackleton provided the first, high resolution, high precision record of long-term global environmental change using deep ocean cores. Their work, published in 1976, used oxygen isotope measurements from the tests (or shells) of deep sea organisms called foraminifera to track changes in global ice volume over the last 450,000 years. This was used as an indicator of global climate changes over this time period, and provided strong evidence in support of the Milankovitch theory.
Eccentricity (100,000 year cycle)
The term eccentricity refers to the shape of the Earth’s orbit, as a measure of the degree to which it departs from a circular shape. The orbit typically varies from near circular (low eccentricity: 0.005), to near elliptical (high eccentricity: 0.058). The mean eccentricity is 0.028, and at present is approximately 0.017. Changes in the orbital shape arise due to a combination of factors, which combine to produce a periodicity of change over approximately 100,000 years. The dominant influence on the shape of the orbit is the interaction with the gravitational fields of Jupiter and Saturn. The shape of the Earth’s orbit determines the amount of incoming solar radiation due to changes in the distance from the Earth to the sun. The shortest distance is termed the perihelion, the longest distance is the aphelion. When the orbit is at its most elliptical, the amount of solar radiation at perihelion is c.23% more than at aphelion.
Obliquity – the tilt of the axis (41,000 year cycle)
This term refers to the angle of the Earth’s axial tile in relation to its orbit. It is also referred to as the ‘obliquity of the ecliptic’. Obliquity varies from 22.1° to 24.5° (an angle of 2.4°) and back again, over a time period of approximately 41,000 years. When obliquity increases (i.e. the earth is tilted at a greater angle) there is a greater variation between Winter and Summer insolation – in Summer there is more solar radiation, and in Winter there is less. This solar radiation is not equally distributed, however, due to the shape of the Earth’s surface. With an increase in obliquity, high latitudes (towards the poles) receive an increase in insolation, while lower latitudes receive a decrease in insolation. The Earth is currently tilted at approximately 23.44°, so it is half way through its cycle of tilt from 24.5° to 22.1°.
Precession – the direction of the axis (26,000 year cycle)
Precession is the direction of the Earth’s rotation (or ‘wobble’) on its axis. This motion occurs due to the tidal forces exerted by the Moon and the Sun on the Earth.
The impacts of precession are largely felt at perihelion (when the Earth is at its closest proximity to the Sun) due to increased solar radiation. For example, when the North Pole is directed towards the Sun, the northern hemisphere receives much more insolation in Summer, and experiences a colder Winter. Meanwhile the southern hemisphere would experience milder seasons. In contrast, when the South Pole is directed at the Sun, it is the South Pole that experiences the larger seasonal variations. In its current state, perihelion is reached during the southern hemisphere summer, and aphelion is reached during the southern hemisphere winter. This is why the southern hemisphere often experiences greater seasonal extremes than the northern hemisphere.